Background: Intermittent fasting encompasses alternating periods of food intake and dietary restriction 1. This practice has gained prevalence, motivated by cultural, religious, and health objectives 1, yet endocrine adaptations across different protocols remain incompletely characterized 1,2. The heterogeneity of fasting protocols (short-term, alternate-day, periodic, and time-restricted feeding 2) contributes to variable endocrine responses 3. Objective: This study aimed to systematically review endocrine responses to intermittent fasting by examining appetite-regulating hormones (leptin, ghrelin), metabolic hormones (insulin), gastrointestinal hormones (GLP-1, PYY, CCK), and thyroid axis hormones across different protocols and populations 2,3. Methods: A systematic review of 9 peer-reviewed articles (2020–2025) was conducted to examine hormonal responses to intermittent fasting 4,5. Studies analyzed leptin and ghrelin secretion 4, insulin sensitivity 6,7, gastrointestinal hormones, and thyroid hormone metabolism with emphasis on hypothalamic–pituitary–thyroid axis regulation 8. Results: Ghrelin increased during fasting 4,5, while leptin decreased, reflecting reduced adipose energy status 4. Leptin reduction was associated with decreased thyroid-stimulating hormone and triiodothyronine, promoting energy conservation 8. Insulin responses remained inconclusive, dependent on protocol, duration, meal timing, and individual characteristics 6,7,9. Gastrointestinal hormones showed modest, context-dependent changes, varying across populations 3,10,11. Thyroid hormone activity decreased peripherally, while the central axis remained stable, representing coordinated adaptive responses 1,8. Conclusions: Intermittent fasting induces coordinated endocrine changes 1. Ghrelin elevation and leptin reduction are consistent markers 4,5, while insulin responses show variability 6,7,9. Heterogeneity reflects differences in fasting duration, protocols, and populations 3. Time-restricted eating shows promise when aligned with circadian rhythms 2. Future research with standardized methodologies is essential 1.
Nogueira et al. (2026) studied this question.