Many studies that track healthy people for years have reached the same conclusion: “People with higher levels of vitamin D have a lower risk of developing type 2 diabetes in the future,” says Tufts Medical Center’s Anastassios Pittas, who is also a professor of medicine at Tufts University’s Sackler School of Graduate Biomedical Sciences.
But those studies can’t tell if something else about people with low vitamin D increases their risk. So researchers have looked further.
“We and others have done short term intervention studies looking at insulin sensitivity, insulin secretion, or similar outcomes,” says Pittas. “Some have not shown any difference, but others show promise.”
For example, Pittas and his colleagues gave 92 overweight or obese adults with prediabetes vitamin D (2,000 IU a day), calcium (400 mg twice a day), both, or a placebo for four months. Those taking vitamin D (either alone or with calcium) had better pancreatic beta cell function than those who didn’t take vitamin D.
“Beta cell function is one of the best measures we have for predicting the future risk of getting type 2 diabetes,” notes Pittas.
How might vitamin D work?
“There’s preliminary evidence that it may improve insulin secretion,” says Pittas.
“The enzyme that converts vitamin D to its active form is expressed in the pancreas’ beta cells, which means that the beta cell needs an adequate vitamin D supply to function. Only a few organs outside of the kidney have the ability to activate vitamin D so they don’t have to depend on the circulating levels of the active vitamin.”
Many trials have revolved around the question, “What is Vitamin D good for?”
Vitamin D and your skeleton
Bess Dawson-Hughes, director of the Bone Metabolism Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston, Massachusetts, and a professor of medicine at Tufts, tells us that vitamin D is essential for absorbing calcium. People with low vitamin D blood levels have more rapid bone loss and more fractures.
“Many have found that taking both vitamin D and calcium lowers the risk of fractures. It’s hard to tell if vitamin D alone would lower the risk, because trials that used higher doses of vitamin D also used calcium. You can’t tell if it’s the combination or the higher dose of vitamin D that did it,” says Dawson-Hughes.
Vitamin D lower the risk of falls
“We know that vitamin D lowers the risk of falling,” says Dawson-Hughes. “Falls are almost always the precipitating cause of hip fractures. So lowering the risk of falls is hugely important.”
Vitamin D helps build muscle. “We did one study on healthy but frail older women with low starting vitamin D levels. We gave them vitamin D or a placebo for four months. We saw that the women who got vitamin D had larger cross-sectional muscle area than the placebo takers, suggesting that vitamin D increased their muscle mass. We and others have also found vitamin D receptors in muscle biopsy studies. That also suggests that vitamin D matters for muscles,” says Dawson-Hughes.
How to get more vitamin D
Dawson-Hughes says, “There’s not much vitamin D in our diet. It’s in foods that people don’t tend to eat daily, like salmon.” It’s also in other fatty fish, and from fortified foods like milk, breakfast cereals, and some brands of yogurt, margarine, and orange juice.
“So supplements are pretty much the way to go,” she says. “And I would take them with food, not on an empty stomach. Also, if you take vitamin D with a meal that has some fat in it, your absorption is greater than if the meal is fat-free.”
What to do
Shoot for the Recommended Dietary Allowance for vitamin D (600 IU a day for adults up to age 70 and 800 IU a day for people over 70).
Sources: Am. J. Clin. Nutr. 94: 486, 2011.. doi:10. J. Clin. Endocrin. Metab. 98: E1927, 2013.