Obesity and hypertension are increasing problems worldwide, resulting in an enormous economic burden to the society. The metabolic syndrome is a combination of medical abnormalities, including central (intra-abdominal) obesity, dyslipidemia, hyperglycemia and hypertension. It is estimated that around 25% of the world's adult population have the metabolic syndrome. The ultimate aim of metabolic syndrome clustering is to identify individuals who are at a high risk of developing cardiovascular disease and type-2 diabetes.1 This clustering of metabolic abnormalities that occur in the same individual seems to present a considerably higher cardiovascular risk compared with the sum of the risks associated with each abnormality. Although having lower prevalence than in the United States, overweight and metabolic syndrome have become increasing problems in many Asian countries, which might be attributed to economic development and the consequential changes in lifestyle and diet.2

Although the metabolic syndrome has existed in various forms and definitions for several decades, only in the past 5 years has real controversy about its definition and significance emerged.3 There are currently two major definitions for the metabolic syndrome, provided by the International Diabetes Federation (IDF)4 and the revised National Cholesterol Education Program (NCEP)5 (Table 1). The clinical diagnosis is based on clustering of simple measurements of waist circumference (WC), circulating levels of triglyceride (TG) and high-density lipoprotein-cholesterol (HDL-C), and measurements of fasting glucose and blood pressure.

Table 1 Diagnostic criteria for metabolic syndrome as proposed by different organizations
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The NCEP uses only one definition for central obesity, whereas the IDF uses geography-specific cut points for WC. Moreover, according to the IDF, if BMI is greater than 30 kg m−2, central obesity can be assumed, and WC does not need to be measured. For the definition and diagnosis of the metabolic syndrome in Japan, a somewhat different standard has been proposed by the Japanese Society of Internal Medicine (JSIM).6

In the present issue of Hypertension Research, Kamezaki et al. have performed an interesting study, in which they investigated seasonal variation in prevalence of the metabolic syndrome in 1202 male Japanese workers (age 44±10 years). The concept of seasonal variation in metabolic parameters has been demonstrated in several studies, for example, by way of showing summer-to-winter differences in blood pressure7 and serum cholesterol levels.8 However, little is known about how the seasonal changes may affect the prevalence of the syndrome. In the present study, the winter season was associated with elevated levels of HDL-C (+3 mg per 100 ml), blood pressure (systolic: +4 mm Hg, diastolic: +8 mm Hg) and fasting glucose (+2 mg per 100 ml), compared with the corresponding values during summer. The WC and TG values were also somewhat higher during winter, but these changes were not found to be significantly different. Interestingly, seasonal changes markedly affected the prevalence of the metabolic syndrome (Table 2). As pointed out by the authors, seasonal variations in metabolic parameters should be taken into consideration during annual health checkups, as they may affect the clinical diagnosis and management. Furthermore, the frequency of the metabolic syndrome could be influenced by age, and the authors clearly demonstrated that the prevalence and seasonal changes were higher in subjects aged ⩾40 years (Table 2).

Table 2 Seasonal variation in metabolic syndrome prevalence in male Japanese workers
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The prevalence was also dependent on the definitions used. As shown in Table 2, there is seasonal variation in the prevalence of metabolic syndrome regardless of whether the guidelines were given by NCEP, IDF or JSIM. However, the different criteria used for diagnosing the metabolic syndrome highly affect the prevalence. The definitions provided by IDF and JSIM are fairly similar, whereas NCEP defines central obesity as WC >102 mm for males, which most likely explains the large difference. Emerging evidence suggests that lowering the threshold for defining central obesity in Asian populations increases the prevalence of the metabolic syndrome. Consequently, this will capture more patients who are at a higher risk for developing cardiovascular disease and/or type-2 diabetes.9

There are some limitations in the study conducted by Kamezaki et al. For example, the study period was limited to 1 year, only male subjects were included, and seasonal changes in diets, physical activity or psychotropic medications were not investigated. However, several lines of evidence now show that there is seasonal variation in the parameters used for diagnosing metabolic syndrome, but the mechanisms are not clear.

There are several factors changing during the year that may influence the parameters used for diagnosing the metabolic syndrome. Some of them can be modified ourselves (that is, exercise and dietary properties), whereas factors associated with climate conditions are difficult to change (that is, outdoor temperature and light exposure; Figure 1). As obesity or abdominal fat is suggested to be an important risk factor for developing cardiovascular disease, and is one of the required criteria for metabolic syndrome according to the IDF and JSIM, increased daily exercise is highly recommended for individuals who are at risk. One hypothesis is that the metabolic syndrome is an outgrowth of insulin resistance and mild-to-moderate weight reduction can provide a strategy for management as it may improve glucose metabolism.3

Figure 1
figure 1

Schematic illustration of the link between metabolic syndrome and increased cardiovascular morbidity and mortality. Central obesity, dyslipidemia, hyperglycemia and hypertension are all risk factors for the metabolic syndrome. The prevalence of the syndrome has been demonstrated to be linked to seasonal variations, but male or female gender, increasing age and genetic predisposition are also important factors.

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When it comes to seasonal variations in dietary properties, the amount of food (or caloric intake) is not necessarily changed, but the composition of food may be different during winter and summer. Clinical trials and experimental studies have shown that diet rich in vegetables and with a low sodium content can lower blood pressure, and therefore reduce the risk of cardiovascular disease.10 The mechanisms for this are not clear, but reduced oxidative stress and increased nitric oxide availability have been suggested.11 The effect of dietary changes on the seasonal variations in metabolic parameters, observed by Kamezaki et al., was not investigated and future studies are warranted.

During winter, a lower outdoor temperature has been demonstrated to increase blood pressure. The mechanisms for this are not yet clear, but changes in sympathetic tone and plasma volume have been suggested.12 In addition, daily light exposure is markedly reduced during winter, which may affect important physiological parameters such as vitamin D levels, mood and behavior, and in turn appetite and weight. Increasing evidence suggest that vitamin D deficiency is strongly associated with overweight, abdominal obesity and a higher cardiovascular risk.3 Furthermore, seasonal variations in mood and behavior, and inevitably appetite and weight, have been linked to changes in the metabolic syndrome.13

Other factors that are not related to seasonal changes, but are still important for the prevalence of metabolic syndrome are gender, age and genetic variations. In the present study by Kamezaki et al., females were not investigated, but studies have shown that the prevalence of the metabolic syndrome is similar between men and women in the United States, whereas the InterASIA trial demonstrated much higher prevalence in women.2 This difference needs further investigation, but might be explained by different cutoff values when defining central obesity. Increasing age is often associated with aggregation of abdominal obesity, impaired glucose metabolism, dyslipidemia and/or hypertension, and is therefore coupled with a higher frequency of metabolic syndrome. This link was confirmed by Kamezaki et al.; however, whether the seasonal variations in metabolic parameters are associated with an increased risk of cardiovascular events remains to be investigated. Finally, genetic predisposition also relates to the metabolic syndrome. Studies have suggested that gene variants related to the circadian clock (NPAS2 and PER2),14 HDL-receptor regulation (PDZK1)15 or impaired cellular adipose tissue regulation1 are linked to the risk factors of the metabolic syndrome.

Taken together, the metabolic syndrome is associated with seasonal changes, and is driving the global epidemics of cardiovascular disease and type-2 diabetes. The mechanisms and clinical role for the seasonal variations in metabolic risk factors remain to be further elucidated. Early identification of individuals who are at risk is important, so that lifestyle interventions and medical treatment that can reduce or prevent the disease in later life may be implemented.