Vitamin D intake cannot represent the extent of vitamin D deficiency or insufficiency: Comment on the article by Hiraki et al

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Vitamin D intake cannot represent the extent of vitamin D deficiency or insufficiency: Comment on the article by Hiraki et al

To the Editor:

We read with great interest the article by Hiraki et al published recently in Arthritis Care & Research (1). The authors reported some interesting findings regarding vitamin D intake and the incidence of systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA) in a large population. However, we believe that some concerns about the conclusions made by the authors should be mentioned.

Recently, several interesting articles have been published examining the associations between vitamin D intake and SLE and RA (2–8); however, vitamin D intake cannot represent the extent of vitamin D deficiency or insufficiency in the human body, which is mostly defined by the measurement of serum vitamin D (25[OH]D level <20–30 ng/ml) (2–5, 7–9). As we know, serum vitamin D deficiency is related to dietary intake and sun avoidance, and 20% of vitamin D is obtained from food (eggs, fish, animal livers, and dairy products), while 80% is obtained from skin synthesis as a product of skin 7-dehydrocholesterol transformations induced by ultraviolet radiation (10, 11). Therefore, vitamin D intake might have been unreliable for determining the associations between vitamin D status and incidence of SLE and RA, even though the authors adjusted for sun sensitivity and sunscreen use. It is obvious that sun exposure should be taken into consideration as a covariant, and also the serum vitamin D level should be measured for subgroup studies, as several previous studies reported (7, 8, 10). We argue that many readers would want to know about these confounding factors that were well discussed in the article.

Acknowledgements

Authors Liao, Liu, and Feng contributed equally to this work.

Mingzhi Liao PhD*, Guiyou Liu MSc†, Rennan Feng PhD‡, Liangcai Zhang MSc‡, * Northwest A&F University, Shaanxi, China, † Tianjin Institute of Industrial Biotechnology, Chinese Academy of Sciences, Tianjin, China, ‡ Harbin Medical University, Harbin, China.

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