NEW YORK – Low levels of vitamin D don't put older women at greater risk for type 2 diabetes, a large study of U.S. women suggests.
The findings may further temper the enthusiasm for vitamin D that built up in recent years, as studies linked it to lower risks of everything from diabetes, to severe asthma, heart disease, certain cancers and depression.
The problem with those studies is that they were observational, which means that researchers simply looked at people's vitamin D intake, or their blood levels of the vitamin, and whether or not they developed a given health condition.
Those types of studies cannot prove that vitamin D was the reason for any lower disease risk.
"You can't make dietary recommendations based on observational studies," said Dr. Jennifer G. Robinson, a professor of epidemiology and medicine at the University of Iowa and the lead researcher on the new study.
In an interview, she pointed to the recent report on vitamin D from the Institute of Medicine (IOM), an independent scientific body that advises the U.S. government. The report said the evidence didn't show that vitamin D has any health benefits beyond building and maintaining strong bones.
It also said average Americans already have vitamin D blood levels at or above the amount that's needed for good bone health.
The new study, published online by the journal Diabetes Care, involved women participating in the Women's Health Initiative, a large government project that looked at the health effects of hormone therapy, diet changes, and vitamin D and calcium supplements on women age 50 and older.
Of 5,140 women who were free of type 2 diabetes at the start of the trial, 6 percent developed the disorder over an average of 7 years.
Robinson's team found no clear link between the women's blood levels of vitamin D at the outset and their risk of developing type 2 diabetes later on.
Initially, there was some evidence of an association. But it disappeared when the researchers accounted for factors like body weight, exercise levels and certain diet habits, like fiber intake, which are key in the risk of developing type 2 diabetes.
"Look at how you get vitamin D," Robinson said. She noted that the main sources include sun exposure, fatty fish like salmon and mackerel, and vitamin D-fortified dairy products. "People who get those things are a lot different from people who don't."
And it's those factors, Robinson said, that may account for the link between vitamin D and lower diabetes risk researchers thought they had found in older studies.
Still, the current study, like past ones, was observational, so it doesn't disprove a role for vitamin D in diabetes risk. So-called randomized studies are what's needed to prove whether the vitamin curbs disease risks, Robinson pointed out.
In that type of trial, participants are randomly assigned to different treatment groups, which helps ensure the results are accurate and not just an effect of chance.
A large, government-sponsored randomized trial is currently underway. The study will involve 20,000 U.S. adults who are randomly assigned to take vitamin D, omega-3 fatty acids (together or alone) or placebo pills -pills that look exactly like the others, for comparison.
For now, Robinson suggests that people focus on lowering their diabetes risk by looking at their overall lifestyle, eating a balanced diet, getting regular exercise and maintaining a healthy weight.
She noted that exercising outside and eating foods rich in vitamin D will also ensure that you're getting enough of the vitamin.
The IOM recommends that adults in their 70s and up get 800 IU of vitamin D per day, while everyone else older than 12 months should get 600 IU.