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January 20, 2023

Obesity blunts the benefits of vitamin D supplements, research shows

A new study sheds light on why supplementation has helped reduce cancer deaths among people with lower body mass indexes – but not others

Adult Health Obesity
obesity and vitamin D Source/Image licensed from Ingram Image

New research suggests people with obesity have reduced responses to vitamin D supplements, which have been shown to help prevent cancer and heart disease. This finding may help explain why some people are more likely to develop cancer, diabetes and autoimmune diseases.

The complicated relationship between vitamin D insufficiency and obesity has been well-documented, but researchers are still not clear on which condition comes first. It is a bit of a chicken or egg situation.

New research, however, offers a deeper explanation of how vitamin D levels can be affected by body weight. Researchers found people with high body mass indexes have reduced responses to vitamin D supplements. This finding suggests people with obesity metabolize Vitamin D differently, and may help explain why some people are more likely to develop cancer, diabetes and autoimmune diseases.

"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, suggesting it may be possible to achieve benefits across the population with more personalized dosing of vitamin D," said researcher Dr. JoAnn E. Manson, chief of the division of preventive medicine at Brigham and Women's Hospital in Boston. "These nuances make it clear that there's more to the vitamin D story."

The human body needs vitamin D to absorb minerals such as calcium and magnesium. It is essential for bone health, but the body cannot make it on its own. People absorb vitamin D from sunlight and certain foods, such as fortified milk and cereals, some seafoods and egg yolks.

A lack of vitamin D has been linked to various health conditions, including cardiovascular disease, type 2 diabetes, cancer, multiple sclerosis, depression and dementia. Previous studies have suggested vitamin D supplements may help reduce the risk of cancer and heart disease – and slow their progression. 

The latest study was based on data from 16,515 people in the VITAL trial, a large study that examined whether taking vitamin D or marine omega-3 supplements reduced the risk of cancer, heart disease or stroke. 

Researchers found that vitamin D supplements offered little benefit at preventing cancer, heart attack or stroke. But they found that people with higher BMIs were more likely to develop cancer and autoimmune diseases, and to die of cancer. Other studies have found a similar link to type 2 diabetes. 

The researchers also found that vitamin D supplements increased most of the biomarkers associated with vitamin D metabolism, regardless of a person's weight. But people with elevated BMIs had significantly smaller increases than the other participants.

"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," said researcher Deirdre K. Tobias, an epidemiologist at Brigham. 

The researchers said their findings are a call to further explore the connection between obesity and vitamin D deficiency. 

Previous research has shown that people with obesity and those with normal weights have similar amounts of vitamin D, but that the nutrient is distributed into a larger volume in obese people, resulting in lower concentrations in the blood. That's why many scientists recommend vitamin D supplements be adjusted for body size. A 2021 study suggested that patients with obesity need higher doses of vitamin D than the recommended dosages for others. 

Some studies have shown that losing weight can help people with obesity improve their vitamin D levels. Scientists have observed an inverse relationship between obesity and vitamin D. Some studies suggest that low levels of vitamin D could be a contributing factor to the development of obesity and not just a consequence of it. 

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