Studies on metabolic syndrome have been actively conducted in recent decades due to its association with cardiovascular diseases. In 1923, Kylin understood hypertension, hyperglycemia, and gout as components of a syndrome. In 1988, Gerald Reaven introduced hypertension, hyperglycemia, hypertriglyceridemia, and low HDL as Syndrome X, which was a risk factor of cardiovascular diseases . Since then, central obesity and insulin resistance have been understood as the core symptoms of metabolic syndrome. World Health Organization (WHO) suggested that the syndrome be called metabolic syndrome in 1998. Each metabolic syndrome factor is known to increase the prevalence of type 2 diabetes and cardiovascular diseases, and their mortality. When these factors are combined, the situation becomes much worse .
Findings of a few studies have indicated that long time shift-work is associated with metabolic syndrome. A study conducted for female workers in fabric plants showed positive association between shift-workers and metabolic syndrome in comparison with daytime workers . A cross-sectional study conducted for female hospital nurses and male manual workers in Korea also showed an association of shift-work with metabolic syndrome . Therefore, to exclude the effect of shift-work on metabolic syndrome, we targeted only workers without shift work as subjects. Self-employed workers and unpaid family workers were excluded from this study because there are differences in health behaviors and working conditions among self-employed workers, unpaid family workers, and wage workers.
The results of this study suggest that long working hours may not be associated with metabolic syndrome, with no significant odds difference in increasing weekly working hours. These insignificant results remained unchanged after adjustments for gender, age, smoking, alcohol drinking habit, and exercise. In stratified analysis by gender, no clear association was observed between long working hours and risk of metabolic syndrome in male group and in female group as well.
These findings are in accordance with those of a cross-sectional study of Japanese employees , showing a negative association between long working hours and the prevalence of diabetes. Another meta-analysis  also did not support the evidence that long working hours are directly associated with an increased risk of type 2 diabetes. A cross-sectional study on the association between daily working hours and metabolic syndrome in male Japanese workers reported significant outcome in subjects aged 40 or more, but, no significant association was observed between working hours and prevalence of metabolic syndrome when stratified by age . However, a longitudinal study conducted for male workers in local public institutions of Japan suggested that long working hours increase the onset risk of hypertriglycemia . In addition, a recent study conducted for Korean adults  demonstrated that long working hours are significantly related to risk of coronary heart disease. These discordant results among the studies are not clearly explained, but a plausible assumption might include the differences in the characteristics of each subjects and inconsistent categorization of working hours.
There are some possible explanations on the finding of our study. First, this study is based on the hypothesis that long working hours can be a potent negative health effect factor. But according to the socioeconomic status of subjects, for example, who are engaged in the specialized or professional job, long working hours can be voluntary and motivational or long working hours may not be hazardous factors of health. Therefore, there is suspicion that long working hours is not a unilateral negative health factor . Second, so-called healthy worker selection effect may exert influence on the result of this study. That is, it is possible that subjects who have health problems cannot perform long hour working and vice versa, subjects who can handle long working hours without health problems survive. Future studies on long working hours and health effect are required in order to take it into account. Third, there is a possibility that metabolic syndrome may be a weaker predictor for detecting the risk of cardiovascular diseases than other predictors such as type 2 diabetes. Fourth, in our analysis, adjustments of some potent factors that could affect the prevalence of metabolic syndrome are missing such as dietary status and socioeconomic status of subjects.
To the extent of our knowledge, this is the first study investigating the association between long working hours and metabolic syndrome in Korea. And the IDF definition to evaluate metabolic syndrome in this study would be an advantage. Several criteria of metabolic syndrome have been reported by various institutions, and it has been an increasing argument as to which criteria are more appropriate as an effective predictor of cardiovascular disease. In some studies, the IDF criteria showed a fair level of agreement with WHO criteria ,. Finally, this study includes population-based large scale data, which would be one of the strengths of our study.
However, this study is not without limitations. First, as the study is based on cross-sectional analysis, it is possible to make causal inferences from the findings of this study, even though it is unlikely that subjects who meet the criteria of metabolic syndrome tend to be work long hours. Second, this study did not distinguish between manual labor workers and white-collar workers. Many research studies have indicated the disparities in the prevalence of metabolic syndrome between manual workers and non-manual workers ,, but this consideration was not reflected in our study. Third, there is a possibility that whole life working duration would affect the results of this study, but data available in our study did not include items on history of working time of subjects. Fourth, the classification of working hour group is not based on strict evidence, and categorization of weekly working hours may cause loss of information. Fifth, because weekly working hours were assessed by self-reported questionnaire in this study, we cannot rule out informational error caused by misclassification.