MetS and its components are risk factors that can elevate the prevalence and severity of leading causes of death such as cardiovascular and cerebrovascular diseases and type 2 diabetes mellitus [30, 31]. The adverse effects of living with the condition are not limited to the affected individuals but also have serious ramifications at the societal level. In the case of Indonesia, all citizens are covered by National Health Insurance (JKN) and treatment costs to the state can represent millions of US dollars each year. It is widely known that BMI, physical activity, and eating habits play critical roles in the development of MetS. This study identified the risk factors for MetS based on six years of follow-up in a cohort of individuals who were not affected by the condition at baseline.
In this population-based cohort study, we investigated the association between risk factors and the survival rate for MetS, defined as the probability of living free from MetS. We demonstrated several principal findings, which are summarised as follows: during the 6-year follow-up period (1) the risk of developing MetS rose with increasing BMI. Obesity reduced the probability of living free from MetS by more than 50% and led to a 4.4-fold increased risk of developing MetS, (2) a sedentary lifestyle significantly increased MetS risk by 1.5 times and reduced the probability of living free from MetS by around 20%, (3) we observed that low and moderate fat intake had a protective effect against MetS and carbohydrate intake was a risk factor for developing MetS. However, this result needs to be carefully interpreted as the lower consumption of fat may be a consequence of having higher consumption of carbohydrate.
Previous studies have consistently reported higher BMI as predictor for MetS [32–34]. Other variables that have been reported in addition to BMI include sedentary lifestyle [35, 36], waist-to-hip ratio [37], older age, being female, and lower education level [34]. A population cohort in Korean adults suggested that dietary aspects, such as dietary diversity, was associated with the risk of MetS [38]. Diet and MetS association also supported by a cross-sectional study reported low fruit and vegetable intakes and high alcohol consumption as dietary factors that contribute to MetS risk, alongside high BMI and a sedentary lifestyle [35]. The findings of the present study are, in general, aligned with previous findings and suggested theories. While overweight-obesity and sedentary lifestyle were consistently reported in another population study, this study’s findings in relation to carbohydrate and fat intake as predictors of the future risk of MetS have not been widely reported in other studies. Different dietary patterns and types of carbohydrate and fat consumed in various populations allow diverse outcomes in the risk of MetS [35].
Previous observations related to fat intake and MetS were inconclusive. While some study proposed that high dietary fat intakes are associated with higher risk of MetS [39, 40], some other studies suggested the opposite [41, 42]. Based on a systematic review, the difference was due to the type of fat consumed, as higher polyunsaturated fatty acids (PUFA) was reported to be inversely associated with adiposity and obesity, as well as lower triglyceride levels, increased HDL, and an overall lower risk of MetS [40]. The current analysis shows that low and moderate fat intake lowers the risk of developing MetS. However, this result needs to be carefully interpreted as it could be depending to the type of fat (fatty acids) consumed, which this study did not measure, or considering the overall energy intake proportion, the protective effect of fat may be a consequence of having higher consumption of carbohydrate.
A dose–response meta-analysis of observational studies suggested that carbohydrate intake had a linear association with MetS risk, with a 2.6% increase in the risk of MetS per 5% energy from carbohydrate intake [43]. In addition, carbohydrate intake from starchy foods with a high glycaemic index (GI > 65) has been shown to contribute more strongly to metabolic disorders and hyperlipidaemia [44]. High consumption of carbohydrates has been consistently associated with a reduced HDL cholesterol level and increased plasma triglyceride levels [45, 46] mainly due to a higher triglyceride content in very low-density lipoprotein (VLDL) particles and overproduction of VLDL particles [47]. In addition to triglyceride and HDL levels, abdominal obesity is also strongly associated with carbohydrate intake [46]. This condition was likely relevant to participants in this study, who were, due to the prevalence of overweight and obesity, already at greater risk of abdominal obesity.
The abdominal obesity component was the most prevalent feature of MetS in this population and another reported MetS population [48]. It is representative of visceral adiposity which is a known risk factor for cardiovascular diseases. Although BMI does not represent adiposity, in the general population, overweight and obese BMI were correlated with increased levels of body fat [49]. Therefore, an individual with overweight-obesity was already at higher risk of abdominal obesity. Furthermore, increased adiposity has been theoretically linked to increased inflammation and metabolism suppression. The biochemical cascade was linked to glucose intolerance and hyperinsulinemia, as well as endothelial dysfunction, which is the predisposing factor for dyslipidaemia and vascular diseases [50, 51].
The present study showed that inadequate exercise had unfavourable effects on MetS risk. Although on the basis of their self-reported data, most participants had sufficient physical activity levels due to their employment as household assistants, when we clustered activity based on intentional physical exercise (playing sports or recreational exercise rather than activity associated with their employment), most participants did not meet recommended levels. Increased sedentary behaviour is associated with an increased risk of high levels of triglycerides, HDL-cholesterol, and fasting glucose [52, 53]. Conversely, around 30 minutes of moderate-intensity daily exercise improves factors related to MetS [54]. Men who have more than 4 times per week of vigorous, moderate, and strength exercise and more than 6 times per week of walking had a triglyceride level lower than 150 mg/dL [55, 56]. Furthermore, diastolic blood pressure [57] and fasting blood glucose [55] were reported to improve as a result of moderate exercise, particularly aerobic exercise [57].
In observing the development of MetS, we worked on several variables to avoid biases in measurement and analysis. We excluded participants who had limited or incomplete data on blood pressure, fasting blood glucose, HDL, and triglycerides and who were taking antihypertensive, antidiabetic, and cholesterol-lowering medications. We also excluded participants who did not fast prior to blood glucose testing. By doing this, we expected to better understand the incidence of MetS during six years of follow-up with observations every two years. However, we tried to involve as many relevant variables as possible to obtain a more accurate model.
The retrospective study design and the use of secondary data were among the limitations of this study. There is some potential that the data is inaccurate due to the self-reporting of some information or human error in data input. However, we used a cleaning procedure to exclude missing information from the MetS criteria, such as instances, when a number was either too small or too excessive, as was the case with blood glucose levels, waist circumference, lipid profiles, and blood pressure, the smaller sample sizes were a result of the stringent data inclusion criteria. The strengths of the present study are the large number of study subjects, and adjustment for potential confounding factors, including dietary intake and physical activity. However, the inclusion of data on changing levels of physical activity during the study period would have been beneficial. The anthropometric and clinical measurements were recorded by trained operators and are more accurate than self-reported measurements. After weighted adjustment, this study is a sound reflection of the epidemic characteristics of MetS among residents in Bogor District, Indonesia.