Your Weight Could Impact How Well Your Body Absorbs Vitamin D, According to New Research

It's no secret that vitamin D is a critical nutrient for supporting good health. It's involved in everything from building strong bones and teeth to fighting infections and reducing inflammation. While you can get some of the vitamin D you need from sunlight and certain foods, it's common to take vitamin D supplements to help ensure adequate intake.

However, a new study published January 17, 2023, in Nutrition, Obesity, and Exercise of the JAMA Network indicates that your body weight may impact the efficacy of vitamin D supplements. Specifically, the study found that overweight and obese individuals process and absorb vitamin D differently, which could prevent them from reaping vitamin D's many health benefits.

"The analysis [...] found that vitamin D supplementation correlated with positive effects on several health outcomes, but only among people with a body mass index (BMI) under 25," said study author Deirdre Tobias, ScD, an associate epidemiologist in Brigham's Division of Preventive Medicine, in a media release. "There seems to be something different happening with vitamin D metabolism at higher body weights, and this study may help explain diminished outcomes of supplementation for individuals with an elevated BMI."

These findings are timely, considering 42% of U.S. adults are considered obese and over one-third of American adults have a vitamin D deficiency. Keep reading to learn more about this study that revealed an association between your weight and low vitamin D levels.

What the Study Found
Researchers at the Brigham and Women's Hospital in Boston, MA, analyzed data from the VITAL trial, a comprehensive nationwide clinical trial that looked at whether taking vitamin D or marine omega-3 supplements could lower the risk of developing cancer, heart disease or stroke. To examine the correlation between vitamin D levels and BMI, the researchers looked at baseline blood samples of 16,515 participants from the VITAL trial, plus another 2,742 follow-up blood samples taken after two years. From these samples, they determined the participants' total vitamin D status, along with metabolites, calcium and parathyroid hormone—other biomarkers that help your body use vitamin D effectively.

Participants with a BMI below 25 were less likely to have a vitamin D deficiency. They were also more likely to resolve their vitamin D deficiency through vitamin D supplementation than those with a BMI above 30. The study's authors theorize the blunted vitamin D response in individuals with a higher BMI may be a result of differences in parathyroid hormone activity, a biological process that regulates blood calcium levels by stimulating the release of calcium from your bones. This activity can also stimulate vitamin D production in the kidneys when a person has low levels, as in people with an elevated BMI.

However, this study has some drawbacks. "A main limitation with a study like this with a follow-up of two years is just adherence from participants to taking the supplementation provided," states Sarah Schlichter, M.P.H., RDN, a registered dietitian at Bucket List Tummy. "Other confounding factors may explain the reduced uptake, such as impaired vitamin D receptor sensitivity in individuals with higher BMIs. There may also be other explanations for people with cancer and other chronic diseases that may better explain the lower vitamin D serum levels."

Not to mention, BMI may not be the best indicator of someone's overall health or amount of body fat. Research has found that people are often incorrectly categorized as "healthy" or "unhealthy". by BMI alone. A primary reason for this is because BMI does not reflect muscle mass or fat distribution, which factor in to someone's health status. More research is needed to consider more thorough indicators of health status and body fat percentage in relation to vitamin D absorption, as well as a more diverse sample size with a longer follow up time.

The Bottom Line

A new study conducted at the Brigham and Women's Hospital found that obese individuals (BMI of 30 or greater) are more likely to have a vitamin D deficiency due to malabsorption. In addition, these individuals are less likely to correct a vitamin D deficiency with vitamin D supplements than those with a BMI of 25 or lower. These findings should encourage people to maintain a healthy weight since vitamin D is an essential nutrient critical for many aspects of overall health. If you want to learn more, talk to your doctor or a registered dietitian about healthy weight management and taking vitamin D supplements.