Sports Nutrition: increasing fat intake can improve endurance levels

High-fat Gains for Ultra-endurance Performance

Short-term high-carbohydrate diets (carbo-loading) designed to optimise pre-exercise glycogen stores are the standard fare for endurance athletes in the lead-up to competition and have been shown to improve performance by comparison with high-fat diets.

But when it comes to longer-term eating habits, the evidence doesn’t weigh so clearly in favour of carbs. Indeed chronic high-fat diets (lasting more than five days) have been shown to enhance performance capacity by means of metabolic adaptations that appear to compensate for lower carbohydrate availability. And these adaptations – particularly an increased maximal capacity to generate power from fat oxidation – may be particularly beneficial for athletes taking part in longer-lasting (ultra-endurance) events. To test this theory, researchers from New Zealand compared the effects of high-fat and high-carbohydrate dietary conditions on metabolism and endurance performance in a group of seven highly trained male cyclists and triathletes. Each of the athletes performed a five-hour exercise test following a two-week adaptation to each of the following diets:

  1. High-fat, low-carbohydrate (with 70% of energy from fat and 15% each from protein and carbohydrate);
  2. High-carbohydrate, low-fat (70% from carbohydrate and 15% each from protein and fat);
  3. Fat with carbo-loading (11.5 days on the high-fat diet followed by 2.5 days on the high-carb version).

Although it was obviously impossible for the athletes to be ‘blinded’ to their diets, the order of treatments was randomised and each diet condition was preceded by a two-week standard normal diet (50% carbohydrate, 35% fat and 15% protein) to act as a controlled lead-in. The conditions included a pre-exercise meal of the same composition as the preceding diet. The five-hour exercise test, performed on cycle ergometers, comprised the following:

  • a 15-minute trial in which the cyclists rode as far as possible in the time allowed;
  • a 45-minute recovery ride at 50% of peak power;
  • an incremental test to measure the peak fat-oxidation rate;
  • a 100k trial.

Sports bars and a 5% carbohydrate solution were ingested during the tests. A range of blood and urine analyses were carried out before, during and after the test with these results:

  • the diets had no statistically significant effect on 15-minute performance, although the high-fat condition tended to reduce distance covered by comparison with the fat-with-carbo-loading condition;
  • in the 100k time trial, the high-fat and fat-with-carbo-loading conditions reduced the decline in power output seen in the high-carbohydrate condition, although the corresponding improvement in performance time (3-4%) was again not statistically significant;
  • power output during the final 5k of the time trial in the fat with carbo-loading condition was 1.3 times greater than in the high-carbohydrate condition;
  • overall for every 10% increase in energy derived from fat, 100k mean power increased by 2%;
  • by comparison with the high-carb condition, the high-fat condition resulted in a range of metabolic changes consistent with increased fat breakdown and fuel availability.

‘In conclusion,’ state the researchers, ‘high-fat dietary conditioning increased fat oxidation, and although the main effects were not statistically significant, there was some evidence for enhanced ultra-endurance cycling performance relative to high-carbohydrate… There was no apparent additional performance benefit arising from carbohydrate loading following the high-fat diet.’

However, the researchers believe that one probable reason for the lack of a statistically significant effect on 100k performance was a relatively high degree of test error coupled with the trial’s very small sample size. They speculate that testing more subjects would have shifted the statistical boundaries, giving significance to the main outcomes. Thus further research is required to clarify whether the trend for enhancement of ultra-endurance performance following high-fat diet conditioning is a real phenomenon, offering significant benefits.

Metabolism 2002 Jun 51(6), pp678-90

Isabel Walker

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