Higher-than-recommended vitamin D intakes do not significantly reduce the risk of developing diabetes, contrary to earlier hypotheses, say Finnish researchers.
Previous population-level studies have linked low levels of vitamin D in the body with a higher risk of type 2 diabetes – but these are observational, and evidence from randomised trials on the benefits of vitamin D supplementation is limited.
Experimental studies have demonstrated that consuming significantly higher-than-recommended doses of vitamin D may slightly reduce the risk of developing type 2 diabetes in individuals with prediabetes. Until now, there has been no research into the effects on individuals who do not have this condition.
However, findings from a randomised controlled trial published this month “do not suggest benefits of long-term moderate- or high-dose vitamin D3 supplementation” in otherwise healthy individuals.
“Among generally healthy older adults who are not at high risk for diabetes and who have serum 25(OH)D3 levels that are sufficient for bone health, vitamin D3 supplementation did not significantly reduce the risk of developing diabetes,” the study authors, from the University of Eastern Finland, wrote in the journal Diabetologia.
However, they added: “Our study results do not exclude the possibility that high-dose vitamin D supplementation could be beneficial among vitamin D-deficient populations with an average risk for type 2 diabetes.”
The Finnish Vitamin D Trial (FIND) investigated the effects of vitamin D3 supplementation at two different doses on the incidence of type 2 diabetes in a generally healthy older adult population.
Conducted at the University of Eastern Finland from 2012 to 2018, it included 2,495 men aged 60-plus and women aged 65-plus, who were randomised into three groups: one placebo group, one group supplemented with vitamin D3 at 40 µg/day (1,600 IU/day), and a third group supplemented with 80 µg/day (3,200 IU/day).
For reference, the European Food Safety Authority (EFSA) recommends 15 µg/day of vitamin D for adults, while the NHS recommends just 10 µg/day.
For the recent sub-study, 224 participants who were already using diabetes medications at the start of the study were excluded, while comprehensive information was collected from the remaining participants on their lifestyle, nutrition, diseases, and risk factors. About one-fifth were randomly selected for more detailed examinations, with blood samples taken from them.
Over the five-year study period, 105 participants developed type 2 diabetes: 38 in the placebo group, 31 in the group receiving vitamin D3 at 40 µg/day, and 36 in the group receiving 80 µg/day. There was no statistically significant difference in the number of cases between the groups.
In the more closely studied group of 505 individuals, participants’ blood calcidiol levels, which describe the body's vitamin D status, were 75 nmol/l on average at the start of the study; just nine percent of participants had a low level (that is, below 50 nmol/l). After one year, calcidiol levels were, on average, 100 nmol/l in the group receiving vitamin D3 at 40 µg/day, and 120 nmol/l in the group receiving 80 µg/day. There was no significant change in the placebo group.
No differences were observed between the groups with regards to blood glucose and insulin levels, body mass index, or waist circumference.
The same research group previously found that taking higher-than-recommended doses of vitamin D may reduce the risk of atrial fibrillation in older adults.
Individuals who were given 40 µg/day of vitamin D3 had a 27% lower risk of atrial fibrillation – a known risk factor for stroke and heart failure – than those who received a placebo, while those given 80 µg/day had a 32% lower risk, the study, published in the American Heart Journal, found.
However, commentators warned against reading too much into those findings, noting that the study “was not designed to test the effects of vitamin D on atrial fibrillation – this was an analysis of an existing dataset”.
Consultant nutritionist Dr Carrie Ruxton, from Nutrition Communications, said: “The researchers admitted they don’t know why vitamin D would protect the heart, so we definitely need to repeat this research in another group of adults and establish likely mechanisms of action before we encourage people to start taking vitamin D for their heart health.”
