Carbohydrate loading and supplementation in endurance-trained women runners

Effectiveness of high-fat and high-carbohydrate diets on body composition and maximal strength after 15 weeks of resistance training

PURPOSE

The aim of this study was to compare High Carbohydrates Low Fat (HCLF) and Low Carbohydrate High Fat (LCHF) diets in terms of changes in body composition and maximal strength.

PATIENTS/METHODS

The study involved 48 men aged 25 ​± ​2.5, divided into two groups, one of which (n ​= ​23) was following the LCHF diet and the other (n ​= ​25) the HCLF diet. Both groups performed the same resistance training protocol for 15 weeks. Maximal strength in squat, bench press and deadlift was assessed pre- and post-intervention. Measurements of selected body circumferences and tissue parameters were made using the multifunctional, multi-frequency, direct bioelectric impedance InBody 770 analyzer from InBody Co., Ltd (Cerritos, California, USA). The team with the necessary qualifications and experience in research performed all the measurements and maintained participants' oversight throughout the entire length of the study.

RESULTS

Both nutritional approaches were effective in terms of reducing body fat mass. The HCLF group achieved greater skeletal muscle hypertrophy. Significant decreases in body circumferences, especially in the abdominal area, were observed for both dietary approaches. Maximal strength significantly increased in the HCLF group and decreased in the LCHF group.

CONCLUSION

Holistic analysis of the results led to the conclusion that both dietary approaches may elicit positive adaptations in body composition. The two approaches constitute useful alternatives for both recreational exercisers and physique athletes with body composition goals.

The Impact of Sex, Body Mass Index, Age, Exercise Type and Exercise Duration on Interstitial Glucose Levels during Exercise

The impact of age, sex and body mass index on interstitial glucose levels as measured via continuous glucose monitoring (CGM) during exercise in the healthy population is largely unexplored. We conducted a multivariable generalized estimating equation (GEE) analysis on CGM data (Dexcom G6, 10 days) collected from 119 healthy exercising individuals using CGM with the following specified covariates: age; sex; BMI; exercise type and duration. Females had lower postexercise glycemia as compared with males (92 ± 18 vs. 100 ± 20 mg/dL, p = 0.04) and a greater change in glycemia during exercise from pre- to postexercise (p = 0.001) or from pre-exercise to glucose nadir during exercise (p = 0.009). Younger individuals (i.e., <20 yrs) had higher glucose during exercise as compared with all other age groups (all p < 0.05) and less CGM data in the hypoglycemic range (<70 mg/dL) as compared with those aged 20-39 yrs (p < 0.05). Those who were underweight, based on body mass index (BMI: <18.5 kg/m2), had higher pre-exercise glycemia than the healthy BMI group (104 ± 20 vs. 97 ± 17 mg/dL, p = 0.02) but similar glucose levels after exercise. Resistance exercise was associated with less of a drop in glycemia as compared with aerobic or mixed forms of exercise (p = 0.008) and resulted in a lower percent of time in the hypoglycemic (p = 0.04) or hyperglycemic (glucose > 140 mg/dL) (p = 0.03) ranges. In summary, various factors such as age, sex and exercise type appear to have subtle but potentially important influence on CGM measurements during exercise in healthy individuals.

Effects of Energy Gel Ingestion on Blood Glucose, Lactate, and Performance Measures During Prolonged Cycling

Kozlowski, KF, Ferrentino-DePriest, A, and Cerny, F. Effects of energy gel ingestion on blood glucose, lactate, and performance measures during prolonged cycling. J Strength Cond Res XX(X): 000-000, 2020-Endurance athletes have long used carbohydrate supplementation during prolonged exercise (most recently with energy gels) to enhance performance. The purpose of this study was to determine the effect of carbohydrate energy gel ingestion schedules (e.g., manufacturer's recommendations vs. a more frequent ingestion schedule) during 2 hours of steady-state cycling exercise on (a) blood glucose, (b) blood lactate, and (c) performance of a subsequent 15-minute time trial (TT). Ten trained cyclists (5 men and 5 women, mean age = 28.4 ± 3.66 years; body mass = 68.9 ± 10.63 kg; and V[Combining Dot Above]O2max = 54.57 ± 9.45 mlO2·kg·min) performed 3 exercise trials in a randomized order. One gel was ingested 15 minutes before exercise during all trials. The 3 experimental trials included gel ingestion every 30 minutes (T1), every 45 minutes (T2) during exercise, and no gel ingested during exercise (T3). Subjects cycled at 70% of V[Combining Dot Above]O2max for 2 hours, followed by a 15-minute fixed gear TT. The blood glucose level at 60 minutes of exercise was higher during T1 (125.5 ± 30.96 mg·dl) and T2 (127.6 ± 14.82 mg·dl) compared with T3 (102.8 ± 15.85 mg·dl). Time trial distance was significantly greater for T1 (7.56 ± 0.77 km) and T2 (7.16 ± 0.92 km) than T3 (6.69 ± 0.74 km) (p = 0.003) with moderate to strong effect sizes between trials. There were no differences in blood lactate concentrations across trials. Ingestion of energy gels during prolonged cycling elevates blood glucose levels and enhances subsequent performance, whereas a more frequent ingestion elicits additional performance benefits.

Effects of supplementing with an 18% carbohydrate-hydrogel drink versus a placebo during whole-body exercise in -5 °C with elite cross-country ski athletes: a crossover study

Background

Whilst the ergogenic effects of carbohydrate intake during prolonged exercise are well-documented, few investigations have studied the effects of carbohydrate ingestion during cross-country skiing, a mode of exercise that presents unique metabolic demands on athletes due to the combined use of large upper- and lower-body muscle masses. Moreover, no previous studies have investigated exogenous carbohydrate oxidation rates during cross-country skiing. The current study investigated the effects of a ¹³C-enriched 18% multiple-transportable carbohydrate solution (1:0.8 maltodextrin:fructose) with additional gelling polysaccharides (CHO-HG) on substrate utilization and gastrointestinal symptoms during prolonged cross-country skiing exercise in the cold, and subsequent double-poling time-trial performance in ~ 20 °C.

Methods

Twelve elite cross-country ski athletes (6 females, 6 males) performed 120-min of submaximal roller-skiing (69.3 ± 2.9% of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \dot{\mathrm{V}} $$\end{document}V˙O₂peak) in −5 °C while receiving either 2.2 g CHO-HG·min^− 1 or a non-caloric placebo administered in a double-blind, randomized manner. Whole-body substrate utilization and exogenous carbohydrate oxidation was calculated for the last 60 min of the submaximal exercise. The maximal time-trial (2000 m for females, 2400 m for males) immediately followed the 120-min submaximal bout. Repeated-measures ANOVAs with univariate follow-ups were conducted, as well as independent and paired t-tests, and significance was set at P < 0.05. Data are presented as mean ± SD.

Results

Exogenous carbohydrate oxidation contributed 27.6 ± 6.6% to the total energy yield with CHO-HG and the peak exogenous carbohydrate oxidation rate reached 1.33 ± 0.27 g·min^− 1. Compared to placebo, fat oxidation decreased by 9.5 ± 4.8% with CHO-HG, total carbohydrate oxidation increased by 9.5 ± 4.8% and endogenous carbohydrate utilization decreased by 18.1 ± 6.4% (all P < 0.05). No severe gastrointestinal symptoms were reported in either trial and euhydration was maintained in both trials. Time-trial performance (8.4 ± 0.4 min) was not improved following CHO-HG compared to placebo (− 0.8 ± 3.5 s; 95% confidence interval − 3.0 to 1.5 s; P = 0.46). No sex differences were identified in substrate utilization or relative performance.

Conclusions

Ingestion of an 18% multiple-transportable carbohydrate solution with gelling polysaccharides was found to be well-tolerated during 120 min of submaximal whole-body exercise, but did not improve subsequent maximal double-poling performance.

Does Carbohydrate Intake During Endurance Running Improve Performance? A Critical Review

Wilson, PB. Does carbohydrate intake during endurance running improve performance? A critical review. J Strength Cond Res 30(12): 3539-3559, 2016-Previous review articles assessing the effects of carbohydrate ingestion during prolonged exercise have not focused on running. Given the popularity of distance running and the widespread use of carbohydrate supplements, this article reviewed the evidence for carbohydrate ingestion during endurance running. The criteria for inclusion were (a) experimental studies reported in English language including a performance task, (b) moderate-to-high intensity exercise >60 minutes (intermittent excluded), and (c) carbohydrate ingestion (mouth rinsing excluded). Thirty studies were identified with 76 women and 505 men. Thirteen of the 17 studies comparing a carbohydrate beverage(s) with water or a placebo found a between-condition performance benefit with carbohydrate, although heterogeneity in protocols precludes clear generalizations about the expected effect sizes. Additional evidence suggests that (a) performance benefits are most likely to occur during events >2 hours, although several studies showed benefits for tasks lasting 90-120 minutes; (b) consuming carbohydrate beverages above ad libitum levels increases gastrointestinal discomfort without improving performance; (c) carbohydrate gels do not influence performance for events lasting 16-21 km; and (d) multiple saccharides may benefit events >2 hours if intake is ≥1.3 g·min Given that most participants were fasted young men, inferences regarding women, adolescents, older runners, and those competing in fed conditions are hampered. Future studies should address these limitations to further elucidate the role of carbohydrate ingestion during endurance running.

Nighttime feeding likely alters morning metabolism but not exercise performance in female athletes

The timing of morning endurance competition may limit proper pre-race fueling and resulting performance. A nighttime, pre-sleep nutritional strategy could be an alternative method to target the metabolic and hydrating needs of the early morning athlete without compromising sleep or gastrointestinal comfort during exercise. Therefore, the purpose of this investigation was to examine the acute effects of pre-sleep chocolate milk (CM) ingestion on next-morning running performance, metabolism, and hydration status. Twelve competitive female runners and triathletes (age, 30 ± 7 years; peak oxygen consumption, 53 ± 4 mL·kg(-1)·min(-1)) randomly ingested either pre-sleep CM or non-nutritive placebo (PL) ∼30 min before sleep and 7-9 h before a morning exercise trial. Resting metabolic rate (RMR) was assessed prior to exercise. The exercise trial included a warm-up, three 5-min incremental workloads at 55%, 65%, and 75% peak oxygen consumption, and a 10-km treadmill time trial (TT). Physiological responses were assessed prior, during (incremental and TT), and postexercise. Paired t tests and magnitude-based inferences were used to determine treatment differences. TT performances were not different ("most likely trivial" improvement with CM) between conditions (PL: 52.8 ± 8.4 min vs CM: 52.8 ± 8.0 min). RMR was "likely" increased (4.8%) and total carbohydrate oxidation (g·min(-1)) during exercise was "possibly" or likely increased (18.8%, 10.1%, 9.1% for stage 1-3, respectively) with CM versus PL. There were no consistent changes to hydration indices. In conclusion, pre-sleep CM may alter next-morning resting and exercise metabolism to favor carbohydrate oxidation, but effects did not translate to 10-km running performance improvements.

Carbohydrate electrolyte solutions enhance endurance capacity in active females

The purpose of the present study was to investigate the effects of supplementation with a carbohydrate-electrolyte solution (CES) in active females during a prolonged session of submaximal running to exhaustion. Eight healthy active females volunteered to perform a session of open-ended running to exhaustion at 70% of their maximal oxygen consumption on a treadmill during the follicular phase of their menstrual cycle on two occasions. During each run, the subjects consumed either 3mL·kg⁻¹ body mass of a 6% CES or a placebo drink (PL) every 20 min during exercise. The trials were administered in a randomized double-blind, cross-over design. During the run, the subjects ingested similar volumes of fluid in two trials (CES: 644 ± 75 mL vs. PL: 593 ± 66 mL, p > 0.05). The time to exhaustion was 16% longer during the CES trial (106.2 ± 9.4 min) than during the PL trial (91.6 ± 5.9 min) (p < 0.05). At 45 min during exercise, the plasma glucose concentration in the CES trial was higher than that in PL trial. No differences were observed in the plasma lactate level, respiratory exchange ratio, heart rate, perceived rate of exertion, sensation of thirst, or abdominal discomfort between the two trials (p > 0.05). The results of the present study confirm that CES supplementation improves the moderate intensity endurance capacity of active females during the follicular phases of the menstrual cycle. However, the exogenous oxidation of carbohydrate does not seem to explain the improved capacity after CES supplementation.

Personal food systems of male collegiate football players: a grounded theory investigation

CONTEXT

Factors that affect food choices include the physical and social environments, quality, quantity, perceived healthfulness, and convenience. The personal food choice process was defined as the procedures used by athletes for making food choices, including the weighing and balancing of activities of daily life, physical well-being, convenience, monetary resources, and social relationships.

OBJECTIVE

To develop a theoretical model explaining the personal food choice processes of collegiate football players.

DESIGN

Qualitative study.

SETTING

National Collegiate Athletic Association Division II football program.

PATIENTS OR OTHER PARTICIPANTS

Fifteen football players were purposefully sampled to represent various positions, years of athletic eligibility, and ethnic backgrounds.

DATA COLLECTION AND ANALYSIS

For text data collection, we used predetermined, open-ended questions. Data were analyzed using the constant comparison method. The athletes' words were used to label and describe their interactions and experiences with the food choice process. Member checks and an external audit were conducted by a qualitative methodologist and a nutrition specialist, and the findings were triangulated with the current literature to ensure trustworthiness of the text data.

RESULTS

Time was the core category and yielded a cyclic graphic of a theoretical model for the food choice system. Planning hydration, macronutrient strategies, snacks, and healthful food choices emerged as themes.

CONCLUSIONS

The athletes planned meals and snacks around their academic and athletic schedules while attempting to consume foods identified as healthful. Healthful foods were generally lower in fat but high in preferred macronutrients. High-protein foods were the players' primary goal; carbohydrate consumption was secondary. The athletes had established plans to maintain hydration. Professionals may use these findings to implement educational programs on food choices for football players.

Carbohydrate ingestion during endurance exercise improves performance in adults

This study was a systematic review with meta-analysis examining the efficacy of carbohydrate (CHO) ingestion compared with placebo (PLA) on endurance exercise performance in adults. Relevant databases were searched to January 2011. Included studies were PLA-controlled, randomized, crossover designs in which CHO ingestion not exceeding 8% and between 30 and 80 g/h during exercise of ≥1 h was evaluated via time trial (TT) or exercise time to exhaustion (TTE). The between-trial standardized mean differences [effect size (ES)] and pooled estimates of the effect of CHO ingestion were calculated. Of the 41,175 studies from the initial search, 50 were included. The ES for submaximal exercise followed by TT was significant (ES = 0.53; 95% CI = 0.37-0.69; P < 0.001) as was the ES for TT (ES = 0.30; 95% CI = 0.07-0.53; P = 0.011). The weighted mean improvement in exercise performance favored CHO ingestion (7.5 and 2.0%, respectively). TTE (ES = 0.47; 95% CI = 0.32-0.62; P < 0.001) and submaximal exercise followed by TTE (ES = 0.44; 95% CI = 0.08-0.80; P = 0.017) also showed significant effects, with weighted mean improvements of 15.1 and 54.2%, respectively, with CHO ingestion. Similar trends were evident for subanalyses of studies using only male or trained participants, for exercise of 1-3 h duration, and where CHO and PLA beverages were matched for electrolyte content. The data support that ingestion of CHO between 30 and 80 g/h enhances endurance exercise performance in adults.

Carbohydrate administration and exercise performance: what are the potential mechanisms involved?

It is well established that carbohydrate (CHO) administration increases performance during prolonged exercise in humans and animals. The mechanism(s), which could mediate the improvement in exercise performance associated with CHO administration, however, remain(s) unclear. This review focuses on possible underlying mechanisms that could explain the increase in exercise performance observed with the administration of CHO during prolonged muscle contractions in humans and animals. The beneficial effect of CHO ingestion on performance during prolonged exercise could be due to several factors including (i) an attenuation in central fatigue; (ii) a better maintenance of CHO oxidation rates; (iii) muscle glycogen sparing; (iv) changes in muscle metabolite levels; (v) reduced exercise-induced strain; and (vi) a better maintenance of excitation-contraction coupling. In general, the literature indicates that CHO ingestion during exercise does not reduce the utilization of muscle glycogen. In addition, data from a meta-analysis suggest that a dose-dependent relationship was not shown between CHO ingestion during exercise and an increase in performance. This could support the idea that providing enough CHO to maintain CHO oxidation during exercise may not always be associated with an increase in performance. Emerging evidence from the literature shows that increasing neural drive and attenuating central fatigue may play an important role in increasing performance during exercise with CHO supplementation. In addition, CHO administration during exercise appears to provide protection from disrupted cell homeostasis/integrity, which could translate into better muscle function and an increase in performance. Finally, it appears that during prolonged exercise when the ability of metabolism to match energy demand is exceeded, adjustments seem to be made in the activity of the Na+/K+ pump. Therefore, muscle fatigue could be acting as a protective mechanism during prolonged contractions. This could be alleviated when CHO is administered resulting in the better maintenance of the electrical properties of the muscle fibre membrane. The mechanism(s) by which CHO administration increases performance during prolonged exercise is(are) complex, likely involving multiple factors acting at numerous cellular sites. In addition, due to the large variation in types of exercise, durations, intensities, feeding schedules and CHO types it is difficult to assess if the mechanism(s) that could explain the increase in performance with CHO administration during exercise is(are) similar in different situations. Experiments concerning the identification of potential mechanism(s) by which performance is increased with CHO administration during exercise will add to our understanding of the mechanism(s) of muscle/central fatigue. This knowledge could have significant implications for improving exercise performance.

Carbohydrate supplementation and sex differences in fuel selection during exercise

PURPOSE

To compare the effects of a high-CHO diet (80% CHO) and glucose ingestion (2 g x kg(-1)) during exercise (120 min, 57% VO2max) on fuel selection in women taking (W+OC) or not (W-OC) oral contraceptives and in men (six in each group).

METHODS

Substrate oxidation was measured using indirect respiratory calorimetry in combination with a tracer technique to compute the oxidation of exogenous (13C-glucose) and endogenous CHO.

RESULTS

In the control situation (mixed diet with water ingestion during exercise), the percent contribution to the energy yield (%En) of CHO oxidation was higher in men than in women (62 vs 53 %En). The high-CHO diet and glucose ingestion during exercise separately increased the %En from CHO oxidation in both men (+12%) and women (+24%), and the sex difference observed in the control situation disappeared. However, the increase in the %En from total CHO oxidation observed when glucose was ingested during exercise and when combined with a high-CHO diet was larger in women than in men (+47 vs +17 %En). This was not attributable to a higher %En from exogenous glucose oxidation in women, for which no sex difference was observed (25 and 27 %En in men and women), but was attributable to a smaller decrease in endogenous glucose oxidation. No significant difference in fuel selection was observed between W+OC and W-OC.

CONCLUSIONS

The increase in total CHO oxidation after the high-CHO diet was not different between sexes. Glucose ingestion during exercise, separately and combined to the high-CHO diet, had a greater effect in women than in men; this was mostly attributable to the smaller reduction in endogenous CHO oxidation.

Glutamine and carbohydrate supplements reduce ammonemia increase during endurance field exercise

Blood ammonia concentration increases during endurance exercise and has been proposed as a cause for both peripheral and central fatigue. We examined the impact of glutamine and (or) carbohydrate supplementation on ammonemia in high-level runners. Fifteen men in pre-competitive training ran 120 min (~34 km) outdoors on 4 occasions. On the first day, the 15 athletes ran without the use of supplements and blood samples were taken every 30 min. After that, each day for 4 d before the next 3 exercise trials, we supplemented the athletes’ normal diets in bolus with carbohydrate (1 g·kg–1·d–1), glutamine (70 mg·kg–1·d–1), or a combination of both in a double-blind study. Blood ammonia level was determined before the run and every 30 min during the run. During the control trial ammonia increased progressively to approximately 70% above rest concentration. Following supplementation, independent of treatment, ammonia was not different (p > 0.05) for the first 60 min, but for the second hour it was lower than in the control (p < 0.05). Supplementation in high-level, endurance athletes reduced the accumulation of blood ammonia during prolonged, strenuous exercise in a field situation.

Metabolismo energético em atletas de endurance é diferente entre os sexos

Atletas de endurance têm suas necessidades energéticas aumentadas devido ao seu alto gasto energético durante o exercício. Contudo, ainda não estão claros quais são as diferenças do metabolismo energético entre os sexos e se essas diferenças vão implicar em mudanças em suas dietas. Sendo assim, o objetivo deste trabalho foi reunir informações da literatura sobre as diferenças entre os sexos em relação ao metabolismo energético em atletas de endurance. Alguns estudos demonstram que, durante exercício de longa duração, as mulheres utilizam como fonte de energia maior quantidade de lipídeos e menor de carboidratos e proteínas, quando comparadas aos homens. Já outros autores não encontraram diferença entre os sexos na proporção dos substratos metabolizados, mas sim nos tipos de lipídeos utilizados. Essa diferença na utilização de substratos vem sendo relacionada a distintas concentrações de alguns hormônios entre homens e mulheres, como: 17beta-estradiol, progesterona, testosterona, epinefrina, norepinefrina, hormônio de crescimento, insulina e glucagon. Também foi observado que os atletas aumentam seus estoques de glicogênio quando consomem dieta com sobrecarga glicídica, enquanto que as atletas apenas têm seus estoques de glicogênio aumentados quando ingerem dieta com sobrecarga glicídica e hiperenergética. Esse achado é bastante relevante, pois a concentração de glicogênio está diretamente ligada ao rendimento do atleta durante o exercício de endurance. Portanto, a literatura sugere que há diferenças entre os sexos quanto à utilização de substratos em atletas. Porém, novos estudos são necessários para melhor esclarecimento do metabolismo energético dos atletas de endurance possibilitando, assim, a adequação de suas recomendações nutricionais.

Biochemical effects of carbohydrate supplementation in a simulated competition of short terrestrial duathlon

The purpose of the present study was to investigate the biochemical effects of carbohydrate supplementation in a simulated competition of short terrestrial duathlon. Ten duathletes participated in a simulated competition of short terrestrial duathlon 30 minutes after the ingestion of a 6% (30 g/500 ml) maltodextrin solution (MALT) or a placebo (PLA). This solution was also ingested every 15 minutes during the competition (12 g/200 ml); and immediately after the competition (18 g/300 ml). Samples of blood were collected at 3 time points: 1) at rest 1 hour before the beginning of the competition; 2) during the competition (approximately 1 hour and 45 minutes after the 1st collection); 3) immediately after the competition. Blood was analyzed for blood glucose, lactate, insulin and cortisol. Significant differences were observed in relation to blood glucose levels between MALT and PLA in the post-competition phase. There was also a significant difference in the lactate levels observed between MALT and PLA during the competition phase. Similarly, a significant difference in the cortisol concentrations during and after the competition phases (MALT and PLA) were observed. We conclude that maltodextrin supplementation appears to be beneficial during short terrestrial duathlon competition as evidenced by biochemical markers.

Gender differences in carbohydrate metabolism and carbohydrate loading

Prior to endurance competition, many endurance athletes participate in a carbohydrate loading regimen in order to help delay the onset of fatigue. The "classic" regimen generally includes an intense glycogen depleting training period of approximately two days followed by a glycogen loading period for 3–4 days, ingesting approximately 60–70% of total energy intake as carbohydrates, while the newer method does not consist of an intense glycogen depletion protocol. However, recent evidence has indicated that glycogen loading does not occur in the same manner for males and females, thus affecting performance. The scope of this literature review will include a brief description of the role of estradiol in relation to metabolism and gender differences seen in carbohydrate metabolism and loading.

Muscle triglyceride and glycogen in endurance exercise: implications for performance

The importance of muscle glycogen as a metabolic substrate in sustaining prolonged exercise is well acknowledged. Being stored in proximity to the site of contraction and able to sustain high rates of adenosine diphosphate (ADP) phosphorylation, glycogen is viewed as the primary fuel for the maintenance of exercise of a moderate to intense nature. As such, to ensure optimal exercise performance, endurance athletes are encouraged to maximise the availability of muscle glycogen through the ingestion of a high carbohydrate (CHO) diet prior to competition. The skeletal muscle cell also contains significant quantities of triglyceride. Recent improvements in the ability to measure these intramyocellular triglyceride (IMTG) stores have confirmed that IMTG acts as a significant fuel substrate during prolonged exercise. While early research of the role of muscle glycogen in endurance exercise provided clear prescriptive information for the endurance-trained athlete, no such direction for optimising exercise performance is yet apparent from research concerning IMTG. In this article, we review the processes of muscle glycogen and triglyceride storage and metabolism. Attention is given to the effects of short-term alterations in diet on muscle substrate, particularly IMTG storage, and the implications of this to endurance exercise performance and competition preparation. We demonstrate that like glycogen, IMTG formation may be relatively rapid, and its storage predominates under conditions that promote minimal glycogen formation. This observation suggests that the role of IMTG is to maintain a readily available substrate to ensure that physical activity of a moderate nature can be performed when glycogen availability is not optimal. Under these conditions, IMTG may offer a similar availability of energy as glycogen in the endurance-trained athlete. Given the potential value of this substrate, the possibility of maximising IMTG storage without compromising glycogen availability prior to competition is considered.