Study population
Participants in the present study were recruited from two population-based cohorts: the Ansung and Ansan cohorts from the KoGES [10]. Eligibility criteria for the KoGES included age 40–69 years and residence within the survey area for 6 months or longer prior to enrolment. A total of 10,038 participants were included in the KoGES cohorts (5018 from a farming community for the Ansung cohort and 5020 from an industrial community for the Ansan cohort). Baseline examinations were performed in 2001 and 2002, and 2-yearly follow-up examinations continued through 2014. We excluded from the analysis participants with unknown glucose status (n = 91), a previous history of diabetes (n = 572) and incident diabetes at the baseline examination (n = 635). After excluding these 1298 participants, 8740 participants underwent repeat examinations every 2 years. Informed written consent was obtained from all participants. Demographic information was collected at the baseline examination using a standard questionnaire that was administered during face-to-face interviews. The study protocol was approved by the Ethics Committee of the Korea Center for Disease Control and the Ajou University School of Medicine Institutional Review Board.
Diagnosis of type 2 diabetes
For both baseline and 2-yearly follow-up evaluations, all participants underwent a 75 g OGTT after an overnight fast of at least 8 h, and biochemical assays were performed at a central laboratory (Seoul Clinical Laboratories, Seoul, Korea). A previous diagnosis of diabetes was identified by self-report, the use of oral hypoglycaemic agents and/or insulin, or OGTT results at baseline. At subsequent follow-up examinations, newly diagnosed type 2 diabetes was defined as a fasting glucose concentration of ≥7 mmol/l or a post-load glucose concentration of ≥11 mmol/l after a 75 g OGTT based on the WHO criteria [14].
Body composition and laboratory assessments
All participants attended a community clinic for clinical assessments at each follow-up visit. BMI was calculated as weight in kg divided by the square of the height in metres, with participants in light clothing and barefoot. Lean body mass and body fat were assessed by multifrequency bioelectrical impedance analysis (MF-BIA; InBody 3.0, Biospace, Seoul, Korea). Unlike conventional bioelectrical impedance analysis (BIA) equipment that often takes only partial measurements and relies on formulas to estimate whole body composition, the MF-BIA technique assumes that the human body is composed of five interconnecting cylinders and takes direct impedance measurements from these bodily compartments. Using a tetrapolar eight-point tactile electrode system, measurements of impedance were taken at four specific frequencies (5, 50, 250 and 500 kHz) in five segments (right arm, left arm, trunk, right leg and left leg) and used to calculate a value for segmental lean body mass by determining the intracellular and extracellular water components of the total amount of water in the body. The MF-BIA machine can provide valid and accurate estimates of lean body mass and body fat that are closely associated with those measured using dual-energy x-ray absorptiometry (DXA) across ranges of age, volume status and BMI [15–17]. The Asian Working Group of Sarcopenia supports using BIA for evaluation of body composition in community-based assessments because of its simplicity and portability [18].
Lean body mass was estimated by dividing the total amount of water in the body by 0.73, a formula that was validated in previous studies [15, 19]. At the limb level, lean body mass is synonymous with skeletal muscle mass, and the appendicular skeletal muscle mass (ASM) calculation was performed based on the sum of the lean body mass in all four limbs [20]. In our study, the muscle mass index (MMI) was derived by dividing ASM by weight in kg × 100, which is suggested to be a better predictor of insulin resistance and diabetes risk than ASM or height-adjusted ASM [11, 13, 21–23].
Main covariates
Covariates including baseline age, sex, urban or rural residence, family history of diabetes, hypertension, smoking status, education level, monthly income, physical activity, alcohol consumption and diet were evaluated based on self-reports. Physical activity was classified into two categories: none and regular exercise (≥1 session/week). A single exercise session was defined as exercising for at least 30 min. Total caloric intake was estimated from a food frequency questionnaire by trained dietitians [24]. Hypertension was defined as systolic BP of >140 mmHg, diastolic BP of >90 mmHg or the use of antihypertensive medication. Fat mass, BMI and waist circumference were also controlled for as categorical variables. Obesity was defined as a BMI of ≥25 kg/m2 or a waist circumference of ≥90 cm for men and ≥85 cm for women [25, 26].
Statistical analysis
All data were expressed as means and SD or as numbers and percentages. One-way ANOVA and linear-by-linear association tests were used for comparing baseline characteristics according to sex-specific tertiles of MMI.
We calculated the cumulative incidence of type 2 diabetes and HRs using multivariable-adjusted Cox proportional hazard regression models to assess the risk of developing diabetes according to sex-specific tertiles of MMI at baseline and as a continuous variable per 1 SD decline in MMI. Each participant was followed from the date of the baseline visit until the first follow-up visit at which diabetes was ascertained, the date of the last informative contact or the end date of the study. Incidence rates were calculated by dividing the number of incident cases by the number of person-years of follow-up in each MMI tertile.
Multivariable models were adjusted for the main covariates. Log-minus-log plots were produced for each variable to verify the assumption of proportional hazards. Colinearities between fat mass, BMI, or waist circumference and MMI were assessed using variance inflation factors, with variance inflation factors of >10 indicating model instability. Statistical interaction effects by age, sex and menopause at baseline and the risk of diabetes associated with tertiles of MMI were assessed by including interaction terms in the models. As part of the sensitivity analysis, participants diagnosed with diabetes within the first 2 years of follow-up were excluded to limit the effect of reverse causation between low muscle mass and diabetes.
Cox regression models were modelled by jointly classifying participants by each sex-specific tertile for MMI at a given BMI to evaluate the predictive value of MMI in addition to BMI for diabetes risk. The population-attributable fraction was calculated as p([HR − 1]/HR), where p is the proportion of total cases in the population arising from the specified exposure category. All p values were based on two-sided tests, and p < 0.05 was taken to indicate statistical significance. Analyses were performed using SPSS version 18.0 (SPSS, Chicago, IL, USA).