Effects of Short-Term (20-Day) Alternate-Day Modified Fasting and Time-Restricted Feeding on Fasting Glucose and IGF-1 in Obese Young Women

Background: Obesity is a metabolic condition that may impair insulin sensitivity and disrupt glucose homeostasis. Since insulin and glucose affect insulin-like growth factor-1 (IGF-1), disruptions in this axis may elevate the risk of chronic diseases. Intermittent fasting (IF) modulates metabolic parameters, but the impacts on glucose regulation and IGF-1 remain underexplored. This study aimed to assess the short-term effects of two IF types, time-restricted feeding (TRF) and alternate-day modified fasting (ADMF), on fasting blood glucose (FBG) and IGF-1 in obese young women. Methods: A quasi-experimental pretest–posttest control group design was conducted over 20 days. The 31 subjects were allocated into ADMF (n = 10), TRF (n = 11), and Control (n = 10). After excluding dropouts and outliers, the final sample consisted of 22 subjects (ADMF = 7, TRF = 8, Control = 7). FBG and IGF-1 serum were measured pre- and post-intervention. Results: The FBG post-intervention significantly increased in TRF (p = 0.001) and ADMF (p = 0.036) groups, but not in Controls. Only the TRF group showed a significant reduction in IGF-1 levels (p < 0.001). Nevertheless, the ADMF group exhibited substantial decreases in body weight (p = 0.047) and visceral fat (p = 0.017). Conclusions: A 20-day IF in obese young women induced distinct metabolic effects: TRF lowered IGF-1, ADMF reduced adiposity, and both
regimens increased FBG. These findings suggest that early changes in glucose regulation are highly dependent on the specific dietary regimen used. Specifically, TRF predominantly influences endocrine regulation (IGF-1 axis), while ADMF favours adiposity reduction. The concurrent rise in FBG may reflect a transient shift in glucose homeostasis during the early stages of fasting.