Daily vitamin D supplements do not stop people with pre-diabetes from developing type 2 diabetes, a randomised controlled trial has found.

However, supplements might have a beneficial effect on pre-diabetic people who have insufficient insulin secretion, the study authors said, but further research was needed to clarify the results.

Writing in The BMJ, the researchers said observational studies had shown vitamin D deficiency was associated with insulin resistance and an increased risk of future diabetes.

Results of randomised controlled trials of vitamin D supplementation for preventing type 2 diabetes had been inconsistent, but one previous study had suggested supplementation was beneficial for vitamin D deficient people with pre-diabetes.

‘Although lifestyle modifications may delay the development of type 2 diabetes, maintaining long term behavioural change is difficult. Therefore, new strategies to reduce the incidence of type 2 diabetes are needed for people with impaired glucose tolerance,’ they wrote.

In the study, 1,256 Japanese adults with impaired glucose tolerance were randomised to receive a 0.75 microgram daily dose of eldecalcitol (an active vitamin D analogue) or a placebo and assessed for type 2 diabetes every three months over three years of follow-up.

The mean age of participants was 61.3 years, 571 were women (45.5%) and 742 (59.1%) had a family history of type 2 diabetes.

Over the follow-up period, 12.5% in the eldecalcitol group developed type 2 diabetes compared with 14% in the placebo group, but this was not a significant reduction, the researchers wrote.

Treatment with eldecalcitol also failed to significantly increase the rate of regression to normoglycaemia - where the concentration of sugar in the blood is normal - compared with placebo (23% in the eldecalcitol group and 20% in the placebo group).

However, after adjusting for potential confounders, including age, sex, blood pressure, BMI, and family history of diabetes, the results suggested that eldecalcitol might prevent type 2 diabetes in pre-diabetic patients with insufficient insulin secretion.

Treatment with eldecalcitol was effective in increasing bone mineral densities and serum osteocalcin concentrations, the study authors noted, and there was no significant difference in serious adverse events between groups.

In an attached editorial, Tatiana Christides, senior lecturer in medical science at Barts and the London School of Medicine and Dentistry, Queen Mary University of London, said the trial was well conducted, with rigorously defined and tested diagnostic criteria, and of sufficient duration, but it might have been underpowered to detect a small effect of supplementation on type 2 diabetes.

Future studies should address whether vitamin D supplementation might be more effective for particular groups, such as people of colour or those with severe vitamin D deficiency (≤25 mmol/L 25-hydroxyvitamin D), and whether longer duration of treatment or younger age at initiation might be more beneficial.

‘Until further data are available from high quality randomised trials, healthcare professionals should continue to discuss with patients the musculoskeletal health benefits of vitamin D and support them to achieve and maintain lifestyle changes that, although challenging to sustain, are known to decrease development of type 2 diabetes,’ she concluded.