Metabolic flexibility and insulin resistance

Insulin: Trigger and Target of Renal Functions

Kidney function in metabolism is often underestimated. Although the word “clearance” is associated to “degradation”, at nephron level, proper balance between what is truly degraded and what is redirected to de novo utilization is crucial for the maintenance of electrolytic and acid–basic balance and energy conservation. Insulin is probably one of the best examples of how diverse and heterogeneous kidney response can be. Kidney has a primary role in the degradation of insulin released in the bloodstream, but it is also incredibly susceptible to insulin action throughout the nephron. Fluctuations in insulin levels during fast and fed state add another layer of complexity in the understanding of kidney fine-tuning. This review aims at revisiting renal insulin actions and clearance and to address the association of kidney dysmetabolism with hyperinsulinemia and insulin resistance, both highly prevalent phenomena in modern society.

PPARδ and FOXO1 Mediate Palmitate-Induced Inhibition of Muscle Pyruvate Dehydrogenase Complex and CHO Oxidation, Events Reversed by Electrical Pulse Stimulation

The mechanisms behind the reduction in muscle pyruvate dehydrogenase complex (PDC)-controlled carbohydrate (CHO) oxidation during chronic high-fat dietary intake are poorly understood, as is the basis of CHO oxidation restoration during muscle contraction. C2C12 myotubes were treated with (300 μM) palmitate or without (control) for 16 h in the presence and absence of electrical pulse stimulation (EPS, 11.5 V, 1 Hz, 2 ms). Compared to control, palmitate reduced cell glucose uptake (p < 0.05), PDC activity (p < 0.01), acetylcarnitine accumulation (p < 0.05) and glucose-derived mitochondrial ATP production (p < 0.01) and increased pyruvate dehydrogenase kinase isoform 4 (PDK4) (p < 0.01), peroxisome proliferator-activated receptor alpha (PPARα) (p < 0.01) and peroxisome proliferator-activated receptor delta (PPARδ) (p < 0.01) proteins, and reduced the whole-cell p-FOXO1/t-FOXO1 (Forkhead Box O1) ratio (p < 0.01). EPS rescued palmitate-induced inhibition of CHO oxidation, reflected by increased glucose uptake (p < 0.01), PDC activity (p < 0.01) and glucose-derived mitochondrial ATP production (p < 0.01) compared to palmitate alone. EPS was also associated with less PDK4 (p < 0.01) and PPARδ (p < 0.01) proteins, and lower nuclear p-FOXO1/t-FOXO1 ratio normalised to the cytoplasmic ratio, but with no changes in PPARα protein. Collectively, these data suggest PPARδ, and FOXO1 transcription factors increased PDK4 protein in the presence of palmitate, which limited PDC activity and flux, and blunted CHO oxidation and glucose uptake. Conversely, EPS rescued these metabolic events by modulating the same transcription factors.

Sphingosine-1-Phosphate Metabolism in the Regulation of Obesity/Type 2 Diabetes

Obesity is a pathophysiological condition where excess free fatty acids (FFA) target and promote the dysfunctioning of insulin sensitive tissues and of pancreatic β cells. This leads to the dysregulation of glucose homeostasis, which culminates in the onset of type 2 diabetes (T2D). FFA, which accumulate in these tissues, are metabolized as lipid derivatives such as ceramide, and the ectopic accumulation of the latter has been shown to lead to lipotoxicity. Ceramide is an active lipid that inhibits the insulin signaling pathway as well as inducing pancreatic β cell death. In mammals, ceramide is a key lipid intermediate for sphingolipid metabolism as is sphingosine-1-phosphate (S1P). S1P levels have also been associated with the development of obesity and T2D. In this review, the current knowledge on S1P metabolism in regulating insulin signaling in pancreatic β cell fate and in the regulation of feeding by the hypothalamus in the context of obesity and T2D is summarized. It demonstrates that S1P can display opposite effects on insulin sensitive tissues and pancreatic β cells, which depends on its origin or its degradation pathway.

Differential changes to splanchnic and peripheral protein metabolism during the diet-induced development of metabolic syndrome in rats

Little is known about the effects of the development of metabolic syndrome (MS) on protein and amino acid (AA) metabolism. During this study, we took advantage of the variability in interindividual susceptibility to high fat diet-induced MS to study the relationships between MS, protein synthesis, and AA catabolism in multiple tissues in rats. After 4 mo of high-fat feeding, an MS score (Z_MS) was calculated as the average of the z-scores for individual MS components [weight, adiposities, homeostasis model for the assessment of insulin resistance (HOMA-IR), and triglycerides]. In the small intestine, liver, plasma, kidneys, heart, and muscles, tissue protein synthesis was measured by ²H₂O labeling, and we evaluated the proportion of tissue AA catabolism (relative to protein synthesis) and nutrient routing to nonindispensable AAs in tissue proteins using natural nitrogen and carbon isotopic distances between tissue proteins and nutrients (Δ¹⁵N and Δ¹³C), respectively. In the liver, protein mass and synthesis increased, whereas the proportion of AA catabolism decreased with Z_MS. By contrast, in muscles, we found no association between Z_MS and protein mass, protein synthesis (except for a weak positive association in the gastrocnemius muscle only), and proportion of AA catabolism. The development of MS was also associated with altered metabolic flexibility and fatty acid oxidation, as shown by less routing of dietary lipids to nonindispensable AA synthesis in liver and muscle. In conclusion, MS development is associated with a greater gain of both fat and protein masses, with higher protein anabolism that mainly occurs in the liver, whereas muscles probably develop anabolic resistance due to insulin resistance.

Sleep-Disordered Breathing and Free Fatty Acid Metabolism

BACKGROUND

Sleep-disordered breathing (SDB) is independently associated with insulin resistance, glucose intolerance, and type 2 diabetes mellitus. However, data on whether SDB alters the metabolism of free fatty acids (FFAs) are lacking.

RESEARCH QUESTION

The primary objective of the current study was to characterize alterations in FFA metabolism across the spectrum of SDB severity.

STUDY DESIGN AND METHODS

The study sample included 118 participants with and without SDB who underwent full-montage polysomnography, the frequently sampled IV glucose tolerance test (FSIGTT), and body composition measurements including determination of percent body fat. Parameters of lipolysis suppression, time to FFA nadir, and FFA rebound after an IV glucose challenge were derived using a mathematical model. Multivariable regression analyses were used to characterize the independent associations between SDB severity and parameters of FFA metabolism.

RESULTS

SDB severity, as assessed by the apnea-hypopnea index, was associated with adipocyte insulin resistance, a decrease in the glucose- and insulin-mediated suppression of lipolysis, a longer duration to reach a nadir in FFA levels during the FSIGTT, and a sluggish rebound in FFA levels after suppression. Severity of SDB-related hypoxemia was independently associated with adipocyte insulin resistance and the time to reach the FFA nadir during the FSIGTT. Finally, a higher percentage of stage N3 sleep was positively associated with greater suppression of lipolysis and a faster rebound in the FFA levels during the FSIGTT.

INTERPRETATION

Independent of adiposity, SDB is associated with impairments in FFA metabolism, which may contribute to the development of glucose intolerance and type 2 diabetes in SDB.

Mitochondrial Utilization of Competing Fuels Is Altered in Insulin Resistant Skeletal Muscle of Non-obese Rats (Goto-Kakizaki)

Aim

Insulin-resistant skeletal muscle is characterized by metabolic inflexibility with associated alterations in substrate selection, mediated by peroxisome-proliferator activated receptor δ (PPARδ). Although it is established that PPARδ contributes to the alteration of energy metabolism, it is not clear whether it plays a role in mitochondrial fuel competition. While nutrient overload may impair metabolic flexibility by fuel congestion within mitochondria, in absence of obesity defects at a mitochondrial level have not yet been excluded. We sought to determine whether reduced PPARδ content in insulin-resistant rat skeletal muscle of a non-obese rat model of T2DM (Goto-Kakizaki, GK) ameliorate the inhibitory effect of fatty acid (i.e., palmitoylcarnitine) on mitochondrial carbohydrate oxidization (i.e., pyruvate) in muscle fibers.

Methods

Bioenergetic function was characterized in oxidative soleus (S) and glycolytic white gastrocnemius (WG) muscles with measurement of respiration rates in permeabilized fibers in the presence of complex I, II, IV, and fatty acid substrates. Mitochondrial content was measured by citrate synthase (CS) and succinate dehydrogenase activity (SDH). Western blot was used to determine protein expression of PPARδ, PDK isoform 2 and 4.

Results

CS and SDH activity, key markers of mitochondrial content, were reduced by ∼10-30% in diabetic vs. control, and the effect was evident in both oxidative and glycolytic muscles. PPARδ (p < 0.01), PDK2 (p < 0.01), and PDK4 (p = 0.06) protein content was reduced in GK animals compared to Wistar rats (N = 6 per group). Ex vivo respiration rates in permeabilized muscle fibers determined in the presence of complex I, II, IV, and fatty acid substrates, suggested unaltered mitochondrial bioenergetic function in T2DM muscle. Respiration in the presence of pyruvate was higher compared to palmitoylcarnitine in both animal groups and fiber types. Moreover, respiration rates in the presence of both palmitoylcarnitine and pyruvate were reduced by 25 ± 6% (S), 37 ± 6% (WG) and 63 ± 6% (S), 57 ± 8% (WG) compared to pyruvate for both controls and GK, respectively. The inhibitory effect of palmitoylcarnitine on respiration was significantly greater in GK than controls (p < 10^-3).

Conclusion

With competing fuels, the presence of fatty acids diminishes mitochondria ability to utilize carbohydrate derived substrates in insulin-resistant muscle despite reduced PPARδ content.

Direct Relationship Between Metabolic Flexibility Measured During Glucose Clamp and Prolonged Fast in Men

OBJECTIVE

This study aimed to determine the relationship between metabolic flexibility (MetFlex) measured during a euglycemic-hyperinsulinemic clamp and a prolonged fast. This study also analyzed the association between MetFlex and metabolic health.

METHODS

Eighteen healthy men (mean [SD]: 22 [2] years old; BMI: 22 [1] kg/m² ) performed two sessions: (1) euglycemic-hyperinsulinemic clamp (2 mIU/kg of insulin per minute) and (2) ~20-hour fast. Clamp MetFlex corresponded to the change in (Δ) respiratory quotient (RQ) (ΔRQ = postchallenge RQ - prechallenge RQ) adjusted for M value and prechallenge RQ. Prolonged fast MetFlex corresponded to the ΔRQ adjusted for the Δβ-hydroxybutyrate and prechallenge RQ.

RESULTS

MetFlex during the clamp related directly with MetFlex during prolonged fast (r = 0.59, P = 0.014). Using the median of MetFlex for each challenge, this study split participants into high or low MetFlex. Participants with high or low MetFlex to both challenges were identified. Participants with high MetFlex had 3% lower serum low-density lipoprotein cholesterol than participants with low MetFlex (P = 0.021).

CONCLUSIONS

Measuring MetFlex during a clamp or a prolonged fast produces similar results, despite challenging the oxidation of different substrates. An impaired MetFlex in response to these challenges may be an early event in the development of abnormal lipid metabolism.

Variance in respiratory quotient among daily activities and its association with obesity status

BACKGROUND/OBJECTIVES

The ability to adapt the level of daily fuel utilization to substrate availability is given by the respiratory quotient (RQ), which its variance is an indicator of metabolic flexibility. Metabolic inflexibility is associated with many pathologies including obesity, but evidence relies on bed-rest studies and exercise-based interventions. Our goal was to examine the associations for RQ variance in response to daily living activities with obesity, in healthy adults.

SUBJECTS/METHODS

Participants (N = 50; 25 women), 20-64 years were lying for 60 min and randomly performed three conditions for 10 min each (sitting, standing, 1 sit/stand/sit transition min^-1). RQ was measured by indirect calorimetry and fat mass (FM), trunk FM (TFM), and fat-free mass (FFM) by DXA.

RESULTS

RQ variance among the three conditions was inversely associated with BMI (ß = 0.005; p < 0.001), FM (ß = 0.007; p = 0.024), and TFM (ß = 0.008; p = 0.026). A positive association was found between RQ variance and FFM (ß = -0.007; p = 0.024). No interactions for sex were found (p ≥ 0.05).

CONCLUSIONS

These findings suggest that a higher RQ variance in response to daily living metabolic challenges such as transitioning between sitting and standing is associated with lower overall and central obesity, as well as with a higher FFM, in healthy adults. Thus, RQ variance may work as an indication of metabolic flexibility, but these findings were obtained in a young and non-obese adult population without considering their fitness levels. Thus, further research in this field is warranted.

Cardiorespiratory Fitness, Physical Activity, and Insulin Resistance in Children

PURPOSE

Few studies have investigated the independent and joint associations of cardiorespiratory fitness (CRF) and body fat percentage (BF%) with insulin resistance in children. We investigated the independent and combined associations of CRF and BF% with fasting glycemia and insulin resistance and their interactions with physical activity (PA) and sedentary time among 452 children age 6 to 8 yr.

METHODS

We assessed CRF with a maximal cycle ergometer exercise test and used allometrically scaled maximal power output (Wmax) for lean body mass (LM) and body mass (BM) as measures of CRF. The BF% and LM were measured by dual-energy X-ray absorptiometry, fasting glycemia by fasting plasma glucose, and insulin resistance by fasting serum insulin and Homeostatic Model Assessment for Insulin Resistance (HOMA-IR). The PA energy expenditure, moderate-to-vigorous PA (MVPA), and sedentary time were assessed by combined movement and heart rate sensor.

RESULTS

Wmax/LM was not associated with glucose (β = 0.065, 95% confidence interval [CI] = -0.031 to 0.161), insulin (β = -0.079, 95% CI = -0.172 to 0.015), or HOMA-IR (β = -0.065, 95% CI = -0.161 to 0.030). Wmax/BM was inversely associated with insulin (β = -0.289, 95% CI = -0.377 to -0.200) and HOMA-IR (β = -0.269, 95% CI = -0.359 to -0.180). The BF% was directly associated with insulin (β = 0.409, 95% CI = 0.325 to 0.494) and HOMA-IR (β = 0.390, 95% CI = 0.304 to 0.475). Higher Wmax/BM, but not Wmax/LM, was associated with lower insulin and HOMA-IR in children with higher BF%. Children with higher BF% and who had lower levels of MVPA or higher levels of sedentary time had the highest insulin and HOMA-IR.

CONCLUSIONS

Children with higher BF% together with less MVPA or higher levels of sedentary time had the highest insulin and HOMA-IR. Cardiorespiratory fitness appropriately controlled for body size and composition using LM was not related to insulin resistance among children.

Postprandial skeletal muscle metabolism following a high-fat diet in sedentary and endurance-trained males

Our objective was to determine the influence of a high-fat diet (HFD) on fasting and postprandial skeletal muscle substrate metabolism in endurance-trained (ET) compared with sedentary (SED) humans. SED (n = 17) and ET (n = 7) males were control-fed a 10-day moderate-fat diet followed by a 5-day isocaloric HFD (55% fat, 30% carbohydrate). Skeletal muscle biopsies were taken in the fasted condition and 4 h after a high-fat meal (820 kcals; 63% fat and 25% carbohydrate). Palmitate-induced suppression of pyruvate oxidation, an indication of substrate preference, and oxidation of fat and glucose were measured in homogenized skeletal muscle in fasted and fed states. Postprandial responses were calculated as percent changes from fasting to fed states. Postprandial suppression of pyruvate oxidation was maintained after the HFD in ET, but not SED skeletal muscle, suggesting greater adaptability to dietary intake changes in the former. Fasting total fat oxidation increased due to the HFD in ET skeletal muscle (P = 0.006), which was driven by incomplete fat oxidation (P = 0.008). Fasting fat oxidation remained unchanged in skeletal muscle of SED individuals. Yet, postprandial fat oxidation was similar between groups. Fasting glucose oxidation was elevated after the HFD in ET (P = 0.036), but not SED, skeletal muscle. Postprandial glucose oxidation was reduced due to the HFD in SED (P = 0.002), but not ET, skeletal muscle. These findings provide insight into differing substrate metabolism responses between SED and ET individuals and highlight the role that the prevailing diet may play in modulating fasting and postprandial metabolic responses in skeletal muscle.NEW & NOTEWORTHY The relationship between high dietary fat intake and physical activity level and their combined effect on skeletal muscle substrate metabolism remains unclear. We assessed the influence of the prevailing diet in modulating substrate oxidation in skeletal muscle of endurance-trained compared with sedentary humans during a high-fat challenge meal. Collectively, our findings demonstrate the adaptability of skeletal muscle in endurance-trained individuals to high dietary fat intake.

Decreased Metabolic Flexibility in Skeletal Muscle of Rat Fed with a High-Fat Diet Is Recovered by Individual CLA Isomer Supplementation via Converging Protective Mechanisms

Energy balance, mitochondrial dysfunction, obesity, and insulin resistance are disrupted by metabolic inflexibility while therapeutic interventions are associated with improved glucose/lipid metabolism in skeletal muscle. Conjugated linoleic acid mixture (CLA) exhibited anti-obesity and anti-diabetic effects; however, the modulatory ability of its isomers (cis9, trans11, C9; trans10, cis12, C10) on the metabolic flexibility in skeletal muscle remains to be demonstrated. Metabolic inflexibility was induced in rat by four weeks of feeding with a high-fat diet (HFD). At the end of this period, the beneficial effects of C9 or C10 on body lipid content, energy expenditure, pro-inflammatory cytokines, glucose metabolism, and mitochondrial efficiency were examined. Moreover, oxidative stress markers, fatty acids, palmitoyletanolamide (PEA), and oleyletanolamide (OEA) contents along with peroxisome proliferator-activated receptors-alpha (PPARα), AKT, and adenosine monophosphate-activated protein kinase (AMPK) expression were evaluated in skeletal muscle to investigate the underlying biochemical mechanisms. The presented results indicate that C9 intake reduced mitochondrial efficiency and oxidative stress and increased PEA and OEA levels more efficiently than C10 while the anti-inflammatory activity of C10, and its regulatory efficacy on glucose homeostasis are associated with modulation of the PPARα/AMPK/pAKT signaling pathway. Our results support the idea that the dissimilar efficacy of C9 and C10 against the HFD-induced metabolic inflexibility may be consequential to their ability to activate different molecular pathways.

Impaired Metabolic Flexibility in the Osteoarthritis Process: A Study on Transmitochondrial Cybrids

Osteoarthritis (OA) is the most frequent joint disease; however, the etiopathogenesis is still unclear. Chondrocytes rely primarily on glycolysis to meet cellular energy demand, but studies implicate impaired mitochondrial function in OA pathogenesis. The relationship between mitochondrial dysfunction and OA has been established. The aim of the study was to examine the differences in glucose and Fatty Acids (FA) metabolism, especially with regards to metabolic flexibility, in cybrids from healthy (N) or OA donors. Glucose and FA metabolism were studied using D-[¹⁴C(U)]glucose and [1-¹⁴C]oleic acid, respectively. There were no differences in glucose metabolism among the cybrids. Osteoarthritis cybrids had lower acid-soluble metabolites, reflecting incomplete FA β-oxidation but higher incorporation of oleic acid into triacylglycerol. Co-incubation with glucose and oleic acid showed that N but not OA cybrids increased their glucose metabolism. When treating with the mitochondrial inhibitor etomoxir, N cybrids still maintained higher glucose oxidation. Furthermore, OA cybrids had higher oxidative stress response. Combined, this indicated that N cybrids had higher metabolic flexibility than OA cybrids. Healthy donors maintained the glycolytic phenotype, whereas OA donors showed a preference towards oleic acid metabolism. Interestingly, the results indicated that cybrids from OA patients had mitochondrial impairments and reduced metabolic flexibility compared to N cybrids.

Association between sleep quality and time with energy metabolism in sedentary adults

The aim of the present study was to investigate the relationship of sleep quality and time with basal metabolic rate (BMR) and fuel oxidation in basal conditions and during exercise in sedentary middle-aged adults. We also studied the mediation role of dietary intake and adherence to the traditional Mediterranean Diet in the relationship between sleep parameters and energy metabolism parameters.A secondary analysis of the FIT-AGEING study was undertaken. 70 middle-aged sedentary adults (40-65 years old) participated in the present study. Sleep quality was assessed using the Pittsburgh Sleep Quality Index (PSQI) and wrist accelerometers (ActiSleep, Actigraph, Pensacola, Florida, USA) for 7 consecutive days. BMR was measured with indirect calorimetry and fuel oxidation was estimated through stoichiometric equations. Maximal fat oxidation was determined by a walking graded exercise test and dietary intake with 24 h recalls. Adherence to the traditional Mediterranean diet was assessed through the PREDIMED questionnaire. PSQI global score (poor sleep quality) was associated with lower basal fat oxidation (BFox), both expressed in g/min and as a percentage of BMR, independently of confounders. We did not find any association between other sleep and energy metabolism parameters. No mediating role of the dietary intake or PREDIMED global score was observed in the association of PSQI and BFox. In conclusion, our study showed that a subjective poor sleep quality was associated with lower BFox, which is not mediated by dietary intake in sedentary adults.

Metabolic flexibility is impaired in women who are pregnant and overweight/obese and related to insulin resistance and inflammation

CONTEXT

Maternal obesity is a significant public health concern that contributes to unfavorable outcomes such as inflammation and insulin resistance. Women with obesity may have impaired metabolic flexibility (i.e. an inability to adjust substrate metabolism according to fuel availability). Impaired metabolic flexibility during pregnancy may mediate poor pregnancy outcomes in women with obesity.

PURPOSE

The purposes of this study were to: 1) compare metabolic flexibility between overweight/obese and lean women; and 2) determine the relationships between metabolic flexibility, inflammation following a high-fat meal, and maternal metabolic health outcomes (i.e. gestational weight gain and insulin resistance).

PROCEDURES

This interventional physiology study assessed lipid oxidation rates via indirect calorimetry before and after consumption of a high-fat meal. The percent change in lipid metabolism was calculated to determine 'metabolic flexibility.' Maternal inflammatory profiles (CRP, IL-6, IL-8, IL-10, IL-12, TNF-α) and insulin resistance (HOMA-IR) were determined via plasma analyses.

MAIN FINDINGS

64 women who were pregnant (lean = 35, overweight/obese = 29) participated between 32 and 38 weeks gestation. Lean women had significantly higher metabolic flexibility compared to overweight/obese women (lean 48.0 ± 34.1% vs overweight/obese 29.3 ± 34.3%, p = .035). Even when controlling for pre-pregnancy BMI, there was a negative relationship between metabolic flexibility and percent change in CRP among the overweight/obese group (r = -0.526, p = .017). Metabolic flexibility (per kg fat free mass) was negatively correlated with postprandial HOMA-IR (2 h: r = -0.325, p = .016; 4 h: r = -0.319, p = .019).

CONCLUSIONS

Overweight and obese women who are pregnant are less 'metabolically flexible' than lean women, and this is related to postprandial inflammation and insulin resistance.

Nutrition and Nonalcoholic Fatty Liver Disease: Current Perspectives

Nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis are diseases in their own right as well as modifiable risk factors for cardiovascular disease and type 2 diabetes. With expanding knowledge on NAFLD pathogenesis, insights have been gleaned into molecular targets for pharmacologic and nonpharmacologic approaches. Lifestyle modifications constitute a cornerstone of NAFLD management. This article reviews roles of key dietary macronutrients and micronutrients in NAFLD pathogenesis and their effects on molecular targets shared with established or emerging pharmacotherapies. Based on current evidence, a recommendation for a dietary framework as part of the comprehensive management strategy for NAFLD is provided.

No Influence of Overweight/Obesity on Exercise Lipid Oxidation: A Systematic Review

Compared to lean counterparts, overweight/obese individuals rely less on lipid during fasting. This deficiency has been implicated in the association between overweight/obesity and blunted insulin signaling via elevated intramuscular triglycerides. However, the capacity for overweight/obese individuals to use lipid during exercise is unclear. This review was conducted to formulate a consensus regarding the influence of overweight/obesity on exercise lipid use. PubMed, ProQuest, ISI Web of Science, and Cochrane Library databases were searched. Articles were included if they presented original research on the influence of overweight/obesity on exercise fuel use in generally healthy sedentary adults. Articles were excluded if they assessed older adults, individuals with chronic disease, and/or exercise limitations or physically-active individuals. The search identified 1205 articles with 729 considered for inclusion after duplicate removal. Once titles, abstracts, and/or manuscripts were assessed, 24 articles were included. The preponderance of evidence from these articles indicates that overweight/obese individuals rely on lipid to a similar extent during exercise. However, conflicting findings were found in eight articles due to the outcome measure cited, participant characteristics other than overweight/obesity and characteristics of the exercise bout(s). We also identified factors other than body fatness which can influence exercise lipid oxidation that should be controlled in future research.

Metabolic rate in sedentary adults, following different exercise training interventions: The FIT-AGEING randomized controlled trial

BACKGROUND & AIMS

This study compares the influence of different exercise training programs on basal metabolic rate (BMR) and fat oxidation, in basal conditions (BFox) and during exercise (MFO), in sedentary, middle-aged adults.

METHODS

The study subjects of this 12 week-long, randomised controlled trial, were 71 middle-aged adults (age 53.5 ± 4.9 years; 52% women). Subjects were randomly assigned to one of the following groups: (1) no exercise, (2) concurrent training based on international physical activity recommendations (PAR group), (3) high intensity interval training (HIIT group), and (4) high intensity interval training plus whole-body electromyostimulation (HIIT + EMS group). Subject BMR, BFox and MFO were determined by indirect calorimetry before and after the intervention.

RESULTS

The HIIT + EMS subjects showed significant increases in BFox following the intervention compared with the control group (all P = 0.043); no such differences were seen in the PAR and HIIT compared with the control group (all P ≥ 0.1). A significant increase in post-intervention MFO was noted for the HIIT and HIIT + EMS group compared to the non-exercise control group (P < 0.05); no such difference was seen in the PAR group compared to the control group (all P ≥ 0.05).

CONCLUSIONS

Twelve weeks of high intensity interval training plus whole-body electromyostimulation may increase the BFox and MFO of middle-aged sedentary adults. These findings have important clinical implications; a well-designed high-intensity interval training program plus whole-body electromyostimulation might be followed to help combat the appearance of chronic metabolic diseases characterized by metabolic inflexibility in middle-aged sedentary adults, though it will be necessary to determine how long the effects last.

Inflexibility of the plasma miRNA response following a high-carbohydrate meal in overweight insulin-resistant women

Context

Metabolic inflexibility is a characteristic of insulin resistance, limiting the ability to transiently regulate oxidative metabolism and gene expression in response to nutrient availability. Little is known of the flexibility of post-transcriptional regulation, including circulatory miRNAs (c-miRNAs).

Design

The abundances of targeted c-miRNAs, with reported functions in metabolic regulation, were analysed in response to a high-carbohydrate meal in healthy weight insulin-sensitive (IS) and overweight insulin-resistant (IR) women.

Participants

Age-matched healthy weight IS (n = 20, BMI = 24.3 ± 0.70) and overweight IR (n = 20, BMI = 28.6 ± 0.67) women.

Methods

An abundance of c-miRNAs was quantified prior to and following a high-carbohydrate breakfast meal (2500 kJ; 50% carbohydrate, 20% fat and 27% protein). Target genes of the differentially regulated c-miRNA were measured in RNA extracted from circulatory peripheral blood mononuclear cells (PBMCs).

Results

In healthy weight IS women, both miR-15a-5p (p = 0.03) and miR-17-5p (p < 0.01) levels were halved at 4 h post-meal. These miRNA remained unaltered following the same meal in the overweight IR women. Furthermore, amongst genes targeted by these miRNA, CPT1A (p = 0.01) and IL8 (p = 0.03) had also reduced expression 4 h post-meal only in the healthy weight IS women.

Conclusions

The study findings provide preliminary evidence for a possible extension of metabolic inflexibility to include c-miRNAs.

Trial registration

The clinical trial is registered with Australian New Zealand Clinical Trials Registry under Trial registration: ANZCTR: ACTRN12615001108505. Registered on 21 October 2015.

Impaired Metabolic Flexibility to High-Fat Overfeeding Predicts Future Weight Gain in Healthy Adults

The ability to switch fuels for oxidation in response to changes in macronutrient composition of diet (metabolic flexibility) may be informative of individuals' susceptibility to weight gain. Seventy-nine healthy, weight-stable participants underwent 24-h assessments of energy expenditure and respiratory quotient (RQ) in a whole-room calorimeter during energy balance (EBL) (50% carbohydrate, 30% fat) and then during 24-h fasting and three 200% overfeeding diets in a crossover design. Metabolic flexibility was defined as the change in 24-h RQ from EBL during fasting and standard overfeeding (STOF) (50% carbohydrate, 30% fat), high-fat overfeeding (HFOF) (60% fat, 20% carbohydrate), and high-carbohydrate overfeeding (HCOF) (75% carbohydrate, 5% fat) diets. Free-living weight change was assessed after 6 and 12 months. Compared with EBL, RQ decreased on average by 9% during fasting and by 4% during HFOF but increased by 4% during STOF and by 8% during HCOF. A smaller decrease in RQ, reflecting a smaller increase in lipid oxidation rate, during HFOF but not during the other diets predicted greater weight gain at both 6 and 12 months. An impaired metabolic flexibility to acute HFOF can identify individuals prone to weight gain, indicating that an individual's capacity to oxidize dietary fat is a metabolic determinant of weight change.

Effects of a whole diet approach on metabolic flexibility, insulin sensitivity and postprandial glucose responses in overweight and obese adults - A randomized controlled trial

BACKGROUND & AIMS

Metabolic flexibility is the ability to adapt fuel oxidation to fuel availability. Metabolic inflexibility has been associated with obesity, the metabolic syndrome and insulin resistance, and can be improved by exercise or weight loss. Dietary changes can modulate metabolic flexibility; however, the effect of a whole diet approach on metabolic flexibility has never been studied. Therefore, our objective was to assess the effect of a healthy diet (HD), as compared to a typical Western diet (WD), on several fasting and postprandial markers of metabolic flexibility and insulin sensitivity.

METHODS

In this parallel randomized trial, overweight or obese men and women (50-70 years; BMI 25-35 kg/m²) consumed a healthy diet (HD; high in fruits and vegetables, pulses, fibers, nuts, fatty fish, and low in high-glycemic carbohydrates; n = 19) or a typical Western diet (WD; n = 21) for six weeks, following a two-week run-in period. The change in respiratory quotient upon insulin stimulation (ΔRQ), and insulin sensitivity, expressed as the M-value, were both determined with a hyperinsulinemic euglycemic clamp. Additionally, other fasting and postprandial markers of metabolic flexibility were assessed during a 5-h high-fat high-glycemic mixed meal challenge.

RESULTS

ΔRQ (p = 0.730) and insulin sensitivity (p = 0.802) were not significantly affected by diet. Postprandial RQ did also not show significant differences (p = 0.610), whereas postprandial glucose excursions were significantly higher in the HD group at T30 (p = 0.014) and T45 (p = 0.026) after mixed meal ingestion (p = 0.037). Fasting glucose (p = 0.530) and HbA1c (p = 0.124) remained unchanged, whereas decreases in fasting insulin (p = 0.038) and the HOMA-IR (p = 0.050) were significantly more pronounced with the HD.

CONCLUSION

A healthy diet for six weeks, without further life-style changes, did not improve metabolic flexibility and whole-body insulin sensitivity, when compared to a Western-style diet. It remains to be determined whether the short time increase in postprandial glucose is physiologically relevant or detrimental to metabolic health. This trial was registered at clinicaltrials.gov as NCT02519127.

Aberrant PDK4 Promoter Methylation Preceding Hyperglycemia in a Mouse Model

Diabetic prevalence is at speedy increase globally. Previous studies stated that other than genetics, factors such as environment, lifestyle, and paternal-maternal condition play critical roles in diabetes through DNA methylation in specific areas of the genome. The purpose of this study is to investigate the methylation pattern of the PDK4 promoter in streptozotocin-induced diabetic mice until the 12th week of the observation. The methylation pattern in the blood samples was analyzed periodically, while the pattern in the muscle sample was only analyzed at the end of the experiment using the blood of the sacrificed animals. Three methylated CpG site 1, CpG site 6, and CpG site 7 were analyzed and quantified based on the band density using bisulfite treatment and methylation-specific polymerase chain reaction (PCR). The hyperglycemia period was developed at the 9th week of experiment. However, there was a significant increase of methylation, specifically on CpG site 6 started from week 6 to week 12. This peculiar methylation on CpG site 6 of PDK4 promoter in the blood sample before the hyperglycemic period might serve as a potential biomarker for early detection of diabetes in the patients. No significant difference was found between the methylation level of streptozotocin (STZ)-treated mice and of the control group in the muscle sample.

High Dose of Dietary Nicotinamide Riboside Induces Glucose Intolerance and White Adipose Tissue Dysfunction in Mice Fed a Mildly Obesogenic Diet

Nicotinamide riboside (NR) is a nicotinamide adenine dinucleotide (NAD⁺) precursor vitamin. The scarce reports on the adverse effects on metabolic health of supplementation with high-dose NR warrant substantiation. Here, we aimed to examine the physiological responses to high-dose NR supplementation in the context of a mildly obesogenic diet and to substantiate this with molecular data. An 18-week dietary intervention was conducted in male C57BL/6JRccHsd mice, in which a diet with 9000 mg NR per kg diet (high NR) was compared to a diet with NR at the recommended vitamin B3 level (control NR). Both diets were mildly obesogenic (40 en% fat). Metabolic flexibility and glucose tolerance were analyzed and immunoblotting, qRT-PCR and histology of epididymal white adipose tissue (eWAT) were performed. Mice fed with high NR showed a reduced metabolic flexibility, a lower glucose clearance rate and aggravated systemic insulin resistance. This was consistent with molecular and morphological changes in eWAT, including sirtuin 1 (SIRT1)-mediated PPARγ (proliferator-activated receptor γ) repression, downregulated AKT/glucose transporter type 4 (GLUT4) signaling, an increased number of crown-like structures and macrophages, and an upregulation of pro-inflammatory gene markers. In conclusion, high-dose NR induces the onset of WAT dysfunction, which may in part explain the deterioration of metabolic health.

Low capacity to oxidize fat and body weight

For a given positive energy balance, a low capacity to oxidize fat could contribute to weight gain (low fat oxidation hypothesis). This hypothesis is based on the arguments that for a given stable diet and food quotient (FQ), the respiratory quotient (RQ) is higher in obesity prone (OP) than in obesity resistant individuals (OR) and that a high RQ predicts higher future weight gain. A review of 42 studies shows that there is no convincing experimental support to these arguments and thus for the low fat oxidation hypothesis. A power analysis also shows that this hypothesis might be impossible to experimentally confirm because very large numbers of subjects would be needed to reject the null hypotheses that the 24-h RQ is not different in OP and OR or that future weight gain is not different in individuals with a low and high 24-h RQ at baseline. A re-examination of the significance of the 24-hour and fasting RQ also shows that the assumption underlying the low fat oxidation hypothesis that a high RQ reflects a low capacity to oxidize fat is not valid: For a stable diet, the 24-h RQ entirely depends on FQ and energy balance, and the fasting RQ mainly depends on the FQ and energy balance and on the size of glycogen stores.

Changes in substrate utilization rates during 40 min of walking within the Fatmax range

BACKGROUND AND AIMS

The aim of this study was to evaluate changes in fat oxidation rate during 40 min of continuous exercise and identify the intensity at the highest fat oxidation rate (Fatmax).

METHODS

A total of 14 sedentary males with age, body height, weight, and BMI averages of 29.3 ± 0.7 years, 178.3 ± 1.7 cm, 81.1 ± 3.9 kg, and 25.4 ± 0.9 kg/m², respectively, were included in the study. Fatmax was determined using an indirect calorimeter with an incremental treadmill walking test at least after 12 h of fasting. On a separate day, at least after 12 h of fasting, the participants walked for 40 min within their predetermined individual Fatmax heart rate and speed ranges.

RESULTS

The initial fat oxidation rate was not sustained within the first 16 min of exercise and was reduced; however, carbohydrate oxidation reached a stable level after nearly 10 min.

CONCLUSIONS

In sedentary individuals, during low-intensity physical activity, fat oxidation rates may not be sustainable as expected from Fatmax testing. Therefore, when exercise is prescribed, one should consider that the fat oxidation rate might decrease in sedentary overweight individuals.

A mathematical model of calcium dynamics: Obesity and mitochondria-associated ER membranes

Multiple cellular organelles tightly orchestrate intracellular calcium (Ca²⁺) dynamics to regulate cellular activities and maintain homeostasis. The interplay between the endoplasmic reticulum (ER), a major store of intracellular Ca²⁺, and mitochondria, an important source of adenosine triphosphate (ATP), has been the subject of much research, as their dysfunction has been linked with metabolic diseases. Interestingly, throughout the cell’s cytosolic domain, these two organelles share common microdomains called mitochondria-associated ER membranes (MAMs), where their membranes are in close apposition. The role of MAMs is critical for intracellular Ca²⁺ dynamics as they provide hubs for direct Ca²⁺ exchange between the organelles. A recent experimental study reported correlation between obesity and MAM formation in mouse liver cells, and obesity-related cellular changes that are closely associated with the regulation of Ca²⁺ dynamics. We constructed a mathematical model to study the effects of MAM Ca²⁺ dynamics on global Ca²⁺ activities. Through a series of model simulations, we investigated cellular mechanisms underlying the altered Ca²⁺ dynamics in the cells under obesity. We predict that, as the dosage of stimulus gradually increases, liver cells from obese mice will reach the state of saturated cytosolic Ca²⁺ concentration at a lower stimulus concentration, compared to cells from healthy mice.

It is well known that intracellular Ca²⁺ oscillations carry encoded signals in their amplitude and frequency to regulate various cellular processes, and accumulating evidence supports the importance of the interplay between the ER and mitochondria in cellular Ca²⁺ homeostasis. Miscommunications between the organelles may be involved in the development of metabolic diseases. Based on a recent experimental study that spotlighted a correlation between obesity and physical interactions of the ER and mitochondria in mouse hepatic cells, we constructed a mathematical model as a tool to probe the effects of the cellular changes linked with obesity on global cellular Ca²⁺ dynamics. Our model successfully reproduced the experimental study that observed a positive correlation between an increase in ER-mitochondrial junctions and the magnitude of mitochondrial Ca²⁺ responses. We postulate that hepatic cells from lean animals exhibit Ca²⁺ oscillations that are more robust under higher concentrations of stimulus, compared to cells from obese animals.

Maternal Overfeeding during Lactation Impairs the Metabolic Response to Fed/Fasting Changing Conditions in the Postweaning Offspring

SCOPE

The metabolic response to fed/fasting changing conditions at early age in rats with different predisposition to obesity-related alterations due to maternal conditions during the perinatal period is studied.

METHODS AND RESULTS

Offspring of dams made obese by a cafeteria diet and moved to a normal-fat diet 1 month before gestation (O-PCaf, with an apparently normal phenotype in adulthood), and offspring of cafeteria diet-fed dams during lactation (O-CAF, with a thin-outside-fat inside phenotype), together with the offspring of control dams (O-C), are studied at early age. Fasting is associated with downregulation of lipogenesis-related genes in liver and rpWAT, and upregulation of genes related to lipolysis and fatty acid uptake in rpWAT in O-C animals. The response to fed/fasting conditions is impaired in O-CAF, but not in O-PCaf animals. The fasting-induced increase in the expression of Prkaa1 in liver and rpWAT, and the corresponding increase of hepatic AMPKα1 protein levels of O-C animals are attenuated in O-CAF rats, while no alterations are found in O-PCaf animals versus controls.

CONCLUSION

Maternal intake of a cafeteria diet during lactation causes early alterations in the offspring, impairing their metabolic flexibility in response to fed/fasting changing conditions, which may contribute to hindering energy homeostasis maintenance.

Muscle-generated BDNF is a sexually dimorphic myokine that controls metabolic flexibility

The ability of skeletal muscle to switch between lipid and glucose oxidation for ATP production during metabolic stress is pivotal for maintaining systemic energy homeostasis, and dysregulation of this metabolic flexibility is a dominant cause of several metabolic disorders. However, the molecular mechanism that governs fuel selection in muscle is not well understood. Here, we report that brain-derived neurotrophic factor (BDNF) is a fasting-induced myokine that controls metabolic reprograming through the AMPK/CREB/PGC-1α pathway in female mice. Female mice with a muscle-specific deficiency in BDNF (MBKO mice) were unable to switch the predominant fuel source from carbohydrates to fatty acids during fasting, which reduced ATP production in muscle. Fasting-induced muscle atrophy was also compromised in female MBKO mice, likely a result of autophagy inhibition. These mutant mice displayed myofiber necrosis, weaker muscle strength, reduced locomotion, and muscle-specific insulin resistance. Together, our results show that muscle-derived BDNF facilitates metabolic adaption during nutrient scarcity in a gender-specific manner and that insufficient BDNF production in skeletal muscle promotes the development of metabolic myopathies and insulin resistance.

Extended indirect calorimetry with isotopic CO2 sensors for prolonged and continuous quantification of exogenous vs. total substrate oxidation in mice

Indirect calorimetry (InCa) estimates whole-body energy expenditure and total substrate oxidation based on O₂ consumption and CO₂ production, but does not allow for the quantification of oxidation of exogenous substrates with time. To achieve this, we incorporated ¹³CO₂ and ¹²CO₂ gas sensors into a commercial InCa system and aimed to demonstrate their performance and added value. As a performance indicator, we showed the discriminative oscillations in ¹³CO₂ enrichment associated with food intake in mice fed diets containing naturally low (wheat) vs high (maize) ¹³C enrichment. To demonstrate the physiological value, we quantified exogenous vs total carbohydrate and fat oxidation continuously, in real time in mice varying in fat mass. Diet-induced obese mice were fed a single liquid mixed meal containing ¹³C-isotopic tracers of glucose or palmitate. Over 13 h, ~70% glucose and ~48% palmitate ingested were oxidised. Exogenous palmitate oxidation depended on body fat mass, which was not the case for exogenous glucose oxidation. We conclude that extending an InCa system with ¹³CO₂ and ¹²CO₂ sensors provides an accessible and powerful technique for real-time continuous quantification of exogenous and whole-body substrate oxidation in mouse models of human metabolic physiology.

Endothelial Dysfunction: Is There a Hyperglycemia-Induced Imbalance of NOX and NOS?

NADPH oxidases (NOX) are enzyme complexes that have received much attention as key molecules in the development of vascular dysfunction. NOX have the primary function of generating reactive oxygen species (ROS), and are considered the main source of ROS production in endothelial cells. The endothelium is a thin monolayer that lines the inner surface of blood vessels, acting as a secretory organ to maintain homeostasis of blood flow. The enzymatic production of nitric oxide (NO) by endothelial NO synthase (eNOS) is critical in mediating endothelial function, and oxidative stress can cause dysregulation of eNOS and endothelial dysfunction. Insulin is a stimulus for increases in blood flow and endothelium-dependent vasodilation. However, cardiovascular disease and type 2 diabetes are characterized by poor control of the endothelial cell redox environment, with a shift toward overproduction of ROS by NOX. Studies in models of type 2 diabetes demonstrate that aberrant NOX activation contributes to uncoupling of eNOS and endothelial dysfunction. It is well-established that endothelial dysfunction precedes the onset of cardiovascular disease, therefore NOX are important molecular links between type 2 diabetes and vascular complications. The aim of the current review is to describe the normal, healthy physiological mechanisms involved in endothelial function, and highlight the central role of NOX in mediating endothelial dysfunction when glucose homeostasis is impaired.

The RNA binding protein HuR influences skeletal muscle metabolic flexibility in rodents and humans

BACKGROUND

Metabolic flexibility can be assessed by changes in respiratory exchange ratio (RER) following feeding. Though metabolic flexibility (difference in RER between fasted and fed state) is often impaired in individuals with obesity or type 2 diabetes, the cellular processes contributing to this impairment are unclear.

MATERIALS AND METHODS

From several clinical studies we identified the 16 most and 14 least metabolically flexible male and female subjects out of >100 participants based on differences between 24-hour and sleep RER measured in a whole-room indirect calorimeter. Global skeletal muscle gene expression profiles revealed that, in metabolically flexible subjects, transcripts regulated by the RNA binding protein, HuR, are enriched. We generated and characterized mice with a skeletal muscle-specific knockout of the HuR encoding gene, Elavl1 (HuR^m-/-).

RESULTS

Male, but not female, HuR^m-/- mice exhibit metabolic inflexibility, with mild obesity, impaired glucose tolerance, impaired fat oxidation and decreased in vitro palmitate oxidation compared to HuR^fl/fl littermates. Expression levels of genes involved in mitochondrial fatty acid oxidation and oxidative phosphorylation are decreased in both mouse and human muscle when HuR is inhibited.

CONCLUSIONS

HuR inhibition results in impaired metabolic flexibility and decreased lipid oxidation, suggesting a role for HuR as an important regulator of skeletal muscle metabolism.

Upregulated PDK4 expression is a sensitive marker of increased fatty acid oxidation

Fatty acid oxidation is a central fueling pathway for mitochondrial ATP production. Regulation occurs through multiple nutrient- and energy-sensitive molecular mechanisms. We explored if upregulated mRNA expression of the mitochondrial enzyme pyruvate dehydrogenase kinase 4 (PDK4) may be used as a surrogate marker of increased mitochondrial fatty acid oxidation, by indicating an overall shift from glucose to fatty acids as the preferred oxidation fuel. The association between fatty acid oxidation and PDK4 expression was studied in different contexts of metabolic adaption. In rats treated with the modified fatty acid tetradecylthioacetic acid (TTA), Pdk4 was upregulated simultaneously with fatty acid oxidation genes in liver and heart, whereas muscle and white adipose tissue remained unaffected. In MDA-MB-231 cells, fatty acid oxidation increased nearly three-fold upon peroxisome proliferator-activated receptor α (PPARα, PPARA) overexpression, and four-fold upon TTA-treatment. PDK4 expression was highly increased under these conditions. Further, overexpression of PDK4 caused increased fatty acid oxidation in these cells. Pharmacological activators of PPARα and AMPK had minor effects, while the mTOR inhibitor rapamycin potentiated the effect of TTA. There were minor changes in mitochondrial respiration, glycolytic function, and mitochondrial biogenesis under conditions of increased fatty acid oxidation. TTA was found to act as a mild uncoupler, which is likely to contribute to the metabolic effects. Repeated experiments with HeLa cells supported these findings. In summary, PDK4 upregulation implies an overarching metabolic shift towards increased utilization of fatty acids as energy fuel, and thus constitutes a sensitive marker of enhanced fatty acid oxidation.

Optimizing Maximal Fat Oxidation Assessment by a Treadmill-Based Graded Exercise Protocol: When Should the Test End?

Maximal fat oxidation during exercise (MFO) and the exercise intensity eliciting MFO (Fatmax) are considered important factors related to metabolic health and performance. Numerous MFO and Fatmax data collection and analysis approaches have been applied, which may have influenced their estimation during an incremental graded exercise protocol. Despite the heterogeneity of protocols used, all studies consistently stopped the MFO and Fatmax test when the respiratory exchange ratio (RER) was 1.0. It remains unknown however whether reaching a RER of 1.0 is required to have an accurate, reliable, and valid measure of MFO and Fatmax. We aimed to investigate the RER at which MFO and Fatmax occurred in sedentary and trained healthy adults. A total of 166 sedentary adults aged between 18 and 65 years participated in the study. MFO and Fatmax were calculated by an incremental graded exercise protocol before and after two exercise-based interventions. Our findings suggest that a graded exercise protocol aiming to determine MFO and Fatmax could end when a RER = 0.93 is reached in sedentary healthy adults, and when a RER = 0.90 is reached in trained adults independently of sex, age, body weight status, or the Fatmax data analysis approach. In conclusion, we suggest reducing the RER from 1.0 to 0.95 to be sure that MFO is reached in outliers. This methodological consideration has important clinical implications, since it would allow to apply smaller workload increments and/or to extend the stage duration to attain the steady state, without increasing the test duration.

Lipotoxicity in Kidney, Heart, and Skeletal Muscle Dysfunction

Dyslipidemia is a common nutritional and metabolic disorder in patients with chronic kidney disease. Accumulating evidence supports the hypothesis that prolonged metabolic imbalance of lipids leads to ectopic fat distribution in the peripheral organs (lipotoxicity), including the kidney, heart, and skeletal muscle, which accelerates peripheral inflammation and afflictions. Thus, lipotoxicity may partly explain progression of renal dysfunction and even extrarenal complications, including renal anemia, heart failure, and sarcopenia. Additionally, endoplasmic reticulum stress activated by the unfolded protein response pathway plays a pivotal role in lipotoxicity by modulating the expression of key enzymes in lipid synthesis and oxidation. Here, we review the molecular mechanisms underlying lipid deposition and resultant tissue damage in the kidney, heart, and skeletal muscle, with the goal of illuminating the nutritional aspects of these pathologies.

The effect of high compared with low dairy consumption on glucose metabolism, insulin sensitivity, and metabolic flexibility in overweight adults: a randomized crossover trial

BACKGROUND

Dairy products contain many nutritious components that may benefit metabolic health. There are indications that glucose metabolism and insulin sensitivity, which are generally disturbed in overweight and obese individuals, may improve by increased dairy intake. This may also affect one's metabolic flexibility.

OBJECTIVE

The aim of this study was to investigate the effects of high compared with low dairy intake on glucose metabolism, insulin sensitivity, and metabolic flexibility in overweight adults (aged 45-65 y).

METHODS

In this randomized intervention study, subjects consumed a high- and a low-dairy diet [HDD (5-6 dairy portions) and LDD (≤1 dairy portion), respectively] for 6 wk in a crossover design, with a washout period of 4 wk. Dairy portions were 200 g semi-skimmed yoghurt, 30 g reduced-fat (30+) cheese, and 250 mL semiskimmed milk and buttermilk. After 6 wk, a 75-g oral-glucose-tolerance test (13C-labeled) and a subsequent fasting challenge were performed. Metabolic flexibility was studied by determining the respiratory quotient (RQ) using indirect calorimetry. Fasting and postprandial plasma concentrations of glucose and insulin were analyzed. The dual isotope technique enabled calculation of glucose kinetics.

RESULTS

The study was completed by 45 overweight men and postmenopausal women [age 58.9 ± 4.3 y, BMI 27.9 ± 1.9 kg/m2 (mean ± SD)]. Fasting RQ and ΔRQ, reflecting metabolic flexibility, did not differ after both diets. Fasting glucose concentrations were similar, whereas fasting insulin concentrations were lower after the LDD (LDD: 8.1 ± 2.8 mU/L; HDD: 8.9 ± 3.3 mU/L; P = 0.024). This resulted in a higher HOMA-IR after the HDD (P = 0.027). Postprandial glucose and insulin responses as well as glucose kinetics were similar after both diets.

CONCLUSIONS

The amount of dairy intake during a 6-wk period had a neutral effect on metabolic flexibility or postprandial glucose metabolism in middle-aged overweight subjects. More trials are needed to study the effects of specific dairy types and to differentiate between metabolic subgroups. This trial was registered at trialregister.nl as NTR4899.

Systemic PPARγ deletion in mice provokes lipoatrophy, organomegaly, severe type 2 diabetes and metabolic inflexibility

BACKGROUND

The peroxisome proliferator-activated receptor γ (PPARγ) is a ligand-dependent transcription factor involved in many aspects of metabolism, immune response and development. Numerous studies relying on tissue-specific invalidation of the Pparg gene have shown distinct facets of its activity, whereas the effects of its systemic inactivation remain unexplored due to embryonic lethality. By maintaining PPARγ expression in the placenta, we recently generated a mouse model carrying Pparg full body deletion (Pparg^Δ/Δ), which in contrast to a previously published model is totally deprived of any form of adipose tissue. Herein, we propose an in-depth study of the metabolic alterations observed in this new model.

METHODS

Young adult mice, both males and females analyzed separately, were first phenotyped for their gross anatomical alterations. Systemic metabolic parameters were analyzed in the blood, in static and in dynamic conditions. A full exploration of energy metabolism was performed in calorimetric cages as well as in metabolic cages. Our study was completed by expression analyses of a set of specific genes.

MAIN FINDINGS

Pparg^Δ/Δ mice show a striking complete absence of any form of adipose tissue, which triggers a complex metabolic phenotype including increased lean mass with organomegaly, hypermetabolism, urinary energy loss, hyperphagia, and increased amino acid metabolism. Pparg^Δ/Δ mice develop severe type 2 diabetes, characterized by hyperglycemia, hyperinsulinemia, polyuria and polydispsia. They show a remarkable metabolic inflexibility, as indicated by the inability to shift substrate oxidation between glucose and lipids, in both ad libitum fed state and fed/fasted/refed transitions. Moreover, upon fasting Pparg^Δ/Δ mice enter a severe hypometabolic state.

CONCLUSIONS

Our data comprehensively describe the impact of lipoatrophy on metabolic homeostasis. As such, the presented data on Pparg^Δ/Δ mice gives new clues on what and how to explore severe lipodystrophy and its subsequent metabolic complications in human.

Metabolic Determinants of Weight Gain in Humans

One of the fundamental challenges in obesity research is to identify subjects prone to weight gain so that obesity and its comorbidities can be promptly prevented or treated. The principles of thermodynamics as applied to human body energetics demonstrate that susceptibility to weight gain varies among individuals as a result of interindividual differences in energy expenditure and energy intake, two factors that counterbalance one another and determine daily energy balance and, ultimately, body weight change. This review focuses on the variability among individuals in human metabolism that determines weight change. Conflicting results have been reported about the role of interindividual differences in energy metabolism during energy balance in relation to future weight change. However, recent studies have shown that metabolic responses to acute, short-term dietary interventions that create energy imbalance, such as low-protein overfeeding or fasting for 24 hours, may reveal the underlying metabolic phenotype that determines the degree of resistance to diet-induced weight loss or the propensity to spontaneous weight gain over time. Metabolically "thrifty" individuals, characterized by a predilection for saving energy in settings of undernutrition and dietary protein restriction, display a minimal increase in plasma fibroblast growth factor 21 concentrations in response to a low-protein overfeeding diet and tend to gain more weight over time compared with metabolically "spendthrift" individuals. Similarly, interindividual variability in the causal relationship between energy expenditure and energy intake ("energy sensing") and in the metabolic response to cold exposure (e.g., brown adipose tissue activation) seems, to some extent, to be indicative of individual propensity to weight gain. Thus, an increased understanding and the clinical characterization of phenotypic differences in energy metabolism among individuals (metabolic profile) may lead to new strategies to prevent weight gain or improve weight-loss interventions by targeted therapies on the basis of metabolic phenotype and susceptibility to obesity in individual persons.

Dietary Macronutrient Management to Treat Mitochondrial Dysfunction in Parkinson's Disease

Mitochondrial dysfunction has been demonstrated to play an important role in the pathogenesis of Parkinson’s disease (PD). The products of several PD-associated genes, including alpha-synuclein, parkin, pink1, protein deglycase DJ-1, and leucine rich repeat kinase 2, have important roles in mitochondrial biology. Thus, modifying mitochondrial function could be a potential therapeutic strategy for PD. Dietary management can alter mitochondrial function as shifts in dietary macronutrients and their ratios in food can alter mitochondrial energy metabolism, morphology and dynamics. Our studies have established that a low protein to carbohydrate (P:C) ratio can increase lifespan, motor ability and mitochondrial function in a parkin mutant Drosophila model of PD. In this review, we describe mitochondrial dysfunction in PD patients and models, and dietary macronutrient management strategies to reverse it. We focus on the effects of protein, carbohydrate, fatty acids, and their dietary ratios. In addition, we propose potential mechanisms that can improve mitochondrial function and thus reverse or delay the onset of PD.

Dynamic mitochondrial responses to a high-fat diet in Drosophila melanogaster

Mitochondria can utilize different fuels according to physiological and nutritional conditions to promote cellular homeostasis. However, during nutrient overload metabolic inflexibility can occur, resulting in mitochondrial dysfunctions. High-fat diets (HFDs) are usually used to mimic this metabolic inflexibility in different animal models. However, how mitochondria respond to the duration of a HFD exposure is still under debate. In this study, we investigated the dynamic of the mitochondrial and physiological functions in Drosophila melanogaster at several time points following an exposure to a HFD. Our results showed that after two days on the HFD, mitochondrial respiration as well as ATP content of thorax muscles are increased, likely due to the utilization of carbohydrates. However, after four days on the HFD, impairment of mitochondrial respiration at the level of complex I, as well as decreased ATP content were observed. This was associated with an increased contribution of complex II and, most notably of the mitochondrial glycerol-3-phosphate dehydrogenase (mG3PDH) to mitochondrial respiration. We suggest that this increased mG3PDH capacity reflects the occurrence of metabolic inflexibility, leading to a loss of homeostasis and alteration of the cellular redox status, which results in senescence characterized by decreased climbing ability and premature death.

Exercise and Omentin: Their Role in the Crosstalk Between Muscle and Adipose Tissues in Type 2 Diabetes Mellitus Rat Models

This study aims to analyze the effects of resisted, aerobic, and combined exercises on omentin levels in visceral adipose tissue and muscle of rats with experimental diabetes to verify whether these adipokines are related to the glucose pathway and inflammation process in this model. Male Wistar rats received a high-fat diet for 4 weeks and a low dose of streptozotocin (35 mg/kg) to induce experimental diabetes. After inducing diabetes, the animals were divided into 4 experimental groups (n = 10): diabetic control (C); resistance training (RT); aerobic training (AT); and combined training (CT). The groups were exercised for 12 weeks, 3 times/week, where: RT means the stair climbing protocol until exhaustion; AT is the 30 min/day reaching 20 m/min protocol, and CT is the combination of RT and AT. The AT group showed reduced retroperitoneal and mesenteric adipose tissue and abdominal fat deposits. Our study also showed a possible control of blood glucose, as well as decreased Interleukin 6 (IL-6) and C-reactive protein, increased circulating adiponectin and increased omentin in visceral adipose tissue. In addition, the AT group affected the glucose pathway by stimulating phosphorylation of Akt in muscle tissue. Omentin also showed a strong positive correlation with adiponectin and a moderate negative correlation with IL-6. Thus, our findings indicated that omentin in type 2 diabetes is changed by AT. Furthermore, increased omentin levels had a close association with the glucose pathway by stimulating phosphorylation of Akt in muscle tissue and with IL-6 in serum, suggesting that omentin is likely to have anti-inflammatory and protective action in experimental diabetes.

Temporal dynamics of liver mitochondrial protein acetylation and succinylation and metabolites due to high fat diet and/or excess glucose or fructose

Dietary macronutrient composition alters metabolism through several mechanisms, including post-translational modification (PTM) of proteins. To connect diet and molecular changes, here we performed short- and long-term feeding of mice with standard chow diet (SCD) and high-fat diet (HFD), with or without glucose or fructose supplementation, and quantified liver metabolites, 861 proteins, and 1,815 protein level-corrected mitochondrial acetylation and succinylation sites. Nearly half the acylation sites were altered by at least one diet; nutrient-specific changes in protein acylation sometimes encompass entire pathways. Although acetyl-CoA is an intermediate in both sugar and fat metabolism, acetyl-CoA had a dichotomous fate depending on its source; chronic feeding of dietary sugars induced protein hyperacetylation, whereas the same duration of HFD did not. Instead, HFD resulted in citrate accumulation, anaplerotic metabolism of amino acids, and protein hypo-succinylation. Together, our results demonstrate novel connections between dietary macronutrients, protein post-translational modifications, and regulation of fuel selection in liver.

Sex-specific effects of maternal and postweaning high-fat diet on skeletal muscle mitochondrial respiration

Exposure to maternal over-nutrition in utero is linked with developmental programming of obesity, metabolic syndrome and cardiovascular disease in offspring, which may be exacerbated by postnatal high-fat (HF) diet. Skeletal muscle mitochondrial function contributes to substrate metabolism and is impaired in metabolic disease. We examined muscle mitochondrial respiration in male and female mice exposed to maternal HF diet in utero, followed by postweaning HF diet until middle age. After in utero exposure to maternal control (Con) or HF diet (45% kcal fat; 39.4% lard, 5.5% soybean oil), offspring were weaned to Con or HF, creating four groups: Con/Con (male/female (m/f), n=8/8), Con/HF (m/f, n=7/4), HF/Con (m/f, n=9/6) and HF/HF (m/f, n=4/4). Oxidative phosphorylation (OXPHOS) and electron transfer system (ETS) capacity were measured in permeabilized gastrocnemius bundles. Maternal HF diet increased fasting glucose and lean body mass in males and body fat percentage in both sexes (P⩽0.05). Maximal adenosine diphosphate-stimulated respiration (complex I OXPHOS) was decreased by maternal HF diet in female offspring (-21%, P=0.053), but not in male (-0%, P>0.05). Sexually divergent responses were exacerbated in offspring weaned to HF diet. In females, OXPHOS capacity was lower (-28%, P=0.041) when weaned to high-fat (HF/HF) v. control diet (HF/Con). In males, OXPHOS (+33%, P=0.009) and ETS (+42%, P=0.016) capacity increased. Our data suggest that maternal lard-based HF diet, rich in saturated fat, affects offspring skeletal muscle respiration in a sex-dependent manner, and these differences are exacerbated by HF diet in adulthood.

Exercise as 'precision medicine' for insulin resistance and its progression to type 2 diabetes: a research review

Type 2 diabetes and obesity epidemics are in effect in the United States and the two pathologies are linked. In accordance with the growing appreciation that ‘exercise is medicine,’ it is intuitive to suggest that exercise can play an important role in the prevention and/or treatment of these conditions. However, if exercise is to truly be considered as a viable alternative to conventional healthcare prevention/treatment strategies involving pharmaceuticals, it must be prescribed with similar scrutiny. Indeed, it seems reasonable to posit that the recent initiative calling for ‘precision medicine’ in the US standard healthcare system should also be applied in the exercise setting. In this narrative review, we consider a number of explanations that have been forwarded regarding the pathological progression to type 2 diabetes both with and without the concurrent influence of overweight/obesity. Our goal is to provide insight regarding exercise strategies that might be useful as ‘precision medicine’ to prevent/treat this disease. Although the etiology of type 2 diabetes is complex and cause/consequence characteristics of associated dysfunctions have been debated, it is well established that impaired insulin action plays a critical early role. Consequently, an exercise strategy to prevent/treat this disease should be geared toward improving insulin sensitivity both from an acute and chronic standpoint. However, research suggests that a chronic improvement in insulin sensitivity only manifests when weight loss accompanies an exercise intervention. This has resonance because ectopic fat accumulation appears to represent a central component of disease progression regardless of whether obesity is also part of the equation. The cause/consequence characteristics of the relationship between insulin resistance, pathological fat deposition and/or mobilsation, elevated and/or poorly-distributed lipid within myocytes and an impaired capacity to use lipid as fuel remains to be clarified as does the role of muscle mitochondria in the metabolic decline. Until these issues are resolved, a multidimensional exercise strategy (e.g., aerobic exercise at a range of intensities and resistance training for muscular hypertrophy) could provide the best alternative for prevention/treatment.

Brown Adipose Tissue and Skeletal Muscle 18F-FDG Activity After a Personalized Cold Exposure Is Not Associated With Cold-Induced Thermogenesis and Nutrient Oxidation Rates in Young Healthy Adults

Cold induced thermogenesis (CIT) in humans results mainly from the combination of both brown adipose tissue (BAT) and skeletal muscle thermogenic activity. The relative contribution of both tissues to CIT and to cold induced nutrient oxidation rates (CI-NUTox) remains, however, to be elucidated. We investigated the association of BAT and skeletal muscle activity after a personalized cold exposure with CIT and CI-NUTox in 57 healthy adults (23.0 ± 2.4 years old; 25.1 ± 4.6 kg/m²; 35 women). BAT and skeletal muscle (paracervical, sternocleidomastoid, scalene, longus colli, trapezius, parathoracic, supraspinatus, subscapular, deltoid, pectoralis major, and triceps brachii) metabolic activity were assessed by means of a ¹⁸Fluorodeoxyglucose positron emission tomography-computed tomography scan preceded by a personalized cold exposure. The cold exposure consisted in remaining in a mild cold room for 2 h at 19.5–20°C wearing a water perfused cooling vest set at 3.8°C above the individual shivering threshold. On a separate day, we estimated CIT and CI-NUTox by indirect calorimetry under fasting conditions for 1 h of personalized cold exposure. There was no association of BAT volume or activity with CIT or CI-NUTox (all P > 0.2). Similarly, the skeletal muscle metabolic activity was not associated either with CIT or CI-NUTox (all P > 0.2). The results persisted after controlling for sex, the time of the day, and the date when CIT was assessed. Our results suggest that human BAT activity and skeletal muscle ¹⁸F-FDG activity are not associated to CIT in young healthy adults. Inherent limitations of the available radiotracers for BAT detection and muscle activity quantification may explain why we failed to detect a physiologically plausible association.

Associations Among Dietary Fat Oxidation Responses to Overfeeding and Weight Gain in Obesity-Prone and Resistant Adults

OBJECTIVE

This study tested the hypothesis that 3 days of overfeeding (OF) decreases dietary fat oxidation and predicts longitudinal weight change in adults classified as obesity prone (OP) and obesity resistant (OR) based on self-identification and personal and family weight history. Changes in diurnal profiles of plasma metabolites and hormones were measured to probe mechanisms.

METHODS

Adults identified as OP (n = 22; BMI: 23.9  ±  2.4 kg/m2 ) and OR (n = 30; BMI: 20.5  ±  2.2 kg/m2 ) completed 3 days of eucaloric (EU) feeding and 3 days of OF. On day 3, the 24-hour total and dietary fat oxidation was measured using room calorimetry and an oral 14 C tracer. Plasma glucose, insulin, triglycerides, and nonesterified fatty acid (NEFA) concentrations were frequently sampled over 24 hours. Body composition was measured annually for 4.0  ±  1.4 years in a subsample (n = 19 OP and 23 OR).

RESULTS

Dietary fat oxidation over 24 hours was not altered by OF versus EU (P = 0.54). Weight gain in OP correlated with lower nocturnal NEFA concentrations during OF (r  = -0.60; P = 0.006) and impaired fuel selection over 24 hours (metabolic inflexibility, wake respiratory quotient-sleep respiratory quotient) (r = -0.48; P  = 0.04).

CONCLUSIONS

Short-term OF did not alter dietary fat oxidation. Lower nocturnal NEFA availability and metabolic inflexibility to overfeeding may be factors contributing to weight gain.

Exercise counteracts lipotoxicity by improving lipid turnover and lipid droplet quality

The incidence of obesity and metabolic disease, such as type 2 diabetes mellitus (T2D), is rising globally. Dietary lipid over supply leads to lipid accumulation at ectopic sites, such as skeletal muscle. Ectopic lipid storage is highly correlated with insulin resistance and T2D, likely due to a loss of metabolic flexibility - the capacity to switch between fat and glucose oxidation upon insulin stimulation - and cellular dysfunction because of lipotoxicity. However, muscle lipid levels are also elevated in endurance-trained athletes, presenting a paradoxical phenotype of increased intramuscular lipids along with high insulin sensitivity - the 'athletes' paradox'. This review focuses on recent human data to characterize intramuscular lipid species in order to elucidate some of the underlying mechanisms driving skeletal muscle lipotoxicity. There is evidence that lipotoxicity is characterized by an increase in bioactive lipid species, such as ceramide. The athletes' paradox supports the notion that regular physical exercise has health benefits that might originate from the alleviation of lipotoxicity. Indeed, exercise training alleviates intramuscular ceramide content in obese individuals without a necessary decrease in ectopic lipid storage. Furthermore, evidence shows that exercise training elevates markers of lipid droplet dynamics such as the PLIN proteins, and triglyceride lipases ATGL and HSL, as well as mitochondrial efficiency, potentially explaining the improved lipid turnover and a reduction in the accumulation of lipotoxic intermediates observed with the athelets' paradox.

Glucose metabolism and metabolic flexibility in blood platelets

Essentials The metabolic integration processes required for platelet activation are unclear. The metabolic plasticity of human platelets were investigated. Activated platelets exhibit a glycolytic phenotype while preserving mitochondrial function. Platelets can switch freely between glucose/glycogen and fatty acids to support aggregation. SUMMARY: Background Platelet activation is an energy-dependent process, but the type and integrated use of metabolic fuels required to drive activation remain unclear. Objective To dissect the metabolic fuel and pathway plasticity required for platelet activation. Methods Platelet oxygen consumption rate and extracellular acidification rate were measured as markers of oxidative phosphorylation (OXPHOS) and glycolysis, respectively. Glucose and glycogen were quantified by enzyme-coupled fluorometric assay. Results Blood platelets switched freely between glycolysis and OXPHOS, using either glucose or fatty acids at rest. The transition of platelets from a quiescent to an activated state promoted rapid uptake of exogenous glucose, associated with a shift to a predominantly glycolytic phenotype coupled with a minor rise in mitochondrial oxygen consumption. Consistent with this metabolic plasticity, under nutrient-limiting conditions, platelets utilized glucose, glycogen or fatty acids independently to support activation. Importantly, the glycolytic switch occurred even in the absence of extracellular glucose, originating from endogenous glycogen. Focusing on the relative flexibility of mitochondrial fuel oxidation of glucose and fatty acids, we found that inhibition of oxidation of a single fuel was compensated for by increased oxidation of the other, but, when oxidation was inhibited, glycolysis was upregulated. Glutamine made little contribution to mitochondrial oxygen consumption. Analysis of platelet functional dependency on ATP from different pathways demonstrated that inhibition of both fuel oxidation and glycolysis were required to prevent agonist-driven platelet activation. Conclusion Platelets have significant metabolic fuel and pathway flexibility, but preferentially use glycolysis for ATP generation when activated.

Metabolic inflexibility in women with PCOS is similar to women with type 2 diabetes

Background

An ability to switch between primarily oxidizing fat in the fasted state to carbohydrate in the fed state, termed metabolic flexibility, is associated with insulin sensitivity. Metabolic flexibility has been explored previously in women with polycystic ovary syndrome (PCOS), yet the independent or synergistic contributions of androgen excess and/or insulin resistance is not yet known. Therefore, the purpose of this article was to characterize metabolic flexibility in women with PCOS compared to women of normal BMI, obesity, or type 2 diabetes (T2DM).

Methods

Eighty-six weight-stable women; thirty with either PCOS (n = 30), or fifty-six with obesity (n = 12), T2DM (n = 27), or normal BMI (n = 17) underwent a hyperinsulinemic euglycemic clamp and indirect calorimetry to measure insulin sensitivity and substrate oxidation via indirect calorimetry, respectively.

Results

All analyses were adjusted for differences in age, ethnicity, and BMI between groups. Women with PCOS were less metabolically flexible compared to healthy women with obesity (p < 0.0001), normal BMI (p < 0.0001), but after controlling for glucose disposal rate, were similar to women with T2DM (p = 0.99). When dividing women with PCOS above and below the mean cutoff for insulin resistance, the insulin resistant women with PCOS had lower rates of non-oxidative glucose metabolism (p = 0.0001), higher levels of percent free testosterone (p = 0.04), a higher free androgen index (p = 0.006), more visceral adipose tissue (p = 0.02), and were less metabolically flexible (p = 0.007).

Conclusions

Women with T2DM were as metabolically inflexible as women with PCOS. When stratifying women with PCOS into those who are metabolically flexible and inflexible, the women who are inflexible display greater amounts of visceral fat and androgen excess. The inability to alter substrate use given the physiological stimulus may lead to subsequent increases in adiposity in women with PCOS thereby further worsening the insulin resistance.

Trial registration number

Clinical Trials.gov, NCT01482286. Registered 30 November 2011.

Obesity in Type 1 Diabetes: Pathophysiology, Clinical Impact, and Mechanisms

There has been an alarming increase in the prevalence of obesity in people with type 1 diabetes in recent years. Although obesity has long been recognized as a major risk factor for the development of type 2 diabetes and a catalyst for complications, much less is known about the role of obesity in the initiation and pathogenesis of type 1 diabetes. Emerging evidence suggests that obesity contributes to insulin resistance, dyslipidemia, and cardiometabolic complications in type 1 diabetes. Unique therapeutic strategies may be required to address these comorbidities within the context of intensive insulin therapy, which promotes weight gain. There is an urgent need for clinical guidelines for the prevention and management of obesity in type 1 diabetes. The development of these recommendations will require a transdisciplinary research strategy addressing metabolism, molecular mechanisms, lifestyle, neuropsychology, and novel therapeutics. In this review, the prevalence, clinical impact, energy balance physiology, and potential mechanisms of obesity in type 1 diabetes are described, with a special focus on the substantial gaps in knowledge in this field. Our goal is to provide a framework for the evidence base needed to develop type 1 diabetes-specific weight management recommendations that account for the competing outcomes of glycemic control and weight management.