Influence of Morning and Afternoon Exercise on Body Composition and Metabolic Health: A Systematic Review and...

Exercise is widely recognized for its beneficial effects on metabolic health and can serve as a non-pharmacological health intervention. However, the effect of morning versus afternoon exercise on various metabolic indicators remains unclear. This study aimed to systematically analyze the effect of exercise at various times of day on body composition, glucose and lipid metabolism.

Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, this study conducted literature searches in the electronic databases PubMed, Embase, Web of Science, Scopus and SPORTDiscus. Fourteen long-term and five acute studies were analyzed (n = 765 participants) that compared exercise performed in the morning versus the afternoon or evening, and assessed body weight, body mass index, fat mass, fasting blood glucose, hemoglobin A1c (HbA1c), insulin, and/or triglycerides. The risk of bias was assessed using the Cochrane tool (RoB 2). The certainty of evidence was assessed using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) framework.

In the long-term studies, morning training was associated with a greater reduction in HbA1c (mean difference [MD] - 0.11%, 95% confidence intervals [CI] - 0.19 to - 0.03, I² = 0%, p = 0.009), an effect primarily observed in populations with type 2 diabetes (T2D) in subgroup analyses. In contrast, afternoon training was associated with a greater reduction in fasting blood glucose (MD 0.13 mmol/L, 95% CI 0.01 to 0.24, I² = 0%, p = 0.03). No significant timing effects were observed for other body composition or lipid outcomes. In the acute studies, afternoon exercise was associated with greater reductions in glucose levels from pre- to post-exercise (MD 0.53 mmol/L, 95% CI 0.10 to 0.96, I² = 21%, p = 0.02), but no difference was found for fasting blood glucose. The certainty of evidence ranged from moderate to very low across most outcomes, primarily due to risk of bias and imprecision.

Morning and afternoon exercise suggest selective effects on glycemic control, though these findings should be considered preliminary and were primarily observed in populations with T2D. Morning training was associated with a statistically significant reduction in HbA1c, whereas afternoon training was associated with lowering fasting blood glucose. However, these findings should be interpreted with caution, as the result was primarily observed in T2D populations and the overall certainty of evidence was low. No consistent differential effects were observed for body composition or other lipid parameters.