Interaction of training and diet on metabolism and endurance during exercise in man

Importance of training volume during intensified training in elite cyclists: Maintained vs. reduced volume at moderate intensity

INTRODUCTION

Male elite cyclists (average VO2 -max: 71 mL/min/kg, n = 18) completed 7 weeks of high-intensity interval training (HIT) (3×/week; 4-min and 30-s intervals) during the competitive part of the season. The influence of a maintained or lowered total training volume combined with HIT was evaluated in a two-group design. Weekly moderate-intensity training was lowered by ~33% (~5 h) (LOW, n = 8) or maintained at normal volume (NOR, n = 10). Endurance performance and fatigue resistance were evaluated via 400 kcal time-trials (~20 min) commenced either with or without prior completion of a 120-min preload (including repeated 20-s sprints to simulate physiologic demands during road races).

RESULTS

Time-trial performance without preload was improved after the intervention (p = 0.006) with a 3% increase in LOW (p = 0.04) and a 2% increase in NOR (p = 0.07). Preloaded time-trial was not significantly improved (p = 0.19). In the preload, average power during repeated sprinting increased by 6% in LOW (p < 0.01) and fatigue resistance in sprinting (start vs end of preload) was improved (p < 0.05) in both groups. Blood lactate during the preload was lowered (p < 0.001) solely in NOR. Measures of oxidative enzyme activity remained unchanged, whereas the glycolytic enzyme PFK increased by 22% for LOW (p = 0.02).

CONCLUSION

The present study demonstrates that elite cyclists can benefit from intensified training during the competitive season both with maintained and lowered training volume at moderate intensity. In addition to benchmarking the effects of such training in ecological elite settings, the results also indicate how some performance and physiological parameters may interact with training volume.

Does the Paleo diet affect an athlete's health and sport performance?

The aim of the study was to assess the impact of an eight-week Paleo diet on the health status (body composition, haematology and biochemistry of blood and urine) and the level of physical capacity (aerobic and anaerobic) of professional handball players. Fifteen athletes were assigned to two groups: 9 in the experimental group (PD) and 6 in the control group (CD). Significant decreases in body mass (BM), body mass index (BMI), and fat mass (FM) as well as an increase in the fat-free mass (FFM) (%) in both groups were observed. There were no significant differences between groups in particular series during the experiment in all haematological and biochemical indicators of blood and urine. Only HDL-C was significantly higher in the last series in the PD compared to the CD (1.63 mmol/l vs. 1.23 mmol/l). In the Wingate test, there were only single intragroup changes, consisting of a significant decrease in the Wt, MAP and Pmean in the experimental group. There were no significant differences between the groups in individual series or intragroup differences during the experiment, determined by the VO2max, VEmax, VE ∙ VCO2 -1, RER, and the time of the test with a gradually increasing load on a treadmill, except for a significant decrease of maximum tidal volume (TVmax) in the PD. No adverse effect of the Paleo diet on the health status was found. The use of the Paleo diet slightly adversely affects anaerobic capacity and does not affect the level of aerobic capacity.

Preexercise High-Fat Meal Following Carbohydrate Loading Attenuates Glycogen Utilization During Endurance Exercise in Male Recreational Runners

ABSTRACT

Iwayama, K, Tanabe, Y, Yajima, K, Tanji, F, Onishi, T, and Takahashi, H. Preexercise high-fat meal following carbohydrate loading attenuates glycogen utilization during endurance exercise in male recreational runners. J Strength Cond Res 37(3): 661-668, 2023-This study aimed to investigate whether one preexercise high-fat meal can increase glycogen conservation during endurance exercise, as compared with one preexercise high-carbohydrate meal. Ten young male recreational runners (22.0 ± 0.6 years; 171.3 ± 0.9 cm; 58.3 ± 1.9 kg; maximal oxygen uptake [V̇ o2 max], 62.0 ± 1.6 ml·kg -1 ·min -1 ) completed 2 exercise trials after high-carbohydrate loading: eating a high-carbohydrate (CHO; 7% protein, 13% fat, 80% carbohydrate) meal or eating a high-fat (FAT; 7% protein, 42% fat, 52% carbohydrate) meal 3.5 hours before exercise. The order of the 2 trials was randomized, and the interval between trials was at least 1 week. The experimental exercise consisted of running on a treadmill for 60 minutes at 95% of each subject's lactate threshold. Muscle and liver glycogen content were assessed using noninvasive carbon magnetic resonance spectroscopy before the experimental meal as well as before and after exercise; respiratory gases were measured continuously during exercise. The respiratory exchange ratio during exercise was statistically lower in the FAT trial than in the CHO trial ( p < 0.01). In addition, muscle ( p < 0.05) and liver ( p < 0.05) glycogen utilization during exercise was less in the FAT trial than in the CHO trial. Therefore, one high-fat meal following carbohydrate loading reduced muscle and liver glycogen use during the 60-minute exercise. These results suggest that this dietary approach may be applied as a strategy to optimize energy utilization during endurance exercise.

Isonitrogenous low-carbohydrate diet elicits specific changes in metabolic gene expression in the skeletal muscle of exercise-trained mice

With the renewed interest in low-carbohydrate diets (LCDs) in the sports field, a few animal studies have investigated their potential. However, most rodent studies have used an LCD containing low protein, which does not recapitulate a human LCD, and the muscle-specific adaptation in response to an LCD remains unclear. Therefore, we investigated the effects of two types of LCDs, both containing the same proportion of protein as a regular diet (isonitrogenous LCD; INLCD), on body composition, exercise performance, and metabolic fuel selection at the genetic level in the skeletal muscles of exercise-trained mice. Three groups of mice (n = 8 in each group), one fed a regular AIN-93G diet served as the control, and the others fed either of the two INLCDs containing 20% protein and 10% carbohydrate (INLCD-10%) or 20% protein and 1% carbohydrate (INLCD-1%) had a regular exercise load (5 times/week) for 12 weeks. Body weight and muscle mass did not decrease in either of the INLCD-fed groups, and the muscle glycogen levels and endurance capacity did not differ among the three groups. Only in the mice fed INLCD-1% did the plasma ketone concentration significantly increase, and gene expression related to glucose utilization significantly declined in the muscles. Both INLCD-1% and INLCD-10% consumption increased gene expression related to lipid utilization. These results suggest that, although INLCD treatment did not affect endurance capacity, it helped maintain muscle mass and glycogen content regardless of the glucose intake restrictions in trained mice. Moreover, an INLCD containing a low carbohydrate content might present an advantage by increasing lipid oxidation without ketosis and suppressing muscle glucose utilization.

Factors Influencing Substrate Oxidation During Submaximal Cycling: A Modelling Analysis

BACKGROUND

Multiple factors influence substrate oxidation during exercise including exercise duration and intensity, sex, and dietary intake before and during exercise. However, the relative influence and interaction between these factors is unclear.

OBJECTIVES

Our aim was to investigate factors influencing the respiratory exchange ratio (RER) during continuous exercise and formulate multivariable regression models to determine which factors best explain RER during exercise, as well as their relative influence.

METHODS

Data were extracted from 434 studies reporting RER during continuous cycling exercise. General linear mixed-effect models were used to determine relationships between RER and factors purported to influence RER (e.g., exercise duration and intensity, muscle glycogen, dietary intake, age, and sex), and to examine which factors influenced RER, with standardized coefficients used to assess their relative influence.

RESULTS

The RER decreases with exercise duration, dietary fat intake, age, VO_2max, and percentage of type I muscle fibers, and increases with dietary carbohydrate intake, exercise intensity, male sex, and carbohydrate intake before and during exercise. The modelling could explain up to 59% of the variation in RER, and a model using exclusively easily modified factors (exercise duration and intensity, and dietary intake before and during exercise) could only explain 36% of the variation in RER. Variables with the largest effect on RER were sex, dietary intake, and exercise duration. Among the diet-related factors, daily fat and carbohydrate intake have a larger influence than carbohydrate ingestion during exercise.

CONCLUSION

Variability in RER during exercise cannot be fully accounted for by models incorporating a range of participant, diet, exercise, and physiological characteristics. To better understand what influences substrate oxidation during exercise further research is required on older subjects and females, and on other factors that could explain additional variability in RER.

The Effect of a Ketogenic Low-Carbohydrate, High-Fat Diet on Aerobic Capacity and Exercise Performance in Endurance Athletes: A Systematic Review and Meta-Analysis

A low-carbohydrate, high-fat (LCHF) diet has been proposed to enhance the fat utilization of muscle and the aerobic capacity of endurance athletes, thereby improving their exercise performance. However, it remains uncertain how the macronutrient intake shift from carbohydrate to fat affects endurance exercise training and performance. This study performed a systematic review and meta-analysis to explore the effects of a ketogenic low-carbohydrate, high-fat (K-LCHF) diet on aerobic capacity and exercise performance among endurance athletes. Searches were carried out in five electronic databases, and we followed the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) guidelines. The search included studies using an LCHF diet as an intervention protocol and compared data on factors such as maximum oxygen uptake (VO2max) and rating of perceived exertion (RPE) from the graded exercise test. In this case, 10 studies met the criteria and were included in the meta-analysis. We did not find a significant effect of K-LCHF diet interventions on VO2max, time to exhaustion, HRmax or RPE. However, a significant overall effect in the substrate oxidation response to respiratory exchange rate was observed. The meta-analysis showed that K-LCHF diets did not affect aerobic capacity and exercise performance. Therefore, high-quality interventions of a K-LCHF diet are needed to illustrate its effect on various endurance training programs.

Variation of skeletal muscle ultrasound imaging intensity in horses after treadmill exercise: a proof of concept for glycogen content estimation

Background

Glycogen in skeletal muscle is a major source of energy during exercise and an important determinant of endurance capacity, so that its measurement may provide a meaningful marker of athletes’ preparation and a possible predictor of performance, both in humans and in equines. Gold standard of glycogen concentration measurement is the histochemical and biochemical analysis of biopsy-derived muscle tissue, an invasive and potentially injuring procedure. Recently, high-frequency ultrasound (US) technology is being exploited in human sports medicine to estimate muscle glycogen content. Therefore, aim of the present study is to evaluate the feasibility of US assessment of muscle glycogen in equines.

Results

US images of gluteus medius (GL) and semitendinosus (ST) muscles were obtained on eight healthy horses (3–10 years) before and after a steady-state exercise on treadmill (velocity: 4.0–12.5 m/s; duration: 2–20 min; heart rate: 137–218 b/min). Average image greyscale intensity was significantly different between GL and ST, both before and after exercise (p < 0.001). Comparing baseline and post-exercise US images, significant increase in greyscale intensity has been observed in ST (p < 0.001), but not in GL (p = 0.129). The volume of the exercise was significantly correlated with exercise-dependent change in image intensity (R² = 0.891), consistent with a reduction of glycogen muscle stores resulting from aerobic activity.

Conclusions

US technique evidences also in horses muscle changes possibly associated to glycogen utilisation during exercise. Present results on a small sample need to be further confirmed and provide preliminary data warranting future validation by direct glycogen measurement through biopsy technique.

Influences of Ketogenic Diet on Body Fat Percentage, Respiratory Exchange Rate, and Total Cholesterol in Athletes: A Systematic Review and Meta-Analysis

(1) Background: The purpose of the current meta-analysis was to investigate any positive or negative effects of ketogenic diets in athletes and provide an assessment of the size of these effects. (2) Methods: Databases were used to select relevant studies up to January 2021 regarding the effects of ketogenic diets in athletes. Inclusion criteria were as follows: data before and after ketogenic diet use, being randomized controlled trials and presenting ketogenic diets and assessments of ketone status. Study subjects were required to be professional athletes. Review studies, pilot studies, and studies in which non-athletes were included were excluded from this meta-analysis. The outcome effect sizes in these selected studies were calculated by using the standardized mean difference statistic. (3) Results: Eight studies were selected for this meta-analysis. Athletes who consumed the ketogenic diet had reduced body fat percentages, respiratory exchange rates, and increased total cholesterol compared to athletes who did not consume this diet. However, body mass index, cardiorespiratory fitness, heart rate, HDL cholesterol, glucose level, and insulin level were unaffected by the diet. (4) Conclusions: Ketogenic diets had a beneficial effect by decreasing body fat percentage, but athletes with high total cholesterol level need to be monitored when instituting a ketogenic diet. Our study sample size was limited; therefore, additional studies may be needed to confirm the current findings. Further studies need to be conducted on changes in LDL cholesterol, HDL cholesterol, total cholesterol and ratio of LDL to HDL cholesterol.

Is a Four-Week Ketogenic Diet an Effective Nutritional Strategy in CrossFit-Trained Female and Male Athletes?

This single-arm interventional study examined the effect of a 4-week ketogenic diet (KD) on aerobic capacity and discipline-specific performance in female (n = 11) and male (n = 11) CrossFit-trained athletes. The participants performed incremental cycling (ICT) and Fight Gone Bad (FGB) tests after consuming a customary diet and a KD. Pre- and post-ICT exercise blood samples were also analysed. Consuming a KD had a slight impact on aerobic capacity and no relevant effect on CrossFit-specific performance. In females, consuming a KD led to an 10.4% decrease in peak oxygen uptake during the ICT (p = 0.027) and resulted in certain alterations in haematological parameters (haemoglobin (HGB), mean corpuscular HGB, and mean corpuscular HGB concentration). Furthermore, in males, alanine aminotransferase activity increased with a simultaneous improvement in the post-ICT blood acid–base balance after consuming a KD. The pre-exercise bilirubin concentration was also elevated in the entire group after consuming a KD. In conclusion, female CrossFit-trained athletes seem to be prone to aerobic performance decrements and increased risk of developing haematological disturbances when consuming a KD. In males who consumed a KD, there was an undesirable alanine aminotransferase elevation and a small tendency towards improved acid–base status. Moreover, consuming a KD had no effect on discipline-specific performance in CrossFit-trained athletes.

Determinants of Peak Fat Oxidation Rates During Cycling in Healthy Men and Women

This study explored lifestyle and biological determinants of peak fat oxidation (PFO) during cycle ergometry, using duplicate measures to account for day-to-day variation. Seventy-three healthy adults (age range: 19-63 years; peak oxygen consumption [V˙O2peak]: 42.4 [10.1] ml·kg BM-1·min-1; n = 32 women]) completed trials 7-28 days apart that assessed resting metabolic rate, a resting venous blood sample, and PFO by indirect calorimetry during an incremental cycling test. Habitual physical activity (combined heart rate accelerometer) and dietary intake (weighed record) were assessed before the first trial. Body composition was assessed 2-7 days after the second identical trial by dual-energy X-ray absorptiometry scan. Multiple linear regressions were performed to identify determinants of PFO (mean of two cycle tests). A total variance of 79% in absolute PFO (g·min-1) was explained with positive coefficients for V˙O2peak (strongest predictor), FATmax (i.e the % of V˙O2peak that PFO occurred at), and resting fat oxidation rate (g·min-1), and negative coefficients for body fat mass (kg) and habitual physical activity level. When expressed relative to fat-free mass, 64% of variance in PFO was explained: positive coefficients for FATmax (strongest predictor), V˙O2peak, and resting fat oxidation rate, and negative coefficients for male sex and fat mass. This duplicate design revealed that biological and lifestyle factors explain a large proportion of variance in PFO during incremental cycling. After accounting for day-to-day variation in PFO, V˙O2peak and FATmax were strong and consistent predictors of PFO.

Tuning fatty acid oxidation in skeletal muscle with dietary fat and exercise

Both the consumption of a diet rich in fatty acids and exercise training result in similar adaptations in several skeletal muscle proteins. These adaptations are involved in fatty acid uptake and activation within the myocyte, the mitochondrial import of fatty acids and further metabolism of fatty acids by β-oxidation. Fatty acid availability is repeatedly increased postprandially during the day, particularly during high dietary fat intake and also increases during, and after, aerobic exercise. As such, fatty acids are possible signalling candidates that regulate transcription of target genes encoding proteins involved in muscle lipid metabolism. The mechanism of signalling might be direct or indirect targeting of peroxisome proliferator-activated receptors by fatty acid ligands, by fatty acid-induced NAD⁺-stimulated activation of sirtuin 1 and/or fatty acid-mediated activation of AMP-activated protein kinase. Lactate might also have a role in lipid metabolic adaptations. Obesity is characterized by impairments in fatty acid oxidation capacity, and individuals with obesity show some rigidity in increasing fatty acid oxidation in response to high fat intake. However, individuals with obesity retain improvements in fatty acid oxidation capacity in response to exercise training, thereby highlighting exercise training as a potential method to improve lipid metabolic flexibility in obesity.

Carbohydrate supplementation: a critical review of recent innovations

PURPOSE

To critically examine the research on novel supplements and strategies designed to enhance carbohydrate delivery and/or availability.

METHODS

Narrative review.

RESULTS

Available data would suggest that there are varying levels of effectiveness based on the supplement/supplementation strategy in question and mechanism of action. Novel carbohydrate supplements including multiple transportable carbohydrate (MTC), modified carbohydrate (MC), and hydrogels (HGEL) have been generally effective at modifying gastric emptying and/or intestinal absorption. Moreover, these effects often correlate with altered fuel utilization patterns and/or glycogen storage. Nevertheless, performance effects differ widely based on supplement and study design. MTC consistently enhances performance, but the magnitude of the effect is yet to be fully elucidated. MC and HGEL seem unlikely to be beneficial when compared to supplementation strategies that align with current sport nutrition recommendations. Combining carbohydrate with other ergogenic substances may, in some cases, result in additive or synergistic effects on metabolism and/or performance; however, data are often lacking and results vary based on the quantity, timing, and inter-individual responses to different treatments. Altering dietary carbohydrate intake likely influences absorption, oxidation, and and/or storage of acutely ingested carbohydrate, but how this affects the ergogenicity of carbohydrate is still mostly unknown.

CONCLUSIONS

In conclusion, novel carbohydrate supplements and strategies alter carbohydrate delivery through various mechanisms. However, more research is needed to determine if/when interventions are ergogenic based on different contexts, populations, and applications.

Skeletal muscle energy metabolism during exercise

The continual supply of ATP to the fundamental cellular processes that underpin skeletal muscle contraction during exercise is essential for sports performance in events lasting seconds to several hours. Because the muscle stores of ATP are small, metabolic pathways must be activated to maintain the required rates of ATP resynthesis. These pathways include phosphocreatine and muscle glycogen breakdown, thus enabling substrate-level phosphorylation ('anaerobic') and oxidative phosphorylation by using reducing equivalents from carbohydrate and fat metabolism ('aerobic'). The relative contribution of these metabolic pathways is primarily determined by the intensity and duration of exercise. For most events at the Olympics, carbohydrate is the primary fuel for anaerobic and aerobic metabolism. Here, we provide an overview of exercise metabolism and the key regulatory mechanisms ensuring that ATP resynthesis is closely matched to the ATP demand of exercise. We also summarize various interventions that target muscle metabolism for ergogenic benefit in athletic events.

Intramuscular Mechanisms Mediating Adaptation to Low-Carbohydrate, High-Fat Diets during Exercise Training

Interest in low-carbohydrate, high-fat (LCHF) diets has increased over recent decades given the theorized benefit of associated intramuscular adaptations and shifts in fuel utilization on endurance exercise performance. Consuming a LCHF diet during exercise training increases the availability of fat (i.e., intramuscular triglyceride stores; plasma free fatty acids) and decreases muscle glycogen stores. These changes in substrate availability increase reliance on fat oxidation for energy production while simultaneously decreasing reliance on carbohydrate oxidation for fuel during submaximal exercise. LCHF diet-mediated changes in substrate oxidation remain even after endogenous or exogenous carbohydrate availability is increased, suggesting that the adaptive response driving changes in fat and carbohydrate oxidation lies within the muscle and persists even when the macronutrient content of the diet is altered. This narrative review explores the intramuscular adaptations underlying increases in fat oxidation and decreases in carbohydrate oxidation with LCHF feeding. The possible effects of LCHF diets on protein metabolism and post-exercise muscle remodeling are also considered.

Heterogeneity in subcellular muscle glycogen utilisation during exercise impacts endurance capacity in men

KEY POINTS

When muscle biopsies first began to be used routinely in research on exercise physiology five decades ago, it soon become clear that the muscle content of glycogen is an important determinant of exercise performance. Glycogen particles are stored in distinct pools within the muscles, but the role of each pool during exercise and how this is affected by diet is unknown. Here, the effects of diet and exercise on these pools, as well as their relation to endurance during prolonged cycling were examined. We demonstrate here that an improved endurance capacity with high carbohydrate loading is associated with a temporal shift in the utilisation of the distinct stores of glycogen pools and is closely linked to the content of the glycogen pool closest to actin and myosin (intramyofibrillar glycogen). These findings highlight the functional importance of distinguishing between different subcellular microcompartments of glycogen in individual muscle fibres.

ABSTRACT

In muscle cells, glycogen is stored in three distinct subcellular pools: between or within myofibrils (inter- and intramyofibrillar glycogen, respectively) or beneath the sarcolemma (subsarcolemmal glycogen) and these pools may well have different functions. Here, we investigated the effect of diet and exercise on the content of these distinct pools and their relation to endurance capacity in type 1 and 2 muscle fibres. Following consumption of three different diets (normal, mixed diet = MIX, high in carbohydrate = HIGH, or low in carbohydrate = LOW) for 72 h, 11 men cycled at 75% of V ̇ O 2 max until exhaustion. The volumetric content of the glycogen pools in muscle biopsies obtained before, during, and after exercise were quantified by transmission electron micrographs. The mean (SD) time to exhaustion was 150 (30), 112 (22), and 69 (18) minutes in the HIGH, MIX and LOW trials, respectively (P < 0.001). As shown by multiple regression analyses, the intramyofibrillar glycogen content in type 1 fibres, particularly after 60 min of exercise, correlated most strongly with time to exhaustion. In the HIGH trial, intramyofibrillar glycogen was spared during the initial 60 min of exercise, which was associated with levels and utilisation of subsarcolemmal glycogen above normal. In all trials, utilisation of subsarcolemmal and intramyofibrillar glycogen was more pronounced than that of intermyofibrillar glycogen in relative terms. In conclusion, the muscle pool of intramyofibrillar glycogen appears to be the most important for endurance capacity in humans. In addition, a local abundance of subsarcolemmal glycogen reduces the utilisation of intramyofibrillar glycogen during exercise.

A 28-Day Carbohydrate-Restricted Diet Improves Markers of Cardiometabolic Health and Performance in Professional Firefighters

Waldman, HS, Smith, JW, Lamberth, J, Fountain, BJ, and McAllister, MJ. A 28-day carbohydrate-restricted diet improves markers of cardiometabolic health and performance in professional firefighters. J Strength Cond Res 33(12): 3284-3294, 2019-We investigated the effects of a 4-week ad-libitum, nonketogenic, carbohydrate-restricted (<25% of calories) diet (CRD) on cardiometabolic and performance markers in firefighters (FF). Subjects (n = 15) completed 9 sessions (trials 1-3 [familiarization], trials 4-6 [baseline], and trials 7-9 [post-CRD]). Following habitual western diet, anthropometric measures were assessed, glucose tolerance measured, and then completed a graded cycling test, maximal Wingate test, and conducted their FF physical performance assessment (FPPA) to measure performance while metabolic variables and perceptual responses were recorded. Subjects then adhered to a CRD for a 4-week duration and returned for repeat testing. Body fat as measured by BodPod, and 7-site skinfold thickness decreased (p < 0.01), and a decrease was observed in blood pressure (BP) (p < 0.01; ∼5 mm Hg) after CRD. There were no differences found for glucose tolerance, but an increase was found for fat oxidation rates (p < 0.01; ∼0.07 g·min) and a decrease in carbohydrate oxidation rates across a range of intensities (p < 0.01; ∼0.24 g·min). Finally, the 2.41-km run and pull-up performance during the FPPA improved (p < 0.01; ∼41 second and 3 repetitions, respectively) and with no differences observed between treatments regarding the Wingate test. To date, this is the first CRD implemented with FF and resulted in decreased fat mass (∼2.4 kg), BP, and improvements to performance on the FPPA while preserving high-intensity exercise. These data suggest that a 28-day CRD can benefit markers of health in professional FF without detriments to occupational performance.

Ergogenic Properties of Ketogenic Diets in Normal-Weight Individuals: A Systematic Review

Ketogenic diets (KDs) have received increasing attention among athletes and physically active individuals. However, the question as to whether and how the diet could benefit this healthy cohort remains unclear.Purpose: This study was designed to systematically review the existing evidence concerning the effect of KDs on body composition, aerobic and anaerobic capacity, muscle development, and sports performance in normal-weight individuals including athletes.Methods: A systematic search of English literature was conducted through electronic databases including PubMed, EBSCOhost, and Google Scholar. Upon the use of search criteria, 23 full-text original human studies involving non-obese participants were included in this review. For more stratified and focused analysis, these articles were further categorized based on the outcomes being examined including 1) body mass (BM) and %fat, 2) substrate utilization, 3) blood substrate and hormonal responses, 4) aerobic capacity and endurance performance, and 5) strength, power, and anaerobic capacity.Results: Our review indicates that a non-calorie-restricted KD carried out for ≥3 weeks can produce a modest reduction in BM and %fat, while maintaining fat-free mass. This diet leads to augmented use of fat as fuel, but this adaptation doesn't seem to improve endurance performance. Additionally, ad libitum KDs combined with resistance training will pose no harm to developing strength and power, especially when protein intake is increased modestly.Conclusions: It appears that a non-calorie-restricted KD provides minimal ergogenic benefits in normal-weight individuals including athletes, but can be used for optimizing BM and body composition without compromising aerobic and anaerobic performance. Key teaching pointsKetogenic diets have received increasing attention among athletes and physically active individuals.It remains elusive as to whether ketogenic diets could confer ergogenic benefits for those who are normal weight but want to use the diet to improve fitness and performance.An interesting dilemma exists in that ketogenic diets can reduce body mass and %fat and increase fat oxidation, but they can also decrease glycogen stores and limit sports performance.This review concludes that a non-calorie-restricted ketogenic diet provides minimal ergogenic benefits in normal-weight individuals, but can be used to optimize body mass and composition without compromising athletic performance.This finding can be important for esthetic or weight-sensitive athletes because the diet may allow them to reach a target body mass without having to sacrifice athletic performance.The ketogenic diet-induced metabolic adaptations require a state of ketosis, and thus caution should be taken because an excessive increase in ketone bodies can be detrimental to health.

Effect of a Low-Carbohydrate High-Fat Diet and a Single Bout of Exercise on Glucose Tolerance, Lipid Profile and Endothelial Function in Normal Weight Young Healthy Females

Low-carbohydrate-high-fat (LCHF) diets are efficient for weight loss, and are also used by healthy people to maintain bodyweight. The main aim of this study was to investigate the effect of 3-week energy-balanced LCHF-diet, with >75 percentage energy (E%) from fat, on glucose tolerance and lipid profile in normal weight, young, healthy women. The second aim of the study was to investigate if a bout of exercise would prevent any negative effect of LCHF-diet on glucose tolerance. Seventeen females participated, age 23.5 ± 0.5 years; body mass index 21.0 ± 0.4 kg/m², with a mean dietary intake of 78 ± 1 E% fat, 19 ± 1 E% protein and 3 ± 0 E% carbohydrates. Measurements were performed at baseline and post-intervention. Fasting glucose decreased from 4.7 ± 0.1 to 4.4 mmol/L (p < 0.001) during the dietary intervention whereas fasting insulin was unaffected. Glucose area under the curve (AUC) and insulin AUC did not change during an OGTT after the intervention. Before the intervention, a bout of aerobic exercise reduced fasting glucose (4.4 ± 0.1 mmol/L, p < 0.001) and glucose AUC (739 ± 41 to 661 ± 25, p = 0.008) during OGTT the following morning. After the intervention, exercise did not reduce fasting glucose the following morning, and glucose AUC during an OGTT increased compared to the day before (789 ± 43 to 889 ± 40 mmol/L∙120min^–1, p = 0.001). AUC for insulin was unaffected. The dietary intervention increased total cholesterol (p < 0.001), low-density lipoprotein (p ≤ 0.001), high-density lipoprotein (p = 0.011), triglycerides (p = 0.035), and free fatty acids (p = 0.021). In conclusion, 3-week LCHF-diet reduced fasting glucose, while glucose tolerance was unaffected. A bout of exercise post-intervention did not decrease AUC glucose as it did at baseline. Total cholesterol increased, mainly due to increments in low-density lipoprotein. LCHF-diets should be further evaluated and carefully considered for healthy individuals.

The impact of keto-adaptation on exercise performance and the role of metabolic-regulating cytokines

The ketogenic diet (KD) is a normocaloric diet composed of high-fat, low-carbohydrate, and adequate protein that induces fasting-like effects and results in the production of ketone bodies. Initially used widely for children with refractory epilepsy, the KD gained popularity due to its beneficial effects on weight loss, diabetes, and cancer. In recent years, there has been a resurgence in interest surrounding the KD and exercise performance. This review provides new insights into the adaptation period necessary for enhancement in skeletal muscle fat and ketone oxidation after sustained nutritional ketosis. In addition, this review highlights metabolically active growth factors and cytokines, which may function as important regulators of keto-adaptation in the setting of exercise and the KD.

Dietary Fuels in Athletic Performance

Focusing on daily nutrition is important for athletes to perform and adapt optimally to exercise training. The major roles of an athlete's daily diet are to supply the substrates needed to cover the energy demands for exercise, to ensure quick recovery between exercise bouts, to optimize adaptations to exercise training, and to stay healthy. The major energy substrates for exercising skeletal muscles are carbohydrate and fat stores. Optimizing the timing and type of energy intake and the amount of dietary macronutrients is essential to ensure peak training and competition performance, and these strategies play important roles in modulating skeletal muscle adaptations to endurance and resistance training. In this review, recent advances in nutritional strategies designed to optimize exercise-induced adaptations in skeletal muscle are discussed, with an emphasis on mechanistic approaches, by describing the physiological mechanisms that provide the basis for different nutrition regimens.

Priority use of medium-chain fatty acids during high-intensity exercise in cross-country skiers

Background

One of the topics discussed in sports science is the use of medium-chain saturated fat as an energy-saving nutrient additive when approaching high-intensity exercise. The purpose of this study was to compare the blood concentrations of medium-chain and long-chain fatty acids (FAs) across different intensity loads.

Methods

Fifteen male highly trained athletes from the Russian cross-country skiing team participated in the study. Blood samples were drawn at rest, at the peak of veloergometric test with a growing exercise load till exhaustion (97–100% VO2max), and after competitions. The plasma FA profile was determined using gas-liquid chromatography.

Results

We observed a substantial increase in the concentrations of capric acid (С10:0) (+ 164.1%), lauric acid (С12:0) (+ 223.9%), and myristic acid (С14:0) (+ 130.2%) in skiers after a sprint distance (1.3 km). A less intense increase in the concentrations of these acids (p < 0.05) was observed after a middle length distance or cycle exercise «until exhaustion». No significant differences in long-chain saturated FA content relative to baselines during exercise tests or competitions were revealed.

Conclusions

In conclusion, the obtained results demonstrate activation of the lipolysis and the oxidation of medium-chain FA involved in the energy supply for highly trained athletes at maximum exercise loads.

Metabolic adaptations to endurance training and nutrition strategies influencing performance

Endurance performance is the result of optimal training targeting cardiovascular, metabolic, and peripheral muscular adaptations and is coupled to effective nutrition strategies via the use of macronutrient manipulations surrounding training and potential supplementation with ergogenic aids. It is important to note that training and nutrition may differ according to the individual needs of the athlete and can markedly impact the physiological response to training. Herein, we discuss various aspects of endurance training adaptations, nutritional strategies and their contributions to towards performance.

Nutrient Intake and Physical Exercise Significantly Impact Physical Performance, Body Composition, Blood Lipids, Oxidative Stress, and Inflammation in Male Rats

BACKGROUND

Humans consuming a purified vegan diet known as the "Daniel Fast" realize favorable changes in blood lipids, oxidative stress, and inflammatory biomarkers, with subjective reports of improved physical capacity.

OBJECTIVE

We sought to determine if this purified vegan diet was synergistic with exercise in male rats.

METHODS

Long⁻Evans rats (n = 56) were assigned to be exercise trained (+E) by running on a treadmill three days per week at a moderate intensity or to act as sedentary controls with normal activity. After the baseline physical performance was evaluated by recording run time to exhaustion, half of the animals in each group were fed ad libitum for three months a purified diet formulated to mimic the Daniel Fast (DF) or a Western Diet (WD). Physical performance was evaluated again at the end of month 3, and body composition was assessed using dual-energy x-ray absorptiometry. Blood was collected for measurements of lipids, oxidative stress, and inflammatory biomarkers.

RESULTS

Physical performance at the end of month 3 was higher compared to baseline for both exercise groups (p < 0.05), with a greater percent increase in the DF + E group (99%) than in the WD + E group (51%). Body fat was lower in DF than in WD groups at the end of month 3 (p < 0.05). Blood triglycerides, cholesterol, malondialdehyde, and advanced oxidation protein products were significantly lower in the DF groups than in the WD groups (p < 0.05). No significant differences were noted in cytokines levels between the groups (p > 0.05), although IL-1β and IL-10 were elevated three-fold and two-fold in the rats fed the WD compared to the DF rats, respectively.

CONCLUSIONS

Compared to a WD, a purified diet that mimics the vegan Daniel Fast provides significant anthropometric and metabolic benefits to rats, while possibly acting synergistically with exercise training to improve physical performance. These findings highlight the importance of macronutrient composition and quality in the presence of ad libitum food intake.

Dietary Manipulations Concurrent to Endurance Training

The role of an athlete's dietary intake (both timing and food type) goes beyond simply providing fuel to support the body's vital processes. Nutritional choices also have an impact on the metabolic adaptations to training. Over the past 20 years, research has suggested that strategically reducing carbohydrate (CHO) availability during an athlete's training can modify the metabolic responses in lieu of simply maintaining a high CHO diet. Several methods have been explored to manipulate CHO availability and include: Low-carb, high-fat (LCHF) diets, performing two-a-day training without glycogen restoration between sessions, and a "sleep-low" approach entailing a glycogen-depleting session in the evening without consuming CHO until after a morning training session performed in an overnight fasted state. Each of these methods can confer beneficial metabolic adaptations for the endurance athlete including increases in mitochondrial enzyme activity, mitochondrial content, and rates of fat oxidation, yet data showing a direct performance benefit is still unclear.

Quantitative proteomic characterization of cellular pathways associated with altered insulin sensitivity in skeletal muscle following high-fat diet feeding and exercise training

Regular exercise elicits advantageous metabolic adaptations in skeletal muscle, such as improved insulin sensitivity. However, the underpinning molecular mechanisms and the effect of diet on muscle exercise training benefits are unclear. We therefore characterized the skeletal muscle proteome following exercise training (ET) in mice fed chow or high-fat diet (HFD). ET increased exercise performance, lowered body-weight, decreased fat mass and improved muscle insulin action in chow- and HFD-fed mice. At the molecular level, ET regulated 170 muscle proteins in chow-fed mice, but only 29 proteins in HFD-fed mice. HFD per se altered 56 proteins, most of which were regulated in a similar direction by ET. To identify proteins that might have particular health-related bearing on skeletal muscle metabolism, we filtered for differentially regulated proteins in response to ET and HFD. This yielded 15 proteins, including the major urinary protein 1 (MUP1), which was the protein most decreased after HFD, but increased with ET. The ET-induced Mup1 expression was absent in mouse muscle lacking functional AMPK. MUP1 also potentiated insulin-stimulated GLUT4 translocation in cultured muscle cells. Collectively, we provide a resource of ET-regulated proteins in insulin-sensitive and insulin-resistant skeletal muscle. The identification of MUP1 as a diet-, ET- and AMPK-regulated skeletal muscle protein that improves insulin sensitivity in muscle cells demonstrates the usefulness of these data.

Uncoupling of glycolysis from glucose oxidation accompanies the development of heart failure with preserved ejection fraction

BACKGROUND

Alterations in cardiac energy metabolism contribute to the development and severity of heart failure (HF). In severe HF, overall mitochondrial oxidative metabolism is significantly decreased resulting in a reduced energy reserve. However, despite the high prevalence of HF with preserved ejection fraction (HFpEF) in our society, it is not clear what changes in cardiac energy metabolism occur in HFpEF, and whether alterations in energy metabolism contribute to the development of contractile dysfunction.

METHODS

We directly assessed overall energy metabolism during the development of HFpEF in Dahl salt-sensitive rats fed a high salt diet (HSD) for 3, 6 and 9 weeks.

RESULTS

Over the course of 9 weeks, the HSD caused a progressive decrease in diastolic function (assessed by echocardiography assessment of E'/A'). This was accompanied by a progressive increase in cardiac glycolysis rates (assessed in isolated working hearts obtained at 3, 6, and 9 weeks of HSD). In contrast, the subsequent oxidation of pyruvate from glycolysis (glucose oxidation) was not altered, resulting in an uncoupling of glucose metabolism and a significant increase in proton production. Increased glucose transporter (GLUT)1 expression accompanied this elevation in glycolysis. Decreases in cardiac fatty acid oxidation and overall adenosine triphosphate (ATP) production rates were not observed in early HF, but both significantly decreased as HF progressed to HF with reduced EF (i.e. 9 weeks of HSD).

CONCLUSIONS

Overall, we show that increased glycolysis is the earliest energy metabolic change that occurs during HFpEF development. The resultant increased proton production from uncoupling of glycolysis and glucose oxidation may contribute to the development of HFpEF.

Gluconeogenesis during endurance exercise in cyclists habituated to a long-term low carbohydrate high-fat diet

KEY POINTS

Blood glucose is an important fuel for endurance exercise. It can be derived from ingested carbohydrate, stored liver glycogen and newly synthesized glucose (gluconeogenesis). We hypothesized that athletes habitually following a low carbohydrate high fat (LCHF) diet would have higher rates of gluconeogenesis during exercise compared to those who follow a mixed macronutrient diet. We used stable isotope tracers to study glucose production kinetics during a 2 h ride in cyclists habituated to either a LCHF or mixed macronutrient diet. The LCHF cyclists had lower rates of total glucose production and hepatic glycogenolysis but similar rates of gluconeogenesis compared to those on the mixed diet. The LCHF cyclists did not compensate for reduced dietary carbohydrate availability by increasing glucose synthesis during exercise but rather adapted by altering whole body substrate utilization.

ABSTRACT

Endogenous glucose production (EGP) occurs via hepatic glycogenolysis (GLY) and gluconeogenesis (GNG) and plays an important role in maintaining euglycaemia. Rates of GLY and GNG increase during exercise in athletes following a mixed macronutrient diet; however, these processes have not been investigated in athletes following a low carbohydrate high fat (LCHF) diet. Therefore, we studied seven well-trained male cyclists that were habituated to either a LCHF (7% carbohydrate, 72% fat, 21% protein) or a mixed diet (51% carbohydrate, 33% fat, 16% protein) for longer than 8 months. After an overnight fast, participants performed a 2 h laboratory ride at 72% of maximal oxygen consumption. Glucose kinetics were measured at rest and during the final 30 min of exercise by infusion of [6,6-(2) H2 ]-glucose and the ingestion of (2) H2 O tracers. Rates of EGP and GLY both at rest and during exercise were significantly lower in the LCHF group than the mixed diet group (Exercise EGP: LCHF, 6.0 ± 0.9 mg kg(-1)  min(-1) , Mixed, 7.8 ± 1.1 mg kg(-1)  min(-1) , P < 0.01; Exercise GLY: LCHF, 3.2 ± 0.7 mg kg(-1)  min(-1) , Mixed, 5.3 ± 0.9 mg kg(-1)  min(-1) , P < 0.01). Conversely, no difference was detected in rates of GNG between groups at rest or during exercise (Exercise: LCHF, 2.8 ± 0.4 mg kg(-1)  min(-1) , Mixed, 2.5 ± 0.3 mg kg(-1)  min(-1) , P = 0.15). We conclude that athletes on a LCHF diet do not compensate for reduced glucose availability via higher rates of glucose synthesis compared to athletes on a mixed diet. Instead, GNG remains relatively stable, whereas glucose oxidation and GLY are influenced by dietary factors.

Genetic Variation of Fatty Acid Oxidation and Obesity, A Literature Review

Modulation of fat metabolism is an important component of the etiology of obesity as well as individual response to weight loss program. The influence of lipolysis process had receives many attentions in recent decades. Compared to that, fatty acid oxidation which occurred after lipolysis seems to be less exposed. There are limited publications on how fatty acid oxidation influences predisposition to obesity, especially the importance of genetic variations of fatty acid oxidation proteins on development of obesity. The aim of this review is to provide recent knowledge on how polymorphism of genes related fatty acid oxidation is obtained. Studies in human as well as animal model showed that disturbance of genes related fatty acid oxidation process gave impact on body weight and risks to obesity. Several polymorphisms on CD36, CPT, ACS and FABP had been shown to be related to obesity either by regulating enzymatic activity or directly influence fatty acid oxidation process.

The life history of whole-organism performance

For almost 40 years, studies of whole-organism performance have formed a cornerstone of evolutionary physiology. Although its utility as a heuristic guide is beyond question, and we have learned much about morphological evolution from its application, the ecomorphological paradigm has frequently been applied to performance evolution in ways that range from unsatisfactory to inappropriate. More importantly, the standard ecomorphological paradigm does not account for tradeoffs among performance and other traits, nor between performance traits that are mediated by resource allocation. A revised paradigm that includes such tradeoffs, and the possible ways that performance and fitness-enhancing traits might affect each other, could potentially revivify the study of phenotypic evolution and make important inroads into understanding the relationships between morphology and performance and between performance and Darwinian fitness. We describe such a paradigm, and discuss the various ways that performance and key life-history traits might interact with and affect each other. We emphasize both the proximate mechanisms potentially linking such traits, and the likely ultimate factors driving those linkages, as well as the evolutionary implications for the overall, multivariate phenotype. Finally, we highlight several research directions that will shed light on the evolution and ecology of whole-organism performance and related life-history traits.

Effect of short-duration lipid supplementation on fat oxidation during exercise and cycling performance

The effect of intramyocellular lipids (IMCLs) on endurance performance with high skeletal muscle glycogen availability remains unclear. Previous work has shown that a lipid-supplemented high-carbohydrate (CHO) diet increases IMCLs while permitting normal glycogen loading. The aim of this study was to assess the effect of fat supplementation on fat oxidation (Fox) and endurance performance. Twenty-two trained male cyclists performed 2 simulated time trials (TT) in a randomized crossover design. Subjects cycled at ∼53% maximal voluntary external power for 2 h and then followed 1 of 2 diets for 2.5 days: a high-CHO low-fat (HC) diet, consisting of CHO 7.4 g·kg(-1)·day(-1) and fat 0.5 g·kg(-1)·day(-1); or a high-CHO fat-supplemented (HCF) diet, which was a replication of the HC diet with ∼240 g surplus fat (30% saturation) distributed over the last 4 meals of the diet period. On trial morning, fasting blood was sampled and Fox was measured during an incremental exercise; a ∼1-h TT followed. Breath volatile compounds (VOCs) were measured at 3 time points. Mental fatigue, measured as reaction time, was evaluated during the TT. Plasma free fatty acid concentration was 50% lower after the HCF diet (p < 0.0001), and breath acetone was reduced (p < 0.05) "at rest". Fox peaked (∼0.35 g·kg(-1)) at ∼42% peak oxygen consumption, and was not influenced by diet. Performance was not significantly different between the HCF and HC diets (3369 ± 46 s vs 3398 ± 48 s; p = 0.39), nor were reaction times to the attention task and VOCs (p = NS for both). In conclusion, the short-term intake of a lipid supplement in combination with a glycogen-loading diet designed to boost intramyocellular lipids while avoiding fat adaptation did not alter substrate oxidation during exercise or 1-hour cycling performance.

Nutrition for sports performance: issues and opportunities

Diet can significantly influence athletic performance, but recent research developments have substantially changed our understanding of sport and exercise nutrition. Athletes adopt various nutritional strategies in training and competition in the pursuit of success. The aim of training is to promote changes in the structure and function of muscle and other tissues by selective modulation of protein synthesis and breakdown in response to the training stimulus. This process is affected by the availability of essential amino acids in the post-exercise period. Athletes have been encouraged to eat diets high in carbohydrate, but low-carbohydrate diets up-regulate the capacity of muscle for fat oxidation, potentially sparing the limited carbohydrate stores. Such diets, however, do not enhance endurance performance. It is not yet known whether the increased capacity for fat oxidation that results from training in a carbohydrate-deficient state can promote loss of body fat. Preventing excessive fluid deficits will maintain exercise capacity, and ensuring adequate hydration status can also reduce subjective perception of effort. This latter effect may be important in encouraging exercise participation and promoting adherence to exercise programmes. Dietary supplement use is popular in sport, and a few supplements may improve performance in specific exercise tasks. Athletes must be cautious, however, not to contravene the doping regulations. There is an increasing recognition of the role of the brain in determining exercise performance: various nutritional strategies have been proposed, but with limited success. Nutrition strategies developed for use by athletes can also be used to achieve functional benefits in other populations.

Factors regulating fat oxidation in human skeletal muscle

In modern societies, oversupply of calories leads to obesity and chronic metabolic stress, which may lead to development of disease. Oversupply of calories is often associated with elevated plasma lipid concentrations and accumulation of lipids in skeletal muscle leading to decreased insulin sensitivity. Consequently, enhanced fat oxidation might be beneficial in counteracting lipid accumulation. Exercise is the most effective way to increase fat oxidation, because it increases metabolic rate. Lipid metabolism can also be altered by dietary manipulations. Thus, a fat rich diet leads to increased potential for fat oxidation by increasing the content of several of the proteins in the fat oxidative pathway. The regulation of both exercise and diet induced lipid oxidation will be discussed in this review.

Carbohydrates for training and competition

An athlete's carbohydrate intake can be judged by whether total daily intake and the timing of consumption in relation to exercise maintain adequate carbohydrate substrate for the muscle and central nervous system ("high carbohydrate availability") or whether carbohydrate fuel sources are limiting for the daily exercise programme ("low carbohydrate availability"). Carbohydrate availability is increased by consuming carbohydrate in the hours or days prior to the session, intake during exercise, and refuelling during recovery between sessions. This is important for the competition setting or for high-intensity training where optimal performance is desired. Carbohydrate intake during exercise should be scaled according to the characteristics of the event. During sustained high-intensity sports lasting ~1 h, small amounts of carbohydrate, including even mouth-rinsing, enhance performance via central nervous system effects. While 30-60 g · h(-1) is an appropriate target for sports of longer duration, events >2.5 h may benefit from higher intakes of up to 90 g · h(-1). Products containing special blends of different carbohydrates may maximize absorption of carbohydrate at such high rates. In real life, athletes undertake training sessions with varying carbohydrate availability. Whether implementing additional "train-low" strategies to increase the training adaptation leads to enhanced performance in well-trained individuals is unclear.

High-fat diet overrules the effects of training on fiber-specific intramyocellular lipid utilization during exercise

In this study, we compared the effects of endurance training in the fasted state (F) vs. the fed state [ample carbohydrate intake (CHO)] on exercise-induced intramyocellular lipid (IMCL) and glycogen utilization during a 6-wk period of a hypercaloric (∼+30% kcal/day) fat-rich diet (HFD; 50% of kcal). Healthy male volunteers (18-25 yrs) received a HFD in conjunction with endurance training (four times, 60-90 min/wk) either in F (n = 10) or with CHO before and during exercise sessions (n = 10). The control group (n = 7) received a HFD without training and increased body weight by ∼3 kg (P < 0.001). Before and after a HFD, the subjects performed a 2-h constant-load bicycle exercise test in F at ∼70% maximal oxygen uptake rate. A HFD, both in the absence (F) or presence (CHO) of training, elevated basal IMCL content by ∼50% in type I and by ∼75% in type IIa fibers (P < 0.05). Independent of training in F or CHO, a HFD, as such, stimulated exercise-induced net IMCL breakdown by approximately twofold in type I and by approximately fourfold in type IIa fibers. Furthermore, exercise-induced net muscle glycogen breakdown was not significantly affected by a HFD. It is concluded that a HFD stimulates net IMCL degradation by increasing basal IMCL content during exercise in type I and especially IIa fibers. Furthermore, a hypercaloric HFD provides adequate amounts of carbohydrates to maintain high muscle glycogen content during training and does not impair exercise-induced muscle glycogen breakdown.

High-fat feeding inhibits exercise-induced increase in mitochondrial respiratory flux in skeletal muscle

Twenty one healthy untrained male subjects were randomized to follow a high-fat diet (HFD; 55-60E% fat, 25-30E% carbohydrate, and 15E% protein) or a normal diet (ND; 25-35E% fat, 55-60E% carbohydrate, and 10-15E% protein) for 2(1/2) wk. Diets were isocaloric and tailored individually to match energy expenditure. At 2(1/2) wk of diet, one 60-min bout of bicycle exercise (70% of maximal oxygen uptake) was performed. Muscle biopsies were obtained before and after the diet, immediately after exercise, and after 3-h recovery. Insulin sensitivity (hyperinsulinemic-euglycemic clamp) and intramyocellular triacylglycerol content did not change with the intervention in either group. Indexes of mitochondrial density were similar across the groups and intervention. Mitochondrial respiratory rates, measured in permeabilized muscle fibers, showed a 31 ± 11 and 26 ± 9% exercise-induced increase (P < 0.05) in state 3 (glycolytic substrates) and uncoupled respiration, respectively. However, in HFD this increase was abolished. At recovery, no change from resting respiration was seen in either group. With a lipid substrate (octanoyl-carnitine with or without ADP), similar exercise-induced increases (31-62%) were seen in HFD and ND, but only in HFD was an elevated (P < 0.05) respiratory rate seen at recovery. With HFD complex I and IV protein expression decreased (P < 0.05 and P = 0.06, respectively). A fat-rich diet induces marked changes in the mitochondrial electron transport system protein content and in exercise-induced mitochondrial substrate oxidation rates, with the effects being present hours after the exercise. The effect of HFD is present even without effects on insulin sensitivity and intramyocellular lipid accumulation. An isocaloric high-fat diet does not cause insulin resistance.

Nutritional modulation of training-induced skeletal muscle adaptations

Skeletal muscle displays remarkable plasticity, enabling substantial adaptive modifications in its metabolic potential and functional characteristics in response to external stimuli such as mechanical loading and nutrient availability. Contraction-induced adaptations are determined largely by the mode of exercise and the volume, intensity, and frequency of the training stimulus. However, evidence is accumulating that nutrient availability serves as a potent modulator of many acute responses and chronic adaptations to both endurance and resistance exercise. Changes in macronutrient intake rapidly alter the concentration of blood-borne substrates and hormones, causing marked perturbations in the storage profile of skeletal muscle and other insulin-sensitive tissues. In turn, muscle energy status exerts profound effects on resting fuel metabolism and patterns of fuel utilization during exercise as well as acute regulatory processes underlying gene expression and cell signaling. As such, these nutrient-exercise interactions have the potential to activate or inhibit many biochemical pathways with putative roles in training adaptation. This review provides a contemporary perspective of our understanding of the molecular and cellular events that take place in skeletal muscle in response to both endurance and resistance exercise commenced after acute and/or chronic alterations in nutrient availability (carbohydrate, fat, protein, and several antioxidants). Emphasis is on the results of human studies and how nutrient provision (or lack thereof) interacts with specific contractile stimulus to modulate many of the acute responses to exercise, thereby potentially promoting or inhibiting subsequent training adaptation.

Beneficial metabolic adaptations due to endurance exercise training in the fasted state

Training with limited carbohydrate availability can stimulate adaptations in muscle cells to facilitate energy production via fat oxidation. Here we investigated the effect of consistent training in the fasted state, vs. training in the fed state, on muscle metabolism and substrate selection during fasted exercise. Twenty young male volunteers participated in a 6-wk endurance training program (1-1.5 h cycling at ∼70% Vo(₂max), 4 days/wk) while receiving isocaloric carbohydrate-rich diets. Half of the subjects trained in the fasted state (F; n = 10), while the others ingested ample carbohydrates before (∼160 g) and during (1 g·kg body wt⁻¹·h⁻¹) the training sessions (CHO; n = 10). The training similarly increased Vo(₂max) (+9%) and performance in a 60-min simulated time trial (+8%) in both groups (P < 0.01). Metabolic measurements were made during a 2-h constant-load exercise bout in the fasted state at ∼65% pretraining Vo(₂max). In F, exercise-induced intramyocellular lipid (IMCL) breakdown was enhanced in type I fibers (P < 0.05) and tended to be increased in type IIa fibers (P = 0.07). Training did not affect IMCL breakdown in CHO. In addition, F (+21%) increased the exercise intensity corresponding to the maximal rate of fat oxidation more than did CHO (+6%) (P < 0.05). Furthermore, maximal citrate synthase (+47%) and β-hydroxyacyl coenzyme A dehydrogenase (+34%) activity was significantly upregulated in F (P < 0.05) but not in CHO. Also, only F prevented the development exercise-induced drop in blood glucose concentration (P < 0.05). In conclusion, F is more effective than CHO to increase muscular oxidative capacity and at the same time enhances exercise-induced net IMCL degradation. In addition, F but not CHO prevented drop of blood glucose concentration during fasting exercise.

Influence of high- and low-carbohydrate diet following glycogen-depleting exercise on heart rate variability and plasma catecholamines

The purpose of this study was to investigate the effects of a short-term low- or high-carbohydrate (CHO) diet consumed after exercise on sympathetic nervous system activity. Twelve healthy males underwent a progressive incremental test; a control measurement of plasma catecholamines and heart rate variability (HRV); an exercise protocol to reduce endogenous CHO stores; a low- or high-CHO diet (counterbalanced order) consumed for 2 days, beginning immediately after the exercise protocol; and a second resting plasma catecholamine and HRV measurement. The exercise and diet protocols and the second round of measurements were performed again after a 1-week washout period. The mean (+/-SD) values of the standard deviation of R-R intervals were similar between conditions (control, 899.0+/-146.1 ms; low-CHO diet, 876.8+/-115.8 ms; and high-CHO diet, 878.7+/-127.7 ms). The absolute high- and low-frequency (HF and LF, respectively) densities of the HRV power spectrum were also not different between conditions. However, normalized HF and LF (i.e., relative to the total power spectrum) were lower and higher, respectively, in the low-CHO diet than in the control diet (mean+/-SD, 17+/-9 normalized units (NU) and 83+/-9 NU vs. 27+/-11 NU and 73+/-17 NU, respectively; p<0.05). The LF/HF ratio was higher with the low-CHO diet than with the control diet (mean+/-SD, 7.2+/-6.2 and 4.2+/-3.2, respectively; p<0.05). The mean values of plasma catecholamines were not different between diets. These results suggest that the autonomic control of the heart rate was modified after a short-term low-CHO diet, but plasma catecholamine levels were not altered.

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.

Chronic AMP-activated protein kinase activation and a high-fat diet have an additive effect on mitochondria in rat skeletal muscle

Factors that stimulate mitochondrial biogenesis in skeletal muscle include AMP-activated protein kinase (AMPK), calcium, and circulating free fatty acids (FFAs). Chronic treatment with either 5-aminoimidazole-4-carboxamide riboside (AICAR), a chemical activator of AMPK, or increasing circulating FFAs with a high-fat diet increases mitochondria in rat skeletal muscle. The purpose of this study was to determine whether the combination of chronic chemical activation of AMPK and high-fat feeding would have an additive effect on skeletal muscle mitochondria levels. We treated Wistar male rats with a high-fat diet (HF), AICAR injections (AICAR), or a high-fat diet and AICAR injections (HF + AICAR) for 6 wk. At the end of the treatment period, markers of mitochondrial content were examined in white quadriceps, red quadriceps, and soleus muscles, predominantly composed of unique muscle-fiber types. In white quadriceps, there was a cumulative effect of treatments on long-chain acyl-CoA dehydrogenase, cytochrome c, and peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) protein, as well as on citrate synthase and beta-hydroxyacyl-CoA dehydrogenase (beta-HAD) activity. In contrast, no additive effect was noted in the soleus, and in the red quadriceps only beta-HAD activity increased additively. The additive increase of mitochondrial markers observed in the white quadriceps may be explained by a combined effect of two separate mechanisms: high-fat diet-induced posttranscriptional increase in PGC-1alpha protein and AMPK-mediated increase in PGC-1alpha protein via a transcriptional mechanism. These data show that chronic chemical activation of AMPK and a high-fat diet have a muscle type specific additive effect on markers of fatty acid oxidation, the citric acid cycle, the electron transport chain, and transcriptional regulation.

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.

Effects of a low carbohydrate weight loss diet on exercise capacity and tolerance in obese subjects

Dietary restriction and increased physical activity are recommended for obesity treatment. Very low carbohydrate diets are used to promote weight loss, but their effects on physical function and exercise tolerance in overweight and obese individuals are largely unknown. The aim of this study was to compare the effects of a very low carbohydrate, high fat (LC) diet with a conventional high carbohydrate, low fat (HC) diet on aerobic capacity, fuel utilization during submaximal exercise, perceived exercise effort (RPE) and muscle strength. Sixty subjects (age: 49.2+/-1.2 years; BMI: 33.6+/-0.5 kg/m2) were randomly assigned to an energy restricted (approximately 6-7 MJ, 30% deficit), planned isocaloric LC or HC for 8 weeks. At baseline and week 8, subjects performed incremental treadmill exercise to exhaustion and handgrip and isometric knee extensor strength were assessed. Weight loss was greater in LC compared with HC (8.4+/-0.4% and 6.7+/-0.5%, respectively; P=0.01 time x diet). Peak oxygen uptake and heart rate were unchanged in both groups (P>0.17). Fat oxidation increased during submaximal exercise in LC but not HC (P<0.001 time x diet effect). On both diets, perception of effort during submaximal exercise and handgrip strength decreased (P<or=0.03 for time), but knee extensor strength remained unchanged (P>0.25). An LC weight loss diet shifted fuel utilization toward greater fat oxidation during exercise, but had no detrimental effect on maximal or submaximal markers of aerobic exercise performance or muscle strength compared with an HC diet. Further studies are required to determine the interaction of LC diets with regular exercise training and the long-term health effects.