Metabolic syndrome (MS) is a cluster of risk factors for type 2 diabetes mellitus (T2DM) and cardiovascular diseases (CVDs), which includes obesity, insulin resistance, hypertension and dyslipidemia. A recent large cohort study on school children revealed the significant association of higher MS risks during childhood with increased risks for CVDs in adulthood [1]. This implies not only that the origins of T2DM and CVDs begin in childhood and adolescence, but also that MS risks in childhood and adolescence adversely affect health conditions in later life. Given that the prevalence of MS in children and adolescents has been increasing at an alarming rate [2, 3], the worldwide epidemic of MS could further accelerate in the near future. To prevent such a public health crisis, it is critically important to understand MS risks in children and adolescents as well as adults. However, little is known about health status and MS risks of Asian young population, although those in European and American children and adolescents have been reported [1, 2, 4].

Our world-wide epidemiological study collaborating with WHO was designated Cardiovascular Diseases and Alimentary Comparison (CARDIAC) study, and has been carried out over the past 25 years in 61 populations in 25 countries to determine the relationship between their MS risks and dietary habits [5–8]. In each population, about 100 males and 100 females aged 48 to 56 years were randomly selected, with the number of participants being over 14,000 in all. The CARDIAC study yielded interesting findings. Namely, the populations with higher sodium (Na) excretion in 24-h urine, correlated with high salt intake, had increased risk for stroke compared with those with lower, whereas the populations with higher taurine excretion, correlated with high seafood intake, had decreased risk for coronary heart disease compared with those with lower [7, 9, 10]. Also, increased excretion of urinary potassium (K), an indicator for vegetables and fruits intakes, was reported to inversely correlate with body mass index (BMI) and blood pressure (BP) [11]. These findings indicate that dietary habit beneficially or adversely affect health status and MS risks. Thus, the characterization of dietary habits may provide a clue to properly control MS risks. Considering that the recording of accurate dietary history is difficult for participants, 24-h urine analysis would be a practical and reliable tool for the characterization of dietary habits. In this study, we conducted a health survey of 243 healthy school girls aged 13-18 years at Mukogawa High School in Nishinomiya, Japan, to obtain baseline data on the health status of Japanese adolescent females, and then investigated the potential relationship between their MS risks and dietary habits by 24-h urine analysis in combination with anthropometric measurements and blood chemical analysis.

Recent studies demonstrate that the prevalence of MS in children and adolescents living in Europe and North America has dramatically increased in parallel with that in adults [2, 3]. There is only limited information available on the prevalence of MS in the Asian young generation, including Japanese, who differs from European and American ones in the component and pathologic mechanisms of MS. These prompted us to survey health status of Japanese school girls and to investigate how their dietary habits affect MS risks.

Anthropometric and blood chemical analyses showed that half of JHS girls and one-quarter of SHS girls had at least one MS risk. The quite common MS risk among JHS and SHS girls was hGlc. These features may reflect excess consumption of sugar-sweetened soft drinks and snacks, decreased physical activity or both. With respect to BMI, the rate of overweight (10%) in SHS girls was found to be nearly equal to the rates of Hong Kong's (9.8%) and British (11.7%) girls, and much lower than those of American (24.0%) and Brazilian ones (15.2%) [13, 14], although it seems hard to directly compare their overweight rates because of some differences in age and study period. Given the rate of obesity in JHS and SHS groups were as small as 0% and 1%, respectively, overweight and obesity are likely to be minor risk factors in Japanese female adolescents compared with American and Brazilian ones with a high prevalence of obesity. Also, the substantial difference in the rates of hBP between JHS (22%) and SHS (4%) girls might be attributed to the application of different criterion for hBP established by the Japanese government. Alternatively, such a difference could be accounted for by a higher intake of salty foods in JHS group, because their daily intake of NaCl and Na/K ratio were larger than those of SHS.

Thinness during adolescence and young adulthood is associated with adverse health consequences in later life, such as osteoporosis and unfavorable pregnancy outcomes, which is regarded as another important health concern different from MS [15, 16]. Thus, we also investigated the prevalence of thinness and confirmed the non-negligible proportion in Japanese school girls. Other groups also demonstrated the increasing trend of thinness particularly in young Japanese women aged 15-29 years [17]. Because such a national trend is attributed mainly to an excess desire for slim body shape [18], nutritional education on risks of unhealthy weight loss as well as on benefits of healthy diets would serve to prevent the unfavorable epidemic.

Our group and other groups reveal that urinary excretions of particular minerals and food factors are associated with dietary habit. For instance, increased urinary excretions of Na, K, and taurine significantly correlate with intakes of salty foods, vegetables and fruits, and seafood, respectively [7, 8, 10, 19]. Thus, 24-h urine analysis, in combination with anthropometric and blood chemical analyses, enables us to link between MS risks and dietary habit. Indeed, a significant reduction in serum triglyceride level and increase in serum HDL level were observed in higher urinary K excreters, indicating that the higher group had a lower risk for dyslipidemia. Such an improvement on serum triglyceride level was shown in the higher urinary taurine excretion group. Intakes of vegetables and fruits rich in K, and seafood rich in taurine are likely useful for getting a better lipid profile of Japanese pubertal girls. Furthermore, we confirmed a decreased tendency in insulin level and HOMA-IR in higher urinary taurine excreters, although the differences did not reach statistical significance possibly due to a small studied population. It points out the possibility that taurine could prevent the future development of insulin resistance, a major cause of T2DM and CVDs.

It is widely accepted that the increased intakes of vegetables and fruits or seafood lead to improvement in anthropometric parameters, particularly BP, in adults [20, 21]. Hence, one might ask why a significant decrease in anthropometric parameters, such as BP and BMI, was not observed in higher K and taurine excretion groups. Although the reason remains unclear, it could be explained by a small studied population, high rates of non-obese and normotensive participants, or both. Further studies are needed to clarify the correlations between dietary habit and each MS risk in Japanese female adolescents.

We here demonstrate that hGlc is quite a common MS risk in Japanese JHS and SHS girls aged 13-18 years, whereas overweight and obesity are minor risk factors. These findings indicate that control of plasma glucose level rather than body weight is a crucial task in Japanese pubertal girls. We also confirm that dietary habit rich in K and taurine, namely adequate intakes of vegetables, fruits and seafood, can improve their lipid profile. Given that vegetables, fruits and seafood abundantly contain multifunctional dietary factors other than K and taurine, such as folate and n-3 fatty acids, nutritional education promoting healthy dietary habit would help to improve health status and to prevent the future development of MS in Japanese female adolescents.