Vitamin D supplements may be less effective in overweight people, study finds

• Overweight people might metabolize vitamin D supplements more slowly, a new study suggests.
• The study found people with a BMI of over 25 had smaller increase in vitamin D after taking supplements for 2 years.

People who are overweight or obese may metabolize vitamin D supplements slower than people with lower weights, a new study suggests.

Researchers gave vitamin D supplements to around 16,000 participants and checked up two years later to see how much of the nutrient their bodies absorbed.

Overweight people — defined in the study as a body mass index (BMI) of over 25 — had smaller increases in vitamin D compared to their starting levels, while people with a lower BMI had absorbed more vitamin D through supplementation.

People with higher BMIs also had lower vitamin D levels at the start of the study, the researchers stated.

The study was part of the VITamin D and OmegA-3 TriaL (VITAL), an effort within Brigham and Women's Hospital in Boston to study whether vitamin D or omega-3 supplements reduce the risk for chronic diseases. It's the largest clinical trial of these two supplements, with more than 25,000 participants, according to Harvard Medical School, which is affiliated with Brigham and Women's.

Study author author and epidemiologist Deirdre K. Tobias said in a release that a previous analysis of VITAL data showed vitamin D supplementation helped reduce certain cancer risks, but only for people with BMIs under 25. Researchers then began to look at the relationship between BMI and vitamin D absorption more closely.

"This study sheds light on why we're seeing 30 to 40% reductions in cancer deaths, autoimmune diseases, and other outcomes with vitamin D supplementation among those with lower BMIs but minimal benefit in those with higher BMIs," JoAnn E. Manson, another study author, said in the release.

Researchers are still studying whether vitamin D supplements help your health

Most doctors and nutritionists say supplements should only be used if you have a diagnosed deficiency or other medical problem, Insider previously reported.

Vitamin D is an essential nutrient the body uses to keep bones strong and strengthen the immune system. Dietitians recommend getting the nutrient through foods like salmon, mushrooms, and eggs.

When taken as a supplement, some small studies have found evidence that vitamin D could help with depression and reduce cancer deaths over time, but large-scale trials have not found evidence the pills help your bones or prevent heart disease.

Plus, too much vitamin D can be a problem. Since vitamin D helps the body absorb calcium, too much of it can cause blood calcium levels to become too high. Too much blood calcium, a condition called hypercalcemia, can cause kidney stones and heart damage.

Insider's Catherine Schuster Bruce recently reported on a man who kept vomiting and lost 28 pounds in three months after taking 400 times the recommended amount of vitamin D.