This transcript has been edited for clarity.
Hello. This is Dr JoAnn Manson, professor of medicine at Harvard Medical School and Brigham and Women’s Hospital. As a director of the Vitamin D and Omega-3 trial (VITAL), the largest randomized clinical trial in the world, I’m often asked, “How much vitamin D do we need, and should I take a vitamin D supplement?” I want to review the findings from recent randomized clinical trials and the implications for practice.
For a long time, vitamin D has been perceived as a magic bullet, a panacea, and a cure-all for many chronic health conditions such as cancer, cardiovascular disease, diabetes, bone fractures, cognitive decline, and depression. Many of the findings, though, have been from observational studies where a higher blood level of 25-hydroxy vitamin D has been linked to a lower risk for these health conditions.
We know in epidemiology that correlation doesn’t prove causation. Other factors could be involved; for example, people who have higher blood levels of vitamin D may have healthier diets, or they may be spending more time outdoors, being physically active and exposed to the sun. Some of these other factors could be lowering their risk.
When the randomized trials began to emerge, in many of these large-scale trials, the findings were generally neutral or null for cardiovascular disease, total cancer, diabetes, cognitive decline, depression, and many other health outcomes, including fracture. So, the question was asked, does this mean that vitamin D is not important to health?
To the contrary, these findings suggest that vitamin D is so essential to health that we need only small to moderate amounts of vitamin D. Vitamin D is very tightly regulated in the body – the metabolism and function of vitamin D. Even small to moderate amounts will meet the requirements for vitamin D and bone health and many other outcomes.
This is what the National Academy of Medicine, U.S. Preventive Services Task Force, and many other professional organizations have advised, that widespread screening for vitamin D deficiency and blanket universal supplementation with vitamin D would not be indicated.
The randomized trials of vitamin D, including the VITAL study, have generally not shown reductions in the major health outcomes. We found two exceptions in VITAL. We saw promising signals, including a 22% reduction in autoimmune conditions (rheumatoid arthritis and psoriasis) and a 17% reduction in advanced (metastatic or fatal) cancers. In meta-analyses of other large-scale randomized trials, the findings were a signal for a reduction in advanced cancers, even with very small doses of vitamin D (400-800 IUs daily). We tested 2,000 IUs daily in VITAL.
Overall, it’s recommended that small to moderate amounts of vitamin D are adequate, and among the healthy population, most people do not need screening or supplements.
The reduction in autoimmune diseases suggests that vitamin D may play a role in tamping down inflammation. The question has been raised about whether vitamin D is beneficial in reducing the severity of COVID illness, the need for hospitalization, and long COVID. We are looking at this question in a separate trial called VIVID (Vitamin D for COVID Trial) which tests a higher dose (> 3,000 IUs daily) of vitamin D. Those results will be available at the end of this year or early next year.
In other randomized trials of COVID and vitamin D, the results have been mixed and inconsistent, with no clear answer. During the COVID pandemic, I have generally advised that it’s reasonable to take 1,000-2,000 IUs of vitamin D daily as a form of insurance. This dose is known to be very safe. Over 5.3 years in the VITAL trial we saw that a dose of 2,000 IUs was very safe.
But it’s not essential to take a supplement. And overall, aside from some high-risk groups, most people do not need a supplement. The high-risk groups include patients in nursing homes who may have restricted diets and limited time out of doors. For people with malabsorption conditions such as Crohn’s disease, celiac disease, post–gastric bypass surgery, and those with osteoporosis who are on medications for osteoporosis, it’s still quite reasonable to prescribe calcium and vitamin D.
Recommendations for vitamin D in the generally healthy population really should focus on a healthy diet. The United States has a fortified food supply. Vitamin D is added to many foods, dairy products, and cereals, as well as beverages. Natural sources of vitamin D include fatty fish and wild mushrooms.
We should be looking at food labels (which now include vitamin D content) and try to get adequate vitamin D from our diet, and also do our best to spend time outdoors, being physically active, because it is of great benefit to our health. The general principle is that a dietary supplement will never be a substitute for a healthy diet or healthy lifestyle. And those other behaviors really should be the focus at this time.
Dr. Manson is professor of medicine and the Michael and Lee Bell Professor of Women’s Health, Harvard Medical School, and chief of the division of preventive medicine at Brigham and Women’s Hospital, both in Boston. She has received infrastructure support from Mars Symbioscience for the COSMOS trial.
A version of this article first appeared on Medscape.com.