Introduction
The World Health Organization (WHO) defines obesity or overweight as abnormal or excessive fat accumulation that presents a risk to health [
1]. The prevalence of obesity is continuously increasing because of increased sedentary lifestyles and unhealthy lifestyles and eating habits in westernized societies as well as developing countries [
2,
3]. In 2015, 39% of the world’s population was reportedly obese or overweight. In particular, the prevalence of obesity has been reported to be higher in women aged ≥19 years than in men aged ≥19 years [
4]. Taken together, obesity is a major public health issue in the twenty-first century and is emerging as an unresolved problem [
1].
Obesity is a major cause of diabetes, and it increases insulin resistance [
5] and the incidence of various chronic diseases [
6,
7]. In particular, obesity considerably influences the risk factors for metabolic syndrome (Mets) [
8]. In addition, Mets is related to women’s menstruation, smoking, and drinking [
9‐
11]. Mets increases the risk of various diseases such as diabetes, stroke, and myocardial infarction, and it greatly affects the quality of life and causes health problems [
12,
13]. Regular physical activity or exercise lowers the risk of development of various chronic diseases such as obesity and Mets, and it has protective effects against risk factors affecting the prevalence of various chronic diseases [
14,
15]. Delgado-Floody et al., reported that a 20-week resistance exercise training program significantly reduced waist circumference (WC), systolic blood pressure (SBP), and triglycerides (TG) in 21 subjects with obesity [body mass index (BMI) ≥ 35.0–39.9 kg/m
2] or severe obesity (BMI ≥ 40 kg/m
2). In particular, significant decreases in diastolic blood pressure (DBP) and increases in high-density lipoprotein-cholesterol (HDL-C) were observed in subjects with severe obesity [
16].
Resistance exercise was reported to have a positive effect on Mets and various diseases [
17]. Resistance exercise training is also effective in increasing muscle mass [
18] and, consequently, muscle strength [
19]. There are various studies regarding the association between Mets and muscle strength [
20,
21]. de Lima et al., reported that Mets and muscle strength were directly related, and that the higher the strength level, the lower the risk of Mets [
22]. Atlantis et al. reported that the prevalence of Mets was 2.15 times higher in the low grip force group than in the high grip force group [
23]. Also, Lopez-Lopez et al. reported that the risk of metabolic syndrome was 1.39 times higher in the low grip strength group than in the high grip strength group [
24]. Tomeleri et al., evaluated the risk factors for Mets in 53 older individuals and found that, compared with the values in the group without resistance training, fasting glucose (FG), WC, and SBP were significantly reduced in the group that participated in a 12-week resistance training program. Moreover, in the resistance training group, FG and SBP after the 12-week exercise program significantly reduced relative to those before the program [
25]. As such, it has been reported that increased muscle strength through resistance exercise training has protective effects against risk factors for Mets. Therefore, resistance exercise training is essential to lower the risk factors and prevalence of Mets.
Various published studies regarding Mets associated with obesity in adult women have been conducted in foreign countries [
26,
27]. However, the number of related studies in Korea is limited; in particular, large-scale cohort studies involving Korean adult women are scarce. Therefore, the aim of this study was to investigate the effects of BMI and muscle strength assessed by handgrip strength, on risk factors for Mets in Korean adult women, using data from the Korea National Health and Nutrition Examination Survey.
Discussion
In this study, we investigated the effects of BMI and muscle strength on risk factors for Mets and its prevalence in Korean adult women aged 19 years or older using data from the 2019 Korea National Health and Nutrition Examination Survey. The results showed that WC, TG and FG were significantly lower, while HDL-C was significantly higher, in the high muscle strength group than in the low muscle strength group. The muscle strength was evaluated by measuring handgrip strength in this study. It has been reported that muscle strength assessed by handgrip strength is closely associated with the prevalence of and mortality from various diseases [
36]. In particular, it has been reported that the level of muscle strength is closely associated with risk factors for Mets [
21], and that lower muscle strength increases the risk factors for Mets and mortality associated with Mets [
20,
37]. In addition, Agner et al., reported that high muscle strength is an important factor in lowering the risk factors and prevalence of Mets [
38].
Ji et al., evaluated 2521 women and analyzed the association between risk factors for Mets and muscle strength assessed by the relative handgrip strength according to body weight, after adjusting for age, race, drinking, smoking, education level, income, energy intake, and physical activity. As a result, when muscle strength was low, WC, TG, FG, SBP, and DBP were high. Also, HDL-C was low [
39]. In this regard, it is known that muscle strength is closely associated with muscle mass [
40], and an increase in muscle mass can be improved through resistance exercise training; this indicates that increased muscle strength protects against the risk factors for Mets [
19]. Oliveira et al., reported that a 1-week resistance exercise training program had protective effects against total cholesterol (TC), TG, and WC in 22 menopausal women [
41].
When subjects were stratified by BMI, we found that WC, SBP, DBP, TG, and FG were significantly lower, while HDL-C was significantly higher, in the low BMI group than in the high BMI group. High BMI is negatively affects health and is known to be a major risk factor for various diseases, including Mets [
42,
43]. Balgoon et al., evaluated the association between BMI and risk factors for Mets in 165 women and reported that risk factors such as WC, SBP, and DBP were significantly higher in women with a high BMI (≥25 kg/m
2) than in those with a low BMI (< 25 kg/m
2), and that the risk of Mets increased with BMI [
44]. In addition, Choromańska et al., evaluated 44 women and reported that WC, BP, FG, and TG were significantly lower, while HDL-C was significantly higher, in non-obese women than in obese women with Mets. These results indicate that a high BMI negatively affects Mets and increases its prevalence [
45]. Therefore, adequate control of BMI can have protective effects against risk factors for Mets.
Finally, in this study, we found that WC, SBP, DBP, TG, and FG were significantly lower, while HDL-C was significantly higher, in subjects with low BMI and high muscle strength than in those with high BMI and low muscle strength. Moreover, the prevalence of Mets was 10.46 times higher in the high BMI & low muscle strength group than in the low BMI & high muscle strength group; after adjusting for age, menstruation status, smoking and drinking rates, the prevalence was 7.38 times higher in the high BMI & low muscle strength group. Similarly, Lu et al., analyzed the risk of Mets development according to muscle strength and obesity after adjusting for age, sex, smoking, and drinking and reported that subjects with low BMI and obesity had an 11.93 times higher risk than did subjects with normal weight [
46]. In addition, Takayama et al., reported that the prevalence of Mets was 3.12 times higher in subjects with low muscle strength and high BMI than in those with normal muscle strength and weight after adjusting for age and sex [
47]. Song et al., evaluated the association between the prevalence of Mets and handgrip strength and investigated the prevalence of Mets according to handgrip strength and BMI in 542 elderly women. They found that with a decrease in handgrip strength and increase in BMI, there was a 2.25-fold increase in the prevalence of Mets. This study also showed that lower handgrip strength and a higher BMI were associated with a 2.12-fold increase in the prevalence of Mets after adjusting for age, smoking status, occupation, education level, family income, nutritional status, and physical activity [
21]. As such, it can be said that muscle strength and BMI have a strongly associated with Mets risk factors and prevalence. Therefore, it is thought that the prevalence of Mets can be lowered by lowering BMI through regular exercise and physical activity and increasing muscle strength through resistance exercise training.
This study has some limitations. First, because the subjects were only Korean adult women, the results cannot be generalized to adult women of different races and adult men. Second, because the subjects were aged ≥19 years, the results cannot be applied to adolescents and children aged < 19 years old. Third, because BMI levels was Asian standards (BMI ≥ 23 kg/m
2), the results cannot be extended to countries other than Asian countries. Fourth, BMI is calculated by dividing weight by the square meter of height (BMI = kg/m
2) and is used as an indicator of obesity, but it is difficult to obtain accurate body composition including muscle. Fifth, since this study did not consider the participation rate of resistance exercise to improve muscle strength related to muscle mass, additional research is needed on the prevalence of metabolic syndrome according to resistance exercise participation and strength level and BMI level. Sixth, WC and Mets are closely related [
48,
49], but in this study, the effect of WC was not analyzed because it was a study to investigate the effect of muscle strength level and BMI level on metabolic syndrome. Therefore, studies related to WC will be analyzed later. Finally, because this study determined the association of BMI and muscle strength based on handgrip strength with risk factors for Mets in Korean adult women, it is difficult to broadly apply the results to determine a causal association. Despite these limitations, this study is significant in that it comparatively analyzed a large number of samples using data from the first year of the 8th Korea National Health and Nutrition Examination Survey in 2019 and confirmed the effects of BMI and muscle strength based on handgrip strength on risk factors for Mets and its prevalence.
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