High-dose vitamin D may prevent pre-diabetes becoming type 2 diabetes

An analysis of three trials found that taking a high dose of vitamin D prevents high blood sugar levels from progressing into type 2 diabetes. But this dose has been linked with kidney stones and other health issues

By Clare Wilson

Regularly taking a high dose of vitamin D may slightly lower the rate at which people with high blood sugar levels progress to having type 2 diabetes, an analysis of existing trials has found. But high-dose vitamin D can also lead to kidney stones and other health problems, so people should only take such a course after discussing it with their doctor, researchers have said.

In type 2 diabetes, which usually starts in midlife and is linked with being overweight, people’s cells become less responsive to insulin, a hormone that regulates blood sugar levels.

Some people have “pre-diabetes”, which means their blood sugar levels are higher than what would be considered normal, but aren’t high enough to be classed as diabetes. Those affected usually progress to having type 2 diabetes, unless they lose weight.

Vitamin D encourages cells in the pancreas to make more insulin, so it has been investigated to see if high doses can help prevent type 2 diabetes. But three randomised trials of people with pre-diabetes who took vitamin D or an artificial form of it, either once a week or every day depending on the trial, found only small effects on their rate of progression to type 2 diabetes. Statistical tests found these effects could have arisen by chance.

Now, Anastassios Pittas at Tufts Medical Center in Boston and his colleagues have combined the results of the three trials, a standard technique in medical research. This suggests that taking a high dose of vitamin D leads to a 15 per cent lower rate of pre-diabetes progressing to type 2 diabetes. The larger number of people in the combined analysis – more than 4000 participants – meant that this result was classed as statistically significant and not due to chance.

But the amount of vitamin D used in the trials – equivalent to about 70 micrograms a day – was much higher than the 10 micrograms a day recommended by dietary guidelines in the UK and the 15 micrograms recommended by those in the US.

Higher levels of vitamin D boost calcium absorption from the gut, which can lead to kidney stones and kidney damage due to dehydration. Such side effects weren’t seen in these three studies at a significantly higher rate than in the placebo groups, but the trials may have been too small to show up these relatively rare adverse events, says Malachi Mc Kenna at University College Dublin, Ireland, who wrote an editorial accompanying the analysis.

Pittas says high-dose vitamin D may need to be considered as a treatment that is overseen by a doctor, rather than a supplement that someone buys and takes at home. “All medical interventions carry some risk,” he says.