2.1. Study population
We analyzed data from the third year (2018) of the KNHANES VII. The KNHANES is a nationwide, population-based, cross-sectional health examination and survey that has been conducted annually since 1998 by the Division of Chronic Disease Surveillance of the Korea Centers for Disease Control and Prevention in the Ministry of Health and Welfare to monitor the general health and nutritional status of the non-institutionalized civilian population of South Korea.
In 2018, the surveys were completed by 7,992 participants. Of these, those aged ≥20 years (n = 6,424) were selected for analysis. We excluded those who were pregnant (n = 23) and had missing data on measurements, including HGS (n = 565), serum UA (n = 159), body mass index (BMI) (n = 29), waist circumference (WC) (n = 3), blood pressure (n = 24), high-density lipoprotein cholesterol (HDL-C) (n = 3), and creatinine ≥1.5 mg/dL (n = 39). Finally, a total of 5.579 participants (2,486 men and 3,093 women), with a weighted total of 38,423,027 participants (19,557,503 men and 18,865,524 women), were included in the analysis (Figure 1). All the KNHANES participants provided written informed consent for the data to be used in the study. This study was approved by the Institutional Review Board of the Korea Centers for Disease Control and Prevention (2018-01-03-P-A) and the Institutional Review Board of Pusan National University Yangsan Hospital, which waived the requirement for approval (IRB No. 05-2022-100).
KNHANES, Korea National Health and Nutrition Examination Survey; HGS, handgrip strength; UA, uric acid; BMI, body mass index; WC, waist circumference; BP, blood pressure; HDL-C, high-density lipoprotein cholesterol.
2.2. Data collection
Health examinations included medical history taking, physical examination, administration of a questionnaire on health-related behaviors, and anthropometric and biochemical measurements. All individuals over the age of 20 were subjected to a physical examination and blood sampling by trained medical personnel who followed standardized procedures. Participants were asked about their health-related behaviors, including cigarette smoking, alcohol consumption, and regular exercise. Smoking status was indicated as “yes” when the participants had smoked more than five packs of cigarettes (100 cigarettes) during their lifetime and were smoking at the time of the survey. A standard drink was defined as a single glass of beer, wine, liquor, or traditional Korean distilled liquor, Soju. One beer bottle (355 mL) was counted as 1.6 standard drinks. The amount of alcohol per standard drink was calculated to be 10 g. Heavy alcohol consumption was indicated as “yes” when the participant had at least seven drinks at one time for men (at least five drinks for women) more than twice a week. Regular exercise was indicated as “yes” when the participant regularly performed moderate (>2 hours and 30 minutes/week, causing slightly increased respiration and heart rate) or strenuous (>1 hour and 15 minutes/week, causing rapid respiration and a substantial increase in heart rate) exercise, regardless of indoor or outdoor activity; or when the participant walked for a minimum of 30 minutes each day for 5 days/week. The completed questionnaires were reviewed by trained staff, and the records were entered into a database.
2.3. Anthropometric and biochemical data
The height and weight were measured to the nearest 0.1 cm and 0.1 kg, respectively, with participants wearing light clothing and barefooted. BMI was calculated as weight in kilograms divided by the square of height in meters. Blood pressure was measured in the right arm using a standard mercury sphygmomanometer (Baumanometer Wall Unit 33 (0850), W.A. Baum Co. Inc., Copiague, NY, USA), with the participant in a sitting position. Systolic and diastolic blood pressure (SBP and DBP, respectively) readings were recorded twice at five-minute intervals and averaged for analysis. WC was measured at the midpoint between the lower margin of the last palpable rib and the top of the iliac crest at the end of normal expiration with the arms relaxed at the sides.
Blood samples were collected from the antecubital vein of each participant in the morning following an overnight fast of at least 8 hours. The samples were processed, transferred to cold storage (2–8 °C) at the central laboratory of Neodin Medical Institute (Seoul, Korea), and analyzed within 24 hours. Measurements of serum UA, fasting blood glucose (FBG), triglyceride (TG), HDL-C, and creatinine levels were performed using a Hitachi automatic analyzer 7600-210 (Hitachi Ltd., Tokyo, Japan).
Hyperuricemia was defined as a serum UA level ≥7.0 mg/dL in men and ≥6.0 mg/dL in women [16, 17]. Serum UA levels were classified into the following quartiles according to their distribution: first quartile (1Q); 2.0–5.0, second quartile (2Q); 5.1–5.9, third quartile (3Q); 6.0–6.8, and fourth quartile (4Q); 6.9–11.2 mg/dL and 1Q; 1.7–3.8, 2Q; 3.9–4.4, 3Q; 4.5–5.1, and 4Q: 5.2–10.1 mg/dL in men and women, respectively.
2.4. HGS measurement
The HGS was measured using a digital hand dynamometer (Digital Grip Dynamometer, TKK 5401, Takei Scientific Instruments Co., Ltd., Tokyo, Japan). Grip strength was measured with the participant in a standing position, with the arms fully extended at the sides without touching the body. The participants were asked to squeeze the dynamometer with as much force as possible for <3 seconds, three times with each hand alternately. A rest interval of at least 30 seconds was allowed between the trials. Absolute HGS was calculated as the summation of the maximal reading from each hand and was expressed in kilograms. Relative HGS was defined as the absolute HGS divided by BMI.
2.5. Statistical analysis
Statistical analyses were performed using SPSS Statistics for Windows, version 18.0 (SPSS Inc., Chicago, IIL., USA), using sampling weights from the KNHANES to acquire nationally representative estimates. In this study, the data were presented as weighted means with standard errors for continuous variables and percentages for categorical. Data for men and women were separated for further analysis owing to significant differences in serum UA and HGS by sex. In the general linear model, the t-test and analysis of variance (ANOVA) were used for continuous variables, and the chi-square test was used for categorical variables. ANOVA was performed using a general linear model approach to determine the association between serum UA levels and relative HGS with progressive levels of adjustment for age and MetS components. The primary independent variable was the serum UA level (categorized into sex-specific quartiles) in each model, and the dependent variable was relative HGS. Model 1 was corrected for age; model 2 was corrected for age, BMI, and WC; and model 3 was corrected for age, BMI, WC, SBP, DBP, FBG, TG, and HDL-C. All statistical tests were two-tailed, and statistical significance was defined as p <0.05.