Vitamin D is widely promoted for better health, but if you're overweight, you might not reap the benefits.
In a new study, researchers found a 30% to 40% reduction in cancer, cancer deaths and autoimmune diseases among people with a lower body mass index (BMI) who took vitamin D supplements, but only a small benefit among those with higher BMIs.
"Patients with obesity, despite taking the same amount of supplement, had a lower response," said lead researcher Deirdre Tobias, an assistant professor in the department of nutrition at the Harvard School of Public Health, in Boston.
The cutoff was a BMI of less than 25, which is considered a healthy weight, the study authors noted.
It's not known why being overweight or obese affects levels of the so-called "sunshine vitamin," but low absorption of vitamin D could be widespread, given that more than 40% of Americans are obese, according to the U.S. Centers for Disease Control and Prevention.
"It's not clear whether it's due to their body weight, per se, or maybe some other factor related to an individual's body weight. It may be due to adiposity itself. Having a lower body weight may lead to a higher internal dose of vitamin D," Tobias said. Adiposity is having too much fatty tissue in the body.
The next step in the research is to try and tease out just what it is about weight that affects the metabolism of vitamin D, she noted.
Tobias also said it's not clear if overweight and obese people can counter the lower effect of vitamin D supplements by taking higher doses.
"This is not the type of vitamin where you can take unlimited amounts. You mostly excrete it in your urine if you take too much," she said. "So taking a higher dose just to be on the safe side is not something this study is suggesting or would recommend."
For the study, Tobias and her colleagues used data from the Vitamin D and Omega-3 Trial (VITAL), which randomly assigned nearly 26,000 older adults to vitamin D supplements or placebo. Although the trial showed little benefit in taking vitamin D supplements to prevent cancer, heart attack or stroke, there appeared to be a correlation between body weight and the risk for cancer, cancer deaths and autoimmune disease.
The researchers decided to dig a little deeper into the data. They looked at about 16,500 participants who provided blood samples at the start of the trial and nearly 3,000 who gave follow-up blood samples two years later.
The investigators found that signs of vitamin D metabolism were seen in all the participants regardless of weight, but were much less in overweight or obese people.
"Vitamin D has been shown to reduce the risk of osteoporosis and other chronic diseases," said Emma Laing, director of dietetics at the University of Georgia and a national spokesperson for the Academy of Nutrition and Dietetics.
Scientists have suggested a few ways that might make vitamin D supplementation less effective among people with larger bodies, Laing noted. "As vitamin D is a fat-soluble vitamin, it's possible that people with higher levels of adipose tissue store more vitamin D in these tissues so less is detectable in the blood. Another hypothesis is that adipose tissue suppresses enzymes and receptors that are responsible for vitamin D's effectiveness in the body," she said.
Laing doesn't advise taking vitamin D supplements without consulting with your doctor first as supplements can have side effects. "Adverse events, ranging from less serious to life-threatening, could occur if you take more than the suggested dose, if you take a combination of supplements, or if you are taking a supplement that interacts negatively with your medications," she said.
Still, supplements are helpful in certain instances, Laing noted.
Taking a vitamin D supplement might be appropriate if you can't get the necessary amount through your diet or if sunlight exposure is limited due to climate, skin color or sunscreen use, she said.
"A supplement may be warranted if a person eliminates food groups from their eating pattern, has a diagnosed vitamin or mineral deficiency, or takes medications that affect appetite or interfere with nutrient digestion and absorption," Laing said. In those cases, it might be necessary to get vitamins and minerals from supplements, she suggested.
Also, when food choices are strictly limited due to food allergies or intolerance, strict diets or health conditions like celiac disease, supplementation is usually recommended to prevent vitamin and mineral deficiencies, Laing explained.
In addition, people who have had weight-loss surgery may require supplements. "Competitive athletes and individuals serving in the military are also among those who might need supplements if their physical performance demands make it difficult to meet their nutrition needs from food alone," Laing said.
The report was published online Jan. 17 in JAMA Network Open.