The potential benefits of taking vitamin D supplements may be affected by your weight and height, new research suggests. The study found that overweight and obese people taking these supplements experienced a smaller increase of vitamin levels and other related markers relative to those with a lower body mass index.
The study is a reanalysis of the VITAL trial, a large-scale project that tested whether proactively taking vitamin D or marine omega-3 supplements could reduce older people’s risk of developing cancer and cardiovascular disease. The randomized, placebo-controlled trial was led by researchers from the Brigham and Women’s Hospital in Massachusetts, which is affiliated with Harvard University. It overall found no significant effect from either type of supplementation on these outcomes. But some data also indicated that vitamin D supplementation was associated with benefits in those with a BMI lower than 25 (BMI between 18.5 to 25 is considered “normal”), specifically a smaller risk of developing cancer and autoimmune disease, as well as a lower cancer mortality.
To better understand this link, some of the same researchers decided to study blood samples taken from over 16,000 volunteers over the age of 50 involved in the trial. These samples allowed them to look at people’s total vitamin D levels as well as other biomarkers of vitamin D, like metabolic byproducts and calcium, before the study began. About 2,700 of these volunteers also came back for follow-up blood tests two years later.
The team found that people’s levels of vitamin D and these biomarkers generally increased following supplementation, no matter their BMI. But this increase was significantly less pronounced in those with a BMI over 25, the threshold for overweight and obesity. This dampening effect was also seen in people who had low levels of vitamin D at baseline, meaning those who would experience the greatest benefit from supplementation. The team’s findings were published Tuesday in JAMA Network Open.
“We observed striking differences after two years, indicating a blunted response to vitamin D supplementation with higher BMI,” said study author Deirdre Tobias, an associate epidemiologist in Brigham’s Division of Preventive Medicine, in a statement from Harvard. “This may have implications clinically and potentially explain some of the observed differences in the effectiveness of vitamin D supplementation by obesity status.”
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As for why this may be happening, the researchers point to two possible theories. It’s possible, for instance, that higher levels of body fat may allow more vitamin D—a fat-soluble vitamin—to be pulled away from a person’s blood circulation and stored away. Obesity-related liver problems might also make it harder for people to metabolize vitamin D as effectively.
This is only one study, however, and more research will be needed to confirm the patterns seen here. But the authors say that their findings might eventually lead to a reevaluation of the positives and negatives of vitamin D supplementation and how to improve its use for people across the board. People with higher BMI, for example, may need higher doses of vitamin D to achieve the same benefits seen in those with lower body weight.