Concurrent physical activity increases fat oxidation during the shift to a high-fat diet

Metabolically Healthy Obesity: Criteria, Epidemiology, Controversies, and Consequences

PURPOSE OF REVIEW

To present a comprehensive overview regarding criteria, epidemiology, and controversies that have arisen in the literature about the existence and the natural course of the metabolic healthy phenotype.

RECENT FINDINGS

The concept of metabolically healthy obesity (MHO) implies that a subgroup of obese individuals may be free of the cardio-metabolic risk factors that commonly accompany obese subjects with adipose tissue dysfunction and insulin resistance, known as having metabolic syndrome or the metabolically unhealthy obesity (MUO) phenotype. Individuals with MHO appear to have a better adipose tissue function, and are more insulin sensitive, emphasizing the central role of adipose tissue function in metabolic health. The reported prevalence of MHO varies widely, and this is likely due the lack of universally accepted criteria for the definition of metabolic health and obesity. Also, the natural course and the prognostic value of MHO is hotly debated but it appears that it likely evolves towards MUO, carrying an increased risk for cardiovascular disease and mortality over time. Understanding the pathophysiology and the determinants of metabolic health in obesity will allow a better definition of the MHO phenotype. Furthermore, stratification of obese subjects, based on metabolic health status, will be useful to identify high-risk individuals or subgroups and to optimize prevention and treatment strategies to compact cardio-metabolic diseases.

In the Footsteps of Wilbur Olin Atwater: The Atwater Lecture for 2019

A central theme of Atwater's research was the development and application of methods to understand how human beings and animals adapt to the nutrients they ingest. The research described in this article also deals with adaptation to nutrition focusing on adaptation to overnutrition, adaptation to undernutrition, adaptation to dietary fat, adaptation to dietary protein, adaptation to micronutrients, and adaptation to sugar and high-fructose corn syrup (HFCS). Studies using overfeeding have shown several things. First, overfeeding did not change the thermic response to ingestion of food nor the coupling of oxidative phosphorylation in muscle to energy expended by muscles during work on a bicycle ergometer between 25 and 100 watts. Second, the response to overfeeding was significantly influenced by the quantity of protein in the diet. During carefully controlled studies of underfeeding of people with obesity, the macronutrient composition of the diet did not affect the magnitude of weight loss. However, baseline genetic and metabolic information could provide guidance for selecting among the lower or higher protein diets, and lower or higher fat diets. Adaptation to an increase in dietary fat from 35% to 50% is slow and variable in healthy sedentary men. Adaptation is more rapid and complete when these same men were physically active. This effect of muscular exercise was traced to changes in the metabolism of glucose in muscles where pathways inhibiting glucose metabolism were activated by exercise. Dietary patterns that increased the intake of calcium, magnesium, and potassium effectively lower blood pressure in individuals with high normal blood pressure. Finally, the intake of sugary beverages was related to the onset of the current epidemic of obesity.

Impact of energy turnover on fat balance in healthy young men during energy balance, energetic restriction and overfeeding

Body weight control is thought to be improved when physical activity and energy intake are both high (high energy turnover (ET)). The aim of the present study was to investigate the short-term impact of ET on fat balance during zero energy balance (EB), energetic restriction (ER) and overfeeding (OF). In a randomised crossover study, nine healthy men (BMI: 23·0 (SD 2·1) kg/m2, 26·6 (SD 3·5) years) passed 3 × 3 d in a metabolic chamber: three levels of ET (low, medium and high; physical activity level = 1·3-1·4, 1·5-1·6 and 1·7-1·8) were performed at zero EB, ER and OF (100, 75 and 125 % of individual energy requirement). Different levels of ET were obtained by walking (4 km/h) on a treadmill (0, 165 and 330 min). Twenty-four-hour macronutrient oxidation and relative macronutrient balance (oxidation relative to intake) was calculated, and NEFA, 24-h insulin and catecholamine secretion were analysed as determinants of fat oxidation. During EB and OF, 24-h fat oxidation increased with higher ET. This resulted in a higher relative fat balance at medium ET (EB: +17 %, OF: +14 %) and high ET (EB: +23 %, OF: +17 %) compared with low ET (all P < 0·05). In contrast, ER led to a stimulation of 24-h fat oxidation irrespective of ET (no differences in relative fat balance between ET levels, P > 0·05). In conclusion, under highly controlled conditions, a higher ET improved relative fat balance in young healthy men during OF and EB compared with a sedentary state.

Metabolically healthy versus metabolically unhealthy obesity

Obesity-related disease complications reduce life quality and expectancy and increase health-care costs. Some studies have suggested that obesity not always entails metabolic abnormalities and increased risk of cardiometabolic complications. Because of the lack of universally accepted criteria to identify metabolically healthy obesity (MHO), its prevalence varies widely among studies. Moreover, the prognostic value of MHO is hotly debated, mainly because it likely shifts gradually towards metabolically unhealthy obesity (MUO). In this review, we outline the differential factors contributing to the metabolic heterogeneity of obesity by discussing the behavioral, genetic, phenotypical, and biological aspects associated with each of the two metabolic phenotypes (MHO and MUO) of obesity and their clinical implications. Particular emphasis will be laid on the role of adipose tissue biology and function, including genetic determinants of body fat distribution, depot-specific fat metabolism, adipose tissue plasticity and, particularly, adipogenesis. Finally, the emerging role of gut microbiota in obesity and adipose tissue dysfunction as well as the search for novel biomarkers for the obesity-related metabolic traits and associated diseases will be briefly presented. A better understanding of the main determinants of a healthy metabolic status in obesity would allow promotion of this favorable condition by targeting the relevant pathways.

Pathways and mechanisms linking dietary components to cardiometabolic disease: thinking beyond calories

Calories from any food have the potential to increase risk for obesity and cardiometabolic disease because all calories can directly contribute to positive energy balance and fat gain. However, various dietary components or patterns may promote obesity and cardiometabolic disease by additional mechanisms that are not mediated solely by caloric content. Researchers explored this topic at the 2017 CrossFit Foundation Academic Conference 'Diet and Cardiometabolic Health - Beyond Calories', and this paper summarizes the presentations and follow-up discussions. Regarding the health effects of dietary fat, sugar and non-nutritive sweeteners, it is concluded that food-specific saturated fatty acids and sugar-sweetened beverages promote cardiometabolic diseases by mechanisms that are additional to their contribution of calories to positive energy balance and that aspartame does not promote weight gain. The challenges involved in conducting and interpreting clinical nutritional research, which preclude more extensive conclusions, are detailed. Emerging research is presented exploring the possibility that responses to certain dietary components/patterns are influenced by the metabolic status, developmental period or genotype of the individual; by the responsiveness of brain regions associated with reward to food cues; or by the microbiome. More research regarding these potential 'beyond calories' mechanisms may lead to new strategies for attenuating the obesity crisis.

Carbon dioxide generation rates for building occupants

Indoor carbon dioxide (CO2 ) concentrations have been used for decades to characterize building ventilation and indoor air quality. Many of these applications require rates of CO2 generation from the building occupants, which are currently based on approaches and data that are several decades old. However, CO2 generation rates can be derived from well-established concepts within the fields of human metabolism and exercise physiology, which relate these rates to body size and composition, diet, and level of physical activity. This paper reviews how CO2 generation rates have been estimated in the past and discusses how they can be characterized more accurately. Based on this information, a new approach to estimating CO2 generation rates is presented, which is based on the described concepts from the fields of human metabolism and exercise physiology. Using this approach and more recent data on body mass and physical activity, values of CO2 generation rates from building occupants are presented along with the variability that may occur based on body mass and activity data.

Sedentary behaviour is a key determinant of metabolic inflexibility

Metabolic flexibility is defined as the ability to adapt substrate oxidation rates in response to changes in fuel availability. The inability to switch between the oxidation of lipid and carbohydrate appears to be an important feature of chronic disorders such as obesity and type 2 diabetes. Laboratory assessment of metabolic flexibility has traditionally involved measurement of the respiratory quotient (RQ) by indirect calorimetry during the fasted to fed transition (e.g. mixed meal challenge) or during a hyperinsulinaemic-euglycaemic clamp. Under these controlled experimental conditions, 'metabolic inflexibility' is characterized by lower fasting fat oxidation (higher fasting RQ) and/or an impaired ability to oxidize carbohydrate during feeding or insulin-stimulated conditions (lower postprandial or clamp RQ). This experimental paradigm has provided fundamental information regarding the role of substrate oxidation in the development of obesity and insulin resistance. However, the key determinants of metabolic flexibility among relevant clinical populations remain unclear. Herein, we propose that habitual physical activity levels are a primary determinant of metabolic flexibility. We present evidence demonstrating that high levels of physical activity predict metabolic flexibility, while physical inactivity and sedentary behaviours trigger a state of metabolic 'inflexibility', even among individuals who meet physical activity recommendations. Furthermore, we describe alternative experimental approaches to studying the concept of metabolic flexibility across a range of activity and inactivity. Finally, we address the promising use of strategies that aim to reduce sedentary behaviours as therapy to improve metabolic flexibility and reduce weight gain risk.

Exercise training improves fat metabolism independent of total energy expenditure in sedentary overweight men, but does not restore lean metabolic phenotype

BACKGROUND

Obesity is a dietary fat storage disease. Although exercise prevents weight gain, effects of chronic training on dietary fat oxidation remains understudied in overweight adults.

OBJECTIVE

We tested whether 2 months of training at current guidelines increase dietary fat oxidation in sedentary overweight adults like in sedentary lean adults.

DESIGN

Sedentary lean (n=10) and overweight (n=9) men trained on a cycle ergometer at 50% VO_2peak, 1 h day^-1, four times per week, for 2 months while energy balance was clamped. Metabolic fate of [d₃₁]palmitate and [1-¹³C]oleate mixed in standard meals, total substrate use, total energy expenditure (TEE), activity energy expenditure (AEE) and key muscle proteins/enzymes were measured before and at the end of the intervention.

RESULTS

Conversely to lean subjects, TEE and AEE did not increase in overweight participants due to a spontaneous decrease in non-training AEE. Despite this compensatory behavior, aerobic fitness, insulin sensitivity and fat oxidation were improved by exercise training. The latter was not explained by changes in dietary fat trafficking but more likely by a coordinated response at the muscle level enhancing fat uptake, acylation and oxidation (FABPpm, CD36, FATP1, ACSL1, CPT1, mtGPAT). ACSL1 fold change positively correlated with total fasting (R²=0.59, P<0.0001) and post-prandial (R²=0.49, P=0.0006) fat oxidation whereas mtGPAT fold change negatively correlated with dietary palmitate oxidation (R²=0.40, P=0.009), suggesting modified fat trafficking between oxidation and storage within the muscle. However, for most of the measured parameters the post-training values observed in overweight adults remained lower than the pre-training values observed in the lean subjects.

CONCLUSION

Independent of energy balance and TEE, exercise training at current recommendations improved fitness and fat oxidation in overweight adults. However the improved metabolic phenotype of overweight adults was not as healthy as the one of their lean counterparts before the 2-month training, likely due to the spontaneous reduction in non-training AEE.

Overweight postmenopausal women with different plasma estradiol concentrations present with a similar pattern of energy expenditure and substrate oxidation rate before and after a fatty meal challenge

Menopause-related withdrawal of ovarian estrogens is associated with reduced energy metabolism and overall impairment of substrate oxidation. Estradiol's withdrawal after menopause is associated with a reduction in energy metabolism and impaired substrate oxidation, which contributes to weight gain and visceral fat accumulation. Here we aimed to investigate the association between plasma estradiol concentrations and energy expenditure (EE)/substrate oxidation in a group of overweight postmenopausal women before and after a fatty meal challenge. Women were divided into three groups according to their plasma estradiol concentrations (E2): group 1 - E₂ ≤ 39, group 2 - 40 ≤ E₂ ≤ 59, and group 3 - E₂ ≥ 60 pg/mL. VO₂ and VCO₂ volumes were collected following indirect calorimetry 5 h following a single lipid overload meal (1100 kcal, 72% of fat). For comparisons between groups and within the same group, a linear regression model with mixed effects was applied (P < 0.05). Forty-four women aged 55 ± 0.7 years-old, 8 ± 1.1 years following menopause, with a BMI of 30.5 ± 0.5 kg/m², and 41.9 ± 0.7% of body fat were enrolled the study. Plasma E2 concentrations were: group 1 - 30.4 ± 1.9, group 2 - 46.9 ± 1.5, and group 3 - 91.3 ± 12.0 pg/mL (P < 0.0001). EE at baseline and in the resting state was 1320 ± 24.3 kcal/d, and increased to 1440 ± 27.0 kcal/d 30 min following ingestion of the fatty meal (P < 0.0001), and rose again to an average of 1475 ± 30.3 kcal/d at the completion of experiment (P < 0.0001). Carbohydrate oxidation (Chox) was 0.155 ± 0.01 g/min at resting, maintained as 0.133 ± 0.00 g/min 30 min after ingestion of the fatty meal, and was 0.123 ± 0.01 g/min at the end of the testing period. Lipid oxidation (Lipox) was 0.041 ± 0.003 g/min at resting, increasing to 0.054 ± 0.003 g/min at 30 min (P = 0.01), and reaching 0.063 ± 0.003 g/min at the end of the experiment (P < 0.0001). There was no difference between groups for EE, Chox or Lipox. Our data suggest that EE and substrate oxidation were modulated following a lipid-meal challenge equally in all groups and this did not differ with plasma E2 concentrations.

Effect of low-glycemic-sugar-sweetened beverages on glucose metabolism and macronutrient oxidation in healthy men

BACKGROUND/OBJECTIVES

Sugar-sweetened-beverages (SSB) provide high amounts of rapidly absorbable sugar and have been shown to impair insulin sensitivity and promote weight gain. We hypothesized that when compared with high-glycemic index (GI) SSB low-GI SSB lead to lower insulin secretion and thus an improved preservation of insulin sensitivity and fat oxidation during an inactive phase.

SUBJECTS/METHODS

In a controlled cross-over dietary intervention 13 healthy men (age: 23.7±2.2 years, body mass index: 23.6±1.9 kg m(-)(2)) consumed low-GI (isomaltulose) or high-GI (75% maltodextrin+25% sucrose, adapted for sweetness) SSBs providing 20% of energy requirement for 7 days. During this phase, participant's habitual high physical activity (11 375±3124 steps per day) was reduced (2363±900 steps per day). The provided ad libitum diet comprised 55% CHO, 30% fat and 15% protein. Glycemic and insulinemic responses were assessed: Day-long (7-day continuous interstitial glucose monitoring, 24-h-urinary c-peptide excretion), during meal test (37 g isomaltulose vs 28 g maltodextrin+9g sucrose) and measures of insulin sensitivity (basal: homeostasis model assessment of insulin resistance (HOMA-IR), postprandial: Matsuda-ISI). Macronutrient oxidation was assessed by non-protein respiratory quotient (npRQ) in the fasted state (npRQfasting) and postprandial as the area under the npRQ-curve during meal test (npRQtAUC-meal).

RESULTS

Day-long glycemia was lower with low-GI compared with high-GI SSB (-5%, P<0.05). Low-GI SSB led to lower insulin secretion during meal test (-28%, P<0.01) and throughout the day (-31%, P<0.01), whereas postprandial glucose levels did not differ between low-GI and high-GI SSBs. Insulin sensitivity deteriorated on inactivity with both SSBs, but was better preserved with low-GI isomaltulose compared with high-GI maltodextrin-sucrose (ΔHOMA-IR: +0.37±0.52 vs +0.85±0.86; ΔMatsuda-ISI: -5.1±5.5 vs -9.6±5.1, both P<0.05). Both, fasting and postprandial fat oxidation declined on inactivity, with no difference between high-GI and low-GI SSBs.

CONCLUSIONS

Compared with high-GI SSB, 7-day consumption of beverages sweetened with low-GI isomaltulose had beneficial effects on inactivity-induced impairment of glucose metabolism without effecting fuel selection.

Increased intramyocellular lipid/impaired insulin sensitivity is associated with altered lipid metabolic genes in muscle of high responders to a high-fat diet

The accumulation of intramyocellular lipid (IMCL) is recognized as an important determinant of insulin resistance, and is increased by a high-fat diet (HFD). However, the effects of HFD on IMCL and insulin sensitivity are highly variable. The aim of this study was to identify the genes in muscle that are related to this inter-individual variation. Fifty healthy men were recruited for this study. Before and after HFD for 3 days, IMCL levels in the tibialis anterior were measured by (1)H magnetic resonance spectroscopy, and peripheral insulin sensitivity was evaluated by glucose infusion rate (GIR) during the euglycemic-hyperinsulinemic clamp. Subjects who showed a large increase in IMCL and a large decrease in GIR by HFD were classified as high responders (HRs), and subjects who showed a small increase in IMCL and a small decrease in GIR were classified as low responders (LRs). In five subjects from each group, the gene expression profile of the vastus lateralis muscle was analyzed by DNA microarray analysis. Before HFD, gene expression profiles related to lipid metabolism were comparable between the two groups. Gene Set Enrichment Analysis demonstrated that five gene sets related to lipid metabolism were upregulated by HFD in the HR group but not in the LR group. Changes in gene expression patterns were confirmed by qRT-PCR using more samples (LR, n = 9; HR, n = 11). These results suggest that IMCL accumulation/impaired insulin sensitivity after HFD is closely associated with changes in the expression of genes related to lipid metabolism in muscle.

Associations among Physical Activity, Diet, and Obesity Measures Change during Adolescence

Background. Obesity in youth is highly prevalent. Physical activity and diet are influential in obesity development. However, there is a knowledge gap regarding links between activity and diet quality and their combined influence on obesity during adolescence. Objectives. We used five years of data from 2379 adolescent girls in the National Heart Lung and Blood Institute Growth and Health Study to evaluate the association between physical activity and diet quality during adolescence and to assess both as correlates of obesity. Design. Diet, activity, and body composition measures were evaluated pairwise for correlation. A canonical correlation analysis was used to evaluate relationships within and between variable groups. All statistics were examined for trends over time. Results. We found positive correlations between physical activity and diet quality that became stronger with age. Additionally we discovered an age-related decrease in association between obesity correlates and body composition. Conclusion. These results suggest that while health behaviors, like diet and activity, become more closely linked during growth, obesity becomes less influenced by health behaviors and other factors. This should motivate focus on juvenile obesity prevention capitalizing on the pliable framework for establishing healthy diet and physical activity patterns while impact on body composition is greatest.

Effect of protein overfeeding on energy expenditure measured in a metabolic chamber

BACKGROUND

Energy expenditure (EE) increases with overfeeding, but it is unclear how rapidly this is related to changes in body composition, increased body weight, or diet.

OBJECTIVE

The objective was to quantify the effects of excess energy from fat or protein on energy expenditure of men and women living in a metabolic chamber.

DESIGN

We conducted a randomized controlled trial in 25 participants who ate ∼40% excess energy for 56 d from 5%, 15%, or 25% protein diets. Twenty-four-hour EE (24EE) and sleeping EE (SleepEE) were measured on days 1, 14, and 56 of overfeeding and on day 57 while consuming the baseline diet (usually day 57). Metabolic and molecular markers of muscle metabolism were measured in skeletal muscle biopsy specimens.

RESULTS

In the low-protein diet group whose excess energy was fat, the 24EE and SleepEE did not increase during the first day of overfeeding. When extra energy contained protein, both 24EE and SleepEE increased in relation to protein intake (r = 0.50, P = 0.02). The 24EE over 8 wk in all 3 groups was correlated with protein intake (r = 0.60, P = 0.004) but not energy intake (r = 0.16; P = 0.70). SleepEE was unchanged by overfeeding in the low-protein diet group, and baseline surface area predicted increased 24EE in this group. Protein and fat oxidation were reciprocally related during overfeeding. Observed 24EE was higher than predicted on days 1 (P ≤ 0.05), 14 (P = 0.0001), and 56 (P = 0.0007). There was no relation between change in fat mass and change in EE.

CONCLUSIONS

Excess energy, as fat, does not acutely increase 24EE, which rises slowly as body weight increases. Excess energy as protein acutely stimulates 24EE and SleepEE. The strongest relation with change in 24EE was the change in energy expenditure in tissue other than muscle or fat-free mass.

Determinants of intramyocellular lipid accumulation after dietary fat loading in non-obese men

Aims/Introduction:  Accumulation of intramyocellular lipid (IMCL) is associated with insulin resistance. However, the factors affecting the change in IMCL remain to be elucidated. The aim of the present study was to determine the factors that influence the change in IMCL level after high‐fat loading.

Materials and Methods:  The study subjects were 37 non‐obese men. Each subject consumed a high‐fat diet for 3 days after a normal‐fat diet for 3 days. After each diet program, IMCL levels in the tibialis anterior (TA‐IMCL) and soleus (SOL‐IMCL) were measured by proton magnetic resonance spectroscopy. Glucose infusion rate (GIR) was evaluated by euglycemic hyperinsulinemic clamp as an index of peripheral insulin sensitivity.

Results:  The high‐fat diet significantly increased TA‐IMCL and SOL‐IMCL by ∼30 and ∼20%, respectively (P < 0.05), whereas it did not significantly alter GIR. The increase in SOL‐IMCL, but not in TA‐IMCL, negatively correlated with serum high molecular weight (HMW)‐adiponectin (r = −0.36, P < 0.05) and HMW‐/total‐adiponectin ratio (r = −0.46, P < 0.05). Although high‐fat diet‐related changes in SOL‐IMCL showed high inter‐individual variations, in subjects doing exercise, changes in SOL‐IMCL (r = 0.55, P < 0.05) and TA‐IMCL (r = 0.61, P < 0.05) positively correlated with daily physical activity level. In contrast, in sedentary subjects, changes in SOL‐IMCL (r = −0.50, P < 0.01) and TA‐IMCL (r = −0.48, P < 0.05) negatively correlated with daily physical activity.

Conclusions:  HMW‐adiponectin and daily physical activity are determinants of IMCL accumulation by a high‐fat diet. Intriguingly, the effect of daily physical activity on the change in IMCL depends on the level of regular exercise. (J Diabetes Invest,doi: 10.1111/j.2040‐1124.2010.00091.x, 2011)

Interaction between dietary fat and exercise on excess postexercise oxygen consumption

The objective of this study was to determine the effect of increased physical activity on subsequent sleeping energy expenditure (SEE) measured in a whole room calorimeter under differing levels of dietary fat. We hypothesized that increased physical activity would increase SEE. Six healthy young men participated in a randomized, single-blind, crossover study. Subjects repeated an 8-day protocol under four conditions separated by at least 7 days. During each condition, subjects consumed an isoenergetic diet consisting of 37% fat, 15% protein, and 48% carbohydrate for the first 4 days, and for the following 4 days SEE and energy balance were measured in a respiration chamber. The first chamber day served as a baseline measurement, and for the remaining 3 days diet and activity were randomly assigned as high-fat/exercise, high-fat/sedentary, low-fat/exercise, or low-fat/sedentary. Energy balance was not different between conditions. When the dietary fat was increased to 50%, SEE increased by 7.4% during exercise (P < 0.05) relative to being sedentary (baseline day), but SEE did not increase with exercise when fat was lowered to 20%. SEE did not change when dietary fat was manipulated under sedentary conditions. Physical activity causes an increase in SEE when dietary fat is high (50%) but not when dietary fat is low (20%). Dietary fat content influences the impact of postexercise-induced increases in SEE. This finding may help explain the conflicting data regarding the effect of exercise on energy expenditure.

Effects of intermittent physical activity on fat utilization over a whole day

PURPOSE

We examined whether continuous and intermittent physical activity (PA) differentially influence fat utilization.

METHODS

This was a randomized crossover study. Nine healthy young male participants performed two 39-h (two nights, three days) PA sessions (continuous and intermittent exercise) in a respiratory chamber to measure energy expenditure (EE) and substrate oxidation. Participants used a stationary cycling ergometer continuously for 40 min and then 45 min in the continuous PA trial and for 5 min every 30 min 17 times in the intermittent PA trial. They consumed high-carbohydrate meals corresponding to predicted daily total EE for 3 d before entering the respiratory chamber and four high-fat meals corresponding to predicted total EE in the chamber.

RESULTS

Twenty-three-hour RER adjusted for sleeping RER on the preceding day was significantly lower in the intermittent PA trial than that in the continuous PA trial (P = 0.021). Twenty-three-hour RER adjusted for sleeping RER on the preceding day was correlated with accumulated consecutive minutes of METs ≤ 1.5 (3 min or more, r = 0.477; 5 min or more, r = 0.510; 10 min or more, r = 0.605).

CONCLUSIONS

The intermittent PA trial induced greater fat utilization than the continuous PA trial. The present study, therefore, suggests that intermittent PA has a beneficial effect on 24-h fat oxidation after consumption of a high-fat meal, which may help prevent weight gain over time.

Activity energy expenditure is a major determinant of dietary fat oxidation and trafficking, but the deleterious effect of detraining is more marked than the beneficial effect of training at current recommendations

BACKGROUND

Previous studies suggested that physical activity energy expenditure (AEE) is a major determinant of dietary fat oxidation, which is a central component of fat metabolism and body weight regulation.

OBJECTIVE

We tested this hypothesis by investigating the effect of contrasted physical activity levels on dietary saturated and monounsaturated fatty acid oxidation in relation to insulin sensitivity while controlling energy balance.

DESIGN

Sedentary lean men (n = 10) trained for 2 mo according to the current guidelines on physical activity, and active lean men (n = 9) detrained for 1 mo by reducing structured and spontaneous activity. Dietary [d31]palmitate and [1-¹³C]oleate oxidation and incorporation into triglyceride-rich lipoproteins and nonesterified fatty acid, AEE, and muscle markers were studied before and after interventions.

RESULTS

Training increased palmitate and oleate oxidation by 27% and 20%, respectively, whereas detraining reduced them by 31% and 13%, respectively (P < 0.05 for all). Changes in AEE were positively correlated with changes in oleate (R² = 0.62, P < 0.001) and palmitate (R² = 0.66, P < 0.0001) oxidation. The d31-palmitate appearance in nonesterified fatty acid and very-low-density lipoprotein pools was negatively associated with changes in fatty acid translocase CD36 (R² = 0.30), fatty acid transport protein 1 (R² = 0.24), and AcylCoA synthetase long chain family member 1 (ACSL1) (R² = 0.25) expressions and with changes in fatty acid binding protein expression (R² = 0.33). The d31-palmitate oxidation correlated with changes in ACSL1 (R² = 0.39) and carnitine palmitoyltransferase 1 (R² = 0.30) expressions (P < 0.05 for all). Similar relations were observed with oleate. Insulin response was associated with AEE (R² = 0.34, P = 0.02) and oleate (R² = 0.52, P < 0.01) and palmitate (R² = 0.62, P < 001) oxidation.

CONCLUSION

Training and detraining modified the oxidation of the 2 most common dietary fats, likely through a better trafficking and uptake by the muscle, which was negatively associated with whole-body insulin sensitivity.

Misconceptions in body weight regulation: implications for the obesity pandemic

Energy is a concept of universal importance. In applying it to body weight regulation, the focus has been on energy balance and how this balance is affected by intakes and expenditures. However, energy is an abstract concept without biological equivalent and applying it to explain body weight regulation has led to various misconceptions and created intellectual obstacles in understanding the obesity problem. When nutrient and substrate interactions are considered, instead, a number of important issues pertaining to body weight regulation and to the obesity epidemic can be much more pertinently addressed.

Effect of contrasted levels of habitual physical activity on metabolic flexibility

The factors regulating the body's ability to switch from fat to carbohydrate oxidation in response to fuel availability changes, or metabolic flexibility (MF), are currently intensively investigated in the context of metabolic diseases. Although numerous metabolic diseases are associated with sedentary behaviors and metabolic inflexibility, the effect of habitual physical activity level (PAL) on MF regulation is surprisingly poorly known. We investigated how PAL affects MF in cross-sectional and interventional studies. MF was assessed in 44 subjects: normal-weight and overweight sedentary men submitted to 2 mo of exercise at current recommendations, normal-weight active men submitted to 1 mo of reduced PAL and normal-weight women submitted to 1 mo of bed rest, with or without exercise. MF was evaluated, before and after interventions, following two standard meals as the relationship between individual mathematical variances in insulin and nonprotein respiratory quotient (NPRQ) daily kinetics. Daily NPRQ and insulin variances differed according to habitual PAL ( P = 0.002 and P = 0.009, respectively); active subjects had higher variances in NPRQ for lower variances in insulin than sedentary subjects, indicating a better MF. Detraining increased insulin variance ( P = 0.009) and decreased NPRQ variance ( P = 0.003), while training tended to have opposite effects. Insulin and NPRQ variances were negatively related along the PAL continuum ( R2 = 0.70, P < 0.001). Variance in NPRQ was also positively related to PAL ( R2 = 0.52, P < 0.001). By assessing MF with mathematical surrogates in conditions of daily pattern in meal's intake, we showed that habitual PAL is associated with MF status, and that MF is modulated by changes in PAL.

Prior Exercise Increases Dietary Oleate, but Not Palmitate Oxidation

OBJECTIVE

Higher levels of physical activity have been associated with body weight maintenance, but previous work in our laboratory suggests that this is not purely related to energy balance. We hypothesize that this may be related to the partitioning of dietary fat between oxidation and storage.

RESEARCH METHODS AND PROCEDURES

Healthy women (age 24 +/- 1 years, BMI = 21.2 +/- 0.4 kg/m2) were recruited to participate in rest (n = 10) or exercise sessions of light (n = 11), moderate (n = 10), and heavy (n = 7) exercise. All exercises (1250 kJ above rest) were performed on a stationary cycle inside of a whole-body calorimeter. [1-(13)C]oleate and [d31]palmitate were given in a liquid meal 30 minutes post-exercise. An additional study was done with identical exercise sessions, but with administration of an oral dose of [1-(13)C]acetate and [d3]acetate 30 minutes post-exercise to determine label sequestration.

RESULTS

Cumulative oxidation of [1-(13)C]oleate was significantly greater after light (45 +/- 3%), moderate (54 +/- 4%), and heavy (51 +/- 4%) exercise than that with rest (33 +/- 3%) (p = 0.0008). Cumulative oxidation of [d31]palmitate did not differ among trials (12 +/- 2%, 14 +/- 1%, 17 +/- 2%, and 14 +/- 2% for rest, light, moderate, and heavy, respectively; p = 0.30).

DISCUSSION

Exercise standardized for energy expenditure increases monounsaturated fat oxidation more than saturated fat oxidation and that the increase occurs regardless of intensity. Recommendations for physical activity for the purposes of weight control may be specific for dietary fat composition.

A 45-minute vigorous exercise bout increases metabolic rate for 14 hours

INTRODUCTION

The magnitude and duration of the elevation in resting energy expenditure after vigorous exercise have not been measured in a metabolic chamber. This study investigated the effects of inserting a 45-min vigorous cycling bout into the daily schedule versus a controlled resting day on 24-h energy expenditure in a metabolic chamber.

METHODS

Ten male subjects (age = 22-33 yr) completed two separate 24-h chamber visits (one rest and one exercise day), and energy balance was maintained for each visit condition. On the exercise day, subjects completed 45 min of cycling at 57% Wmax (mean ± SD = 72.8% ± 5.8% VO(2)max) starting at 11:00 a.m. Activities of daily living were tightly controlled to ensure uniformity on both rest and exercise days. The area under the energy expenditure curve for exercise and rest days was calculated using the trapezoid rule in the EXPAND procedure in the SAS and then contrasted.

RESULTS

The 45-min exercise bout resulted in a net energy expenditure of 519 ± 60.9 kcal (P < 0.001). For 14 h after exercise, energy expenditure was increased 190 ± 71.4 kcal compared with the rest day (P < 0.001).

CONCLUSIONS

In young male subjects, vigorous exercise for 45 min resulted in a significant elevation in postexercise energy expenditure that persisted for 14 h. The 190 kcal expended after exercise above resting levels represented an additional 37% to the net energy expended during the 45-min cycling bout. The magnitude and duration of increased energy expenditure after a 45-min bout of vigorous exercise may have implications for weight loss and management.

Effect of exercise on the diurnal variation in energy substrate use during a high-fat diet

Aerobic exercise increases 24-h fat oxidation following initiation of a high-fat diet. The objective of this study is to examine the time course of increased fat oxidation under exercise and sedentary conditions. Eighteen healthy subjects completed a randomized crossover design (sedentary and exercise visits) staying for five consecutive days in a metabolic chamber each visit. On day 1, 30% of energy intake was from fat; days 2-5 had 50% of energy as fat. During exercise, subjects rode on a stationary cycle at 45% of VO2max for 1 h in the mornings and evenings. Respiratory gases and urinary nitrogen were collected to calculate macronutrient oxidation and non-protein respiratory exchange ratio (NPRER). This data, collected continuously (24-h periods), were subsequently divided into three time segments: (1) exercise + recovery (1000-1200 hours, 2100-2200 hours), (2) sleep (2300-0645 hours), and (3) wake (all remaining hours). NPRER on exercise versus sedentary visits was lower for the sleep segment (0.77 ± 0.01 01 vs. 0.81 ± 0.01, p < 0.001), higher for the exercise + recovery segment (0.88 ± 0.01 vs. 0.86 ± 0.01, p < 0.001), and was not different for the wake segment. Fat oxidation was significantly higher for exercise versus sedentary treatments during sleep (41 ± 2 vs. 31 ± 2 g), wake (62 ± 3 vs. 51 ± 3 g), and exercise + recovery segments (33 ± 3 vs.16 ± 1 g), but so was fat intake by design (171 ± 8 vs. 128 ± 7 g/d). Although exercise showed greater fat oxidation during all segments, dietary fat intake was also higher. Therefore, based on NPRER, the time of day during which the exercise treatment increased the ratio of fat to carbohydrate oxidation was during sleep.

Increasing dietary fat elicits similar changes in fat oxidation and markers of muscle oxidative capacity in lean and obese humans

In lean humans, increasing dietary fat intake causes an increase in whole-body fat oxidation and changes in genes that regulate fat oxidation in skeletal muscle, but whether this occurs in obese humans is not known. We compared changes in whole-body fat oxidation and markers of muscle oxidative capacity differ in lean (LN) and obese (OB) adults exposed to a 2-day high-fat (HF) diet. Ten LN (BMI = 22.5±2.5 kg/m², age = 30±8 yrs) and nine OB (BMI = 35.9±4.93 kg/m², 38±5 yrs, Mean±SD) were studied in a room calorimeter for 24hr while consuming isocaloric low-fat (LF, 20% of energy) and HF (50% of energy) diets. A muscle biopsy was obtained the next morning following an overnight fast. 24h respiratory quotient (RQ) did not significantly differ between groups (LN: 0.91±0.01; OB: 0.92±0.01) during LF, and similarly decreased during HF in LN (0.86±0.01) and OB (0.85±0.01). The expression of pyruvate dehydrogenase kinase 4 (PDK4) and the fatty acid transporter CD36 increased in both LN and OB during HF. No other changes in mRNA or protein were observed. However, in both LN and OB, the amounts of acetylated peroxisome proliferator-activated receptor γ coactivator-1-α (PGC1-α) significantly decreased and phosphorylated 5-AMP-activated protein kinase (AMPK) significantly increased. In response to an isoenergetic increase in dietary fat, whole-body fat oxidation similarly increases in LN and OB, in association with a shift towards oxidative metabolism in skeletal muscle, suggesting that the ability to adapt to an acute increase in dietary fat is not impaired in obesity.

Energy balance: an overview with emphasis on children

Childhood obesity is a significant public health problem, affecting one in five children in the United States. At the crux of this issue is a dysregulation of energy intake and energy expenditure. This review will provide an overview on energy and nutrient balance. We discuss energy balance studies in children using indirect and direct measures, and focus particularly on obesity as a deleterious consequence in childhood survivors of cancer. Obesity affects 11-57% of children with acute lymphoblastic leukemia, probably due to increased energy intake and reduced energy expenditure secondary to reduced habitual activity caused by fatigue. However, most of the studies in children with leukemia are retrospective, use BMI as a measure of obesity, and are inconclusive about the impact of the type of treatment on the development of obesity later in life. To better understand the etiology of obesity in both healthy and sick children, we need to undertake nutrient balance studies with appropriate measures of fat mass and fat distribution while keeping in mind the influence of normal tissue growth and puberty on energy balance.

The relevance of increased fat oxidation for body-weight management: metabolic inflexibility in the predisposition to weight gain

Cells, tissues and organisms have the ability to rapidly switch substrate oxidation from carbohydrate to fat in response to changes in nutrient intake, and to changes in energy demands, environmental cues and internal signals. In healthy, metabolically normal individuals, substrate switching occurs rapidly and completely; in other words, substrate switching is 'flexible'. A growing body of evidence demonstrates that a blunted substrate switching from low- to high-fat oxidation exists in obese individuals, as well as in pre-obese and post-obese, and that this 'metabolic inflexibility' may be a genetically determined trait. A decreased fat oxidation can lead to a positive energy balance under conditions of high-fat feeding, due to depletion of glycogen stores that stimulates appetite and energy intake through glucostatic and glucogenostatic mechanisms, e.g. hepatic sensing of glycogen stores. Several genetic polymorphisms and single-nucleotide polymorphisms have been identified that are associated with low-fat oxidation rates and metabolic inflexibility, and genetic identification of susceptible individuals may lead to personalized prevention of weight gain using fat oxidation stimulants ('fat burners') in the future.

Beyond thriftiness: independent and interactive effects of genetic and dietary factors on variations in fat deposition and distribution across populations

The thrifty genotype hypothesis initiated speculation that feast and famine cycling throughout history may have led to group-specific alterations of the human genome, thereby augmenting the capacity for excessive fat mass accrual when immersed in the modern-day obesogenic environment. Contemporary work, however, suggests alternative mechanisms influencing fuel utilization and subsequent tissue partitioning to be more relevant in the etiology of population-based variation in adipose storage. The objective of this study was to evaluate the independent and interactive contribution of ancestral admixture as a proxy for population-based genetic variation and diet on adipose tissue deposition and distribution in peripubertal children and to identify differences in racial/ethnic and sex groups. Two-hundred seventy-eight children (53% male) aged 7-12 years, categorized by parental self-report as African- (n = 91), European- (n = 110), or Hispanic American (n = 77), participated. Ancestral genetic admixture was estimated using 140 ancestry informative markers. Body composition was evaluated by dual-energy X-ray absorptiometry; energy expenditure by indirect calorimetry and accelerometry; and diet by 24-h-recall. Admixture independently contributed to all adiposity parameters; i.e., estimates of European and Amerindian ancestries were positively associated with all adiposity parameters, whereas African genetic admixture was inversely associated with adiposity. In boys, energy intake was associated with adiposity, irrespective of macronutrient profile, whereas in girls, the relationship was mediated by carbohydrate. We also observed moderating effects of energy balance/fuel utilization of the interaction between ancestral genetic admixture and diet. Interactive effects of genetic and non-genetic factors alter metabolic pathways and underlie some of the present population-based differences in fat storage.

Impact of exercise and dietary fatty acid composition from a high-fat diet on markers of hunger and satiety

To compare the effects of both dietary fatty acid composition and exercise vs. sedentary conditions on circulating levels of hunger and satiety hormones. Eight healthy males were randomized in a 2 × 2 crossover design. The four treatments were 3 days of HF diets (50% of energy) containing high saturated fat (22% of energy) with exercise (SE) or sedentary (SS) conditions, and high monounsaturated fat (30% of energy) with exercise (UE) or sedentary (US) conditions. Cycling exercise was completed at 45% of VO(2)max for 2h daily. On the third HF day, 20 blood samples were drawn over a 24h period for each hormone (leptin, insulin, ghrelin, and peptide YY (PYY)). A visual analog scale (VAS) was completed hourly between 0800 and 2200. Average 24h leptin and insulin levels were lower while 24h PYY was higher during exercise vs. sedentary conditions. FA composition did not differentially affect 24h hormone values. VAS scores for hunger and fullness did not differ between any treatment but did correlate with ghrelin, leptin, and insulin. High saturated or unsaturated fat diets did not differ with respect to markers of hunger or satiety. Exercise decreased 24h leptin and insulin while increasing PYY regardless of FA composition.

Influence of dietary fatty acid composition and exercise on changes in fat oxidation from a high-fat diet

Acute high-fat (HF) diets can lead to short-term positive fat balances until the body increases fat oxidation to match intake. The purpose of this study was to examine the effects of a HF diet, rich in either mono-unsaturated or saturated fatty acids (FAs) and exercise, on the rate at which the body adapts to a HF diet.13C-labeled oleate and 2H-labeled palmitate were also given to determine the contribution of exogenous vs. global fat oxidation. Eight healthy men (age of 18–45 yr; body mass index of 22 ± 3 kg/m2) were randomized in a 2 × 2 crossover design. The four treatments were a high saturated fat diet with exercise (SE) or sedentary (SS) conditions and a high monounsaturated fat diet with exercise (UE) or sedentary (US) conditions. Subjects stayed for 5 days in a metabolic chamber. All meals were provided. On day 1, 30% of energy intake was from fat, whereas days 2–5 had 50% of energy as fat. Subjects exercised on a stationary cycle at 45% of maximal oxygen uptake for 2 h each day. Respiratory gases and urinary nitrogen were collected to calculate fat oxidation. Change from day 1 to day 5 showed both exercise treatments increased fat oxidation (SE: 76 ± 30 g, P = 0.001; UE: 118 ± 31 g, P < 0.001), whereas neither sedentary condition changed fat oxidation (SS: −10 ± 33 g, P = not significant; US: 41 ± 14 g, P = 0.07). No differences for dietary FA composition were found. Exercise led to a faster adaptation to a HF diet by increasing fat oxidation and achieving fat balance by day 5. Dietary FA composition did not differentially affect 24-h fat oxidation.

Exercise improves fat metabolism in muscle but does not increase 24-h fat oxidation

Despite decades of research into the effects of exercise on fat metabolism, there is still no clear understanding of how exercise helps to regulate fat mass. Although exercise improves the capacity of muscle to oxidize fat, our studies suggest that moderate duration exercise (< or =1 h) has little impact on 24-h fat oxidation.

When energy balance is maintained, exercise does not induce negative fat balance in lean sedentary, obese sedentary, or lean endurance-trained individuals

Fat oxidation during exercise is increased by endurance training, and evidence suggests that fat oxidation during exercise is impaired in obesity. Thus the primary aim of this study was to compare the acute effects of exercise on 24-h fat oxidation and fat balance in lean sedentary [LS, n = 10, body mass index (BMI) = 22.5 +/- 6.5 kg/m(2)], lean endurance-trained (LT, n = 10, BMI = 21.2 +/- 1.2 kg/m(2)), and obese sedentary (OS, n = 7, BMI = 35.5 +/- 4.4 kg/m(2)) men and women. Twenty-four-hour energy expenditure and substrate oxidation were measured under sedentary (control; CON) and exercise (EX) conditions while maintaining energy balance. During EX, subjects performed 1 h of stationary cycling at 55% of aerobic capacity. Twenty-four-hour fat oxidation did not differ on the CON or EX day in LS (43 +/- 9 vs. 29 +/- 7 g/day, respectively), LT (53 +/- 8 vs. 42 +/- 5 g/day), or OS (58 +/- 7 vs. 80 +/- 9 g/day). However, 24-h fat balance was significantly more positive on EX compared with CON (P < 0.01). Twenty-four-hour glucose, insulin, and free fatty acid (FFA) profiles were similar on the EX and CON days, but after consumption of the first meal, FFA concentrations remained below fasting levels for the remainder of the day. These data suggest that when exercise is performed with energy replacement (i.e., energy balance is maintained), 24-h fat oxidation does not increase and in fact, may be slightly decreased. It appears that the state of energy balance is an underappreciated factor determining the impact of exercise on fat oxidation.

Daily physical activity and macronutrient distribution of low-calorie diets jointly affect body fat reduction in obese women

Inadequate dietary patterns and sedentary lifestyles are believed to be important factors in predisposing people to obesity. This study analyzed the potential interaction between habitual physical activity and the carbohydrate (CHO)-fat distribution in 2 hypocaloric diets and the impact of such interplay on body composition changes. Forty healthy obese women, 20–50 years old, were randomly assigned to a high- or low-CHO energy-restricted diet, which was low or high in fat, respectively, during 10 weeks. Baseline and final measurements were performed to assess dietary habits, resting metabolic rate, and body composition changes. Physical activity was measured with a triaxial accelerometer and with a questionnaire. There were no significant differences in anthropometric and metabolic variables between both dietary groups at baseline. However, there was a positive correlation between total free-living physical activity and arm muscle preservation after 10 weeks (r = 0.371; p = 0.024). Interestingly, an interaction between macronutrient (CHO–fat distribution) intake and physical activity was found, since less-active subjects with a high-CHO–low-fat diet showed a greater fat loss than those more active with a lower-CHO–high-fat diet, whereas more-active subjects with a high-CHO–low-fat diet showed a smaller fat loss than those receiving a low-CHO–high-fat diet. Physical activity and the macronutrient content of energy-restricted diets, when designed to promote body fat mass reduction, should be considered together to better predict the outcome.

The role of impaired mitochondrial lipid oxidation in obesity

Obesity represents a disruption in balancing fuel intake with energy expenditure in favor of energy conservation. Adiposity is known to be carefully regulated and, over time, highly resistant to major changes, raising questions about how energy homeostasis can become dysregulated in favor of fat accumulation. In obesity, the excess lipid accumulation represents a surfeit of energy, but those who are obese often experience rapid fatigue and decreased physical endurance, reflecting an energy deficiency. To develop an explanation for this apparent contradiction in energy homeostasis and the chronic overeating relative to energy used in obesity, a review of the literature was conducted. The resulting model of obesity is based on a growing body of research demonstrating that altered mitochondrial energy production, particularly in skeletal muscles, is a major anomaly capable of setting off a chain of metabolic events leading to obesity. Alterations in skeletal muscle mitochondria distribution and their oxidative and glycolytic energy capacities in obesity are described. The metabolic responses of obese and normal individuals to exercise are contrasted, and the effects of weight loss on energy production are presented. The effect of altered fat oxidation is considered in relation to energy regulation by the central nervous system and the development of major obesity comorbidities, including systemic inflammation, insulin resistance and diabetes, and cardiovascular disease. Recommendations for clinical intervention and additional research are proposed based on the model presented of impaired mitochondrial function in obesity.

Acutely reduced locomotor activity is a major contributor to Western diet-induced obesity in mice

The aim of the present study was to investigate the short- and long-term effects of a high-fat Western diet (WD) on intake, storage, expenditure, and fecal loss of energy as well as effects on locomotor activity and thermogenesis. WD for only 24 h resulted in a marked physiological shift in energy homeostasis, including increased body weight gain, body fat, and energy expenditure (EE) but an acutely lowered locomotor activity. The acute reduction in locomotor activity was observed after only 3-5 h on WD. The energy intake and energy absorption were increased during the first 24 h, lower after 72 h, and normalized between 7 and 14 days on WD compared with mice given chow diet. Core body temperature and EE was increased between 48 and 72 h but normalized after 21 days on WD. These changes paralleled plasma T(3) levels and uncoupling protein-1 expression in brown adipose tissue. After 21 days of WD, energy intake and absorption, EE, and body temperature were normalized. In contrast, the locomotor activity was reduced and body weight gain was increased over the entire 21-day study period on WD. Calculations based on the correlation between locomotor activity and EE in 2-h intervals at days 21-23 indicated that a large portion of the higher body weight gain in the WD group could be attributed to the reduced locomotor activity. In summary, an acute and persisting decrease in locomotor activity is most important for the effect of WD on body weight gain and obesity in mice.

Differences in postprandial responses to fat and carbohydrate loads in habitual high and low fat consumers (phenotypes)

The present study investigated metabolic responses to fat and carbohydrate ingestion in lean male individuals consuming an habitual diet high or low in fat. Twelve high-fat phenotypes (HF) and twelve low-fat phenotypes (LF) participated in the study. Energy intake and macronutrient intake variables were assessed using a food frequency questionnaire. Resting (RMR) and postprandial metabolic rate and substrate oxidation (respiratory quotient; RQ) were measured by indirect calorimetry. HF had a significantly higher RMR and higher resting heart rate than LF. These variables remained higher in HF following the macronutrient challenge. In all subjects the carbohydrate load increased metabolic rate and heart rate significantly more than the fat load. Fat oxidation (indicated by a low RQ) was significantly higher in HF than in LF following the fat load; the ability to oxidise a high carbohydrate load did not differ between the groups. Lean male subjects consuming a diet high in fat were associated with increased energy expenditure at rest and a relatively higher fat oxidation in response to a high fat load; these observations may be partly responsible for maintaining energy balance on a high-fat (high-energy) diet. In contrast, a low consumer of fat is associated with relatively lower energy expenditure at rest and lower fat oxidation, which has implications for weight gain if high-fat foods or meals are periodically introduced to the diet.

Exercise increases the proportion of fat utilization during short-term consumption of a high-fat diet

BACKGROUND

Increases in energy substrate oxidation occur at different rates after an increase in either fat or carbohydrate intake. Adaptations to increased fat intake are relatively slow and are influenced by activity level.

OBJECTIVE

We tested the hypothesis that increased levels of daily activity, as influenced by added exercise, would have a graded effect on the rate of compensatory adjustment to a short-term high-fat diet.

DESIGN

Daily total energy expenditure and macronutrient oxidation were measured at 3 physical activity levels (PALs) by using a whole-room indirect calorimeter in 10 adult women as they transitioned from a 1-d low-fat (30% of energy) control diet to a 4-d high-fat (50% of energy) diet. The 3 PALs (1.4, 1.6, and 1.8) were provided daily by increases in bicycle ergometer exercise time.

RESULTS

An increase in physical activity led to a greater increase in the nonprotein respiratory exchange ratio (-0.047 +/- 0.02, -0.064 +/- 0.02, and -0.071 +/- 0.02; P < 0.0001) and 24-h fat oxidation (113 +/- 24, 125 +/- 19, and 147 +/- 20 g/d; P < 0.0001) for PALs of 1.4, 1.6, and 1.8, respectively, after the transition from the low-fat control diet to the high-fat diet. Random-effects analysis found a significant (P = 0.003) relation between PAL and the compensatory fat oxidation response to a high-fat diet.

CONCLUSIONS

Amounts of exercise consistent with the Institute of Medicine's recommendations reduce the time required to match fat oxidation to a change in the percentage of fat in the diet. Because short-term consumption of high-fat diets is thought to contribute to excess fat accumulation, regular exercise should be protective and should help maintain a healthy body composition.

High-fat/low-carbohydrate diets regulate glucose metabolism via a long-term transcriptional loop

Insulin sensitivity is characterized by insulin-stimulated glucose metabolism in skeletal muscle. We hypothesized that carbohydrate metabolism and storage might be under transcriptional control. To test this hypothesis, we fed insulin-sensitive males (glucose disposal rate, 14.7 +/- 4.1 mg/kg fat-free mass [FFM] per minute) an isoenergetic high-fat/low-carbohydrate diet (HF/LCD) for 3 days with muscle biopsies before and after intervention. Oligonucleotide microarrays revealed a total of 369 genes of 18861 genes on the arrays were differentially regulated in response to diet (Bonferonni adjusted P < .01). A similar experiment was conducted in mice with a 3-week intervention using a control group and an HF/LCD group to offset the lack of a control group within the human cohort. As part of an analysis of results previously published from this data set, 7 genes in the carbohydrate metabolism pathway changed in response to the HF/LCD, and 3 genes were confirmed by quantitative reverse transcriptase-polymerase chain reaction: fructose-2,6-biphosphatase 3 (PFKFB3), pyruvate dehydrogenase kinase, isoenzyme 4 (PDK4), and glycogen synthase 1 (muscle). In a separate experiment, we fed C57Bl/6J mice an HF/LCD for 3 weeks and found that the same glucose metabolism genes were changed by approximately 70% on average. Fructose-2,6-biphosphatase 3 and pyruvate dehydrogenase kinase, isoenzyme 4 increased and glycogen synthase 1 (muscle) decreased. Combined, these results suggest a mechanism whereby HF/LCD regulates the genes necessary for glucose utilization and storage vis-á-vis transcriptional control.

Dynamic changes in fat oxidation in human primary myocytes mirror metabolic characteristics of the donor

Metabolic flexibility of skeletal muscle, that is, the preference for fat oxidation (FOx) during fasting and for carbohydrate oxidation in response to insulin, is decreased during insulin resistance. The aim of this study was to test the hypothesis that the capacity of myotubes to oxidize fat in vitro reflects the donor's metabolic characteristics. Insulin sensitivity (IS) and metabolic flexibility of 16 healthy, young male subjects was determined by euglycemic hyperinsulinemic clamp. Muscle samples were obtained from vastus lateralis, cultured, and differentiated into myotubes. In human myotubes in vitro, we measured suppressibility (glucose suppression of FOx) and adaptability (an increase in FOx in the presence of high palmitate concentration). We termed these dynamic changes in FOx metabolic switching. In vivo, metabolic flexibility was positively correlated with IS and maximal oxygen uptake and inversely correlated with percent body fat. In vitro suppressibility was inversely correlated with IS and metabolic flexibility and positively correlated with body fat and fasting FFA levels. Adaptability was negatively associated with percent body fat and fasting insulin and positively correlated with IS and metabolic flexibility. The interindividual variability in metabolic phenotypes was preserved in human myotubes separated from their neuroendocrine environment, which supports the hypothesis that metabolic switching is an intrinsic property of skeletal muscle.

A high-fat diet coordinately downregulates genes required for mitochondrial oxidative phosphorylation in skeletal muscle

Obesity and type 2 diabetes have been associated with a high-fat diet (HFD) and reduced mitochondrial mass and function. We hypothesized a HFD may affect expression of genes involved in mitochondrial function and biogenesis. To test this hypothesis, we fed 10 insulin-sensitive males an isoenergetic HFD for 3 days with muscle biopsies before and after intervention. Oligonucleotide microarray analysis revealed 297 genes were differentially regulated by the HFD (Bonferonni adjusted P < 0.001). Six genes involved in oxidative phosphorylation (OXPHOS) decreased. Four were members of mitochondrial complex I: NDUFB3, NDUFB5, NDUFS1, and NDUFV1; one was SDHB in complex II and a mitochondrial carrier protein SLC25A12. Peroxisome proliferator-activated receptor gamma coactivator-1 (PGC1) alpha and PGC1beta mRNA were decreased by -20%, P < 0.01, and -25%, P < 0.01, respectively. In a separate experiment, we fed C57Bl/6J mice a HFD for 3 weeks and found that the same OXPHOS and PGC1 mRNAs were downregulated by approximately 90%, cytochrome C and PGC1alpha protein by approximately 40%. Combined, these results suggest a mechanism whereby HFD downregulates genes necessary for OXPHOS and mitochondrial biogenesis. These changes mimic those observed in diabetes and insulin resistance and, if sustained, may result in mitochondrial dysfunction in the prediabetic/insulin-resistant state.

Beyond Energy Balance: There Is More to Obesity than Kilocalories

Using an epidemiologic model of the interactions between environmental agents and human hosts to explain obesity, we explored food, medications, physical inactivity, toxins, and viruses as environmental agents that interact with a genetically programmed host to disturb energy balance and cause obesity. Large portion sizes, high fat intakes, easy access to calorically sweetened beverages, and lack of any need to be physically active all play a role in the toxic environment that leads to obesity. The genetic and physiologic responses of a host determine whether or not this toxic environment will produce obesity. Reversing the current trends of obesity requires a new look at the limits of the energy balance concept, and a better understanding of how environmental factors acutely and chronically change the responses of susceptible hosts.

Physical activity, obesity and cardiovascular diseases

Sedentary lifestyle and overweight are major public health, clinical, and economical problems in modern societies. The worldwide epidemic of excess weight is due to imbalance between physical activity and dietary energy intake. Sedentary lifestyle, unhealthy diet, and consequent overweight and obesity markedly increase the risk of cardiovascular diseases. Regular physical activity 45-60 min per day prevents unhealthy weight gain and obesity, whereas sedentary behaviors such as watching television promote them. Regular exercise can markedly reduce body weight and fat mass without dietary caloric restriction in overweight individuals. An increase in total energy expenditure appears to be the most important determinant of successful exercise-induced weight loss. The best long-term results may be achieved when physical activity produces an energy expenditure of at least 2,500 kcal/week. Yet, the optimal approach in weight reduction programs appears to be a combination of regular physical activity and caloric restriction. A minimum of 60 min, but most likely 80-90 min of moderate-intensity physical activity per day may be needed to avoid or limit weight regain in formerly overweight or obese individuals. Regular moderate intensity physical activity, a healthy diet, and avoiding unhealthy weight gain are effective and safe ways to prevent and treat cardiovascular diseases and to reduce premature mortality in all population groups. Although the efforts to promote cardiovascular health concern the whole population, particular attention should be paid to individuals who are physically inactive, have unhealthy diets or are prone to weight gain. They have the highest risk for worsening of the cardiovascular risk factor profile and for cardiovascular disease. To combat the epidemic of overweight and to improve cardiovascular health at a population level, it is important to develop strategies to increase habitual physical activity and to prevent overweight and obesity in collaboration with communities, families, schools, work sites, health care professionals, media and policymakers.

Manipulation of dietary carbohydrate and muscle glycogen affects glucose uptake during exercise when fat oxidation is impaired by beta-adrenergic blockade

We have recently reported that, during moderate intensity exercise, low muscle glycogen concentration and utilization caused by a high-fat diet is associated with a marked increase in fat oxidation with no effect on plasma glucose uptake (R(d) glucose). It is our hypothesis that this increase in fat oxidation compensates for low muscle glycogen, thus preventing an increase in R(d) glucose. Therefore, the purpose of this study was to determine whether low muscle glycogen availability increases R(d) glucose under conditions of impaired fat oxidation. Six cyclists exercised at 50% peak O(2) consumption (Vo(2 peak)) for 1 h after 2 days on either a high-fat (HF, 60% fat, 24% carbohydrate) or control (CON, 22% fat, 65% carbohydrate) diet to manipulate muscle glycogen to low and normal levels, respectively. Two hours before the start of exercise, subjects ingested 80 mg of propanolol (betaB), a nonselective beta-adrenergic receptor blocker, to impair fat oxidation during exercise. HF significantly decreased calculated muscle glycogen oxidation (P < 0.05), and this decrease was partly compensated for by an increase in fat oxidation (P < 0.05), accompanied by an increase in whole body lipolysis (P < 0.05), despite the presence of betaB. Although HF increased fat oxidation, plasma glucose appearance rate, R(d) glucose, and glucose clearance rate were also significantly increased by 13, 15, and 26%, respectively (all P < 0.05). In conclusion, when lipolysis and fat oxidation are impaired, in this case by betaB, fat oxidation cannot completely compensate for a reduction in muscle glycogen utilization, and consequently plasma glucose turnover increases. These findings suggest that there is a hierarchy of substrate compensation for reduced muscle glycogen availability after a high-fat, low-carbohydrate diet, with fat being the primary and plasma glucose the secondary compensatory substrate. This apparent hierarchy likely serves to protect against hypoglycemia when endogenous glucose availability is low.

Insulin resistance as a modifier of the relationship between dietary fat intake and weight gain

OBJECTIVE

To investigate whether insulin resistance modifies the rate of weight gain associated with a high percent of energy intake from dietary fat.

DESIGN

Longitudinal, observational population study.

SUBJECTS

A total of 782 nondiabetic Hispanic and non-Hispanic white free-living adult residents of the San Luis Valley in Colorado.

MEASUREMENTS

Subjects were seen up to three times over a 14-y period. Weight, height, fasting insulin and glucose, diet by 24 h recall, and self-reported physical activity were collected at each visit.

RESULTS

Percentage of energy intake from dietary fat was positively associated with weight gain over time (P=0.0103). High intake of dietary fat was more strongly related to weight gain in women than in men, and in those with lower total energy intake levels. The relationship between weight change and relative macronutrient intake also varied by baseline insulin sensitivity (P=0.0025). Weight gain over time in individuals with relative insulin resistance at baseline, as measured by QUICKI, was the greatest among those who consumed a higher percent of energy from fat.

CONCLUSION

Percentage of total intake from dietary fat predicts weight change independent of total energy intake. Nondiabetic, insulin-resistant individuals are particularly susceptible to the weight gain associated with high levels of dietary fat intake. Further investigation into the relationship between insulin resistance, diet, and weight gain is warranted.

High-fat diet elevates resting intramuscular triglyceride concentration and whole body lipolysis during exercise

This study determined the role of intramuscular triglyceride (IMTG) and adipose lipolysis in the elevated fat oxidation during exercise caused by a high-fat diet. In four separate trials, six endurance-trained cyclists exercised at 50% peak O2 consumption for 1 h after a two-day control diet (22% fat, CON) or an isocaloric high-fat diet (60% fat, HF) with or without the ingestion of acipimox, an adipose lipolysis inhibitor, before exercise. During exercise, HF elevated fat oxidation by 72% and whole body lipolysis [i.e., the appearance rate of glycerol in plasma (Ra glycerol)] by 79% compared with CON (P < 0.05), and this was associated with a 36% increase (P < 0.05) in preexercise IMTG concentration. Although acipimox lowered plasma free fatty acid (FFA) availability, HF still increased fat oxidation and Ra glycerol to the same magnitude above control as the increase caused by HF without acipimox (i.e., both increased fat oxidation 13-14 micromol.kg(-1).min(-1)). In conclusion, the marked increase in fat oxidation after a HF diet is associated with elevated IMTG concentration and whole body lipolysis and does not require increased adipose tissue lipolysis and plasma FFA concentration during exercise. This suggests that altered substrate storage in skeletal muscle is responsible for increased fat oxidation during exercise after 2 days of an HF diet.

An evolutionary perspective on human physical activity: implications for health

At present, human genes and human lives are incongruent, especially in affluent Western nations. When our current genome was originally selected, daily physical exertion was obligatory; our biochemistry and physiology are designed to function optimally in such circumstances. However, today's mechanized, technologically oriented conditions allow and even promote an unprecedentedly sedentary lifestyle. Many important health problems are affected by this imbalance, including atherosclerosis, obesity, age-related fractures and diabetes, among others. Most physicians recognize that regular exercise is a critical component of effective health promotion regimens, but there is substantial disagreement about details, most importantly volume: how much daily caloric expenditure, as physical activity, is desirable. Because epidemiology-based recommendations vary, often confusing and alienating the health-conscious public, an independent estimate, arising from a separate scientific discipline, is desirable, at least for purposes of triangulation. The retrojected level of ancestral physical activity might meet this need. The best available such reconstruction suggests that the World Health Organization's recommendation, a physical activity level of 1.75 ( approximately 2.1 MJ (490 kcal)/d), most closely approximates the Paleolithic standard, that for which our genetic makeup was originally selected.

Reduced rate of fat oxidation: a metabolic pathway to obesity in the developing nations

The purpose of this article is to document the metabolic and environmental factors associated with the increased frequency of obesity in the developing nations. While the prevalence of obesity in the developed countries is caused by the increased consumption of calorie-dense foods, in the developing nations, because obesity coexists with undernutrition, additional factors are necessary to account for it. The evidence suggests that an important contributing factor for obesity in the developing nations is a reduced fat oxidation and increased metabolism of carbohydrate that has been brought about by the chronic undernutrition experienced during prenatal and postnatal growth. This shift toward a preferential metabolic use of carbohydrate rather than of fat results in an increased deposition of body fat. This tendency, along with the general decrease of energy expenditure in physical activity associated with urbanization, and the culturally mediated acceptance of fatness leads to obesity among populations from the developing nations. A joint effect of these factors is that in the developing nations obesity is associated with short stature resulting from developmental undernutrition, while in the developed countries obesity is associated with tall stature. It is hoped that future research will address the mechanisms whereby undernutrition increases the tendency toward obesity. Understanding how to modify fat oxidation could affect our ability to prevent weight gain among undernourished populations of the developing nations. Therefore, future research on the interaction of undernutrition and the development of obesity is of prime importance for anthropology concerned with the origins of human variability.