Introduction: There is little evidence on the effects of muscular strength, independent of cardiorespiratory fitness, on the development of type 2 diabetes.
Hypothesis: We hypothesised that muscular strength, independent of cardiorespiratory fitness, has significant benefits in type 2 diabetes prevention.
Methods: Participants were 5,578 men and women aged 18 to 100 years (mean age, 44) who received preventive medical examinations during 1980-2006 in the Aerobics Center Longitudinal Study. Participants were free of myocardial infarction, stroke, cancer, and diabetes at baseline. Total body muscular strength was quantified by combining 1 repetition maximum (1-RM) measures for leg and bench presses and categorized into three groups, lower (weak), middle, and upper (strong), based on the tertiles (thirds) of muscular strength. Cardiorespiratory fitness was measured by a maximal treadmill exercise test. Type 2 diabetes was defined as a fasting plasma glucose level of ≥126 mg/dl, a history of diabetes, or current insulin therapy. Cox regression was used to calculate hazard ratios (HRs) and 95% confidence intervals (CIs) of incident type 2 diabetes by muscular strength after adjusting for baseline age, sex, examination year, body mass index (BMI), current smoking, heavy alcohol drinking, parental history of diabetes, hypertension, hypercholesterolemia, glucose level, and maximum treadmill test time.
Results: During an average follow-up of 8 years, 270 (4.8%) individuals developed type 2 diabetes. Compared with the individuals in the lower third of muscular strength, individuals in the middle third had a 30% lower risk of developing type 2 diabetes (HR: 0.70, 95% CI: 0.51-0.95), whereas no significant benefit was found in individuals in the upper third (HR: 1.16, 95% CI: 0.86-1.57) after adjusting for potential confounders and cardiorespiratory fitness. We found similar results in men and women, young (<50 years) and old (≥50 years), and normal (BMI <25 kg/m2) and overweight/obese (BMI ≥25 kg/m2) individuals. In the combined analysis of muscular strength and cardiorespiratory fitness, we also found similar results that individuals in the middle third of muscular strength showed lower risks of developing type 2 diabetes in both low (lower 50%) and high (upper 50%) cardiorespiratory fitness, compared with individuals in the lower third of muscular strength and low cardiorespiratory fitness.
Conclusions: We found that a moderate level of muscular strength, independent of cardiorespiratory fitness, is associated with a lower risk of developing type 2 diabetes. Additional studies on the dose-response relationship of muscular strength and incident type 2 diabetes are warranted.