Determining the Fasting Length in Time-Restricted Feeding: Analysis of Endocrine Function Among Young, Healthy, Normal Weight, European Men

Abstract

Abstract Objectives Intermittent calorie restriction has emerged as an effective alternative to continuous calorie restriction to impart favorable health benefits. However, there remains a degree of uncertainty about the optimal fasting length to simultaneously improve metabolic flexibility and promote positive behavior-change. While a 16-hr fast is commonly employed, it is possible that a shorter fast could result in similar metabolic benefits and better longer term adherence. Thus, we compared changes in hormone response between two 4-hr time intervals of fasting – 11–15 hrs. vs. 15–19 hrs. Methods Secondary analyses were performed from a 38-hr fasting study that enrolled 24 healthy, European men (18–30 y) with normal body mass index. Participants initiated a fast at 2300, arrived to the laboratory at 0800, and continued fasting in individual rooms. Venous blood was sampled at 13 time-points to measure insulin, glucose, adrenocorticotropin (ACTH), C-peptide, cortisol, and adiponectin via immunoassays. Data were normally distributed without outliers or missing values. Within-group differences were analyzed with paired t-tests. Results Insulin (pmol/L) declined between 11–15 hrs. (7.00; CI = 0.97, 13.02; P = .025), but not 15–19 hrs. (5.67; CI = –0.65, 11.98; P = .076). The decline in C-peptide (nmol/L) was sharper between 11–15 hrs. (0.09; CI = 0.05, 0.13; P = < .001), compared to 15–19 hrs. (0.06; CI = 0.02, 0.09; P = .003). Cortisol (nmol/L) also steeply declined between 11–15 hrs. (60.09; CI = 30.58, 89.59; P = < .001), but slightly increased between 15–19 hrs. (10.75; CI = −33.15, 11.65; P = 0.331). The change in glucose (mmol/L) was not different between 11–15 hrs. (0.02 mmol/L; CI = −0.15, 0.19; P = 0.784), but sharply decreased between 15–19 hrs. (0.24; CI = 0.10, 0.39; P = 0.002). No differences were detected for ACTH or adiponectin. Conclusions Though substrate oxidation was not measured, changes in insulin, C-peptide, cortisol, and glucose were consistent with increased gluconeogenesis during 11–15 hrs. and lipid oxidation during 15–19 hrs. of fasting. Whereas cortisol often declines throughout the day, a slight increase was noted between 15–19 hrs. – evidence of a heightened sympathetic activity. Further study of hormone response during intermittent fasting will inform optimal length to promote health benefits and longer term adherence. Funding Sources None