Afternoon exercise was associated with greater fat loss than morning exercise in overweight/obese adults: an exploratory study of PER3 rs228697 genotype matching.

Objective: Exercise timing may influence metabolic responses and body composition, but the extent to which circadian genetic variation contributes remains unclear. The PER3 rs228697 polymorphism has been linked to chronotype-related traits, although its relevance to exercise timing remains uncertain. This exploratory study examined whether body-composition responses to morning versus afternoon moderate-intensity continuous training (MICT) differed according to PER3 rs228697 genotype in overweight/obese university students. Methods: In this 8-week supervised intervention, 31 overweight/obese university students performed MICT in the morning (7:00-10:00) or afternoon (16:00-19:00). Participants with the CC genotype were allocated to a morning training group (CC-AM) or an afternoon training group (CC-PM), and G-allele carriers were included in an afternoon group (G-PM). Body weight, body mass index (BMI), waist circumference, body fat percentage, total fat mass, and regional fat mass were assessed before and after the intervention. Circadian preference, physical activity, sleep quality, and fatigue were also evaluated. Because the G-PM subgroup was very small, findings for this subgroup were treated as exploratory. Results: Across the overall sample, body weight, BMI, waist circumference, body fat percentage, and total fat mass decreased over the 8-week intervention. In descriptive subgroup analyses, the CC-PM group showed larger reductions in several body-composition outcomes, including body weight, BMI, waist circumference, body fat percentage, and total fat mass, than the CC-AM group. A similar pattern was observed for several regional fat measures, particularly trunk, android, and gynoid fat mass. By contrast, findings for G-allele carriers were difficult to interpret because of the very small subgroup size. Estimated between-group differences were generally modest and imprecise, and behavioral chronotype did not clearly correspond with PER3 genotype. Conclusions: In this exploratory sample of overweight/obese university students, 8 weeks of MICT was associated with favorable changes in several body-composition outcomes. Among CC genotype carriers, afternoon training was associated with descriptively greater improvements than morning training; however, the estimates were imprecise and do not support strong conclusions regarding genotype-based exercise-time matching based on PER3 rs228697 alone. Larger, well-controlled studies with objective circadian phenotyping are needed to clarify the potential role of genotype-informed exercise timing in personalized weight management.