Ask the Experts - Nutrition

Answers from Andrew Hamilton:

Q. I’ve never really seen a good answer for what exactly constitutes a low or high-carbohydrate diet for runners. Also, how does your carbohydrate intake affect the burning of muscle protein for fuel when running? Many thanks

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Your first question is quite straightforward. While there’s no absolute definition for ‘low’ or ‘high-carbohydrate’ diets for runners, a look at the widely accepted ‘Food Guide Pyramid’ guidelines for general healthy eating helps to provide some initial guidance. These recommend that carbohydrates (breads, grains, cereals, rice, beans pasta, fruits, vegetables etc.) should comprise at least 50% of the total calorie intake. However, most runners are considerably more active than sedentary folk, and since carbohydrate (in the form of stored muscle glycogen) is the major source of energy for vigorous exercise, the consensus among sports nutritionists is that runners’ diets should aim to supply around 60% of total calorie intake in the form of carbohydrate, and maybe even more in very high-mileage trainers. For runners, these kind of diets would be considered as high-carbohydrate, while those supplying under 50% of calories as carbohydrate would be considered as low in carbohydrate (not to be confused with the very low-carbohydrate diets advocated on some weight loss diets).

As for the link between carbohydrate intake and protein oxidation (burning) for energy during exercise, it’s important to realise that we all burn a mixture of fat, protein and carbohydrate for energy all the time, regardless of how hard we’re exercising. However, what does change with increasing exercise intensity is the ratio of the fuels utilised. Studies have shown that at high intensities (75% of maximum oxygen uptake [VO2max]), the majority of the energy for muscle contraction comes from carbohydrate with a reducing amount from fat. At higher intensities still (85%VO2max and over), almost all the energy comes from carbohydrate, with minimal amounts from fat. This is why it’s very hard to perform at peak intensity if your muscles are even slightly depleted in glycogen.

Protein oxidation also increases with increasing exercise intensity, but this becomes much more pronounced when muscle glycogen stores are depleted (as is often the case when dietary carbohydrate intakes are too low). In other words, low-carbohydrate diets are known to increase protein oxidation, with an increase in lean tissue loss in endurance athletes who train while consuming low-carbohydrate diets. Moreover, new research has indicated that training on a low-carbohydrate diet increases the secretion of stress hormones in the body such as cortisol. These hormones are involved in the breakdown of tissue for energy during exercise and may also hinder the recovery and repair of damaged muscle tissues after training. For all these reasons, runners and all other endurance athletes should ensure that they consume a high-carbohydrate to support their training.