To our knowledge, the present study is the largest recent epidemiologic study of the incidence of metabolic syndrome among RTA personnel in Thailand. These data provide essential evidence on behavioral risk factors for metabolic syndrome in this population.
Several studies have reported data on the prevalence of metabolic syndrome in diverse populations in different regions of the world. However, the results of these studies are difficult to compare because of variation in the criteria used to define metabolic syndrome, with some relying on the NCEP ATP III, the IDF, and the World Health Organization (WHO)2 criteria.
We found that the overall incidence rate of metabolic syndrome among RTA personnel was 3.7 per 100 person-years. This rate is relatively high compared with results from a nine-year follow-up study in a rural community in Thailand, reporting an incidence rate of metabolic syndrome of 3.5 per 100 person-years25. Furthermore, this incidence rate was higher than that reported in a Japanese7 and a Taiwanese8 cohort indicating a rate of 2.1 and 2.3 per 100 person-years, repectively. This finding may be explained by the institutional structures of the military that convey detrimental behaviors and weaken personnel’s efforts to embrace healthier habits14; for example, in the present study a higher prevalence was found of regular alcohol consumption compared with the study participants in a Japanese7 and a Taiwanese8 cohort.
When considering the association between sex and the incidence of metabolic syndrome, our finding of a higher rate among males than females was consistent with results from a Japanese cohort7, but other studies, including a cohort of US military personnel26, and cohorts from Korea27 and Thailand25 have reported higher rates among females than males. Beliefs about masculinity embedded in the culture of military personnel may establish the behavioral patterns of men in styles affecting their health awareness28. Compared with females, on average, males have higher risk for cardiovascular diseases29, 30 and premature death31. The majority of the RTA population is male; therefore, targeting behaviors, that may mitigate the disparity in metabolic syndrome incidence between males and females has the potential to lower the incidence of sequelae of metabolic syndrome, including NCDs and ASCVD in particular.
In the present study, we intended to determine associations between the incidence rate of metabolic syndrome and behavioral factors at baseline. According to NCEP ATP III (2005), metabolic syndrome consisted of laboratory testing, history of pharmacologic treatment and anthropometric measurements. The RTA personnel, after participating in the annual physical health examination, received their health examination results and may have been advised by healthcare workers to modify their lifestyle. If the RTA personnel improved their health behaviors, it may have positively affected their laboratory testing results and anthropometric measurements during the follow-up period. Therefore, the incidence rates of metabolic syndrome in this population may have been underestimated.
We found that participants older than 35 years exhibited a higher incidence of metabolic syndrome than younger participants with a dose–response relationship in both sexes. Although our findings were consistent with a report from Japan reporting a higher incidence of metabolic syndrome among older than younger adults, a few studies did not find an increase of metabolic syndrome with increasing age25, 27, suggesting that the association with age was not inevitable. Several mechanisms can explain a higher incidence of metabolic syndrome with increasing age. For example, central adiposity, accumulation of fatty infiltration of the liver and sedentary lifestyle have a higher prevalence among older than younger individuals32. Furthermore, an age-related increase in oxidative stress plays a significant role in contributing vascular alterations by triggering the biochemical processes concomitant with metabolic syndrome33,34,35. When stratified by sex, the incidence rate of metabolic syndrome among participants aged ≥ 45 years was higher than those aged < 35 years with adjusted HR 6.34 (95%CI 6.01 to 6.70) for males, and 9.59 (95% CI 7.55 to 12.19) for females. Alterations in circulating female sex hormone levels, such as lower estrogen levels, were associated with distinct changes in adipose distribution patterns, reverting to visceral accumulation and raising the likelihood of increases in WC19, 36. Therefore, our study suggested that preventing the development of metabolic syndrome with increasing age is an essential target for the primary prevention of NCDs and ASCVD later in life.
Excessive alcohol consumption is a known behavioral risk factor for the incidence of metabolic syndrome and NCDs37, 38. This study found that participants reporting alcohol consumption (regular or irregular) were more likely to develop metabolic syndrome than abstainers. A recent report from a contemporary Japanese cohort reported that compared with abstainers, consumption of more than 60 g of alcohol daily was associated with a higher rate of metabolic syndrome. In contrast, less than 20 g of alcohol daily was associated with a lower risk16. In 2019, The 6th National Health Examination Survey (NHES VI) in Thailand reported that the prevalence of alcohol consumption among Thai adults was 44.6, 59.0 and 31.0% in total, and among males and females, respectively16. Compared the NHES VI, our findings indicated that the prevalence of current alcohol consumption was higher among RTA personnel (70.7%) than that of the general Thai population16. Therefore, alcohol consumption may be an important contributor to the incidence of metabolic syndrome, especially among male RTA personnel. According to tradition among males RTA personnel, reduced harmful use of alcohol may constitute a priority39. Therefore, additional pattern and intensity of alcohol consumption should be assessed in the annual physical health examination section; then motivational interventions such as a Brief Negotiated Interview should be offered to support consumers to modify their risky behaviors40.
We found that compared with lifelong nonsmokers, former smokers had a lower incidence of metabolic syndrome. This finding may result from the lifestyle modification among former smokers who may have relatively higher health awareness than lifelong nonsmokers. For example, a related study in China reported that former smokers had significantly more knowledge of all health effects than those who had never smoked41. Stratified by age, younger RTA personnel, aged < 35 years, compared with lifelong non-smokers, current smokers exhibited a lower incidence of metabolic syndrome. This observation was likely due to the well-documented negative relationship between smoking and obesity42, 43 and the finding that current smokers have less subcutaneous and visceral adipose tissue than those who never have smoked38, 44. However, not all studies have found a negative association between smoking and metabolic syndrome. For example, a report from the CARDIA study in the US did not find a significant association between smoking and risk of metabolic syndrome (RR 1.16; 95% CI 0.96 to 1.40)45. Furthermore, a meta-analysis of prospective studies found an overall significantly positive association between active smoking and the incidence of metabolic syndrome46.
In terms of regular exercise, RTA personnel may have more physical activity than the general civilian population16. However, RTA personnel serve in various departments with different characteristics of duty, for instance, military training units, healthcare workers in military hospitals and office workers in headquarters. Hence, the level of physical activity varied across our study population. We found that regular exercise was associated with a lower incidence of metabolic syndrome among RTA personnel. Regular exercise was negatively associated with obesity, a component of metabolic syndrome47. Therefore, our study suggested that regular exercise should be encouraged to lower the incidence of metabolic syndrome and ultimately NCDs, including ASCVD48, 49. Because vigorous physical activity can rarely trigger acute cardiovascular events or heat-related illness50,51,52, physical activity and structured exercise should be performed based on relevant guidelines53.
This study encountered several limitations. First, the present study was conducted among RTA personnel comprising a greater proportion of male participants than that of the general population. However, the results reported the real-world situation in the RTA population.
Because this constituted a retrospective cohort study using previously collected data, some variables were collected very broadly. For example, we did not have detailed data on how many days per week participants consumed alcohol or the number of alcoholic beverages consumed daily. Likewise, we did not have details of the smoking history, such as the current number of cigarettes smoked daily or pack-years of past exposure. We also did not have detailed data on the frequency, type or intensity of physical activity. Because we used collected data, unmeasured confounders such as family history, socioeconomic status, total calorie intake and nutritional status were excluded in the analysis. Due to the nature of an observational study, the information on some variables was unobtainable, including smoking status (5.0%), alcohol consumption (4.5%) and exercise (5.7%). Nonetheless, the available data provided valuable evidence regarding the associations between these health behaviors and the incidence of metabolic syndrome. The information on laboratory testing and anthropometric measurements came from several RTA hospitals nationwide providing the measurements; however, the measurements were performed by trained technicians, and the standard and quality of services of all RTA hospitals were certified by Healthcare Accreditation Institute, Thailand. In the present study, we aimed to determine associations between the incidence rate of metabolic syndrome and behavioral factors at baseline. However, the RTA personnel received their health examination results and may have been advised by healthcare workers to modify their lifestyle; thus, their behaviors may be changed over time. Thus, time-varying covariates may be considered to approach in future research.
Our study also exhibited significant strengths; of the approximately 130,000 RTA personnel, 98,264 (75.6%) participants, without a diagnosis of metabolic syndrome at baseline, were enrolled in the present study, representing a large sample of RTA personnel. Thus, our findings provide valuable insights into the demographics and behavioral and clinical risk factors, for the incidence of metabolic syndrome in this population. These data may contribute to strategies for the primary prevention of NCDs, ASCVD and premature death in Thai populations.
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