Circadian Timing, Rather than Hydration Status, Determines Metabolic Adaptation to Ramadan-like Fasting in Diet-Induced Obese Rats

Background: Ramadan fasting involves daily abstinence from food and water between dawn and sunset, but human studies cannot readily disentangle the effects of fasting timing, circadian alignment, and hydration status on metabolic regulation. Objective: To determine whether fasting timing or hydration status exerts a stronger influence on metabolic outcomes in diet-induced obese rats under controlled Ramadan-like conditions. Methods: Forty diet-induced obese rats were assigned to four Ramadan-like fasting groups differing by timing and hydration: dry morning (DM), wet morning (WM), dry night (DN), or wet night (WN) fasting, in addition to healthy control (HC) and obese control (OC) groups (n = 8 each). Because rats are nocturnal, morning fasting restricted food during the inactive (light) phase, whereas night fasting restricted food during the active (dark) phase. Body weight, glucose, insulin, HOMA-IR, lipid profile, adipokines, and electrolytes were assessed after four weeks. Results: Morning fasting significantly reduced body-weight gain (F(5,42) = 10.72, p < 0.0001; η2 = 0.56) and improved insulin sensitivity, reflected by lower insulin (F(5,30) = 2.98, p = 0.027; η2 = 0.33) and HOMA-IR (F(5,30) = 3.76, p = 0.0092; η2 = 0.39), independent of hydration status. Serum glucose differed across groups (F(5,42) = 5.82, p = 0.00036). After body-weight adjustment, total cholesterol and triglycerides were reduced in fasting groups, whereas hydration primarily influenced fluid and electrolyte parameters without materially altering core metabolic outcomes. Conclusions: Under controlled conditions, fasting timing exerted a stronger influence on metabolic regulation than hydration status. Fasting aligned with the inactive circadian phase was associated with more favorable metabolic outcomes, highlighting circadian alignment as a key determinant of fasting-related metabolic adaptation in obesity.