Letters to the Editor Heritability of serum vitamin D concentrations: twin

studies Dear Sir: It was with interest that we read the article by Karohl et al (1), which uses a classical twin study design. Serum 25-hydroxyvitamin D [25(OH)D] concentrations were examined in American adult twins to estimate the genetic contribution in determining circulating 25(OH)D concentrations. The difference in correlation of a given phenotype within monozygotic compared with dizygotic twin pairs, or the intraclass correlation (ICC), provides insight into the relative contributions of genetic factors to the phenotype (2). Structural equation modeling and other methods are used to estimate herita-bility or the proportion of phenotypic variation that is attributable to genetics (2). Elucidating the genetic components involved in regulat-ing 25(OH)D status is a valuable pursuit given the known associations of suboptimal vitamin D status and a wide range of diseases