A 2 Week Cross-over Intervention with a Low Carbohydrate, High Fat Diet Compared to a High Carbohydrate Diet Attenuates Exercise-Induced Cortisol Response, but Not the Reduction of Exercise Capacity, in Recreational Athletes

Low carbohydrate, high fat (LCHF) diets are followed by athletes, but questions remain regarding effects of LCHF on metabolic adaptation, exercise-induced stress, immune function and their time-course. In this cross-over study, 14 recreational male athletes (32.9 +/- 8.2 years, VO2(max) 57.3 +/- 5.8 mL/kg/min) followed a two week LCHF diet (<10 En% carbohydrates (CHO), similar to 75En% Fat) and a two week HC diet (>50 En% CHO), in random order, with a wash-out period of >2 weeks in between. After 2 days and 2 weeks on either diet, participants performed cycle ergometry for 90 min at 60%W-max. Blood samples for analysis of cortisol, free fatty acids (FFA), glucose and ketones, and saliva samples for immunoglobin A (s-IgA) were collected at different time points before and after exercise. The LCHF diet resulted in higher FFA, higher ketones and lower glucose levels compared to the HC diet (p < 0.05). Exercise-induced cortisol response was higher after 2 days on the LCHF diet (822 +/- 215 nmol/L) compared to 2 weeks on the LCHF diet (669 +/- 243 nmol/L, p = 0.004) and compared to both test days following the HC diet (609 +/- 208 and 555 +/- 173 nmol/L, both p < 0.001). Workload was lower, and perceived exertion higher, on the LCHF diet compared to the HC diet on both occasions. A drop in s-IgA following exercise was not seen after 2 days on the LCHF diet, in contrast to the HC diet. In conclusion, the LCHF diet resulted in reduced workload with metabolic effects and a pronounced exercise-induced cortisol response after 2 days. Although indications of adaptation were seen after 2 weeks on the LCHF diet, work output was still lower.

Automated summary

The study showed that following a LCHF diet resulted in higher levels of free fatty acids and ketones, as well as a decreased level of glucose. It also led to an increased cortisol response after exercise, indicating potential effects on metabolism and exercise performance. Additionally, athletes on a LCHF diet experienced reduced workload and increased perceived exertion during exercise compared to a high carbohydrate diet.