In this study, we demonstrated that elevated blood mercury levels were associated with risks for overweight in Korean children and adolescents after adjusting for possible potential confounders according to data from the KNHANES.
Several studies in adults have shown that BMI was associated with blood mercury levels; however, they showed inconsistent results [5, 6, 9]. Moreover, few studies have been performed on children and adolescents, and these studies also showed inconsistent results . One study showed that obesity is associated with blood mercury levels in children and adolescents, but this previous study had a limitation in regard to a lack of adjustment for seafood consumption, whereas our study performed this adjustment . Another study in children revealed that BMI was inversely related to blood mercury levels, suggesting differences in mercury metabolism related to BMI between adults and children . Our study revealed that blood mercury levels showed a significant positive association with BMI after adjusting for covariates, including seafood consumption. Additionally, participants in the highest quartile of blood mercury levels were at higher risk of overweight than those in the lowest quartile, and the blood mercury levels exhibited acceptable performance in predicting overweight in the ROC analysis. These results suggested that mercury exposure could result in an increased risk of overweight in Korean children and adolescents. Blood mercury levels could be related to age, pattern of seafood consumption, genetics involved in mercury clearance, and race . Our study could provide additional valuable information on blood mercury levels associated with Korean overweight children and adolescents.
The most commonly accepted methods of assessing mercury exposure are to test urine or blood, which usually measure the level of total mercury (elemental, inorganic and organic) . The blood mercury level reflects the burden of organic mercury that has accumulated in the body, whereas urinary mercury reflects exposure to inorganic mercury . For most people, an elevated blood mercury level is associated with eating fish and other seafood containing organic mercury. In a study of a general Korean population, seafood intake during the previous 3 days before blood sampling affected blood mercury levels in both males and females . In line with these results, our study showed that blood mercury levels were significantly higher in the highest seafood consumption quartile than in the lowest seafood consumption quartile in the previous 24 hours. The consumption of fish contaminated with a form of methylmercury is considered to be an important exposure pathway in children and adolescents.
In our study, blood mercury levels were significantly elevated in the overweight male group compared with the levels in the normal group after controlling for covariates, but not in the overweight female group. Possible explanations are that males have more visceral adipose tissue and less subcutaneous adipose tissue than females and that visceral adipose tissue contains higher toxin levels than subcutaneous adipose tissue [25-27]. Our study also showed higher waist circumferences in the overweight male group than in the overweight female group. However, no sex differences were demonstrated in the geometric mean value of blood mercury levels. This finding is consistent with the representative data of blood mercury levels in the Korean general population, which revealed sex differences in blood mercury levels in a population over 20 years old . In rodent studies, sex differences in tissue distribution and clearance of mercury have been demonstrated, and the findings suggest that these differences may be caused by androgens and estrogen [28, 29]. Our result can be explained by the fact that sex hormones are not as present in participants aged 10-18 years.
Some studies of adults have reported that other factors associated with higher blood mercury levels are household income, alcohol consumption, and smoking [19, 30]. In our study, blood mercury levels were not significantly associated with household income or the current status of alcohol consumption and smoking after adjustment for covariates. This inconsistency with previous reports could be related to the different definitions of the status of smoking and alcohol consumption, different populations and small numbers of current alcohol consumers and smokers in our study. Hct is also considered to be a covariate related to blood mercury level because up to 80% of methylmercury binds to red blood cells . Higher blood Hct levels are known to result in greater mercury binding in whole blood and thus a higher mercury concentration in the blood sample . Our results showed that the blood Hct levels in males were higher than those in females, which is related to the report that the differences in hematologic variables between sexes emerge after the onset of menstruation . After adjusting for these possible covariates related to blood mercury levels, our study suggested that blood mercury levels were a significant risk factor for overweight in Korean children and adolescents.
According to the KNHNES 2011-2013, the geometric mean blood mercury level in Korean adults was 3.37 𝜇g/L . These mercury levels are approximately three to four times higher than those of adults in the U.S. (0.86 𝜇g/L), Germany (0.58 𝜇g/L) or Canada (0.76 𝜇g/L) [33-35]. A previous study in a general population noted that as the participants got older, the blood mercury levels increased . Our study, conducted with children and adolescents aged 10-18 years, showed that the geometric means of the blood mercury levels were 2.19 𝜇g/L, 2.18 𝜇g/L, and 2.14 𝜇g/L in all participants, males and females, respectively, after adjusting for covariates. The mercury levels in our study were also much higher than the 0.8 𝜇g/L of the reference value in German children aged 3-14 years old and the 0.3 𝜇g/L of the median blood mercury level in American children and adolescents aged 1-17 years old [36, 37]. Accordingly, the blood mercury level in Koreans is greatly higher than that in individuals in Western countries, including children and adolescents. This difference seems to be related to the fact that seafood is a staple food in Korea and can be a primary exposure pathway of methylmercury . In our study, the mean intake of seafood was 54.18 g/day, which is higher than in Germany (37.5 g/day), Canada (24.7 g/day), and the U.S. (12.8 g/day) [39-41].
Many kinds of fish do not contain high levels of mercury . While balancing concerns for methylmercury exposure, eating a variety of different types of fish is a good way to gain good nutrition because fish contain high-quality protein and omega-3 fatty acids. According to the Environmental Protection Agency (EPA)-FDA advice in the U.S., women of childbearing age (approximately 16-49 years old), pregnant and breastfeeding women, and young children should eat more fish that are low in mercury for important developmental and health benefits . They recommend that those people eat two to three servings (8-12 ounces for adults and children over age 10) of a variety of fish and shellfish each week. Mercury has been widely recognized as a threat to children and adolescents’ health, and a global policy for seafood consumption is required to reduce and prevent mercury exposure.
This cross-sectional study has a limitation in that it is difficult to establish accurate cause-effect relationships between overweight and blood mercury levels. To our knowledge, no prospective studies have noted whether overweight/obesity develops or is aggravated after mercury exposure. To understand the underlying pathogenesis of this association between overweight and blood mercury levels, further studies are required. Additionally, since environmental conditions are changing, reference values are continuously checked and updated if new information becomes available. Recent national data for mercury exposure in the general population are required.
In conclusion, our study demonstrated an association of mercury exposure with the risk of overweight in Korean children and adolescents. Further studies are needed to investigate the mechanism by which high blood mercury levels are associated with overweight and to establish global guidelines for seafood consumption to reduce mercury exposure in children and adolescents.