Benefits of Vitamin D May Depend on Your Body Weight, Says New Study

A new study from Brigham and Women’s Hospital looks at the impact body mass index (BMI) plays in vitamin D metabolization, finding that weight can make a significant difference in health outcomes. Published in JAMA Network Open, the clinical trial shows vitamin D is most effective on people who are at a healthy weight. «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,» says co-author Deirdre K. Tobias, ScD, associate epidemiologist in Brigham’s Division of Preventive Medicine. «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.»

«Most studies like this focus on the total vitamin D blood level,» said senior author JoAnn E. Manson, MD, DrPH, chief of the Division of Preventive Medicine at the Brigham and principal investigator of VITAL. 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.»

So what exactly did the research find? There is no one-size-fits-all formula for vitamin D supplementation, and vitamin D simply doesn’t have the same effect for those with a higher BMI. «We observed striking differences after two years, indicating a blunted response to vitamin D supplementation with higher BMI,» Tobias says. «This may have implications clinically and potentially explain some of the observed differences in the effectiveness of vitamin D supplementation by obesity status.»

«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,» Manson says. «These nuances make it clear that there’s more to the vitamin D story.» Vitamin D is critical for health—here are five benefits vitamin D has for health. Read on—and to ensure your health and the health of others, don’t miss these Sure Signs You’ve Already Had COVID.

Woman going through bone density exam.
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Vitamin D helps the body absorb calcium and phosphorus from food. «You probably already understand that calcium is good for your bones and helps ward off osteoporosis,» says Pauline M. Camacho, MD, FACE

«The nutrient is essentially a building block of bone, and it helps maintain bone strength throughout your lifetime. But calcium can only reach its full bone-building potential if your body has enough vitamin D. Calcium and vitamin D work together to protect your bones—calcium helps build and maintain bones, while vitamin D helps your body effectively absorb calcium. So even if you’re taking in enough calcium, it could be going to waste if you’re deficient in vitamin D.»

Vitamin D is important for helping the immune system fight bacteria and viruses. «While we generally associate vitamin D with musculoskeletal health, it actually has several functions in the body, including the role it plays in immune function and reducing inflammation,» says Jacyln Tolentino, a physician at Parsley Health in Los Angeles.

«Some studies have shown that vitamin D deficiency is even associated with greater risk of self-reported upper respiratory tract infections… low serum levels of calcidiol [a form of vitamin D]  are also associated with higher susceptibility to infections like tuberculosis, influenza, and viral infections of the upper respiratory tract.» Vitamin D activates killer T cells, making it important for fighting illness. «That makes vitamin D especially crucial for maintaining a functioning immune system that’s capable of fighting back foreign pathogens,» Tolentino says.

body inflammation
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Research shows vitamin D has a powerful anti-inflammatory effect. One study published by Purdue University and the National Institutes of Health shows the possibility of vitamin D eventually being used to support immune health against viruses such as COVID-19. «Our work demonstrates a mechanism by which vitamin D reduces inflammation caused by T cells,» says Majid Kazemian, assistant professor in the departments of Computer Science and Biochemistry at Purdue University. «These are important cells of the immune system and implicated as part of the immune response to the infection causing COVID-19. Further research, especially clinical trials, and testing in patients, are necessary before this can be adopted as a treatment option. We do not recommend the use of normal vitamin D off the shelf at the pharmacy. No one should be taking more than the recommended doses of vitamin D in an attempt to prevent or combat COVID infections.»

Researchers say clinical trials are needed to understand the potential efficacy of Vitamin D as treatment. «In normal infections, Th1 cells, a subset of T cells, go through a pro-inflammatory phase,» Kazemian says. «The pro-inflammatory phase clears the infection, and then the system shuts down and goes to anti-inflammatory phase. Vitamin D helps to speed up this transition from pro-inflammatory to the anti-inflammatory phase of the T cells. We don’t know definitively, but theorize the vitamin could potentially help patients with severe inflammation caused by Th1 cells… We found that vitamin D – a specialized form of it, not the form you can get at the drugstore — has the potential to reduce inflammation in the test tube, and we figured out how and why it does that. However, it’s important to understand that we did not carry out a clinical study, and the results of our experiments in the test tube need to be tested in clinical trials in actual patients.»

Doctor with glucometer and insulin pen device talking to male patient at medical office in hospital.
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Vitamin D has been shown to have a positive impact in insulin sensitivity and type 2 diabetes. «There are many scientific studies and clinical trials claiming that vitamin D plays an important role in improving the sensitivity of insulin, which is a hormone required for maintaining blood glucose levels,» says Dr. Sanchari Sinha Dutta, Ph.D. «It is known that maintaining a vitamin D level of 80 nmol/l or above is appropriate to keep normal glucose homeostasis. The effects of vitamin D on type 2 diabetes may be driven by multiple mechanisms. For instance, studies have found that the pancreas contains receptors for the active vitamin D metabolite called 1,25-dihydroxyvitamin D, which is required for the synthesis and secretion of insulin by the pancreatic beta cells.»

Vitamin D may also have an impact on obesity. «There are also some secondary effects of vitamin D related to diabetes management,» Dr. Dutta says. «For example, studies have shown that maintaining an optimal vitamin D level over the long-term is associated with weight loss and decreased risk of obesity, both of which decrease the risk of diabetes. Vitamin D may reduce the risk of obesity in two ways. It can regulate appetite by increasing the blood leptin level, which is required for controlling fat storage and inducing satiety. Moreover, it can reduce the blood level of parathyroid hormone, which in the long run can trigger weight loss mechanisms.»

Smiling young lady looking at her vitamins
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Vitamin D is developed through exposure to sunlight and by consuming certain foods. «Vitamin D naturally occurs in egg yolks, beef liver, fatty fish like salmon, tuna, swordfish or sardines and fish liver oils,» Tolentino says. «Unfortunately, vitamin D isn’t naturally occurring in lots of foods, which is why some foods have vitamin D added to them. Vitamin D is added to cereals, dairy and plant milks and orange juice… It’s not that easy to get your daily recommended intake of vitamin D through food. We’re just not eating large quantities of most of these foods. How much beef liver or sardines are you realistically eating every day?»

Exposure to sunlight (approximately 15 minutes) is sufficient to make vitamin D. «Where you live (your geographic location), sunscreen usage and coverage and the amount of melanin in your skin can all impact vitamin D absorption,» Tolentino says. «That makes it really difficult to provide generalized guidelines for the appropriate amount of sun exposure. What may be a sufficient or healthy amount of time in the sun with no sun protection for one person might not be advisable for another person.»

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