What can—or can’t—vitamin D do?

Results are in from three recent studies testing vitamin D at or above the Recommended Dietary Allowance (RDA)—600 IU a day up to age 70 and 800 IU over 70.

Insulin sensitivity

Researchers randomly assigned 18 people with obesity to a low-calorie diet with either a placebo or a weekly dose of vitamin D equal to 3,570 IU a day. Most had low blood vitamin D levels to start with.

After three months, insulin sensitivity improved only in the vitamin D takers.1 (That is, their insulin worked more efficiently.)

In one small study, insulin sensitivity improved in dieters who took a weekly dose of vitamin D equal to 3,570 IU a day.

Artery stiffness

Researchers randomly assigned 70 overweight or obese African-Americans aged 13 to 45 with low blood vitamin D levels to take either a placebo or monthly doses of vitamin D equal to 600 IU, 2,000 IU, or 4,000 IU a day.

After four months, artery stiffness decreased in the groups that got 2,000 IU or 4,000 IU.2

Diabetes control

Researchers randomly assigned 127 people aged 25 to 75 (average age: 60) with stable type 2 diabetes to take vitamin D (4,000 IU a day) or a placebo. Only 26 percent had low blood vitamin D levels.

After 11 months, insulin secretion, insulin sensitivity, and hemoglobin A1C (a long-term measure of blood sugar) didn’t improve more in the vitamin D takers.3

What to do

It’s too early to tell what high doses of vitamin D can do. But since it’s hard to get enough from food or, for most people, from the sun, take a multivitamin or a supplement with the RDA to play it safe.

You may need more if your vitamin D blood level is low (under 20 ng/mL). Just keep in mind that some studies find a higher risk of falls at 2,000 IU a day. For more information, read our interview with Tufts University bone health expert Bess Dawson-Hughes.


1 Obesity 26: 651, 2018.
2 PLoS ONE 12: e0188424, 2017.
3 J. Endocr. Soc. 2: 310, 2018.

The information in this post first appeared in Nutrition Action Healthletter in May 2018.

Photo: BillionPhotos.com/stock.adobe.com.

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