Carbohydrates beneficial for heavy work

In sports nutrition, an old question is: are carbohydrates or fat better as fuel? A low carbohydrate/high fat diet does provide better fat burning, but it is not recommended for other reasons, including sports performance.

A century ago, in 1920, a publication appeared with the conclusion that physical labor works more efficiently on a diet with a lot of carbohydrates than with a high-fat-diet. It was also more difficult for the subjects to perform heavy exercise on a fatty diet, and they became much more tired than on a diet high in carbohydrates (1). These results are in line with the research of Dr.ir. Rieneke Terink, researcher at Gelderse Vallei Hospital at the Eat2move project and Sports Valley (2).

Study design

Terink divided fourteen male athletes into two groups, and they followed a low-carbohydrate/high-fat diet (< 10 energy% from carbohydrates and about 75 energy% from fat) and a high-carbohydrate (about 50 energy% from carbohydrates and about 35 energy% from fat) for two weeks. All subjects followed both diets and fate determined in this RCT which diet they followed first. Between the two diets there was a so-called washwood period of at least two weeks. Because of this crossover design, the subjects were their own control. The high carbohydrate diet resembled their normal diet. After two days and after two weeks on a diet, there was a test day on which the subjects had to cycle for 90 minutes at 60% of their maximum wattage, which was predetermined with a VO2-max test. Outcome variables were metabolic parameters (e.g., ketone bodies, free fatty acids, and glucose), stress (cortisol), body sizes, and immune response.?

Cycling

"We had expected that a low-carbohydrate diet/high fat (LC) is unfavorable for sportsperformance and that turned out to be the case: The subjects on a high fat diet performed much worse in the bicycle test. They were unable to complete the bike test at 60% of their own maximum wattage. The resistance had to be decreased while cycling on a LC diet to achieve a rotation speed of sixty per minute and I did not expect that. As a result, the total bicycle work was lower in the LC group.", says Terink.

Stress

"In the study, the hypothesis was that the cortisol level in blood would be higher after heavy bicycle work", says Terink. Cortisol is a measure of stress and a signal for the body to release fuel. Heavy physical exercise puts the body under pressure. Terink: "One of my assumptions was: 'Switching to a low carbohydrate will initially increase the stress response; whether adjustment of the metabolism, which takes place after some time, would have an effect on was still a question.”?"The cortisol level was indeed considerably higher due to cycling. Cortisol increased by 80% on the LC diet after two days versus 28% on the high carbohydrate diet (HC). After two weeks, the difference was less: 31% on the LC diet versus 19% on the HC diet. " says Terink.\

Fat burning

Terink: "Due to fewer available carbohydrates, the body quickly switches to fat burning. We already saw that after two days on such a diet. After cycling, the blood glucose level on a LC diet was lower than on a HC diet. The blood also contained more free fatty acids and ketones on a LC diet. Our main indication for fat burning was the ratio of exhaled carbondioxide and inhaled oxygen, also called RER (respiratory exchange ratio). This was lower with an LC diet."

Adaptation

"After two days on the LC diet, the body's metabolism was already somewhat adapted to a diet with few carbohydrates, but the high cortisol-concentration still indicated stress." , according to Terink: "The ketone content in LC was even higher after two weeks than after two days, which indicates that the body is better adapted to fat burning than after two days.?After two weeks on a LC diet was the RER even lower which indicates that during the cycling test the burning of fats is proportionally greater than that of carbohydrates and the lower cortisol level after two weeks indicates less stress. However, the cortisol concentration after two weeks was still significantly higher with a LC diet than with an HC diet."

After two weeks, the heart rate was higher in the LC group and Terink suspects that this is because burning fat costs more oxygen and the heart therefore must beat more often: "To make the same amount of ATP, the universal energy source, more oxygen is needed for fat burning than for burning carbohydrates. "

Advice to athletes

Terink: "With a moderate endurance effort, athletes can rely on their fat burning also because the fat supply of the body is usually greater than the supply of carbohydrates. Still, I would not recommend a LC diet to endurance athletes because you need carbohydrates for high-intensity exercise, both in training and in a competition. Even with a moderate endurance effort, the athlete sometimes must go a little deeper, for which carbohydrates are needed. With few carbohydrates, the fat burning becomes better, but the burning of carbohydrates is worse and that is of course very unfavorable in efforts where the burning of carbohydrates is important." About any adverse health effects of LC, Terink says: "There are indications that intestinal permeability will be higher whereby the risk of low-grade inflammation in the body increases. "

1. Krogh A, Lindhard J. The Relative Value of Fat and Carbohydrate as Sources of Muscular Energy: With Appendices on the Correlation between Standard Metabolism and the Respiratory Quotient during Rest and Work. The Biochemical Journal 1920; 14 (3-4) 290-363.

2. Terink R. Effect of exercise on micronutrient status and stress and immune response. PhD thesis, Wageningen University, Wageningen, the Netherlands (2020) ISBN: 978-94-6395-403-7. DOI: 10.18174/521561

Michiel Lowik