The prevalence of MetS
This study detected MetS in 24·8% of the younger adults and 40·8% of the older adults examined. Studies related to the prevalence of MetS in Korea can be found beginning in the late 1990s. Several of these studies targeted examinees in comprehensive examination centres in one city or one university, but none examined prevalence on a nation-wide scale. In contrast, this survey targeted 690,671 examinees that underwent the health test on life transition periods conducted by the National Health Insurance Corporation, thereby more accurately reflecting the prevalence of MetS in Korea.
A study in Finland that targeted older adults over a period of 14 years reported that the prevalence of MetS (defined by NCEP criteria) was 42·1% (Wang et al. 2008). In the USA, 12·9% of a sample population that had been MetS-free for 15 years was diagnosed with MetS at a 20-year follow-up exam (Paul et al. 2008). In Germany, in a cohort study conducted over 10 years, the prevalence of MetS (using NCEP criteria) was 25·8% (Jacqueline et al. 2005). In Mexico, Rojas et al. (2009) reported that the prevalence was 36·8% (using NCEP-ATP III criteria), while this study showed 41·6% under a criteria presented by the AHA/NHLBI, similar to the prevalence of 40·8% among the 66 year old adults in this study.
Risk factors for metabolic syndrome
Among the 40 year old adults, the MetS prevalence was lower in women than in men, while the prevalence was 1·28-fold higher in women than in men in the 66 year old adults. According to the 2005 National Health and Nutrition Survey, the prevalence of MetS among Korean adults aged 30 or more was 32·9% in men and 31·8% in women; in men, this prevalence gradually increased with age during their 50s and thereafter decreased into their 70s, while the prevalence in women increased steadily with age: at age 70, the prevalence among women was 63·4%, almost twice the 34·1% prevalence in men (Lee et al. 2009). Park et al. (2003a, 2003b) pointed out that while the prevalence of MetS increased with age, the pattern of increase depended on gender. As in our study, Park et al. (2003a, 2003b) reported that men in their 50s had MetS with a higher frequency than did women, while after the age of 60 the opposite pattern was observed. This was attributed to increased risk factors such as abdominal obesity and cardiovascular disease in postmenopausal women (Park et al. 2003a, 2003b). Therefore, women should be pre-educated about MetS and receive intervention before the age of 40.
According to this study, a family history of cardiovascular disease was the most significant risk factor for MetS. Ahn et al. (2010) reported that a family history of high blood pressure had the greatest degree of correlation with MetS: 27·5% in men and 40·9% in women in these groups. Park et al. (2003a, 2003b) reported that women with a family history of high blood pressure, diabetes, hyperlipidaemia, angina, myocardial infarction, or stroke had a 1·4-fold higher risk of MetS (P < 0·05). In our study, family history of stroke, cardiovascular disease, or high blood pressure had statistically significant effects on the prevalence of MetS. Therefore, individuals with such family histories should be screened to detect MetS in advance and should take steps to reduce the risk of MetS such as decreasing BMI, weight control, smoking cessation, and exercise training.
It is known that frequent smoking reduces HDL-cholesterol and increases LDL-cholesterol and triglycerides, resulting in elevated risk of cardiovascular disease; furthermore, abdominal obesity increases with the frequency of smoking. Several studies have reported that smoking may be closely related to MetS (Katano et al. 2010). In our study, the prevalence of MetS was lower in non-smokers than in smokers at the age of 40 but higher in non-smokers at age 66. We conservatively interpret this phenomenon to be due to former smokers being classified as non-smokers. In our study, the prevalence ratio of MetS among smokers was statistically significant at 1·26 and 1·12 at the ages of 40 and 66 years respectively. In previous studies, Choi et al. (2009) reported that the prevalence ratio of MetS among smokers was statistically insignificant at 1·5 (95% CI: 0·8–2·6) and Jung et al. (2002) reported that the comparative risk of MetS prevalence among smokers was 1·9 (95% CI: 1·1–3·7). Our results confirmed once again that smoking could be closely related to MetS.
Concerning drinking, among 40 year old adults, drinking more than once a week increased the prevalence of MetS, while 66 year old who drank less than once per week had a higher prevalence (41·8%) than those who drank more than once a week (38·2%). Previous studies have also reported the effect of drinking on MetS. Our study defined a person who drank more than once a week as a drinker; as with the discordant results seen for smoking, it is possible that this contradiction is due to the classification as non-drinkers of individuals who drank excessively when younger but no longer drank at the age of 66. In addition, the effect of drinking on MetS prevalence was not statistically significant among 40 year old adults. Among the 66 year old adults, however, the risk was 1·02 times higher in drinkers than in non-drinkers. According to a previous study, the prevalence of MetS increased with a cross ratio of 1·15 in those drinking more than two alcoholic beverages a day. It is known that moderate drinking (one alcoholic beverage per day) reduces the risk of MetS, largely by elevating HDL-cholesterol (Hong et al. 2007). Among 40 year old adults, drinking was the only variable that did not show a statistically significant effect on MetS prevalence, confirming once again that drinking has complex effects on cardiovascular disease. Therefore, further research using longitudinal rather than cross-sectional studies is needed to confirm the effects of drinking on the prevalence of MetS.
This study has the limitation that an analysis of the exact relationships among drinking, smoking, and exercise was measured not by collecting exact frequencies of the behaviours but only by distinguishing between ‘less than once a week’ or ‘more than once a week’. Further research that collects more specific measurements of smoking, drinking, and exercise is necessary. Another factor contributing to the prevalence of MetS is reduced physical activity. According to many previous studies, physical activity is somewhat important in preventing chronic diseases such as type II diabetes and cardiovascular disease (Jekal et al. 2009). According to a study conducted by Wamala et al. (1999) in Sweden, the risk of MetS was 3·3 times higher prior to adjustment and 2·8 after adjustment in non-exercisers relative to exercisers and Jung et al. (2002) reported that non-exercisers showed a comparative risk of MetS prevalence of 1·7 (95% CI: 0·9–2·8) compared with those who exercised more than 5 days a week, similar to the results of our study. In the 66 year old adults, the prevalence of MetS was significantly higher in non-exercisers (41·6%) than in exercisers (39·4%), a phenomenon likely due to inaccurate tracking and observation of exercise habits. Given the previous studies, the results indicate that the results for the 40 year old in Table 3, where the exercisers had a marginally higher prevalence of MetS, would be more noteworthy.
In this study, the prevalence of MetS increased with obesity. This was similar to the results from Park et al. (2003a, 2003b): among both younger and older adults, the greater the degree of obesity, the higher the prevalence of MetS. Park et al. (2002) reported that individuals with a normal BMI of 18·5–23 kg/m2 had a MetS prevalence of 10% and our study showed a prevalence of 18·6% in the normal-BMI group (18·5–25 kg/m2) among 40 year old adults and a higher prevalence of 30·8% among 66 year old adults with normal-BMI measurements. This study confirmed once again that in Asian populations, the risk of MetS can be high in Asians even in the presence of relatively low BMI compared with western populations. In other words, MetS risk can be high in this population even when BMI is low because the level of abdominal obesity is high relative to the BMI, similar to other Asian populations (McKeigue et al. 1991). As mentioned above, the effect of obesity on MetS prevalence was much greater than that of smoking, drinking, or exercise, confirming that weight control is an important suggestion for those at risk for MetS and that it is imperative to control weight to reduce the risk of MetS in Korea.