Serum 25OHD concentration as a predictor of haemoglobin A1c among adults living in the USA: NHANES 2003 to 2010

BACKGROUND: Vitamin D status influences glucose metabolism. Serum 25-hydroxyvitamin D (25OHD) concentrations have been inversely associated with type 2 diabetes risk. The optimal serum 25OHD level needed for adequate glycaemic control is unknown. OBJECTIVE: To determine the relationship among serum 25OHD concentrations and degree of glucose regulation using percentage of haemoglobin A1c (HbA1c%). METHODS: Data for adults [Formula: see text] 20 years from the National Health and Nutrition Examination Survey (NHANES) (2003–2010) were included. A binary logistic regression was used for serum 25OHD (nmol/L) as a continuous variable to determine the OR and 95% CI for HbA1c >6.5%, adjusting for sex, race and body mass index (BMI). Measures of serum 25OHD were grouped into quartiles and entered into a binary logistic regression model to determine the OR and 95% CI for HbA1c >6.5% in an adjusted model. RESULTS: Across all NHANES cycles, lower serum 25OHD was associated with greater odds of HbA1c [Formula: see text] 6.5% when adjusting for sex, race, age and BMI (NHANES 2003–2004 (N=4402): OR 0.985, 95% CI 0.979 to 0.990; NHANES 2005–2006 (N=4409): OR 0.976, 95% CI 0.969 to 0.982; NHANES 2007–2008 (N=4525): OR 0.989, 95% CI 0.984 to 0.993; and NHANES 2009–2010 (N=5660): OR 0.988, 95% CI 0.984 to 0.991). In an adjusted model, the lowest quartile of serum 25OHD (0–41 nmol/L, N=4879) was associated with greater odds of HbA1c [Formula: see text] 6.5% compared with the highest quartile (73–260 nmol/L, N=4472), OR 2.37, 95% CI 2.03 to 2.77. The odds of HbA1c [Formula: see text] 6.5% were also greater for adults with serum 25OHD considered to be sufficient compared with the highest quartile, OR 1.68, 95% CI 1.56 to 1.61). CONCLUSION: Lower serum 25OHD concentrations are associated with poor glycaemic control (HbA1c [Formula: see text] 6.5%). Sufficient serum 25OHD levels were also associated with poorer blood glucose control. Further research is needed to investigate an optimal serum concentration or threshold to support adequate blood glucose control.