Aerobic exercise and substrate oxidation

Dietary fats spare carbohydrates during aerobic exercise by increasing fat oxidation and decreasing carbohydrate oxidation. This decrease in carbohydrate oxidation may improve endurance performance by using fats for energy. It has been hypothesized that increasing dietary fat intake may increase fatty acid oxidation, spare carbohydrates, and improve other performance measures. However, current evidence does not support this hypothesis.

Studies have shown that triacylglyceride emulsion infusion or saturated fatty acid ingestion do not affect exercise muscle glycogen levels, performance, or other parameters. In addition, some researchers have used fasting in an attempt to increase fatty acid oxidation relative to carbohydrate during exercise. Although fasting increased fatty acid oxidation during exercise, it did not improve other parameters. The effects of low-carbohydrate and high-fat diets on exercise performance and glycogen stores have been examined. These dietary manipulations have not shown corresponding effects on muscle glycogen stores, performance, or parameters.

At this stage, the effectiveness of short-term dietary manipulation involving fat loading to improve performance by increasing fat oxidation and decreasing carbohydrate oxidation in endurance athletes remains to be demonstrated. On the other hand, long-term adaptation to a high-fat diet may induce metabolic adaptations and/or morphological changes that may in turn impact performance.

Lambert et al. observed that feeding a 76% fat diet compared to a 74% carbohydrate diet to cyclists for 14 days did not impair maximal energy output or time to exhaustion. However, muscle glycogen stores were half as good on the high-fat diet compared to the high-carbohydrate diet, making it difficult to interpret the effect of this dietary manipulation on endurance performance. Helge et al. showed that untrained men on either a high-fat (62% energy) or a high-carbohydrate (65% energy) diet and 40 weeks of training experienced a 9% increase in V02max and increased time to exhaustion on both diets. In conclusion, adaptation to a high-fat diet combined with up to 4 weeks of submaximal training did not impair endurance performance, and a high-fat diet for 7 weeks was associated with a reduction in time to exhaustion compared to the high-carbohydrate diet group, suggesting that the duration of the high-fat diet influences performance.

This adaptation to dietary fat may be associated with fatty acid oxidation enzymes. A strong relationship was found between 3-hydroxyacyl-CoA dehydrogenase activity and fatty acid uptake and oxidation. Despite this adaptation, the training-induced increase in endurance performance with a high-fat diet is not comparable to that observed with a high-carbohydrate diet.

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