Induction Phases of the Atkins Diet and South Beach Diet Decrease Exercise Capacity

Intro The Atkins and South Beach Diets are popular low‐carbohydrate diets. Despite differences between these diets, they share a similar induction phase involving strict removal of carbohydrates from the diet for 2 weeks. Glycogen stores are a major source of fuel for high‐intensity exercise and have been shown to decrease during the diet induction phase. We have previously shown that exercise capacity is diminished after 1 week on these diets. We hypothesized that the exercise capacity diminution would persist into week 2 of the diet and that exercise capacity would normalize on return of carbohydrates to the diet. Methods 17 adults followed the Atkins & South Beach diet induction phases for 7 days. 5 participants continued the induction phase for the 2nd week followed by return to normal diet. Time to exhaustion during exercise, body weight (BW), % body fat, and plasma ketones were measured. Results During the 1st and 2nd weeks of the diet induction phase, the decrease in exercise capacity was persistent (~50% of pre‐diet capacity), ketones remained elevated (200 ‐ 300% of pre‐diet) and BW continued to decrease (3% at 1 wk and 4.5% at 2 wk). After 1 week of return to normal diet, exercise capacity was partially normalized (~75% of pre‐diet) and BW returned to within 1% of pre‐diet values. Conclusions Low‐carb diet induction temporarily decreases BW and high‐intensity exercise capacity, and elevates plasma ketones. It is possible that some of the weight loss during the induction phase may be due to dehydration secondary to glycogen depletion and ketosis induced hyperventilation. These data suggest that people concerned with athletic performance and health should be cautious about beginning a low‐carb diet and that further research is warranted in this area.