The popular belief that high-carbohydrate diets aid weight loss by suppressing appetite has been challenged by the results of a British study examining metabolic and appetite responses to prolonged walking under different dietary conditions.

Eight moderately-trained male hill walkers undertook three 450-minute walks, at intensities varying between 25-30 and 50-55% of VO2max, under each of the following dietary conditions:

1. High-fat, including breakfast cereal with double cream and milk, bread and cheese sandwiches, ice cream, and snack foods like coconut and almonds;
2. High-carbohydrate, including cereal with skimmed milk, banana, white toast and jam, low-fat yoghurt, orange juice and dried fruit;
3. Mixed, including elements of both the others.

Each dietary condition, including breakfast, lunch and two snacks, produced the same total calorie count, even though the contents were radically different.

The researchers had hypothesised, among other things, that at this intensity of exercise a high-fat diet would be associated with decreased ratings of fatigue and perceived exertion through sparing of carbohydrate stores and enhanced fat utilisation.

But in fact there were no significant differences in fatigue ratings between the three dietary conditions. Neither were there any significant differences in any of the following: heart rate, perceived exertion, hunger, fullness, satiety, strength of appetite or desire to eat.

According to the researchers, ‘the absence of any change in heart rate, (rating of perceived exertion) or subjective ratings of fatigue between the dietary manipulations during prolonged exercise is an important…observation. This suggests that dietary composition will not adversely affect physiological and subjective factors over one day’.

And the fact that the three diets resulted in a similar energy deficiency at the end of the day suggests that ‘a wide range of dietary patterns may be acceptable for those trying to lose weight by incorporating moderate-intensity exercise into their routine’.

Although diet influenced the degree of total carbohydrate and fat oxidation, fat was the main source of energy in all three trials.

The main drawback of the high-fat diet, however, was that it resulted in a negative carbohydrate balance over the exercise period, suggesting that it might not allow for further exercise, even at low-to-moderate intensities.

‘Decreases in the glycogen stores…, especially if continued over a few days of walking, would be detrimental to the ability to sustain the activity,’ the researchers point out.

J Appl Physiol 2002 May;92(5), pp2061-70