The relationship between the level of exercise and hemoglobin A1c in patients with type 2 diabetes mellitus: A systematic review and meta-analysis

Background: Several systematic reviews and meta-analyses reported that exercise improved hemoglobin A 1 c (HbA 1c ) in patients with type 2 diabetes mellitus, but the improvement reported was heterogeneous. A difference in the level of exercise might be the reason for that heterogeneity. Objective: The aim of study was to evaluate the relationship between changes in HbA 1c and exercise levels when performing various types of exercise. Methods: The inclusion criteria were randomized controlled trials involving adults with type 2 diabetes mellitus and a mean age of ≥18 years, intervention involving exercise alone, the overall duration of intervention ≥12 weeks, and reporting HbA 1c . The mean difference (defined as difference [intervention group value minus control group value] in change [final value minus baseline value] in the mean value) was calculated. Weighted mean difference (WMD) was defined as the mean difference weighted by the inverse of the squared standard error for each study, and all WMDs were pooled as overall effects. A meta-regression analysis was performed to evaluate the relationship between the exercise level and the WMD in HbA 1c . All analyses were performed using restricted maximum likelihood. Results: Forty-eight studies (2395 subjects) were analyzed. The pooled WMD in HbA 1c decreased significantly (−0.5% [95% confidence intervals: −0.6 to −0.4)]) but contained significant heterogeneity (Q=103.8, P<0.01; I 2 =36.6%). The pooled WMD in body mass index (BMI) decreased significantly (−0.55 kg/m 2 [95% confidence intervals: −0.58 to −0.51]) and did not contain heterogeneity (Q=25.8, P=0.99; I 2 =0.0%). A meta-regression analysis showed that the intensity (metabolic equivalents [METs]), time (min/session), or frequency (sessions/week) of exercise was not associated with the HbA 1c . However, the overall duration of exercise (weeks) was significantly associated with the WMD in HbA 1c (meta-regression coefficient: 0.01 [95% confidence intervals: 0.002 to 0.016]; R 2 =70.0%), and that result did not contain significant heterogeneity (P>0.05; I 2 =14.7%). Even when studies were limited to those involving mean age ≥40 years, mean baseline HbA 1c ≥6.5%, mean duration of type 2 diabetes mellitus ≥5.0 years, mean baseline BMI ≥30 kg/m 2 , calculation of the WMD in BMI, performance of aerobic exercise alone, or no inclusion of a high risk of bias, there was no change in these results. Conclusions: The exercise intervention decreased HbA 1c in type 2 diabetes mellitus patients, but that change likely depended on the overall duration of exercise. HbA 1c may increase if exercise is continued, but at least continuing exercise is not necessarily beneficial for type 2 diabetes mellitus patients. This may be the result of intervention in the form of exercise alone.