Whilst the shelves of your local bookshop are filled with 100’s of books on diet, and regimens to lose weight, careful consideration must be given to whether these strategies are appropriate to individuals undertaking exercise.

Scientific data from the early 1970’s showed that the resting energy requirements were typically 42% of energy from carbohydrate, 41% from fat and 17% from protein (Knoebel, 1971). Therefore if energy is consumed in these ratios and in appropriate quantities, it is sufficient to maintain both health and a constant body weight for sedentary individuals. However, because of the energy stores within the body, the intake of food in these proportions is not sufficient for active or athletic individuals.

Considering that a typical adult male of 80kg (~12st 8lb) and 15% body fat will have the capacity to store carbohydrate in the muscle to a maximum of 300-400g, and a total liver store of 80-100g, this means that when the body is fully loaded with carbohydrate, at best, it will have available 500g of carbohydrate. With each 1g of carbohydrate yielding just over 5 kcal of energy, the total energy available from carbohydrate is approximately 2000 kcal.

In comparison the same individual will be carrying a total of 12 kg of fat, which equates to over 50 times the amount of energy as that stored within carbohydrate (~115,000 kcal). Even if that individual should lose a large proportion of their fat mass, and be carrying just 6% body fat (the equivalent of an elite endurance athlete), their total body mass would fall to 72.3 kg, with a drop of 7.7 kg of body fat, they would still be storing around 40,000 kcal of energy as fat. This fat storage would rise by 10.7 kg and a total of ~215,000 kcal, for someone carrying 25% body fat (this is overweight, but not clinically obese!)

Table 1: Percentage body fat, fat mass, and energy available

Weight (Kg)——% Body Fat——Lean Mass (Kg)——Fat Mass (Kg)——Fat Energy (kcal)
72.3————-6———–68.0———–4.3———–40,000
80.0————15———–68.0———–12.0———–115,000
90.7————25———–68.0———–22.7———–215,000

So with all this energy available from fat, why are the limited stores of carbohydrate of importance? Quite simply, the burning of fat as a fuel is a long slow process, and does not yield as high an energy release as carbohydrate. Oxygen is needed to burn either fat or carbohydrate, unfortunately, there is a limit to how much oxygen the body can take up and use. For an elite endurance athlete this can be as much as five to seven litres of oxygen every minute, but it is unlikely that much more than four litres of oxygen can be extracted from the air and used every minute during sustained exercise. Given that for each litre of oxygen the energy yield from fat is 4.686 kcal, and from carbohydrate it is 5.047, during high intensity work, where the ability to consume oxygen is near its limit, the body will preferentially burn carbohydrate as it provides a greater amount of energy to move the body. The final twist to the release of energy, is that for each litre of oxygen used approximately 0.5 g of fat would be burnt, whereas nearly 1.25 g of carbohydrate is utilised.

So for every minute of exercise, the individual consuming four litres of oxygen would be burning 5 g of carbohydrate. Even with a full carbohydrate store, enough to sustain just 100 minutes of exercise before the energy source is depleted, and exercise has to slow or stop completely. This is the process that occurs when the marathon runner “hits the wall”, both muscle and blood levels are carbohydrate at are a level so low that exercise cannot be continued, and in some cases that full brain function cannot be sustained, and collapse occurs.

The level of dietary carbohydrate has a large impact on the levels of muscle glycogen (the storage form of carbohydrate). In 1967 Bergstrom and his colleagues clearly demonstrated the effects of a low (5%), moderate (40%) and high (82%) carbohydrate diet. When individuals eat the low carbohydrate diet, glycogen levels remained low, and the athletes could only tolerate moderate intensity exercise for 60 minutes before fatigue, whereas with the high carbohydrate intakes, muscle glycogen storage was high, and athletes exercised for over 3 hours before fatigue. This is one of many examples showing, the importance of high dietary carbohydrate for individuals undertaking exercise.

Similarly in 1980, Coggan and Miller published scientific data which suggested that when individuals undertook just an hour of moderate to intense exercise everyday, it took only three days to empty a moderate glycogen store, when consuming a moderate carbohydrate intake (following the recommendations of Knobel). .Whereas if a high carbohydrate diet was consumed, the result was near full repletion of the glycogen stores to allow normal training on subsequent days. Similarly, Backx and Palmer (2002) have shown that with a high carbohydrate intake, competitive performance was maintained in three days of simulated competition, whereas large drops in performance occurred when a moderate carbohydrate diet was consumed.

Thus, the overwhelming wealth of scientific data suggests that to maintain optimal performance, sufficient levels of dietary carbohydrate need to be consumed. In terms daily intake, 60-75% of the daily energy should come from carbohydrates. This will ensure that carbohydrates burnt during exercise are replaced daily. In order to compensate for differing body sizes, the intakes can be related to body mass. Target intake will be between 5 and 10 g of carbohydrate per Kg of body mass. The actual need will be determined by the amount of exercise being undertaken (in terms of hours per day), the intensity of exercise (the higher the intensity, the more carbohydrates are burnt), and also the type of exercise (weight bearing exercise such as running, requires a greater amount of energy than non-weight bearing exercise such as swimming or cycling).

This Article first appeared in London Sport Magazine London Sport Magazine

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