Abstract

This study investigated whether the timing of a 24-hour fast influences indicators of glycemic control throughout the duration of the fast or upon refeeding. Twenty-four adults who have overweight or obesity (13 female, 11 male) completed this randomized crossover study involving three 24-hour 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 hours. Continuous glucose monitors (CGMs) were employed to track interstitial glucose throughout each fast and OGTT. Plasma samples for insulin, glucagon, glucagon-like peptide-1 (GLP-1), and glucose-dependent insulinotropic polypeptide (GIP) were collected at the beginning and end of each fast, and 30 minutes post-OGTT. Capillary beta-hydroxybutyrate (BHB) was only measured at the end of the fasting. The 24-hour fast initiated in the afternoon had lower post-prandial glucose area under the curve (AUC) following a standardized meal and lower mean 24-hour 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, glucagon, and GLP-1 did not differ at any point (p>0.05). GIP was higher in the evening than the afternoon (t=6.99, p<0.0001) or morning fasts (t=5.31, p<0.0001) at baseline and declined from 0 to 24 hours in both the afternoon and evening fasts (t=2.34, p=0.0203; t=7.21, p<0.0001) but did not otherwise differ. The timing of a 24-hour fast alters glycemic control and metabolic switching. The afternoon fast optimized glycemic control and ketosis while the morning fast enhanced glucose tolerance upon refeeding. These effects appear independent of insulin, glucagon, or GLP-1 concentrations, and may be partly mediated by circadian variations in GIP clearance. Aligning fasting protocols with circadian rhythms may improve metabolic outcomes related to glycemic control.

Degree

College and Department

Life Sciences; Exercise Sciences

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