High BMI May Reduce the Benefits of Vitamin D Supplements

Vitamin D metabolism is different in people with a BMI greater than 25, which could diminish the effects of vitamin D supplementation, suggests new research from Brigham and Women’s Hospital. The study is published in JAMA Network Open.

The importance of vitamin D

Vitamin D is a fat-soluble hormone with roles such as maintaining bone health, reducing inflammation and regulating pathways involved in cell growth, neuromuscular and immune functions.

Sometimes called the “sunshine vitamin”, vitamin D is synthesized when our skin is exposed to UV light. It’s also found in a few foods, including oily fish and mushrooms. Most people don’t get the required amount of vitamin D in their diet, so many choose to take supplements.

The VITAL study

The VITAL trial was a nationwide, double-blind, placebo-controlled clinical trial that commenced in 2010 and analyzed the impact of taking an omega-3 or vitamin D supplement, particularly on the risk of developing cancer, heart disease or stroke. Participants were men over the age of 50 and women over the age of 55.

Initial analysis of results from the VITAL trial found that vitamin D supplementation had little effect on the incidence of these health conditions in the overall cohort. However, in people with a BMI under 25, vitamin D supplementation was associated with improved health outcomes with respect to cancer incidence and mortality risk.

In the new study, the researchers took a closer look at this correlation by analyzing blood samples taken from the VITAL trial and during a two-year follow up. They assessed an expanded set of vitamin D biomarkers, including its metabolites and compounds that help the body effectively use the vitamin, as well as the levels of total and free vitamin D.

Senior author Dr. JoAnn Manson, chief of the division of preventative medicine at Brigham, said: “Most studies like this focus on the total vitamin D blood level. The fact that we were able to look at this expanded profile of vitamin D metabolites and novel biomarkers gave us unique insights into vitamin D availability and activity, and whether vitamin D metabolism might be disrupted in some people but not in others.”

Vitamin D is metabolized differently in people with a higher BMI

“The analysis of the original VITAL data found that vitamin D supplementation correlated with positive effects on several health outcomes, but only among people with a BMI under 25,” said first author Dr. Deirdre Tobias, an associate epidemiologist in Brigham’s division of preventive medicine.

When the VITAL data was analyzed with the expanded biomarker panel, the researchers noted that vitamin D supplementation increased the markers of vitamin D metabolism across the cohort, but to a lesser extent in people with a higher BMI, indicating a difference in how vitamin D is metabolized.

“This may have implications clinically and potentially explain some of the observed differences in the effectiveness of vitamin D supplementation by obesity status,” reported Tobias.

“This study sheds light on why we’re seeing 30-40 percent 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, suggesting it may be possible to achieve benefits across the population with more personalized dosing of vitamin D,” said Manson. “These nuances make it clear that there’s more to the vitamin D story.”

Reference: Tobias DK, Luttmann-Gibson H, Mora S, et al. Association of body weight with response to vitamin d supplementation and metabolism. JAMA Netw Open. 2023;6(1):e2250681. doi: 10.1001/jamanetworkopen.2022.50681

This article is a rework of a press release from Brigham and Women’s Hospital. Material has been edited for length and content.