Study design and population
Community-dwelling of local Qingdao residents aged 65 and over were received annual physical check-up in community service center. Of these, 1336 senior citizens in 2 neighborhoods (Fushan and Ningxia Community) were participated in health examination between Mar. and Nov. 2020. After excluding missing data of skeletal muscle (n = 196) and the participants with unreliable 24-hour recall data (n = 17), 1123 participants were available for data analyses, including 494 males and 629 females. A total of 250 MetS patients were selected as the cases, excluding those with severe heart failure, liver and kidney disease, malignancy, or cognitive impairment. Meanwhile, the residents without MetS who underwent physical examination in the same period at the community hospital were selected as the control group. Controls were individually matched to cases by age (± 3 y), gender, with a 1:3 case-to-control ratio. Written informed consent was obtained from all participants, and this research was approved by the Ethics Committee of the Affiliated Hospital of Qingdao University.
According to Diabetes Society of the Chinese Medical Association (CDS), MetS was defined as three or more of the following indexes: (1) overweight or obesity (BMI ≥ 25.0 kg/m2); (2) hyperglycemia (FPG ≥ 6.1 mmol/L or 2 h post-meal glucose ≥ 7.8 mmol/L, or diagnosed before); (3) hypertension (SBP ≥ 140 mmHg, DBP ≥ 90 mmHg or diagnosed before); and (4) dyslipidemia (TG ≥ 1.7 mmol/L, or HDL < 1.0 mmol/L in female, or HDL < 0.9 mmol/L in male) [14].
Questionnaire Interview
A face-to-face questionnaire interview was conducted by well-trained investigators to collect the information on the socio-demographic characteristics, lifestyle habits, family history of chronic disease, current clinical conditions, and medical treatments. In addition, a three-day 24-hour dietary questionnaire (2 working days and 1 weekend) was recorded to monitor the average intake of dietary nutrients.
Anthropometrical Measurements
Anthropometrical data, including height (m), body weight (kg), waistline (cm) and blood pressure (mmHg), were measured by well-trained physicians using calibrated equipment. Body mass index (BMI, kg/m2) was calculated as the participant’s weight (kg) divided by the square of height (m2).
Biochemical Measurement
Fasting blood samples were collected into vacuum tubes for laboratory analyses. Serum was separated by centrifugation (4000 rpm for 10 min at 4°C) for biochemical analysis. Meanwhile, assays of serum fasting glucose and lipid profiles (total cholesterol [TC], triglyceride [TG], high-density lipoprotein cholesterol [HDL-C], low-density lipoprotein cholesterol [LDL-C]) were determined by automatic biochemical analyzer (TBA-40FR, Toshiba, Japan).
Measurement Of Muscle Mass
After an overnight fasting, skeletal muscle mass was estimated via bioelectric impedance analysis (BIA, InBody S10, Korea), an effective tool that was widely applied for assessing body composition. The participants rested for at least 10 minutes in order to achieve a regular distribution of fluid. The measurement was carried out in a sitting position. Inbody S10 performed three different frequencies (5KHz, 50KHz and 250kHz) to measure the impedance of five parts of the body (trunk, left and right arms, left and right legs), and calculate the muscle mass of each part. Then, body weight was adopted to standardized regional muscle mass.
Statistical analysis
The Shapiro-Wilk test was adopted to check the normality of continuous variables. The normal variables were expressed as mean ± standard deviation (SD), whereas the skewed variables were presented as the median (interquartile rage [IQR]). The baseline characteristics of study participants were evaluated by the Chi-test for categorical variables and the Wilcoxon rank sum test for continuous variables.
Conditional logistical regression model was implemented to estimate crude odds ratio (OR) with 95% confidence interval (CI) of MetS risk across quintiles, with the lowest quintile serving as the reference. Meanwhile, multivariate-adjusted OR with 95% CI was also implemented by adjustment for age, gender, duration of education, exercise, smoking status, alcohol drinking status, total energy intake, proteins, fats, carbohydrates, serum fasting glucose and lipid profiles. We used a restricted cubic spline model with 4 knots (at 5th, 35th, 65th and 95th) to explore the shape of the association between skeletal muscle mass index and MetS risk, adjusting for confounding factors [15]. Tests for trends were conducted by assigning the median value for each category and modeling this variable as a continuous variable [16]. All statistical analyses were performed using STATA 15.0 (Stata CORP, College Station, TX). A two-tailed P < 0.05 was regarded as statistically significant.