Low Levels of Vitamin D Don't Put Older Women at Greater Risk for Type 2 Diabetes

But pregnant women with gestational diabetes are likely to have low levels of vitamin D, potentially leading to bone weakness in their babies

A recent report on vitamin D from the Institute of Medicine (IOM), an independent scientific body that advises the U.S. government, 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.

These findings are corroborated a new study ‒ published online by the journal Diabetes Care ‒ which found that low levels of vitamin D don't put older women at greater risk for type 2 diabetes. The findings may further temper the enthusiasm for vitamin D that has built up in recent years.

Many studies have linked Vitamin D deficiency to lower risks of everything from diabetes, to severe asthma, heart disease, certain cancers and depression. But 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.

That’s one reason why people should continue to focus on lowering their diabetes risk by looking at their overall lifestyle ‒ eating a balanced diet, getting regular exercise and maintaining a healthy weight. 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.

The new study 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. But the research 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.

Very few foods in nature contain vitamin D. The flesh of fatty fish (such as salmon, tuna, and mackerel) and fish liver oils are among the best sources. Small amounts of vitamin D are found in beef liver, cheese, and egg yolks. Vitamin D in these foods is primarily in the form of vitamin D3 and its metabolite 25(OH)D3. Some mushrooms provide vitamin D2 in variable amounts. Mushrooms with enhanced levels of vitamin D2 from being exposed to ultraviolet light under controlled conditions are also available.                                                        

Most people meet at least some of their vitamin D needs through exposure to sunlight. And fortified foods, especially dairy products, provide most of the vitamin D in the American diet. People who get those things are a lot different from people who don't and it's those factors that may account for the link between vitamin D and lower diabetes risk researchers thought they had found in older studies. Still, the 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. 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.

In the meanwhile, another research paper published in the latest issue of theMedical Journal of Australia recommends that all pregnant women should be tested for vitamin D deficiency and those found to be deficient should be treated. Pregnant women with gestational diabetes are likely to have low levels of vitamin D, potentially leading to bone weakness in their babies, the research discovered.

Studying 147 women who attended a gestational diabetes clinic between February 2007 and February 2008, excluding those with known pre-pregnancy glucose intolerance, the Australian researchers found that more than 40 percent of the women were found to have inadequate vitamin D levels. They found that while low levels were more common in women with darker skin, more than 25% of women in all groups were deficient.

"Vitamin D insufficiency has a well-established impact on bone density, neonatal vitamin D and calcium status, and childhood rickets (soft or weak bones). The 41 per cent prevalence of inadequate 25(OH)D (25-hydroxyvitamin D) levels in women with GDM (gestational diabetes mellitus) in our study is unacceptable and identifies vitamin D insufficiency as an issue of public health significance," the researchers noted while recommending that further research into the potential link between vitamin D status and gestational diabetes be conducted.

In an accompanying editorial in the journal, Professor Peter Ebeling from the University of Melbourne at Western Health suggested scheduling of lower-cost, higher-dose vitamin D supplements be altered so that more women could afford them. "Those pregnant and breastfeeding women that are most at risk of vitamin D deficiency are often the least likely to be able to afford supplements," Professor Ebeling said.