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imental studies suggest that cobalt may reduce the risk of obesity.Although many previous studies ignored the bioavailability of trace elements and their effects according to the subjects’ gender, the present study examined UCo levels and stratified the effects by the participants’ gender. There was no significant difference in the UCo levels between boys and girls in the present study. However, previous studies have reported higher levels of UCo in women than in men23-25). This difference may be induced by the greater iron demand in women. Cobalt and iron may share a common intes-tinal uptake mechanism39); thus, iron deficiency (a common problem in young women) increases cobalt absorption and urinary excretion in animals40) and humans41). Since the participants of the present study were in early adolescence (56 % of the girls did not experience menarche), the difference in UCo between girls and boys could not be detected at their ages.Pearson’s correlation analysis revealed an inverse Log urinary concentrationaLiNaMgKCaVCrCoNiCuZnAsSeSrMoCdBaTla Adjusted for age, birthweight, average sleep duration, father’s BMI, mother’s BMI, parental smoking, annual household income, father’s education, and mother’s education, by multiple linear regression analysis using the forced-entry methodb Standardized partial regression coefficientTable 4　Relationships of 18 trace elements to BMIBoysp-value0.029 0.093 0.020 0.061 0.501 0.348 0.495 <0.0010.019 0.079 0.079 0.126 0.501 0.227 0.133 0.649 0.170 0.453 Betab−0.08 −0.06 −0.08 −0.07 −0.02 −0.03 −0.02 −0.14 −0.08 −0.06 −0.06 −0.05 0.02 −0.04 −0.05 −0.02 −0.05 −0.03 correlation between log UCo and BMI in both boys and girls. When we adjusted for confounding factors, in the multiple linear regression analysis, a significant correlation was demonstrated only in the boys. These gender-related findings suggest that UCo is a protective factor against childhood obesity, predominantly in boys. The molecular or biochemical mechanisms underlying the reduction of BMI by cobalt have not been clearly understood. Tascilar (2011) found a correlation between plasma cobalt and the insulin resistance index (HOMA-IR), which suggests that cobalt acts as a regulator of glycogen depot by suppressing glucagon signaling and its effect on body weight12). In rats, cobalt administration was reported to result in decreased blood glucose levels, regulated glucose tolerance, and reduced body weight38). In addition, cobalt can decrease obesity risk by altering lipid metabolism, such as increasing leptin; the magni-tude of the effect varies according to gender. For instance, leptin levels in women are higher after the onset of puberty42, 43). Similarly, cobalt increases Girlsp-value0.328 0.910 0.138 0.848 0.046 0.418 0.525 0.106 0.095 0.126 0.051 0.926 0.233 0.536 0.018 0.208 0.785 0.900 Betab−0.04 −0.00 −0.06 0.01 0.08 −0.03 −0.03 −0.06 −0.07 −0.06 −0.08 0.00 0.05 0.02 −0.09 −0.05 −0.01 0.01 257