We thank Liao et al for their interest in our study and their comments. Our study aimed to investigate the association between adolescent vitamin D dietary intake and adult onset RA and SLE utilizing retrospective high school food frequency questionnaires to calculate vitamin D intake during adolescence.
Liao and colleagues emphasize an important point that we have also made in our article, that dietary sources of vitamin D account for only a small proportion (∼20%) of circulating vitamin D. Other sources, particularly skin exposure to ultraviolet B radiation, account for a larger proportion of circulating vitamin D (1). The food frequency questionnaire used to calculate vitamin D intake from food and supplements cannot be extrapolated to estimate effective circulating 25(OH)D levels; rather, it provides a relative ranking of individuals in regard to their dietary intake of vitamin D. One of our more striking observations was that the vast majority of nurses (85–90%) reported vitamin D intake well below the currently recommended 600 IU per day (2).
The studies of vitamin D levels in individuals with SLE cited by Liao et al reported associations after SLE onset (3–6); however, for disease etiology studies, it would be more important to know the vitamin D levels prior to disease onset. In our study, since circulating 25(OH)D levels were not measured during adolescence, we could not address the prospective association between vitamin D levels and SLE; our conclusions were based on the observed lack of an association between reported adolescent dietary vitamin D intake and adult-onset RA and SLE.
As mentioned by Liao et al, we did include all of the available indices associated with sun exposure in the Nurses' Health Study cohorts as possible confounders in our analyses, including adolescent sunscreen use and sun sensitivity, latitude of residence at birth and at age 15 years, and physical activity during adolescence. We observed that these factors did not vary across quintiles of vitamin D intake (Tables 1 and 2 of the article), suggesting that these factors are not confounders of the association between dietary vitamin D intake and RA/SLE.
We hope our study invites further investigation into the role of vitamin D in rheumatic disease.