Lipolysis during fasting. Decreased suppression by insulin and increased stimulation by epinephrine

Lifestyle Optimization Leads to Superior Liver Regeneration in Live Liver Donors and Decreases Early Allograft Dysfunction in Recipients: A Randomized Control Trial

INTRODUCTION

The aim of the current randomized control trial was to assess the efficacy of donor lifestyle optimization on liver regeneration and outcome following live donor liver transplantation.

METHODS

Live liver donors (LLDs) who were fit with no or minimal steatosis were randomized to receive either a customized low-calorie diet with calorie intake equalling their basal requirement along with exercise for 2 weeks before surgery versus to continue their normal routine lifestyle. Primary objectives were the difference in the day of normalization of serum bilirubin and PT-International normalized ratio and the percentage growth of the liver at postoperative day 7 and 14. Secondary objectives were differences in intraoperative liver biopsy, liver-regeneration markers, blood loss, hospital stay, the complication rate in LLDs, and rates of early graft dysfunction (EGD) in recipients.

RESULTS

Sixty-two consecutive LLDs were randomized (28 in intervention vs. 34 in control). Baseline parameters and graft parameters were similar in both groups. LLDs in the intervention arm had significantly decreased calorie intake ( P <0.005), abdominal girth ( P <0.005), BMI ( P =0.05), and weight ( P <0.0005). The mean blood loss ( P =0.038), day of normalization of bilirubin ( P =0.005) and International normalized ratio ( P =0.061), postoperative peak aspartate transaminase ( P =0.003), Alanine transaminase ( P =0.025), and steatosis ( P <0.005) were significantly less in the intervention group. There was significantly higher volume regeneration ( P =0.03) in donors in the intervention arm. The levels of TNF-α, IL-6, and IL-10 levels were significantly higher, while the TGF-β level was lower in donors in the intervention group. The rate of EGD was significantly higher in recipients in the control group ( P =0.043).

CONCLUSION

Lifestyle optimization of LLD is simple to comply with, improves liver regeneration in LLDs, and decreases EGD in recipients, thus can enhance donor safety and outcomes in live donor liver transplantation.

Effects of acute changes in fasting glucose and free fatty acid concentrations on indices of β-cell function and glucose metabolism in subjects without diabetes

Elevated fasting free fatty acids (FFAs) and fasting glucose are additively associated with impaired glucose tolerance (IGT) and decreased β-cell function [quantified as disposition index (DI)]. We sought to examine how changes in fasting FFA and glucose alter islet function. We studied 10 subjects with normal fasting glucose (NFG) and normal glucose tolerance (NGT) on two occasions. On one occasion, Intralipid and glucose were infused overnight to mimic conditions present in IFG/IGT. In addition, we studied seven subjects with IFG/IGT on two occasions. On one occasion, insulin was infused to lower overnight FFA and glucose concentrations to those observed in people with NFG/NGT. The following morning, a labeled mixed meal was used to measure postprandial glucose metabolism and β-cell function. Elevation of overnight fasting FFA and glucose in NFG/NGT did not alter peak or integrated glucose concentrations (2.0 ± 0.1 vs. 2.0 ± 0.1 Mol per 5 h, Saline vs. Intralipid/glucose, P = 0.55). Although overall β-cell function quantified by the Disposition Index was unchanged, the dynamic component of β-cell responsivity (ϕd) was decreased by Intralipid and glucose infusion (9 ± 1 vs. 16 ± 3 10-9, P = 0.02). In people with IFG/IGT, insulin did not alter postprandial glucose concentrations or indices of β-cell function. Endogenous glucose production and glucose disappearance were also unchanged in both groups. We conclude that acute, overnight changes in FFA, and glucose concentrations do not alter islet function or glucose metabolism in prediabetes.NEW & NOTEWORTHY This experiment studied the effect of changes in overnight concentrations of free fatty acids (FFAs) and glucose on β-cell function and glucose metabolism. In response to elevation of these metabolites, the dynamic component of the β-cell response to glucose was impaired. This suggests that in health overnight hyperglycemia and FFA elevation can deplete preformed insulin granules in the β-cell.

Measuring the Rate of Lipolysis in Ex vivo Murine Adipose Tissue and Primary Preadipocytes Differentiated In Vitro

Adipocytes store energy in the form of triglycerides in lipid droplets. This energy can be mobilized via lipolysis, where the fatty acid side chains are sequentially cleaved from the glycerol backbone, resulting in the release of free fatty acids and glycerol. Due to the low expression of glycerol kinase in white adipocytes, glycerol re-uptake rates are negligible, while fatty acid re-uptake is dictated by the fatty acid binding capacity of media components such as albumin. Both glycerol and fatty acid release into media can be quantified by colorimetric assays to determine the lipolytic rate. By measuring these factors at multiple time points, one can determine the linear rate of lipolysis with high confidence. Here, we provide a detailed protocol for the measurement of lipolysis in in vitro differentiated adipocytes and ex vivo adipose tissue from mice. This protocol may also be optimized for other preadipocyte cell lines or adipose tissue from other organisms; considerations and optimization parameters are discussed. This protocol is designed to be useful in determining and comparing the rate of adipocyte lipolysis between mouse models and treatments.

Metabolic effects of alternate-day fasting in males with obesity with or without type 2 diabetes

Alternate-day fasting induces oscillations in energy stores. We hypothesized that repeated oscillations increases insulin secretion and sensitivity, and improve metabolic health in patients with obesity with or without type 2 diabetes (T2DM). Twenty-three male patients fasted every other day for 30 h for 6 weeks. Experiments included resting energy expenditure, continuous glucose monitoring, intravenous glucose tolerance test, euglycemic hyperinsulinemic clamp, body composition, hepatic triglyceride content, muscle biopsies which were performed at baseline, during 3 weeks without allowed weight loss, and after additional 3 weeks with weight loss. Bodyweight decreased ∼1% and further ∼3% during weeks one to three and four to six, respectively (p < 0.05). Only minor changes in fat mass occurred in weeks 1-3. With weight loss, visceral fat content decreased by 13 ± 3% and 12 ± 2% from baseline in patients with and without T2DM, respectively (p < 0.05). Hepatic triglyceride content decreased by 17 ± 9% and 36 ± 9% (with diabetes) and 27 ± 8% and 40 ± 8% (without diabetes) from baseline to week 3 and week 6, respectively (all p < 0.05). Muscle lipid and glycogen content oscillated with the intervention. Glucose homeostasis, insulin secretion and sensitivity was impaired in patients with T2DM and did not change without weight loss, but improved (p < 0.05) when alternate day fasting was combined with weight loss. In conclusion, alternate-day fasting is feasible in patients with obesity and T2DM, and decreases visceral fat and liver fat deposits. Energy store oscillations by alternate-day fasting do not improve insulin secretion or sensitivity per se. Clinical Trial registration: (ClinicalTrials.gov), (ID NCT02420054).

Glucagon receptor signaling at white adipose tissue does not regulate lipolysis

Although the physiological role of glucagon receptor signaling in the liver is well defined, the impact of glucagon receptor (Gcgr) signaling on white adipose tissue (WAT) continues to be debated. Although numerous studies propose that glucagon stimulates WAT lipolysis, we lack evidence that physiological concentrations of glucagon regulate WAT lipolysis. In turn, we performed studies in both wild-type and WAT Gcgr knockout mice to determine if glucagon regulates lipolysis at WAT in the mouse. We assessed the effects of fasting and acute exogenous glucagon administration in wild-type C57BL/6J and Gcgr^Adipocyte+/+ versus Gcgr^Adipocyte-/- mice. Using an ex vivo lipolysis protocol, we further examined the direct effects of glucagon on physiologically (fasted) and pharmacologically stimulated lipolysis. We found that adipocyte Gcgr expression did not affect fasting-induced lipolysis or hepatic lipid accumulation in lean or diet-induced obese (DIO) mice. Acute glucagon administration did not affect serum nonesterified fatty acids (NEFA), leptin, or adiponectin concentration, but did increase serum glucose and FGF21, regardless of genotype. Glucagon did not affect ex vivo lipolysis in explants from either Gcgr^Adipocyte+/+ or Gcgr^Adipocyte-/- mice. Gcgr expression did not affect fasting-induced or isoproterenol-stimulated lipolysis from WAT explants. Moreover, glucagon receptor signaling at WAT did not affect body weight or glucose homeostasis in lean or DIO mice. Our studies have established that physiological levels of glucagon do not regulate WAT lipolysis, either directly or indirectly. Given that glucagon receptor agonism can improve dyslipidemia and decrease hepatic lipid accumulation, it is critical to understand the tissue-specific effects of glucagon receptor action. Unlike the crucial role of hepatic glucagon receptor signaling in maintaining glucose and lipid homeostasis, we observed no metabolic consequence of WAT glucagon receptor deletion.NEW & NOTEWORTHY It has been postulated that glucagon stimulates lipolysis and fatty acid release from white adipose tissue. We observed no metabolic effects of eliminating or activating glucagon receptor signaling at white adipose tissue.

Extended Inter-Meal Interval Negatively Impacted the Glycemic and Insulinemic Responses after Both Lunch and Dinner in Healthy Subjects

This study aimed to investigate the glycemic and insulinemic effects of lunch timing based on a fixed feeding window, and the effects of apple preload on postprandial glucose and insulin responses after nutrient-balanced lunch and the subsequent high-fat dinner in healthy participants. Twenty-six participants completed four randomized, crossover experimental trials: (1) early standardized lunch at 12:00 (12S); (2) apple preload to 12S (12A+S); (3) late standardized lunch at 14:00 (14S); and (4) apple preload to 14S (14A+S); wherein twenty participants’ blood samples were collected for insulin analysis following the lunch trails. In each experimental trial, each participant equipped with a continuous glucose monitor (CGM) was provided with a standardized breakfast and a high-fat dinner to be consumed at 8:00 and 18:00, respectively. The late lunch (14S) resulted in significantly elevated glucose peak, delayed insulin peak time, decreased insulin sensitivity, and increased insulin resistance following the lunch; also decreased glycemic response following the subsequent dinner and larger blood glucose fluctuation over the 24-h period compared with the 12S. The 14A+S significantly reduced the glucose peak, the insulin peak time and the glycemic variability following the lunch, also the 24-h glycemic variability compared with the 14S. The insulin sensitivity was significantly improved in the 12A+S, compared with that of the 12S. In conclusion, the present study found that an extra 2-h inter-meal fasting before and after lunch resulted in elevated glycemic response in both macronutrient-balanced meal and high-fat meal in healthy subjects. The negative impact of a late lunch could be partly reversed by the apple preload, without a trade-off of insulin secretion.

Exploring the Effects of Energy Constraints on Performance, Body Composition, Endocrinological/Hematological Biomarkers, and Immune System among Athletes: An Overview of the Fasting State

The Ramadan fasting period (RFP) means abstaining from consuming food and/or beverages during certain hours of the day—from sunrise to sunset. Engaging in exercise and sports during the RFP leads to the lipolysis of adipose tissue and an increase in the breakdown of peripheral fat, leading to an increase in fat consumption. The effects of the RFP on functional, hematological, and metabolic parameters needs further study as existing studies have reported contradictory results. The differences in the results of various studies are due to the geographical characteristics of Muslim athletes, their specific diets, and their genetics, which explain these variations. In recent years, the attention of medical and sports researchers on the effects of the RFP and energy restrictions on bodily functions and athletic performance has increased significantly. Therefore, this brief article examines the effects of the RFP on the immune system, body composition, hematology, and the functionality of athletes during and after the RFP. We found that most sporting activities were performed during any time of the day without being affected by Ramadan fasting. Athletes were able to participate in their physical activities during fasting periods and saw few effects on their performance. Sleep and nutritional factors should be adjusted so that athletic performance is not impaired.

The β-Hydroxybutyrate-GPR109A Receptor Regulates Fasting-induced Plasticity in the Mouse Adrenal Medulla

During fasting, increased sympathoadrenal activity leads to epinephrine release and multiple forms of plasticity within the adrenal medulla including an increase in the strength of the preganglionic → chromaffin cell synapse and elevated levels of agouti-related peptide (AgRP), a peptidergic cotransmitter in chromaffin cells. Although these changes contribute to the sympathetic response, how fasting evokes this plasticity is not known. Here we report these effects involve activation of GPR109A (HCAR2). The endogenous agonist of this G protein-coupled receptor is β-hydroxybutyrate, a ketone body whose levels rise during fasting. In wild-type animals, 24-hour fasting increased AgRP-ir in adrenal chromaffin cells but this effect was absent in GPR109A knockout mice. GPR109A agonists increased AgRP-ir in isolated chromaffin cells through a GPR109A- and pertussis toxin-sensitive pathway. Incubation of adrenal slices in nicotinic acid, a GPR109A agonist, mimicked the fasting-induced increase in the strength of the preganglionic → chromaffin cell synapse. Finally, reverse transcription polymerase chain reaction experiments confirmed the mouse adrenal medulla contains GPR109A messenger RNA. These results are consistent with the activation of a GPR109A signaling pathway located within the adrenal gland. Because fasting evokes epinephrine release, which stimulates lipolysis and the production of β-hydroxybutyrate, our results indicate that chromaffin cells are components of an autonomic-adipose-hepatic feedback circuit. Coupling a change in adrenal physiology to a metabolite whose levels rise during fasting is presumably an efficient way to coordinate the homeostatic response to food deprivation.

5 days of time-restricted feeding increases fat oxidation rate but not affect postprandial lipemia: a crossover trial

Studies have revealed that time-restricted feeding affects the fat oxidation rate; however, its effects on the fat oxidation rate and hyperlipidemia following high-fat meals are unclear. This study investigated the effects of 5-day time-restricted feeding on the fat oxidation rate and postprandial lipemia following high fat meals. In this random crossover experimental study, eight healthy male adults were included each in the 5-day time-restricted feeding trial and the control trial. The meals of the time-restricted feeding trial were provided at 12:00, 16:00, and 20:00. The meals of the control trial were provided at 08:00, 14:00, and 20:00. The contents of the meals of both trials were the same, and the calories of the meals met the 24-h energy requirement of the participants. After 5 days of the intervention, the participants consumed high-fat meals on the sixth day, and their physiological changes were determined. The fasting fat oxidation rate (p < 0.001) and postprandial fat oxidation rate (p = 0.019) of the time-restricted feeding trial were significantly higher than those of the control trial. The 24-h energy consumption and postprandial triglyceride, blood glucose, insulin, glycerol, and free fatty acid concentrations of the two trials showed no significant differences (p > 0.05). The results revealed that 5 days of time-restricted feeding effectively increased the fasting and postprandial fat oxidation rate, but it did not affect postprandial lipemia.

Physiological responses to acute fasting: implications for intermittent fasting programs

Intermittent fasting (IF) is a dietary strategy that involves alternating periods of abstention from calorie consumption with periods of ad libitum food intake. There is significant interest in the body of literature describing longitudinal adaptations to IF. Less attention has been given to the acute physiological responses that occur during the fasting durations that are commonly employed by IF practitioners. Thus, the purpose of this review was to examine the physiological responses - including alterations in substrate metabolism, systemic hormones, and autophagy - that occur throughout an acute fast. Literature searches were performed to locate relevant research describing physiological responses to acute fasting and short-term starvation. A single fast demonstrated the ability to alter glucose and lipid metabolism within the initial 24 hours, but variations in protein metabolism appeared to be minimal within this time frame. The ability of an acute fast to elicit significant increases in autophagy is still unknown. The information summarized in this review can be used to help contextualize existing research and better inform development of future IF interventions.

Adipose tissue lipolysis is regulated by PAQR11 via altering protein stability of phosphodiesterase 4D

Fat storage and mobilization in adipose tissue play a central role in energy metabolism and are directly linked to the development of obesity. Upon starvation, fat is mobilized from adipose tissue by lipolysis, a process by which triglycerides are hydrolyzed to free fatty acids to be used as an energy source in skeletal muscles and other tissues. However, how lipolysis is activated by starvation is not fully known. In this study, we demonstrate that PAQR11, a member of the progesterone and AdipoQ receptor family, regulates starvation-mediated lipolysis. Paqr11-deleted mice are resistant to high-fat diet-induced obesity. Paqr11 deletion promotes lipolysis in white adipose tissue, characterized by increased phosphorylations of hormone-sensitive lipase (HSL) and perilipin 1 (PLIN1) and elevated serum levels of glycerol and free fatty acids. PKA activity and cAMP levels in white adipose tissue are also increased by Paqr11 deletion, accompanied by accelerated protein degradation of phosphodiesterase 4D (PDE4D). Mechanistically, PAQR11 decreases the interaction of PDE4D with SKP1-CUL1-FBXO2 E3 ligase complex, thus modulating the polyubiquitination/degradation of PDE4D. Fasting decreases the expression of the Paqr11 gene, and starvation-induced lipolysis in white adipose tissue is enhanced by Paqr11 deletion, while insulin-mediated suppression of lipolysis is not affected. Collectively, these results reveal that PAQR11 regulates lipolysis of adipose tissue and affects high-fat diet-induced obesity.

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Paqr11 deletion promotes lipolysis in epididymal white adipose tissue.

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PAQR11 modulates cAMP level by altering protein degradation of PDE4D.

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PAQR11 affects the interaction of PDE4D with SKP1-CUL1-FBXO2 E3 ligase complex.

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PAQR11 regulates starvation-induced lipolysis in adipose tissue.

Unique Metabolic Features of Adults Discordant for Indices of Insulin Resistance

PURPOSE

Homeostatic Model Assessment for Insulin Resistance (HOMA-IR) and Adipose Insulin Resistance index (ADIPO-IR) values are often concordant. In this study we evaluated whether there are groups discordant for HOMA-IR and ADIPOpalmitate-IR and, if so, what are their defining characteristics.

METHODS

The body composition, basal metabolic rate (BMR), fasting plasma lipids, insulin, glucose, and free fatty acid (FFA) palmitate concentrations data of 466 volunteers from previous research studies were abstracted and analyzed. The middle 2 population quartiles for HOMA-IR and Adipose Insulin Resistance index palmitate concentration (ADIPOpalmitate-IR) defined medium HOMA-IR and ADIPOpalmitate-IR (MH and MA), the top and bottom quartiles were defined as high/low HOMA (HH/LH), and high/low ADIPOpalmitate as HA/LA. Because ADIPOpalmitate-IR was significantly greater in women than in men, we established sex-specific quartiles for each index. We identified groups discordant for HOMA-IR and ADIPO-IR (HHMA, LHMA, MHHA, and MHLA).

RESULTS

Body fat and fasting triglycerides (TGs) were significantly greater with higher indices in the concordant groups (HHHA > MHMA > LHLA). MHHA differed from MHLA by visceral fat (P < .01) and fasting TGs (P < .05), whereas HHMA differed (P < .01) from LHMA by BMR. By multivariate regression, the group factor contributed to BMR (P < .01) and visceral fat (P < .05).

CONCLUSIONS

Adults discordant for HOMA-IR and ADIPO-IR have unique features including differences in visceral fat, TGs, and BMR. This suggests different forms of insulin resistance are present, which should be considered when studying insulin resistance in the future.

Euglycemic Ketoacidosis

PURPOSE OF REVIEW

Diabetic ketoacidosis is a life-threatening complication of diabetes characterized by hyperglycemia, acidosis, and ketosis. Ketoacidosis may occur with blood glucose level < 200 mg/dl (improperly defined as euglycemic ketoacidosis, euKA) and also in people without diabetes. The absence of marked hyperglycemia can delay diagnosis and treatment, resulting in potential serious adverse outcomes.

RECENT FINDINGS

Recently, with the wide clinical use of sodium glucose co-transporter 2 inhibitors (SGLT2i), euKA has come back into the spotlight. Use of SGLT2i use can predispose to the development of ketoacidosis with relatively low or normal levels of blood glucose. This condition, however, can occur, in the absence of diabetes, in settings such as pregnancy, restriction on caloric intake, glycogen storage diseases or defective gluconeogenesis (alcohol abuse or chronic liver disease), and cocaine abuse. euKA is a challenging diagnosis for most physicians who may be misled by the presence of normal glycemia or mild hyperglycemia. In this article, we review pathophysiology, etiologies, clinical presentation and the management of euKA.

Recharacterizing the Metabolic State of Energy Balance in Thrifty and Spendthrift Phenotypes

PURPOSE

The human thrifty phenotype hypothesis presupposes that lower 24-hour (24h) energy expenditure (24EE) during famine preserves body mass and promotes survival. The prevailing view defines thrifty individuals as having a lower 24EE during fasting. However, it is also plausible that the greater decline in 24EE during fasting in thrifty individuals is due to higher 24EE during energy balance conditions (ENBAL). Herein, we provide evidence that this is indeed the case.

METHODS

In 108 healthy subjects, 24EE was measured in a whole-room indirect calorimeter both during ENBAL and 24h fasting conditions. Subjects were categorized as thrifty or spendthrift based on the median value (-162 kcal/day) of the difference in 24EE (adjusted for body composition) between fasting and ENBAL conditions. Concomitant 24h urinary catecholamines were assessed by liquid chromatography-mass spectrometry.

RESULTS

Compared to ENBAL, 24EE decreased during 24h fasting by 172 kcal/day (standard deviation = 93; range, -470 to 122). A greater-than-median decrease in 24EE ("thriftier" phenotype) was due to higher 24EE during ENBAL (+124 kcal/day; P < 0.0001) but not to lower 24EE during fasting (P = 0.35). Greater fasting-induced increase in epinephrine was associated with concomitant lower decrease in 24EE (r = 0.27; P = 0.006).

MAIN CONCLUSION

The greater decrease in 24EE during acute fasting (which characterizes the thrifty phenotype) is not due to reduced metabolic rate during fasting but to a relatively higher 24EE during feeding conditions, and this decrease in 24EE during fasting is accompanied by a smaller increase in epinephrine. These results recharacterize the prevailing view of the short-term 24EE responses that define the human metabolic phenotypes. Clinical Trials: NCT00523627, NCT00687115, NCT02939404.

Euglycemic Diabetic Ketoacidosis in a Young Pregnant Woman Precipitated by Urinary Tract Infection

Diabetic ketoacidosis (DKA) is a life-threatening diabetic complication and medical emergency. Euglycemic DKA (EKDA) is a variant of DKA with normal range glucose levels. The condition can be difficult to diagnose due to the misleading euglycemic levels. Pregnancy, infection, low-calorie intake, and use of insulin are some of the common etiologies of EDKA. We report a case of a young, pregnant female, with type I diabetes mellitus, in her third trimester admitted with EKDA. The EKDA was triggered by urinary tract infection (UTI), and the patient had other etiologies that have predisposed her to EKDA. Along with the case presentation, we discuss the common etiologies, pathophysiology, and management of EKDA. Euglycemic DKA is a life-threatening emergency that needs to be recognized early and treated aggressively, especially in pregnant patients, to avoid deleterious effects to maternal and fetal health.

Spatiotemporal contact between peroxisomes and lipid droplets regulates fasting-induced lipolysis via PEX5

Lipid droplets (LDs) are key subcellular organelles for regulating lipid metabolism. Although several subcellular organelles participate in lipid metabolism, it remains elusive whether physical contacts between subcellular organelles and LDs might be involved in lipolysis upon nutritional deprivation. Here, we demonstrate that peroxisomes and peroxisomal protein PEX5 mediate fasting-induced lipolysis by stimulating adipose triglyceride lipase (ATGL) translocation onto LDs. During fasting, physical contacts between peroxisomes and LDs are increased by KIFC3-dependent movement of peroxisomes toward LDs, which facilitates spatial translocations of ATGL onto LDs. In addition, PEX5 could escort ATGL to contact points between peroxisomes and LDs in the presence of fasting cues. Moreover, in adipocyte-specific PEX5-knockout mice, the recruitment of ATGL onto LDs was defective and fasting-induced lipolysis is attenuated. Collectively, these data suggest that physical contacts between peroxisomes and LDs are required for spatiotemporal translocation of ATGL, which is escorted by PEX5 upon fasting, to maintain energy homeostasis.

Lipid droplets are organelles that regulate lipid metabolism but if organellar contacts play a role during lipolysis is unclear. Here, the authors show that peroxisomes and peroxisomal protein PEX5 play pivotal roles in the spatial and temporal regulation of fasting-induced lipolysis by translocating ATGL onto lipid droplets

Tanycytes Regulate Lipid Homeostasis by Sensing Free Fatty Acids and Signaling to Key Hypothalamic Neuronal Populations via FGF21 Secretion

The hypothalamus plays a key role in the detection of energy substrates to regulate energy homeostasis. Tanycytes, the hypothalamic ependymo-glia, are located at a privileged position to integrate multiple peripheral inputs. We observed that tanycytes produce and secrete Fgf21 and are located close to Fgf21-sensitive neurons. Fasting, likely via the increase in circulating fatty acids, regulates this central Fgf21 production. Tanycytes store palmitate in lipid droplets and oxidize it, leading to the activation of a reactive oxygen species (ROS)/p38-MAPK signaling pathway, which is essential for tanycytic Fgf21 expression upon palmitate exposure. Tanycytic Fgf21 deletion triggers an increase in lipolysis, likely due to impaired inhibition of key neurons during fasting. Mice deleted for tanycytic Fgf21 exhibit increased energy expenditure and a reduction in fat mass gain, reminiscent of a browning phenotype. Our results suggest that tanycytes sense free fatty acids to maintain body lipid homeostasis through Fgf21 signaling within the hypothalamus.

Core body temperature, energy expenditure, and epinephrine during fasting, eucaloric feeding, and overfeeding in healthy adult men: evidence for a ceiling effect for human thermogenic response to diet

BACKGROUND

In homeothermic animals, approximately 50% of daily energy expenditure (EE) is spent to maintain a consistent core body temperature (CBT). In humans, little is known about CBT responses to feeding and overfeeding and their relationship to diet-related changes in EE.

OBJECTIVE

To study the effects of feeding and overfeeding on CBT and its association with diet-induced thermogenesis (DIT).

DESIGN

Fifty-three healthy men with normal glucose regulation and a wide range of body composition (mean ± SD, body fat: 25 ± 8%, range: 7-43%) had 24-h EE assessed during fasting in a whole-room indirect calorimeter with concomitant CBT measurement by ingestible capsules and 24-h urinary collection for catecholamine measurements. Changes in 24-h EE (DIT) and CBT compared to fasting were assessed during three normal-protein (20%) diets using a cross-over design: one eucaloric diet (EBL, 50% carbohydrate, n = 37) and two overfeeding diets with 200% energy requirements: a high-fat (FNP, 60% fat, n = 25) and a high-carbohydrate (CNP; 75% carbohydrate, n = 24) diet.

RESULTS

The average 24-h CBT (avgCBT) during fasting was 36.81 ± 0.14 °C (inter-individual CV = 0.4%) and positively correlated with 24-h urinary epinephrine (r = 0.61, p < 0.001), but not with body composition measures (p > 0.05). AvgCBT increased during EBL (Δ = 0.06 ± 0.11 °C, p = 0.002), FNP (Δ = 0.13 ± 0.14 °C, p < 0.001), and CNP (Δ = 0.19 ± 0.13 °C, p < 0.001) and associated with increased DIT during EBL (r = 0.43, p = 0.01, β = 31 kcal/day/0.1 °C) and FNP (r = 0.60, p = 0.002, β = 43 kcal/day/0.1 °C), but not CNP (p = 0.47). A ceiling effect for the increase in CBT, but not in DIT, was observed during feeding and, particularly, overfeeding.

CONCLUSIONS

CBT increases with feeding and is moderately associated with DIT to a different degree depending on the macronutrient composition of the overfeeding diet. There is a ceiling effect such that individuals with a higher CBT during fasting have limited capacity to increase CBT with feeding. Because of body thermoregulatory mechanisms that maintain a constant CBT, these results indicate that CBT has a limited role in the inter-individual variability in DIT.

Euglycemic diabetic ketoacidosis

Euglycemic DKA (eu-DKA) is a life-threatening emergency. It may occur in patients with both type 1 and type 2 DM, and characterized by milder degrees of hyperglycemia with blood glucose level < 200 mg/dl, which can result in delayed diagnosis and treatment with potential for adverse metabolic consequences. Following the wide introduction of the sodium glucose transporter 2 inhibitors (SGLT2i) in therapeutic practice for DM type 2 treatment the amount of eu-DKA increased and therefore, interest to this entity rose. Other causes associated with eu-DKA include pregnancy, decreased caloric intake, heavy alcohol use, insulin use prior to hospital admission, cocaine abuse, pancreatitis, sepsis, chronic liver disease and liver cirrhosis. Patients with eu-DKA as well as with DKA need immediate referral for emergency evaluation and treatment. The treatment includes rapid correction of dehydration, correction electrolyte abnormalities, and use of insulin drip until the anion gap, and bicarbonate levels normalize. Increased glucose administration using higher percentages of dextrose (10 or 20%) are required to facilitate the concomitant administration of the relatively large amounts of insulin that are needed to correct the severe acidosis in these patients.

Errors in measuring plasma free fatty acid concentrations with a popular enzymatic colorimetric kit

OBJECTIVES

Our goal was to test whether an enzymatic, colorimetric assay, the WAKO NEFA kit, provides information equivalent to liquid chromatography (LC) LC-based measures of free fatty acid (FFA).

DESIGN & METHODS

We reanalyzed nadir FFA samples from 109 volunteers from a previous study where we demonstrated that maximal suppression of FFA concentrations predicts metabolic abnormalities in humans; the results from the WAKO NEFA kit, which has been widely used for over three decades, could not replicate our findings. We conducted additional studies to directly compare results from this kit to our LC-mass spectrometry (LC/MS) method that was validated by our LC-UV detection method.

RESULTS

Plasma samples with FFA concentrations ranging from 0.015 to 1.813 mmol/L were measured both by LC-mass spectrometry (LC/MS) and by the WAKO NEFA kit. Despite good overall agreement (R² = 0.86), the slope was significantly different from 1.0 and the intercept was significantly different from zero. The results from the kit were especially discrepant with FFA concentrations <0.200 and >1.000 mmol/L. Some of the discrepancy was related to the use of oleate as the standard solution for the kit and the substrate specificity of the kit enzymes for different fatty acids. Despite attempts to improve the kit by modifying the reaction time, sample volume and the types of standard solutions, we could not obtain a satisfactory agreement between the WAKO NEFA results and LC/MS.

CONCLUSIONS

The WAKO NEFA kit should not be used when high precision and accuracy of FFA concentrations over a wide range is required.

Starvation reduces hyaluronan synthesis by suppressing TGF-β1/IGF-I signaling in rat skin

Although starvation has been reported to influence the functions of various tissues, its effects on the skin are not well understood. In this study, we investigated the effect of starvation on hyaluronan synthesis in rat skin. Starvation reduced hyaluronan synthesis in the skin. Starvation also decreased the skin mRNA expression of transforming growth factor (TGF)-β1, which enhances the gene expression of rhas2 and rhas3. The serum levels of insulin-like growth factor (IGF)-I, which enhances rhas2, rhas3, and TGF-β1 mRNA expression, in the starvation group were considerably lower than those in the control (CO) group. IGF-IR phosphorylation was substantially lower in the starvation group compared with the CO group. These findings suggest that starvation reduces hyaluronan synthesis in the skin by suppressing TGF-β1/IGF-I signaling. Abbreviations: HAS: hyaluronan synthase; IGF-I: insulin-like growth factor-I; IGFBP-1: insulin-like growth factor binding protein-1; TGF-β1: transforming growth factor-β1; TBST: tris buffered saline containing 0.5% (v/v) Tween 20; HABP: hyaluronic acid binding protein; GAPDH: glyceraldehyde-3-phosphate dehydrogenase.

The Role of Leptin in Maintaining Plasma Glucose During Starvation

For 20 years it has been known that concentrations of leptin, a hormone produced by the white adipose tissue (WAT) largely in proportion to body fat, drops precipitously with starvation, particularly in lean humans and animals. The role of leptin to suppress the thyroid and reproductive axes during a prolonged fast has been well defined; however, the impact of leptin on metabolic regulation has been incompletely understood. However emerging evidence suggests that, in starvation, hypoleptinemia increases activity of the hypothalamic-pituitary-adrenal axis, promoting WAT lipolysis, increasing hepatic acetyl-CoA concentrations, and maintaining euglycemia. In addition, leptin may be largely responsible for mediating a shift from a reliance upon glucose metabolism (absorption and glycogenolysis) to fat metabolism (lipolysis increasing gluconeogenesis) which preserves substrates for the brain, heart, and other critical organs. In this way a leptin-mediated glucose-fatty acid cycle appears to maintain glycemia and permit survival in starvation.

Orthodox religious fasting as a medical nutrition therapy for dyslipidemia: where do we stand and how far can we go?

Previous studies have reported inconclusive results regarding the effects of Orthodox religious fasting (OF) on common cardiovascular risk factors, including arterial hypertension and diabetes. Contrariwise, an optimal effect on dyslipidemia has been demonstrated and significant reductions in total cholesterol (TC) and low-density lipoprotein-cholesterol (LDL-C) concentrations, in Orthodox populations without comorbidities, have been consistently reported. This favorable effect on lipid homeostasis is probably facilitated by a broad lifestyle modification related to OF, extended beyond dietary habits. However, several clinical issues require further clarification by future trials including the efficacy and safety of OF in specific groups of patients with dyslipidemia, mainly those with cardiovascular disease and diabetes, the effects of OF on high-density lipoprotein-cholesterol (HDL-C), the dietary limitations of specific vitamins and minerals during fasting, as well as the optimal duration of this dietary intervention. The present article aims to discuss, whether there is a strong enough theoretical background, to currently support the implementation of OF as a medical nutrition therapy for dyslipidemia in the daily clinical setting, with a discourse on available evidence and future research agenda.

High-intensity interval training improves performance in young and older individuals by increasing mechanical efficiency

This study evaluated the effects of 6 weeks of high‐intensity interval training (HIIT) on mechanical efficiency (ME) in young and older groups. Seventeen healthy young adults [26.2(2.4) year], and thirteen healthy older adults [54.5(2.3) year] completed a 6‐week HIIT intervention (three sessions per week) on an electromagnetically braked cycle ergometer. Each HIIT session contained six repetitions of supramaximal exercise intervals (6 seconds each) with 2 min of passive recovery between each repetition. ME (%) were computed in net terms across stages corresponding to ventilator thresholds 1 (VT1) and 2 (VT2) and at 100% of maximal oxygen consumption (VO₂max) of an incremental maximal cycling test. After 6 weeks, the ME values did not differ between the two groups and were significantly higher than the ones at baseline (P < 0.01). In this study, the multiple linear regression analysis demonstrated the increases in maximal power (Pmax) contributed significantly to ME increases over 6 weeks at VT1, VT2 and at 100% of VO_2max. This model accounted respectively for 28, 38, and 42%, of the increases. In older adults, ME determined during incremental maximal cycling test increases at VT1, VT2 and at 100% over 6‐week HIIT intervention, and the increment appeared to be related to increases in Pmax. HIIT can be recommended as a strategy aimed at improving muscle efficiency among older adults.

Variation in adrenal and thyroid hormones with life-history stage in juvenile northern elephant seals (Mirounga angustirostris)

The classical approach to quantifying the impact of stressors on wildlife is through characterization of hormones associated with the generalized stress response. However, interpretation of hormone data can be difficult due to the range of natural variation within a species and potential confounds of individual and life-history variables. Blood adrenal and thyroid hormones were measured in 144 chemically immobilized yearling northern elephant seals (Mirounga angustirostris) to characterize variation between sexes and across semiannual haul-outs. There was no relationship between hormone concentrations and time needed for collecting blood nor evidence of diel patterns, suggesting that collection of samples for baseline values can be accomplished without bias due to handling artifacts or time of day. Serum cortisol concentrations did not vary with gender or across haul-out fasts but increased dramatically during molting. Cortisol was correlated with aldosterone across all measured life-history stages. Thyroid hormone levels were lower in females and decreased with fasting in both sexes during the fall haul-out. Cortisol concentrations were inversely associated with total triiodothyronine (T3) and positively associated with reverse T3 concentrations across all measured life-history stages suggesting an important impact of cortisol on deiodinase enzymes and thyroid function. Epinephrine concentrations increased across fasts and norepinephrine concentrations were higher in males than in females. Significant variation in stress hormone concentrations with gender and life-history stage emphasizes the importance of contextual variables when interpreting serum hormone concentrations.

Effects of Orthodox religious fasting on human health: a systematic review

PURPOSE

Different studies have pointed towards a positive effect of religious fasting on human health. Orthodox fasting (OF) regime could be characterized as a periodical vegetarian diet, demonstrating several common characteristics with the typical Mediterranean diet. The present systematic review aimed to synthesize available results regarding the potential impact of OF on human health.

METHODS

Key biomedical databases were searched to identify studies examining the effects of OF on humans. Following implementation of specific criteria, ten studies were included in the analysis and their results were systematically reported and critically appraised in this review.

RESULTS

According to the available limited results, OF periods are characterized by a restriction in total energy and fat intake, an increase in carbohydrate and fiber consumption, while in terms of protein intake, results are contradictive. The overall effect of OF on lipids profile seems to be optimal, with the reduction of total cholesterol and LDL-C levels, being a consistent finding across studies (up to 17.8 and 31.4%, respectively). However, the effect on HDL-C is still unclear. Conclusions regarding the impact on body weight and glucose homeostasis cannot be drawn, given that relevant data are limited with conflicting results. Any potential negative aspects of OF, mainly attributed to reduced dietary intake of vitamin D and B12 and minerals (mainly calcium), require further investigation.

CONCLUSIONS

Given the limitations of available evidence, more studies are required before reaching definite conclusions about the effects of OF on human health.

AgRP-Expressing Adrenal Chromaffin Cells Are Involved in the Sympathetic Response to Fasting

Fasting evokes a homeostatic response that maintains circulating levels of energy-rich metabolites and increases the drive to eat. Centrally, this reflex activates a small population of hypothalamic neurons that are characterized by the expression of AgRP, a neuropeptide with an extremely restricted distribution. Apart from the hypothalamus, the only other site with substantial expression is the adrenal gland, but there is disagreement about which cells synthesize AgRP. Using immunohistochemistry, flow cytometry, and reverse transcription-polymerase chain reaction, we show AgRP is present in the mouse adrenal medulla and is expressed by neuroendocrine chromaffin cells that also synthesize the catecholamines and neuropeptide Y. Short-term fasting led to an increase in adrenal AgRP expression. Because AgRP can act as an antagonist at MC3/4 receptors, we tested whether melanotan II, an MC3/4 receptor agonist, could regulate pre- and postsynaptic signaling within the adrenal medulla. Melanotan II decreased the paired-pulse ratio of evoked synaptic currents recorded in chromaffin cells; this effect was blocked by exogenous AgRP. In contrast, neither melanotan II nor AgRP altered the optogenetically evoked release of catecholamines from isolated chromaffin cells. These results are consistent with the idea that AgRP regulates the strength of the sympathetic input by modulation of presynaptic MC3/4 receptors located on preganglionic neurons. We conclude that a small population of neuroendocrine cells in the adrenal medulla, and the arcuate nucleus of the hypothalamus, express AgRP and neuropeptide Y and are functionally involved in the systemic response to fasting.

Impact of breakfast skipping compared with dinner skipping on regulation of energy balance and metabolic risk

Background: Meal skipping has become an increasing trend of the modern lifestyle that may lead to obesity and type 2 diabetes.Objective: We investigated whether the timing of meal skipping impacts these risks by affecting circadian regulation of energy balance, glucose metabolism, and postprandial inflammatory responses.Design: In a randomized controlled crossover trial, 17 participants [body mass index (in kg/m²): 23.7 ± 4.6] underwent 3 isocaloric 24-h interventions (55%, 30%, and 15% carbohydrate, fat, and protein, respectively): a breakfast skipping day (BSD) and a dinner skipping day (DSD) separated by a conventional 3-meal-structure day (control). Energy and macronutrient balance was measured in a respiration chamber. Postprandial glucose, insulin, and inflammatory responses in leukocytes as well as 24-h glycemia and insulin secretion were analyzed.Results: When compared with the 3-meal control, 24-h energy expenditure was higher on both skipping days (BSD: +41 kcal/d; DSD: +91 kcal/d; both P < 0.01), whereas fat oxidation increased on the BSD only (+16 g/d; P < 0.001). Spontaneous physical activity, 24-h glycemia, and 24-h insulin secretion did not differ between intervention days. The postprandial homeostasis model assessment index (+54%) and glucose concentrations after lunch (+46%) were, however, higher on the BSD than on the DSD (both P < 0.05). Concomitantly, a longer fasting period with breakfast skipping also increased the inflammatory potential of peripheral blood cells after lunch.Conclusions: Compared with 3 meals/d, meal skipping increased energy expenditure. In contrast, higher postprandial insulin concentrations and increased fat oxidation with breakfast skipping suggest the development of metabolic inflexibility in response to prolonged fasting that may in the long term lead to low-grade inflammation and impaired glucose homeostasis. This trial was registered at clinicaltrials.gov as NCT02635139.

Basal rate tests (24-hour fasts) performed in type-1 diabetic subjects with either absolute fasting or snacks containing negligible carbohydrate amounts result in similar glucose profiles: A randomized controlled prospective trial

AIMS

Basal rate tests (24-hour fasting periods) may be necessary to optimize basal insulin replacement in type 1 diabetes. It was the aim of this study to prospectively compare the allowance of negligible carbohydrate snacks vs absolute fasting.

METHODS

A total of 20 patients with type 1 diabetes (age, 48 ± 15 years (9 women, 11 men); BMI, 28.5 ± 4.5 kg/m² ; HbA1c, 8.8% ± 2.0% (73.0 ± 21.9 mmol/mol); insulin dose, 0.69 ± 0.31 IU/kg body weight and per day) participated in 2 basal rate tests lasting 24 hours in random order with unchanged basal insulin replacement. On 1 occasion, negligible carbohydrate snacks (salads and vegetables, up to 5.1 g carbohydrate and 276.3 kJ per portion) were allowed; during the second test subjects were obliged to fast absolutely. Plasma glucose profiles were determined using an exact laboratory method. Hypoglycaemic episodes (plasma glucose < 70 mg/dL) were compared.

RESULTS

Plasma glucose concentrations during fasting periods, with and without negligible carbohydrate snacks, did not differ significantly ( P = .65) and differences were negligible (95% confidence intervals always included a difference of 0 mmol/L). Also, there was no difference in the number of hypoglycaemic plasma glucose values (P = .40) or in compensatory carbohydrate intake. Basal rate testing with negligible carbohydrate snacks was better tolerated (questionnaire, P = .046) and the desire to discontinue the fasting period was significantly reduced (P = .023).

CONCLUSIONS

Allowing negligible carbohydrate snacks results in unchanged plasma glucose profiles during basal rate testing and is better tolerated by patients with type 1 diabetes.

The Consistency in Macronutrient Oxidation and the Role for Epinephrine in the Response to Fasting and Overfeeding

Context

In humans, dietary vs intraindividual determinants of macronutrient oxidation preference and the role of the sympathetic nervous system (SNS) during short-term overfeeding and fasting are unclear.

Objective

To understand the influence on metabolic changes of diet and SNS during 24 hours of overfeeding.

Design, Setting, Participants, and Interventions

While residing on a clinical research unit, 64 participants with normal glucose regulation were assessed during energy balance, fasting, and four 24-hour overfeeding diets, given in random order. The overfeeding diets contained 200% of energy requirements and varied macronutrient proportions: (1) standard (50% carbohydrate, 20% protein, and 30% fat); (2) 75% carbohydrate; (3) 60% fat; and (4) 3% protein.

Main Outcome Measures

Twenty-four-hour energy expenditure (EE) and macronutrient oxidation rates were measured in an indirect calorimeter during the dietary interventions, with concomitant measurement of urinary catecholamines and free cortisol.

Results

EE decreased with fasting (-7.7% ± 4.8%; P < 0.0001) and increased with overfeeding. The smallest increase occurred during consumption of the diet with 3% protein (2.7% ± 4.5%; P = 0.001) and the greatest during the diet with 75% carbohydrate (13.8 ± 5.7%; P < 0.0001). Approximately 60% of macronutrient oxidation was determined by diet and 20% by intrinsic factors (P < 0.0001). Only urinary epinephrine differed between fasting and overfeeding diets (Δ = 2.25 ± 2.9 µg/24h; P < 0.0001). During fasting, higher urinary epinephrine concentrations correlated with smaller reductions in EE (ρ = 0.34; P = 0.01).

Conclusions

Independent from dietary macronutrient proportions, there is a strong individual contribution to fuel preference that remains consistent across diets. Higher urinary epinephrine levels may reflect the importance of epinephrine in maintaining EE during fasting.

In Vitro lipolysis is associated with whole-body lipid oxidation and weight gain in humans

OBJECTIVE

To assess the association of adipocyte size with cellular lipolysis and between cellular lipolysis and whole-body lipid oxidation. This study also assessed the association between adipocyte size and cellular lipolysis with weight and fat mass gain.

METHODS

Subjects had assessment of percent body fat (%fat) and adipose tissue biopsy for in vitro lipolysis (n = 325), and a subset of subjects had measurement of whole-body lipid oxidation (n = 112). A subset of subjects (n = 243) returned for repeated measurements of body weight and composition (mean follow-up 8.2 ± 5.5 years).

RESULTS

In vitro lipolysis (r = 0.47, P < 0.0001) and adipocyte size (r = 0.49, P < 0.0001) were strongly associated with %fat. In vitro lipolysis (P = 0.04) but not adipocyte size (P = 0.44) was associated with whole-body fat oxidation. Adipocyte size was not associated with rate of percent weight gain (P = 0.20) but was negatively associated with rate of percent fat mass gain (P = 0.01). In vitro lipolysis was negatively associated with rate of percent weight gain (P = 0.02) and had a marginal negative association with rate of percent fat mass gain (P = 0.08).

CONCLUSIONS

These results indicate inherent characteristics of adipocytes, including size and lipolytic activity, may be important determinants of whole-body lipid oxidation and subsequent weight gain.

Mechanical efficiency improvement in relation to metabolic changes in sedentary obese adults

Purpose

Mechanical efficiency (ME) refers to the ability of an individual to transfer energy consumed by external work. This performance indicator is impaired by obesity and is associated with decreased high-intensity exercise performance. However, it is unclear if ME may be improved in response to high intensity training (HIT). This study aimed to determine if ME increases in response to HIT in obese adults and to identify the factors associated with these changes.

Methods

24 obese adults (body mass index=∼33 kg/m²) were randomised into control (n=12) and trained (n=12) groups. Following baseline metabolic, anthropometric, fitness and ME measurements, the participants completed a 6-week exercise intervention that included 18 sessions of six repeats of 6 s supramaximal sprints on an electromagnetically braked cycle ergometer. The metabolic, anthropometric and fitness assessments were repeated postintervention. ME (expressed as a %) was calculated during an incremental maximal cycling test at stages of 25, 50, 75, 100 and 125 W.

Results

ME did not differ across the groups at 25 and 50 W. Following HIT, ME increased significantly at 75, 100 and 125 W (p<0.01, respectively) compared with the control group (p<0.01, respectively). Although no changes in fat-free mass were observed following HIT, the increases in ME at 75, 100 and 125 W correlated positively with both homeostasis model assessment-estimated insulin resistance index decreases (r=0.9; r=0.89 and r=0.88, p<0.01, respectively) and peak power increases (r=0.87, r=0.88 and r=0.9, p<0.01, respectively).

Conclusions

Although there were no changes in the participants’ anthropometric variables, HIT improved ME in obese adults, an enhancement that appears to be related to increases in muscle strength and metabolic adaptations.

Effects of Acute Supramaximal Cycle Exercise on Plasma FFA Concentration in Obese Adolescent Boys

AIMS

The aims of the present study are 1) to evaluate the free fatty acid (FFA) profile and 2) to determine the relative anaerobic and aerobic contributions to total energy consumption during repeated supramaximal cycling bouts (SCE) in adolescent boys with different body weight statuses.

MATERIALS AND METHODS

Normal-weight (NW), overweight (OW), and obese (OB) adolescent boys (n =15 per group) completed a SCE sessions consisted of 6 x 6s maximal sprints with 2 min of passive rest between each repetition. Plasma FFA levels were determined at rest, immediately after a 10 min warm-up, and immediately at the end of SCE. The anaerobic and aerobic contributions (%) were measured via repeated SCE bouts. Insulin resistance was calculated using the homoeostatic model assessment (HOMA-IR) index.

RESULTS

The FFA concentrations measured immediately after SCE were higher in the OB group than in the OW and NW (p<0.01 and p<0.01, respectively) groups. Moreover, the anaerobic contributions to SCE were significantly lower in obese adolescents (p<0.01) and decreased significantly during the 2nd, 3rd and 4th repetitions. The FFA levels were significantly associated with the HOMA-IR index and aerobic contribution among adolescent boys (r=0.83 and r=0.91, respectively, p<0.01).

CONCLUSION

In contrast to the NW and OW groups, there is an increase in lipid mobilization and sift to aerobic energy metabolism during SCE in the OB group.

Effects of weight loss via high fat vs. low fat alternate day fasting diets on free fatty acid profiles

Cardiovascular disease risk is associated with excess body weight and elevated plasma free fatty acid (FFA) concentrations. This study examines how an alternate-day fasting (ADF) diet high (HF) or low (LF) in fat affects plasma FFA profiles in the context of weight loss, and changes in body composition and lipid profiles. After a 2-week weight maintenance period, 29 women (BMI 30-39.9 kg/m(2)) 25-65 years old were randomized to an 8-week ADF-HF (45% fat) diet or an ADF-LF (25% fat) diet with 25% energy intake on fast days and ad libitum intake on feed days. Body weight, BMI and waist circumference were assessed weekly and body composition was measured using dual x-ray absorptiometry (DXA). Total and individual FFA and plasma lipid concentrations were measured before and after weight loss. Body weight, BMI, fat mass, total cholesterol, LDL-C and triglyceride concentrations decreased (P < 0.05) in both groups. Total FFA concentrations also decreased (P < 0.001). In the ADF-LF group, decreases were found in several more FFAs than in the ADF-HF group. In the ADF-HF group, FFA concentrations were positively correlated with waist circumference. Depending on the macronutrient composition of a diet, weight loss with an ADF diet decreases FFA concentrations through potentially different mechanisms.

Contributions of white and brown adipose tissues and skeletal muscles to acute cold-induced metabolic responses in healthy men

KEY POINTS

Both brown adipose tissue (BAT) and skeletal muscle activation contribute to the metabolic response of acute cold exposure in healthy men even under minimal shivering. Activation of adipose tissue intracellular lipolysis is associated with BAT metabolic response upon acute cold exposure in healthy men. Although BAT glucose uptake per volume of tissue is important, the bulk of glucose turnover during cold exposure is mediated by skeletal muscle metabolic activation even when shivering is minimized.

ABSTRACT

Cold exposure stimulates the sympathetic nervous system (SNS), triggering the activation of cold-defence responses and mobilizing substrates to fuel the thermogenic processes. Although these processes have been investigated independently, the physiological interaction and coordinated contribution of the tissues involved in producing heat or mobilizing substrates has never been investigated in humans. Using [U-(13)C]-palmitate and [3-(3)H]-glucose tracer methodologies coupled with positron emission tomography using (11)C-acetate and (18)F-fluorodeoxyglucose, we examined the relationship between whole body sympathetically induced white adipose tissue (WAT) lipolysis and brown adipose tissue (BAT) metabolism and mapped the skeletal muscle shivering and metabolic activation pattern during a mild, acute cold exposure designed to minimize shivering response in 12 lean healthy men. Cold-induced increase in whole-body oxygen consumption was not independently associated with BAT volume of activity, BAT oxidative metabolism, or muscle metabolism or shivering intensity, but depended on the sum of responses of these two metabolic tissues. Cold-induced increase in non-esterified fatty acid (NEFA) appearance rate was strongly associated with the volume of metabolically active BAT (r = 0.80, P = 0.005), total BAT oxidative metabolism (r = 0.70, P = 0.004) and BAT glucose uptake (r = 0.80, P = 0.005), but not muscle glucose metabolism. The total glucose uptake was more than one order of magnitude greater in skeletal muscles compared to BAT during cold exposure (674 ± 124 vs. 12 ± 8 μmol min(-1), respectively, P < 0.001). Glucose uptake demonstrated that deeper, centrally located muscles of the neck, back and inner thigh were the greatest contributors of muscle glucose uptake during cold exposure due to their more important shivering response. In summary, these results demonstrate for the first time that the increase in plasma NEFA appearance from WAT lipolysis is closely associated with BAT metabolic activation upon acute cold exposure in healthy men. In humans, muscle glucose utilization during shivering contributes to a much greater extent than BAT to systemic glucose utilization during acute cold exposure.

Dissecting adipose tissue lipolysis: molecular regulation and implications for metabolic disease

Lipolysis is the process by which triglycerides (TGs) are hydrolyzed to free fatty acids (FFAs) and glycerol. In adipocytes, this is achieved by sequential action of adipose TG lipase (ATGL), hormone-sensitive lipase (HSL), and monoglyceride lipase. The activity in the lipolytic pathway is tightly regulated by hormonal and nutritional factors. Under conditions of negative energy balance such as fasting and exercise, stimulation of lipolysis results in a profound increase in FFA release from adipose tissue (AT). This response is crucial in order to provide the organism with a sufficient supply of substrate for oxidative metabolism. However, failure to efficiently suppress lipolysis when FFA demands are low can have serious metabolic consequences and is believed to be a key mechanism in the development of type 2 diabetes in obesity. As the discovery of ATGL in 2004, substantial progress has been made in the delineation of the remarkable complexity of the regulatory network controlling adipocyte lipolysis. Notably, regulatory mechanisms have been identified on multiple levels of the lipolytic pathway, including gene transcription and translation, post-translational modifications, intracellular localization, protein-protein interactions, and protein stability/degradation. Here, we provide an overview of the recent advances in the field of AT lipolysis with particular focus on the molecular regulation of the two main lipases, ATGL and HSL, and the intracellular and extracellular signals affecting their activity.

Keap1-knockdown decreases fasting-induced fatty liver via altered lipid metabolism and decreased fatty acid mobilization from adipose tissue

AIMS

The purpose of this study was to determine whether Nrf2 activation, via Keap1-knockdown (Keap1-KD), regulates lipid metabolism and mobilization induced by food deprivation (e.g. fasting).

METHODS AND RESULTS

Male C57BL/6 (WT) and Keap1-KD mice were either fed ad libitum or food deprived for 24 hours. After fasting, WT mice exhibited a marked increase in hepatic lipid accumulation, but Keap1-KD mice had an attenuated increase of lipid accumulation, along with reduced expression of lipogenic genes (acetyl-coA carboxylase, stearoyl-CoA desaturase-1, and fatty acid synthase) and reduced expression of genes related to fatty acid transport, such as fatty acid translocase/CD36 (CD36) and Fatty acid transport protein (FATP) 2, which may attribute to the reduced induction of Peroxisome proliferator-activated receptor (Ppar) α signaling in the liver. Additionally, enhanced Nrf2 activity by Keap1-KD increased AMP-activated protein kinase (AMPK) phosphorylation in liver. In white adipose tissue, enhanced Nrf2 activity did not change the lipolysis rate by fasting, but reduced expression of fatty acid transporters--CD36 and FATP1, via a PPARα-dependent mechanism, which impaired fatty acid transport from white adipose tissue to periphery circulation system, and resulted in increased white adipose tissue fatty acid content. Moreover, enhanced Nrf2 activity increased glucose tolerance and Akt phosphorylation levels upon insulin administration, suggesting Nrf2 signaling pathway plays a key role in regulating insulin signaling and enhanced insulin sensitivity in skeletal muscle.

CONCLUSION

Enhanced Nrf2 activity via Keap1-KD decreased fasting-induced steatosis, pointing to an important function of Nrf2 on lipid metabolism under the condition of nutrient deprivation.

Plasma sphingosine-1-phosphate is elevated in obesity

Background

Dysfunctional lipid metabolism is a hallmark of obesity and insulin resistance and a risk factor for various cardiovascular and metabolic complications. In addition to the well known increase in plasma triglycerides and free fatty acids, recent work in humans and rodents has shown that obesity is associated with elevations in the bioactive class of sphingolipids known as ceramides. However, in obesity little is known about the plasma concentrations of sphinogsine-1-phosphate (S1P), the breakdown product of ceramide, which is an important signaling molecule in mammalian biology. Therefore, the purpose of this study was to examine the impact of obesity on circulating S1P concentration and its relationship with markers of glucose metabolism and insulin sensitivity.

Methodology/Principal Findings

Plasma S1P levels were determined in high-fat diet (HFD)-induced and genetically obese (ob/ob) mice along with obese humans. Circulating S1P was elevated in both obese mouse models and in obese humans compared with lean healthy controls. Furthermore, in humans, plasma S1P positively correlated with total body fat percentage, body mass index (BMI), waist circumference, fasting insulin, HOMA-IR, HbA1c (%), total and LDL cholesterol. In addition, fasting increased plasma S1P levels in lean healthy mice.

Conclusion

We show that elevations in plasma S1P are a feature of both human and rodent obesity and correlate with metabolic abnormalities such as adiposity and insulin resistance.

Cellular mechanisms regulating fuel metabolism in mammals: role of adipose tissue and lipids during prolonged food deprivation

Food deprivation in mammals results in profound changes in fuel metabolism and substrate regulation. Among these changes are decreased reliance on the counter-regulatory dynamics by insulin-glucagon due to reduced glucose utilization, and increased concentrations of lipid substrates in plasma to meet the energetic demands of peripheral tissues. As the primary storage site of lipid substrates, adipose tissue must then be a primary contributor to the regulation of metabolism in food deprived states. Through its regulation of lipolysis, adipose tissue influences the availability of carbohydrate, lipid, and protein substrates. Additionally, lipid substrates can act as ligands to various nuclear receptors (retinoid x receptor (RXR), liver x receptor (LXR), and peroxisome proliferator-activated receptor (PPAR)) and exhibit prominent regulatory capabilities over the expression of genes involved in substrate metabolism within various tissues. Therefore, through its control of lipolysis, adipose tissue also indirectly regulates the utilization of metabolic substrates within peripheral tissues. In this review, these processes are described in greater detail and the extent to which adipose tissue and lipid substrates regulate metabolism in food deprived mammals is explored with comments on future directions to better assess the contribution of adipose tissue to metabolism.

Adaptive reciprocity of lipid and glucose metabolism in human short-term starvation

The human organism has tools to cope with metabolic challenges like starvation that are crucial for survival. Lipolysis, lipid oxidation, ketone body synthesis, tailored endogenous glucose production and uptake, and decreased glucose oxidation serve to protect against excessive erosion of protein mass, which is the predominant supplier of carbon chains for synthesis of newly formed glucose. The starvation response shows that the adaptation to energy deficit is very effective and coordinated with different adaptations in different organs. From an evolutionary perspective, this lipid-induced effect on glucose oxidation and uptake is very strong and may therefore help to understand why insulin resistance in obesity and type 2 diabetes mellitus is difficult to treat. The importance of reciprocity in lipid and glucose metabolism during human starvation should be taken into account when studying lipid and glucose metabolism in general and in pathophysiological conditions in particular.

Targeting presynaptic norepinephrine transporter in brown adipose tissue: a novel imaging approach and potential treatment for diabetes and obesity

Brown adipose tissue (BAT) plays a significant role in metabolism. In this study, we report the use of atomoxetine (a clinically applicable norepinephrine reuptake inhibitor) for (18)F-FDG PET imaging of BAT and its effects on heat production and blood glucose concentration. Fasted-male Sprague-Dawley rats were administered with intravenous (18)F-FDG. The same rats were treated with atomoxetine (0.1 mg/kg, i.v.) 30 min before (18)F-FDG administration. To confirm the β-adrenergic effects, propranolol (β-adrenergic inhibitor) 5 mg/kg was given intraperitoneally 30 min prior to atomoxetine administration. The effect of atomoxetine on BAT metabolism was assessed in fasted and non-fasted rats and on BAT temperature and blood glucose in fasted rats. In (18)F-FDG PET/CT images, interscapular BAT (IBAT) and other areas of BAT were clearly visualized. When rats were fasted, atomoxetine (0.1 mg/kg) increased the (18)F-FDG uptake of IBAT by factor of 24 within 30 min. Propranolol reduced the average (18)F-FDG uptake of IBAT significantly. Autoradiography of IBAT and white adipose tissue confirmed the data obtained by PET. When rats were not fasted, atomoxetine-induced increase of (18)F-FDG uptake in IBAT was delayed and occurred in 120 min. For comparison, direct stimulation of β(3)-adrenreceptors in non-fasted rats with CL-316, 243 occurred within 30 min. Atomoxetine-induced IBAT activation was associated with higher IBAT temperature and lower blood glucose. This was mediated by inhibition of norepinephrine reuptake transporters in IBAT leading to increased norepinephrine concentration in the synapse. Increased synaptic norepinephrine activates β(3)-adrenreceptors resulting in BAT hypermetabolism that is visible and quantifiable by (18)F-FDG PET/CT.

Free fatty acid storage in human visceral and subcutaneous adipose tissue: role of adipocyte proteins

OBJECTIVE

Because direct adipose tissue free fatty acid (FFA) storage may contribute to body fat distribution, we measured FFA (palmitate) storage rates and fatty acid (FA) storage enzymes/proteins in omental and abdominal subcutaneous fat.

RESEARCH DESIGN AND METHODS

Elective surgery patients received a bolus of [1-(14)C]palmitate followed by omental and abdominal subcutaneous fat biopsies to measure direct FFA storage. Long chain acyl-CoA synthetase (ACS) and diacylglycerol acyltransferase activities, CD36, fatty acid-binding protein, and fatty acid transport protein 1 were measured.

RESULTS

Palmitate tracer storage (dpm/g adipose lipid) and calculated palmitate storage rates were greater in omental than abdominal subcutaneous fat in women (1.2 ± 0.8 vs. 0.7 ± 0.4 μmol · kg adipose lipid(-1) · min(-1), P = 0.005) and men (0.7 ± 0.2 vs. 0.2 ± 0.1, P < 0.001), and both were greater in women than men (P < 0.0001). Abdominal subcutaneous adipose tissue palmitate storage rates correlated with ACS activity (women: r = 0.66, P = 0.001; men: r = 0.70, P = 0.007); in men, CD36 was also independently related to palmitate storage rates. The content/activity of FA storage enzymes/proteins in omental fat was dramatically lower in those with more visceral fat. In women, only omental palmitate storage rates were correlated (r = 0.54, P = 0.03) with ACS activity.

CONCLUSIONS

Some adipocyte FA storage factors correlate with direct FFA storage, but sex differences in this process in visceral fat do not account for sex differences in visceral fatness. The reduced storage proteins in those with greater visceral fat suggest that the storage factors we measured are not a predominant cause of visceral adipose tissue accumulation.

Fasting, but not exercise, increases adipose triglyceride lipase (ATGL) protein and reduces G(0)/G(1) switch gene 2 (G0S2) protein and mRNA content in human adipose tissue

CONTEXT

Fasting and exercise are characterized by increased lipolysis, but the underlying mechanisms are not fully understood.

OBJECTIVE

The study was designed to test whether fasting and exercise affect mRNA and protein levels of adipose triglyceride lipase (ATGL) and G(0)/G(1) switch gene 2 (G0S2), a recently discovered ATGL inhibitor, in humans.

DESIGN AND PARTICIPANTS

We studied eight healthy men (age, 25.5 ± 4.3 yr) for 6 h (a 4-h basal and a 2-h clamp period) on three occasions in a randomized crossover design: 1) in the basal state and after; 2) 72-h fasting; and 3) 1-h exercise (65% VO(2max)). Subcutaneous abdominal adipose tissue (AT) biopsies were taken at t = 30 and 270 min.

SETTING

The study was conducted at a university hospital research unit.

RESULTS

Circulating free fatty acids and GH were increased, and C-peptide was decreased by both fasting and exercise. During fasting, insulin failed to suppress free fatty acid levels, suggesting AT insulin resistance. ATGL protein was increased 44% (P < 0.001), and G0S2 mRNA and protein were decreased 56% (P = 0.02) and 54% (P = 0.01), respectively, after fasting, but both ATGL and G0S2 were unaffected by exercise. Protein levels of hormone-sensitive lipase and comparative gene identification-58 were unaffected throughout.

CONCLUSIONS

We found increased AT content of ATGL and decreased protein and mRNA content of the ATGL inhibitor G0S2, suggesting increased ATGL activity during fasting, but not after short-term exercise. These findings are compatible with the notion that the ATGL-G0S2 complex is an important long-term regulator of lipolysis under physiological conditions such as fasting in humans.

Galactose promotes fat mobilization in obese lactating and nonlactating women

BACKGROUND

Galactose consumption as the only carbohydrate source results in little increase in plasma glucose and insulin concentrations when compared with fasting. Lower insulin might promote endogenous lipolysis during meal absorption, which may facilitate fat loss.

OBJECTIVE

The objective was to test the hypothesis that consumption of an isocaloric, isonitrogenous galactose drink will result in higher rates of lipolysis and fat oxidation than consumption of a glucose drink in obese lactating and nonlactating women.

DESIGN

Seven healthy, obese, exclusively breastfeeding women and 7 healthy, obese, nonlactating women were studied on 2 occasions according to a randomized, crossover, single-blinded design. Subjects received drinks providing ≈70% of the daily estimated energy requirement, of which 60% was either glucose or galactose. The primary outcomes were the rate of appearance (Ra) of glycerol and palmitate, and the secondary outcomes were glucose Ra, milk production, energy expenditure, and substrate oxidation.

RESULTS

Plasma glucose and insulin concentrations were lower (P < 0.05) and those of glycerol, palmitate, free fatty acids, and triglycerides were higher (P < 0.05) during galactose than during glucose feeding in both nonlactating and lactating women. During galactose feeding, glucose Ra was lower (P < 0.01) and glycerol, palmitate, and free fatty acid Ra were higher (P < 0.01) in both groups. During galactose feeding in all women combined, fat oxidation was higher (P = 0.01) and protein oxidation was lower (P < 0.01). Milk production, energy expenditure, and carbohydrate oxidation were similar between glucose and galactose feeding.

CONCLUSIONS

Galactose consumption is associated with higher endogenous fat mobilization and oxidation during meal absorption. Long-term studies are required to determine whether galactose as an exclusive carbohydrate source would promote body fat loss in obese subjects.

Hypoglycemia: A Possible Link between Insulin Resistance, Metabolic Dyslipidemia, and Heart and Kidney Disease (the Cardiorenal Syndrome)

Resistance to insulin metabolic signaling in adipose tissue contributes to the lipid abnormalities in obese, hyperinsulinemic, insulin-resistant patients who develop the cardiorenal syndrome. These same metabolic dyslipidemic abnormalities can be found in conditions of caloric energy restriction with decreased adiposity or normal insulin levels, such as anorexia, starvation or non-diabetic kidney disease. In this review, we assess hypoglycemia as an alternative physiological explanation for the biochemical and lipid findings in conditions of insulin resistance (IR). Therefore, PubMed databases (1961-2010) were searched for articles on the effect of hypoglycemia and starvation on non-esterified fatty acid (NEFA) elevation and abnormalities in insulin signaling in muscles as well as abnormal kidney metabolism. The search included articles on NEFA and their role in triglyceride (TG) and high-density lipoprotein (HDL) metabolism, as well as kidney and heart disease. Available studies support that hypoglycemia increases NEFA generation from adipose tissue. Elevated levels of NEFA induce increased plasma levels of TG and decreased levels of HDL cholesterol, and may cause direct kidney and myocardial damage. IR of adipose and skeletal muscle tissue, and the elevation in insulin levels in obese, insulin-resistant patients could be explained by an adaptation to their carbohydrate intake. These molecular abnormalities in insulin metabolic signaling can also be found in hypoglycemia or starvation. In conclusion, IR of adipose tissue cannot fully explain the lipid abnormalities observed in the cardiorenal syndrome. Decreased blood glucose levels (e.g. hypoglycemia) occur frequently in patients at risk for this syndrome. Hypoglycemia-induced increases in NEFA levels can promote lipid abnormalities that contribute to IR and the cardiorenal syndrome.