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This study is part of the Korean SO Study (KSOS), which is an ongoing epidemiologic study supported by the Korea Science and Engineering Foundation. This is a prospective, observational cohort study including a total of 591 healthy volunteers 20–88 years of age, who were recruited between September 2007 and August 2008 in Seoul, Korea. The KSOS was intended to examine the prevalence of SO in Korean adults and to evaluate the effects of SO on metabolic disorders and predict likely health outcomes. None of the participants had a history of cardiovascular disease (myocardial infarction, unstable angina, stroke, or cardiovascular revascularization), any type of diabetes, stage 2 hypertension (resting blood pressure, 160/100 mm Hg), malignant disease, severe renal, or hepatic disease. Subjects taking medications that might affect body weight or body composition were excluded. Analyses were conducted on 526 participants (328 women, 198 men) who had complete data available on body composition. Among them, young healthy volunteers (aged 20–40; 145 subjects; 54 men, 91 women) were included in a sex-specific young reference group. All participants provided written informed consent, and in accordance with the Declaration of Helsinki of the World Medical Association, the Korea University Institutional Review Board approved this study protocol.
Anthropometric and laboratory measurements
BMI was calculated as weight in kilograms divided by the square of height in meters. All blood samples were obtained in the morning after a 12-h overnight fast and were immediately stored at −80 °C for subsequent assays. Serum triglycerides and HDL cholesterol were determined enzymatically using a chemistry analyzer (Hitachi 747; Tokyo, Japan). A glucose oxidase method was used to measure plasma glucose.
Definition of metabolic syndrome
Metabolic syndrome was defined according to the criteria established by the National Cholesterol Education Program Adult Treatment Panel III using the adjusted waist circumference for Asians.14 Accordingly, participants with three or more of the following five criteria were defined as having metabolic syndrome: (i) abdominal obesity by waist circumference (defined as Asian-specific waist circumference cutoff values of 90 cm for men and 80 cm for women), (ii) systolic blood pressure 130 mm Hg or diastolic blood pressure 85 mm Hg or on antihypertensive medication, (iii) elevated fasting blood glucose (5.6 mmol l–1), (iv) hypertriglyceridemia (1.7 mmol l–1), and (v) low serum HDL cholesterol (<1.03 mmol l–1 in men and <1.29 mmol l–1 in women).
Dual-energy X-ray absorptiometry
A whole body dual X-ray absorptiometry scan was performed for each patient to measure total and regional lean mass (kg), total body fat (kg), and total body fat percentage (%) using fan-beam technology (Hologic Discovery A, Hologic; Bedford, MA, USA). ASM (kg) was defined as the sum of the lean soft tissue masses for the arms and legs, after the method of Heymsfield et al.15 The ASM adjusted by stature index (ASM/height2) was also computed as described by Baumgartner et al.16 In this study, sex-specific ASM/height2 quintiles were created. For women, the bounds for ASM/height2 quintiles were (I) <6.86 kg/m2, (II) 6.87–7.36 kg/m2, (III) 7.37–7.72 kg/m2, (IV) 7.73–8.30 kg/m2, and (V) >8.31 kg/m2. The corresponding bounds for men were (I) <8.37 kg/m2, (II) 8.38–8.81 kg/m2, (III) 8.82–9.37 kg/m2, (IV) 9.38–10.06 kg/m2, and (V) >10.07 kg/m2. The total skeletal muscle mass (kg) was obtained from ASM by using the predictive equation of Kim et al.17 The SMI (%) (total skeletal muscle mass (kg)/weight (kg) × 100) was obtained by calculating the total skeletal muscle mass adjusted by weight as described by Janssen et al.3 As with ASM/height2, SMI was divided into sex-specific quintiles to facilitate the interpretation of the odds ratios (OR). For women, the bounds for SMI quintiles were (I) <32.40%, (II) 32.41–34.06%, (III) 34.07–35.78%, (IV) 35.79–37.87%, and (V) >37.88%. The corresponding bounds for men were (I) <39.84%, (II) 39.85–41.47%, (III) 41.48–43.10%, (IV) 43.11–45.29%, and (V) >45.30%.
Definitions of sarcopenia and SO
First, sarcopenia was defined as an ASM/height2 less than two s.d. below the sex-specific normal mean of a younger reference group 8 or the two lower quintiles in the study population.9, 10 Alternatively, sarcopenia was defined as an SMI of two s.d. below the sex-specific mean value for a younger reference group from the entire study population.3 Finally, the residual method was defined as the reference values of sex-specific lower 20% of the distribution of residuals between measured ASM and the ASM predicted by linear regression analysis used to model the relationship between ASM as a dependent variable, and age, height (meters), and total fat mass (kg) as independent variables.11 A positive residual indicates a relatively muscular individual, whereas negative residual is indicative of a relatively sarcopenic individual.12
Obesity was defined as values greater than the median total fat percentage for each sex 8 or the upper two quintiles for total body fat percentage of the study population.9, 10 SO was defined as high total body fat percentage and low relative skeletal muscle mass in the same subjects according to other earlier studies.8, 9, 10 We classified four sarcopenia/obesity groups by using the new criterion of SMI of two s.d. below the value of a young reference group and the upper two quintiles for total body fat percentage. The four groups included the following: normal body fat and muscle mass, sarcopenia (and normal body fat), obesity (and normal muscle mass), and SO.
Cutoff points for sarcopenia and obesity
For men, the cutoff values for sarcopenia were 7.40 kg/m2 (ASM/height2) and 35.71% (SMI), defined as less than two s.d. below the sex-specific normal mean for the young reference group. For women, the corresponding limits were 5.14 kg/m2 (ASM/height2) and 30.70% (SMI). The cutoff values for the lower two quintiles of ASM/height2 in our population were 8.81 kg/m2 in men and 7.36 kg/m2 in women, respectively. To calculate the cutoff value for the residual method, we first established a model of predicted ASM using multiple linear regression analysis by adjusting for height and total fat mass. Sex-specific equations were predicted as ASM (kg)=−31.23+32.84 × height (m)+0.24 × total fat mass (kg) for men and predicted as ASM (kg)=−19.10+21.65 × height (m)+0.20 × total fat mass (kg) for women. As a result, the sex-specific cutoff points of the lower 20% of distribution of residuals were −1.87 for men and −1.62 for women.
The cutoff values for obesity, defined as the two highest quintiles of total body fat percentage, were 20.21% for men and 31.71% for women, respectively.
Data are expressed as the mean±s.d., median and inter-quartile range (25%–75%), or as a percentage. Differences between groups were tested using a Student t-test or the Mann–Whitney U-test, and the χ2-test was used to test for differences in the distribution of categorical variables. Each variable was examined for normal distribution and any positively skewed variable was log transformed. OR (95% confidence interval (CI)), predicting metabolic syndrome based on different indices for SO, were obtained from logistic regression models after controlling for potential covariates similar to gender and age. A P-value <0.05 was considered statistically significant in all analyses. All statistical results were based on two-sided tests. Data were analyzed using SPSS for Windows (Version 12.0; SPSS Inc.; Chicago, IL, USA).