If you’re talking vitamin D and expecting it to lower your risk of getting type 2 diabetes when you get older, it’s time to lower your expectations.

A new study, the largest of its kind, has found that taking 4,000 international units (IU) per day, which is on the upper limit of the recommended intake, may double the amount of vitamin D in the blood but it gives most people roughly the same chance of developing blood sugar problems as people who don’t take the vitamin.

After about 2.5 years, diabetes appeared at a rate of 9.4 per cent per year with vitamin D supplements and 10.7 per cent with placebo capsules, an insignificant difference. All the patients were already at high risk for developing type 2 diabetes and 80 per cent already

had adequate levels of the vitamin.

For the 5 per cent of the population “with very low levels of vitamin D, there appears to be a benefit, but we would urge caution and not have people overreact to that,” chief author Anastassios Pittas told Reuters Health in a telephone interview.

Even if the vitamin helped this group to lower their diabetes risk — and the numbers were too small to prove that — “these people would need to take vitamin D anyway so it doesn’t change the recommendations,” said Pittas, co-director of the Diabetes and Lipid Center at Tufts Medical Center in Boston.

Vitamin D also keeps bones healthy and doctors may recommend a supplement for other reasons as well. Many foods add the vitamin.

The study, reported online by The New England Journal of Medicine and at a meeting of the American Diabetes Association in

San Francisco, involved 2,423 volunteers who were at high risk for developing the adult-onset version of the disease, which can often be prevented with diet and exercise.