Introduction This study investigated whether the timing of a 24-h fast influences indicators of glycemic control throughout the duration of the fast or upon refeeding. Methods Twenty-four adults who have overweight or obesity (13 female, 11 male) completed this randomized crossover study involving three 24-h fasts initiated at either 8:00 am, 1:00 pm, or 6:00 pm followed by a 75 g oral glucose tolerance test (OGTT) which lasted 2 h. Continuous glucose monitors (CGMs) were employed to track interstitial glucose throughout each fast and OGTT. Plasma samples for insulin and glucagon were collected at the beginning and end of each fast, and 30 min post-OGTT. Capillary beta-hydroxybutyrate (BHB) was only measured at the end of the fasting. Results The 24-h fast initiated in the afternoon had lower post-prandial glucose area under the curve (AUC) following a standardized meal and lower mean 24-h glucose levels than the morning or evening fasts ( p 0.0001) which did not differ. Glucose AUC above baseline at the conclusion of a fast was optimal after the morning compared to the afternoon or evening fast ( p 0.0001), though BHB was above the ketosis threshold (≥0.5 mmol/L) at the end of the afternoon and evening fasts but not the morning fast. Insulin and glucagon did not differ at any point ( p 0.05). Discussion The timing of a 24-h fast alters glycemic control and metabolic switching. The afternoon fast optimized glycemic outcomes and ketosis, and these effects appear independent of insulin and glucagon. Aligning fasting protocols with circadian rhythms by initiating a 24-h fast in the afternoon may improve metabolic outcomes related to glycemic control during and immediately after the fast.
Gipson et al. (2026) studied this question.