The use of dietary supplements, and the association between supplemental vitamin D and glycaemic control in adult individuals with type 1 diabetes.

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To assess the dietary supplement use in adult individuals with type 1 diabetes, and to study the association between vitamin D supplementation and glycaemic control in an observational cross-sectional study. The study subjects were participants of the Finnish Diabetic Nephropathy Study. Data were included from all individuals with type 1 diabetes with estimated glomerular filtration rate ≥60 mL/min/1.73 m2, who had completed a diet questionnaire. In the questionnaire, the participants reported dietary supplement use for the past 30 days. A thorough investigation with an assessment of the blood panel was conducted at the study visit. Data were available from 1181 individuals (43% men, mean ± SD age 45 ± 13 years). Altogether 62% of the sample reported supplement use; 56% reported some vitamin or mineral and 27% reported non-vitamin and non-mineral supplement use. Supplement use was more frequent among women and those supplementing had better overall health. In the study sample, of the vitamins and minerals, vitamin D (45%) and magnesium (31%), respectively, were the most frequently reported. In the multivariable models, vitamin D supplementation was associated with better glycaemic control. Starting from a daily dose of ≥30 μg, there was evidence of improving glycaemic control with higher doses of supplemental vitamin D (e.g., for 30 μg: B [Wald Confidence Internal], p-value, -2.76 [-5.03 to -0.49], 0.017). Supplement use was frequent in this sample of adult individuals with type 1 diabetes. Due to potential drug-supplement interactions, the attending physicians should be aware of their patients' supplement use. The causality between vitamin D supplementation and glycaemic control should be assessed in a randomized controlled trial.