We read with interest the paper by Costenbader et al on vitamin D intake and risks of systemic lupus erythaematosus (SLE) and rheumatoid arthritis (RA) in women.1

The authors conclude that their prospective study does not lend evidence that to the hypothesis that increasing vitamin D intake can protect against SLE or RA.

In this case, using semiquantitative questionnaires regarding the intake of food and the frequency of supplements to assess vitamin D status may not necessarily be a reliable way to indicate the actual absorption and serum levels. This is due to the fact that several other factors, including altered metabolic pathways and others, regulate the real availability. Therefore, the most reliable way to assess vitamin D …