Vitamin D may not ward off Type 2 diabetes

Vitamin D supplementation is unlikely to improve glycaemic measures in people with prediabetes, show the findings of a randomised trial.

“On the other hand, no major adverse effects were seen”, write researcher Stina Sollid (The University of Tromsø, Norway) and co-workers.

For the trial, which is ongoing for 5 years, the team randomly assigned 511 people with impaired fasting glucose or impaired glucose tolerance to receive vitamin D3 20,000 IU/week or placebo.

Baseline 25-hydroxyvitamin D (25[OH]D) levels were similar, at 59.9 and 61.1 nmol/L in the vitamin D and placebo groups, respectively, and vitamin D supplementation had the expected effect, raising levels by 45.8 nmol/L compared with a 3.4 nmol/L rise in the placebo group.

Baseline fasting glucose levels were 6.12 and 6.08 mmol/L in the vitamin D and placebo groups, respectively. During follow-up, levels fell by 0.07 mmol/L in the vitamin D group and rose by 0.02 mmol/L in the placebo group, with the difference between the two being nonsignificant. Likewise, 24-hour fasting glucose levels were a respective 7.26 and 7.40 mmol/L and these rose by 0.71 and 0.62 mmol/L.

Sollid et al also investigated subgroups of patients with low vitamin D levels at baseline, defined as 25(OH)D levels of less than 40 nmol/L and less than 50 nmol/L. Their reasoning was that these people might derive the greatest benefit from vitamin supplementation, but again the intervention had no noticeable effect on glycaemic measures.

However, the team notes in Diabetes Care that the trial was designed as a 5-year intervention, saying that “because type 2 diabetes develops slowly and small effects on glucose metabolism might accumulate over time, a 1-year intervention might be too short to draw firm conclusions.”

Indeed, the association between genetic determinants of vitamin D and diabetes risk seen in Mendelian randomisation studies suggests that “lifelong endogenous vitamin D is possibly needed for the prevention of diabetes.”

The researchers stress that their findings do not signify that vitamin D is “without importance for glucose regulation and cardiovascular health”, but do indicate that “the conventional definition of vitamin D deficiency as serum 25(OH)D <50 nmol/L in this regard is not relevant.”

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