Vitamin D epimers are associated with circulating haemoglobin levels independently of C-reactive protein.

Vitamin D deficiency has been shown to be associated with anaemia. Circulating 25(OH)D consists of both epimeric and nonepimeric forms. However, the relative roles of epimeric and nonepimeric vitamin D in regulating anaemia and haemoglobin levels remain unknown. Therefore, in this study, we examined the effect of vitamin D, including its epimers, on haemoglobin levels, independently of its effect on circulating high-sensitivity C-reactive protein (hsCRP). This was a cross-sectional study of 1655 subjects from a long-term follow-up cohort at the Electricity Generating Authority of Thailand. Venous blood sample were collected for determination of vitamin D [25(OH)D2, 25(OH)D3, 3'-epi-25(OH)D2, and 3'-epi-25(OH)D3], haemoglobin, and hsCRP levels. Data are presented as mean ± standard deviation. Age, sex, and body mass index (BMI) were significantly associated with circulating haemoglobin levels, while no association was found between total serum 25(OH)D and haemoglobin levels. However, when total 25(OH)D was separated into 3'-epimeric and non-3'-epimeric forms, 3'-epi-25(OH)D was significantly associated with haemoglobin levels, independently of age, sex, and BMI (P < 0.01). No association was found between non-3'-epi-25(OH)D and haemoglobin. When hsCRP was added to the model, the effect 3'-epi-25(OH)D on haemoglobin levels remained significant (P < 0.01). In conclusion, vitamin D epimers are associated with circulating haemoglobin levels, which supports the role of vitamin D in red blood cell and iron physiology.