Six-Day Fasting Causes Temporary Increases in Both Antioxidant Capacity and Oxidative Stress in Healthy Young Men: A Randomized Controlled Trial

The impact of prolonged fasting on human oxidative stress (OS) levels and antioxidant defence mechanisms remains poorly understood. The aim of this current study was to investigate the redox response to a 6-day fast in a cohort of healthy men. Twenty-six participants were randomly allocated to a 6-day complete fasting or a control trial. Sympathetic activity, substrate oxidation, redox status, blood glucose, ketones, and testosterone concentrations were assessed. Throughout the fasting period, ketone concentration and fat oxidation increased, and carbohydrate oxidation and glucose and testosterone concentrations decreased. Heart rate increased on fasting days 2 and 4 and returned to the pre-fasting level on fasting day 6. Malondialdehyde (MDA) concentration increased after fasting days 4 and 6, and this increase was accompanied by an increase in the total antioxidant capacity (TAC), but the TAC/MDA ratio remained constant. Notably, all fasting-evoked changes returned to the baseline values after resumption of the regular diet. Thus, prolonged fasting activated both antioxidant defence and OS, but the redox balance was maintained. Consistent with this response, ketone concentration and sympathetic nervous system activity increased, and testosterone concentration decreased. These variables returned to the pre-fasting state after resumption of the usual eating habits.