A Healthier U


The recommended amount of carbohydrate consumed surrounding an exercise bout depends on the mode, intensity, duration, an athletes training status, as well as recent and long term energy intake. Low carbohydrate diets (definition: less than 5 percent of calories or less than 50g) have become popularized lately as “energy boosters” or “fat blasters.” But are low carbohydrate diets actually beneficial for performance?

When an athlete decreases the amount of one macronutrient (i.e. carbohydrate), they must make up the remaining calories in another macronutrient, which is typically fat and/or protein. We know that when the body has low carbohydrate stores, there is increased mobilization of fat. If a person truly eats low carbohydrate and thus has low carbohydrate stores, through a series of steps in metabolism, the body will produce ketones which the body is able to utilize as energy. These low carbohydrate high fat diets are cited to reduce the reliance on glucose utilization and spare glycogen stores. This is what research tells us:

For a true ketogenic diet – i.e. high fat (85 percent), low carbohydrate (5 percent) and adequate protein (15 percent), athletes were found to:

  • Have no improvements in strength
  • Experience a reduction in max workload that can be performed
  • May have small favorable changes in lean body mass

For a non-ketogenic, but still low carbohydrate diet, athletes may experience:

  • Decreased performance
  • No improvement with high intense exercise
  • Small improvements in submaximal testing

For those who consume a high fat diet for 5-10 days, followed by a carbohydrate restoration, athletes may experience:

  • No difference in time-trial events
  • No benefit for very high intensity exercise
  • Decreased power

Overall, the research does not support a low carbohydrate diet to enhance exercise capacity or performance. Athletes may experience a decrease in training capacity, an increase in perceived effort, increased heart rate at high intensity exercise, and overall impaired performance. Although rare, athletes who may be successful with this type of diet may be those who only perform sub max testing and/or those who experience gastrointestinal distress when consuming carbohydrate during exercise. Thus, exercise recommendations based on performance should include adequate carbohydrate. If you have any further questions, please contact PEAK Health and Fitness Registered Dietitians (RD).

Reference: Burke LM. Re-Examining High-Fat Diets for Sports Performance: Did We Call the “Nail in the Coffin” Too Soon? Sports Medicine (Auckland, N.z). 2015;45(Suppl 1):33-49. doi:10.1007/s40279-015-0393-9.



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