While Dr. Anthony Fauci admits most "so-called immune boosting" supplements being marketed amid Covid-19 mostly do "nothing," he does believe in the benefits of vitamin D.
"If you are deficient in vitamin D, that does have an impact on your susceptibility to infection. So I would not mind recommending, and I do it myself taking vitamin D supplements," Fauci, 79, said during an Instagram Live on Sept. 10.
But figuring out whether you are vitamin D deficient and how much of the supplement you need to take is complicated. In fact, medical professionals have been debating the efficacy of routine vitamin D screenings and supplementation recommendations for years.
"You're wandering into a maze," Dr. Clifford Rosen, an endocrinologist and professor of medicine at Tufts University's School of Medicine, who has studied vitamin D for more than 30 years, tells CNBC Make It.
Here's what you need to know from three experts.
The primary source of vitamin D is through direct sunlight and it can also be obtained through foods such as fatty fish (salmon, tuna and mackerel), mushrooms and milk.
What's more, researchers at the University of Chicago Medicine found a link between vitamin D deficiency and the likelihood of being infected with Covid-19 — those with an untreated deficiency were more likely to test positive, according to the study published in September. (The National Institutes of Health released a statement last updated in July saying "there are insufficient data to recommend either for or against the use of vitamin D for the prevention or treatment of Covid-19.")
Without a blood test (more on that later), it can be hard to tell. Early signs of vitamin D deficiency are subtle if they even exist. You may not show any symptoms at all, according to experts.
But vitamin D deficiency can cause accelerated skin aging and dry skin, according to Dr. Raman Madan, a dermatologist at Northwell Health's Huntington Hospital. And over time, severe deficiency can result in muscle weakness and bone fractures, says Paul Thomas, a registered dietitian nutritionist and scientific consultant at the National Institutes of Health's Office of Dietary Supplements.
Studies have also found that prolonged vitamin D deficiency can cause bone-related diseases in adults and children.
All that said, the only way to truly know if you are vitamin D deficient is to get a blood test through your doctor, says Thomas.
Not all medical professionals, however, think routine testing for vitamin D is a good idea.
Routine testing for vitamin D can be controversial among medical professionals.
The National Endocrine Society (NES) and the Institute of Medicine (IOM), for example, recommend vitamin D testing be limited to specific patients (like those at risk for weakened bones or with certain chronic conditions). Rosen and Madan also say not everyone needs vitamin D supplements, only those who have a confirmed deficiency and experience symptoms, according to Madan.
One of the criticisms of routine vitamin D testing is that it can be time-consuming and costly for insurance companies. Out-of-pocket costs for vitamin D tests could range from $40 to $225, according to Kaiser Health News. And typically, most vitamin D tests are covered by health insurance.
Another issue is that widespread vitamin D testing can result in unnecessary treatment of patients with supplements. "It leads to lots of people taking very high doses of vitamin D," Rosen says.
There are concerns around consuming too much vitamin D — according to the NIH, excess vitamin D can cause nausea, poor appetite, constipation and weight loss. Severe vitamin D toxicity can cause confusion, disorientation and problems with heart rhythm.
Additionally, there is not a lot of scientific evidence that taking vitamin D, if you do not have a deficiency, does anything helpful. It's safe to take in small doses, Rosen says (600 IUs to 800 IUs daily — IUs, or international units, are the measurement in which the tablets are sold), but "whether it's effective is really the question."
To date, the research that has been done around vitamin D supplementation has not shown clear results. Rosen says until researchers do randomized controlled trials on the effects of vitamin D supplementation in the prevention of chronic diseases and acute infections such as Covid 19, its efficacy will not be established.
On the other hand, Dr. Michael Holick, a vitamin D researcher, professor of medicine at Boston University and director of Bone Health Care Clinic, who chaired the expert panel that wrote the NES guidelines, believes in the effectiveness of vitamin D and thinks everyone should take it.
Holick points to his research and other studies that suggest an association between low vitamin D levels and higher rates of various diseases. According to Holick's 2010 book, The Vitamin D Solution," vitamin D deficiency is extremely common and supplements could help many people "avoid the myriad ailments associated with deficiency, including heart disease, cancer, and osteoporosis," he wrote. (Past studies have associated low vitamin D levels with increased risk of cardiovascular events and some cancers.)
Holick references the recommendation from NES, which, to guarantee vitamin D sufficiency, is around 1500-2000 IUs of vitamin D supplementation daily for adults. "For obese people, they need two to three times more," Holick, an endocrinologist, says.
The NIH Office of Dietary Supplements does not recommend whether or not to take supplements, but says the daily upper limit for adults is 4,000 IUs.
Holick has been criticized, however. In 2018, The New York Times reported that Holick received hundreds of thousands of dollars from the vitamin D supplement industry over the years for his research. Holick told the Times that the industry funding "doesn't influence me in terms of talking about the health benefits of vitamin D." And Holick tells CNBC Make It that he stands behind his peer-reviewed, published science and always makes financial disclosures.
According to Thomas, natural vitamin D builds up in your blood when your intake is sufficient from sunlight and/or food. "If your vitamin D blood level is sufficiently good at the end of the summer, it can remain at adequate levels throughout the winter ... if you get some vitamin D from food." And while excessive sun exposure is unhealthy for other reasons, it won't cause vitamin D toxicity because the body naturally limits the amount of vitamin D it produces, according to the NIH.
As for food, "the only foods naturally containing vitamin D are oily fish such as wild caught salmon, which contains about 600-1000 IUs vitamin D, mushrooms exposed to sunlight and cod liver oil. Eight ounces of milk or orange fortified with vitamin D contains 100 IUs," Holick says.
Holick also recommends getting "sensible sun exposure" for "some" vitamin D, but caveats that "you cannot make any significant vitamin D before 10 a.m. and after 3 p.m. no matter where you live in the United States." And if you wear sunscreen with a sun protection factor of 30, it will reduce your ability to make vitamin D in your skin by 97.5%, he says.
Holick helped to develop an app called dminder to help people figure out how much vitamin D they may be getting from the sun.
Thomas says that people should consult their own healthcare provider before taking any vitamin D supplements.