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

Impact of a fat-rich diet on endurance in man: role of the dietary period

OBJECTIVE

The aim of the present study was to evaluate the effect of duration on the interaction between training and a fat-rich or a carbohydrate-rich diet on endurance performance.

METHODS

Fifteen untrained males were randomly assigned to consume a fat-rich (T-FAT) or a carbohydrate-rich diet (T-CHO) while following an endurance training program. Endurance performance at 80% of pretraining VO2max was measured before, after 2 wk, and after 4 wk.

RESULTS

Time to exhaustion, when exercising at the same absolute workload, was similar in T-FAT and T-CHO at all tests and was significantly increased by 166% and 150% in T-FAT and T-CHO, respectively, after 4 wk. Maximal oxygen uptake increased by 9% in both groups (P < 0.05). After 4 wk, RER was significantly lower during exercise in T-FAT both compared with the initial test and with T-CHO, while no changes appeared in T-CHO.

CONCLUSIONS

The present findings showed that endurance performance was enhanced similarly after both 2 and 4 wk of adaptation to training and a fat-rich or a carbohydrate-rich diet.

Influence of diet on the metabolic responses to exercise

The relationship between dietary intake and skeletal-muscle exercise metabolism is central to the interests of exercise physiologists. This area has been examined experimentally for over 100 years. Classic studies with male subjects demonstrated the importance of dietary CHO in maximizing muscle and liver glycogen stores in an attempt to optimize exercise performance. CHO becomes the predominant fuel for exercise at power outputs above 50-60% Vo2max and its availability limits prolonged aerobic exercise at intensities corresponding to 65-85% VO2max. Recent information suggests that female subjects are less able to maximize muscle glycogen stores through dietary means. Contemporary studies have documented in more detail the greater reliance on CHO metabolism following a high-CHO-low-fat and -protein diet and the greater reliance on fat metabolism following a low-CHO-high-fat and protein diet. More emphasis on documenting key enzymic changes in the energy-producing pathways and transport proteins has appeared. However, very little is known regarding the mechanisms that induce these changes over the short or long term in human skeletal muscle. For example, the central role of PDH activity in the selection of intramuscular fuel during exercise and the role of carnitine palmitoyltransferase 1 in the entry of NEFA into the mitochondria, and the effects of diet on these enzymes has received little attention to date. Many research studies have examined extreme diet variations (% total energy; > 85% CHO v. < 5-10% CHO) for short periods of time in an attempt to maximize diet-induced alterations and study the mechanisms responsible for the changes. However, future studies will need to examine less-severe diet alterations for longer periods of time that more accurately reflect what the normal population might experience, such as a diet containing (% total energy) 60 fat, 20 CHO, 20 protein or the recently popular diet with (% total energy) 30 fat, 40 CHO, 30 protein.

Utilization of lipids during exercise in human subjects: metabolic and dietary constraints

During endurance exercise, skeletal muscle relies mainly on both carbohydrate (CHO) and fat oxidation to cover energy needs. Numerous scientific studies have shown that increasing the exercise intensity leads to a progressive utilization of CHO. The latter will induce a state of glycogen depletion which is generally recognized as being a limiting factor for the continuation of strenuous exercise. Different dietary interventions have been proposed to overcome this limitation. A high-CHO diet during periods of intense training and competition, as well as CHO intake during exercise, are known to maintain a high rate of CHO oxidation and to delay fatigue. However, it has been recognized also that enhancing fatty acid (FA) oxidation during exercise induces a reduced rate of glycogen degradation, resulting in an improved endurance capacity. This is most strikingly observed as a result of frequent endurance exercise which improves a number of factors known to govern the FA flux and the oxidative capacity of skeletal muscle. Such factors are: (1) blood flow and capillarization; (2) lipolysis of triacylglycerol (TAG) in adipose tissue and circulating TAG and transport of FA from blood plasma to the sarcoplasm; (3) availability and rate of hydrolysis of intramuscular TAG; (4) activation of the FA and transport across the mitochondrial membrane; (5) the activity of enzymes in the oxidative pathway; (6) hormonal adaptations, i.e. sensitivity to catecholamines and insulin. The observation that the plasma FA concentration is an important factor in determining the rate of FA oxidation, and that some dietary factors may influence the rate of FA supply to muscle as well as to the mitochondria, has led to a number of dietary interventions with the ultimate goal to enhance FA oxidation and endurance performance. It appears that experimental data are not equivocal that dietary interventions, such as a high-fat diet, medium-chain TAG-fat emulsions and caffeine intake during exercise, as well as L-carnitine supplementation, do significantly enhance FA oxidation during exercise. So far, only regular endurance exercise can be classified as successful in achieving adaptations which enhance FA mobilization and oxidation.

Effect of endurance training on fatty acid metabolism: local adaptations

Older studies of humans seem to suggest a correlation between free fatty acid (FFA) turnover and oxidation on the one hand and plasma FFA concentration on the other hand during submaximal exercise. However, recent studies, in which higher concentrations of plasma FFA have been reached during prolonged submaximal exercise, have revealed a levelling off in net uptake in spite of increasing plasma FFA concentrations. Furthermore, this relationship between FFA concentration and FFA uptake and oxidation is altered by endurance training. These recent findings in humans support the notion from other cell types that transmembrane fatty acid transport is not only by simple diffusion, but predominantly carrier-mediated. During prolonged submaximal knee-extension exercise it has been demonstrated that the total oxidation of fatty acids was approximately 60% higher in trained subjects than in nontrained subjects. The training-induced adaptations responsible for this increased utilization of plasma fatty acids by the muscle could be located at several steps from the mobilization of fatty acids to skeletal muscle metabolism in the mitochondria. In this paper regulation at the transport steps and also at various metabolic steps is discussed.