Can eating a high-fat diet increase exercise endurance? During low and moderate-intensity exercise, the body uses fat as its main source of fuel. Only with higher intensity exercise does a carbohydrate become the primary energy source the body uses to fuel muscle movements. For this reason, it would make sense that a high-fat diet would boost the body’s ability to mobilize and use fat during moderate-intensity exercise. If the body adapts to preferentially using fat for fuel, it would also preserve glycogen stores. This could also help to increase endurance and delay fatigue. But does it hold true in practice?
High-Carbohydrate vs. High-Fat Diet and Exercise Endurance
In one study, researchers gave 10 equally fit young men either a high fat-diet (62% fat) or a high-carbohydrate diet (62% carbs) for 7 weeks. Each group worked by pedaling a bicycle ergometer 3 to 4 days per week at a moderate intensity for 7 weeks. The groups then switched diets so the high-carb eaters ate a high-fat diet for 7 weeks and vice versa.
The results? The high-fat diet didn’t improve exercise performance or endurance in either group. The groups eating the high-carbohydrate diet consistently outperformed the group on the high-fat diet.
Research suggests that a high-fat diet increases the body’s ability to use fat as fuel over glycogen, but it doesn’t seem to increase exercise endurance. Why would that be? One theory is when the body adapts to using fat over carbohydrates as fuel, it makes it more difficult for it to break down and use glycogen. Breakdown of muscle glycogen for fuel is important during higher intensity exercise, and when the body adapts to using fat it may not be as efficient at using carbohydrates.
This inability to use carbohydrates as efficiently wouldn’t be a problem during sustained exercise at a moderate-intensity such as long-distance running where fat is the preferred fuel source. But what about running up a hill or short periods of sprinting? During periods of more intense exercise, you need carbohydrates as fuel and a high-fat diet could make it more difficult to mobilize them.
There’s also the issue of the longer-term health effects of eating a high-fat diet. A high-fat diet can raise lipid levels and increase the risk of heart disease, especially in individuals already at higher risk. No one knows the long-term effects of eating a diet where 60% of the calories come from fat.
Aim for Balance in Your Diet to Maximize Exercise Performance
Eating a high-fat diet is unlikely to boost endurance during moderate-intensity exercise such as long distance running, and it’s likely to hurt your performance if you do even short periods of higher intensity exercise. Most endurance athletes will benefit from a diet consisting of 60 to 70% carbohydrates, 10-15% protein and the remainder from fat. This is a diet that’s more consistent with good health, especially if you get most of your carbohydrates from high-fiber, non-processed foods. When it comes to endurance training, stick to a balanced diet to get the most benefits.
References:
Exercise Physiology. Energy, Nutrition, and Human Performance. Fifth Edition. 2001.
J. Physiology. 1996; 492:293.
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