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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.

  • 1
    Hiraki LT, Munger KL, Costenbader KH, Karlson EW. Dietary intake of vitamin D during adolescence and risk of adult-onset systemic lupus erythematosus and rheumatoid arthritis. Arthritis Care Res (Hoboken) 2012; 64: 182936.
  • 2
    Bultink IE. Osteoporosis and fractures in systemic lupus erythematosus. Arthritis Care Res (Hoboken) 2012; 64: 28.
  • 3
    Touma Z, Eder L, Zisman D, Feld J, Chandran V, Rosen CF, et al. Seasonal variation in vitamin D levels in psoriatic arthritis patients from different latitudes and its association with clinical outcomes. Arthritis Care Res (Hoboken) 2011; 63: 14407.
  • 4
    Mok CC, Birmingham DJ, Leung HW, Hebert LA, Song H, Rovin BH. Vitamin D levels in Chinese patients with systemic lupus erythematosus: relationship with disease activity, vascular risk factors and atherosclerosis. Rheumatology (Oxford) 2012; 51: 64452.
  • 5
    Lee YH, Bae SC, Choi SJ, Ji JD, Song GG. Associations between vitamin D receptor polymorphisms and susceptibility to rheumatoid arthritis and systemic lupus erythematosus: a meta-analysis. Mol Biol Rep 2011; 38: 364351.
  • 6
    Cutolo M, Pizzorni C, Sulli A. Vitamin D endocrine system involvement in autoimmune rheumatic diseases. Autoimmun Rev 2011; 11: 847.
  • 7
    Mok CC, Birmingham DJ, Ho LY, Hebert LA, Song H, Rovin BH. Vitamin D deficiency as marker for disease activity and damage in systemic lupus erythematosus: a comparison with anti-dsDNA and anti-C1q. Lupus 2012; 21: 3642.
  • 8
    Stockton KA, Kandiah DA, Paratz JD, Bennell KL. Fatigue, muscle strength and vitamin D status in women with systemic lupus erythematosus compared with healthy controls. Lupus 2012; 21: 2718.
  • 9
    Ruiz-Irastorza G, Gordo S, Olivares N, Egurbide MV, Aguirre C. Changes in vitamin D levels in patients with systemic lupus erythematosus: effects on fatigue, disease activity, and damage. Arthritis Care Res (Hoboken) 2010; 62: 11605.
  • 10
    Marcinowska-Suchowierska E, Walicka M, Talalaj M, Horst-Sikorska W, Ignaszak-Szczepaniak M, Sewerynek E. Vitamin D supplementation in adults: guidelines. Endokrynol Pol 2010; 61: 7239.
  • 11
    Schmajuk G, Yelin E, Chakravarty E, Nelson LM, Panopolis P, Yazdany J. Osteoporosis screening, prevention, and treatment in systemic lupus erythematosus: application of the systemic lupus erythematosus quality indicators. Arthritis Care Res (Hoboken) 2010; 62: 9931001.

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.