Sex differences in iron stores of adolescents: what is normal?

We evaluated iron status and its determinants in healthy adolescents. Fasting morning blood samples from a school-based cross-sectional study were analyzed for serum ferritin (SF), serum iron, total iron-binding capacity, and circulating transferrin receptors. Physical development, chronic disease, medication, dietary intake, and physical activity were assessed using clinical examination, questionnaires, and 7-day records. The risk of having low serum ferritin values was estimated using bivariate and multivariate regression. Subjects were 867 healthy Swedish adolescents, 14- and 17-year-olds (472 boys and 395 girls). SF values increased with pubertal stage in boys but not in girls. Five percent of the boys and 15% of the girls had SF values < 12 micrograms/L. Of the 17-year-old boys, 7% compared to 1% of the 17-year-old girls had SF values > 100 micrograms/L. Forty-one percent of cases with SF values > 12 micrograms/L had serum iron values < 15 microM, and 22% had transferrin saturation values < 16%. Mean total iron intakes of the boys were high [1.6 times recommended daily allowance (RDA)] and mean intakes of the girls were adequate (0.9 times RDA). Low heme iron intakes increased the risk of low iron stores (< 12 micrograms/L) in girls but not in boys. Total iron intake or other dietary factors, physical development, or level of physical activity did not influence the risk of low SF. The findings of this study suggest that the differences in iron status between boys and girls in adolescence results primarily from biological differences other than menstrual bleeding or insufficient iron intake. Furthermore, the results question the role of SF as an indicator of iron deficiency in adolescence, in particular if age and sex are not taken into consideration. We suggest that different reference values for SF, including the cut-off limit for low SF, adjusted for age and sex, should be considered. The high iron intakes and corresponding high SF values found in the older boys are noticeable in light of the possible negative health consequences of iron overload.