Last February, my physician, Connecticut-based Robert Ruxin M.D., informed me that my recent blood work showed a vitamin D level that was a click lower than optimal. “I was not alarmed but I was concerned,” he told me, pointing out that the deficiency combined with my tall stature (and advancing age) could increase my risk of osteoporosis, which causes bone brittleness, fractures, shrinkage and even spinal deformities such as the dreaded “widow’s hump.” Ruxin immediately put me on 4,000 International Units (IUs) of vitamin D a day, roughly six times the recommended daily allowance, to boost me out of the danger zone.

More so resembling a hormone, vitamin D is not technically a vitamin—and our bodies do not produce it on their own; it can only be processed through consuming specific foods, such as fatty fish, egg yolks, and fortified dairy products, and synthesized through our skin with direct exposure to sunlight—UVB rays specifically. Its ability to assist in calcium absorption for good bone health is only one of its many critical biological benefits. “It’s not a panacea, but vitamin D seems to be essential for nearly all organ systems of the body,” says JoAnn Manson, a professor of epidemiology at Harvard’s T. P. Chan School of Public Health, who led a 2018 study that linked vitamin D to a reduction in cancer deaths. Low vitamin D levels have also been connected to depression, anxiety, cardiovascular function, and neurodegenerative disorders. But most recently, the “sunshine vitamin” has been making headlines for something else: its role in possibly preventing and treating COVID-19. “It’s central to a healthy immune system,” says Manson. “But beyond that,” she continues, “vitamin D seems to have a benefit in tamping down the inflammation that can occur with COVID, and it may also have a role in reducing the risk of developing severe illness and a need for hospitalization.”

Early in the pandemic, inspired by older published studies showing a reduction of viral respiratory infections with vitamin D supplementation, David Meltzer M.D., a professor of medicine at The University of Chicago, conducted a retrospective study following patients who were already in the hospital system. The results were staggering: those whose records indicated a vitamin D deficiency in the prior year were 77 percent more likely to test positive for COVID-19. Even allowing for the profound socio-economic determinants of health outcomes, and other pre-existing conditions—such as obesity or diabetes—that might make someone more likely to contract the virus or develop a severe case of it, Meltzer’s study is hard to ignore. It joins a growing body of evidence suggesting vitamin D’s prominence in the fight against COVID-19. “It’s still soon, and no single intervention against COVID has been proven beyond a reasonable doubt, but I think the picture emerging is compelling, and collectively the evidence is pretty robust,” says Vadim Backman, PhD, a researcher and professor of biomedical engineering at Northwestern University, who led another recent study linking vitamin D deficiency with an increase in COVID complications and deaths. The data is so convincing that Scotland has provided the country’s vulnerable population a four-month supply of vitamin D to shore up for the challenging months that lay ahead with the virus, and U.K. Prime Minister Boris Johnson is reportedly weighing a similar move.

New York-based clinical herbalist Daniela Turley is not surprised by the promising results of these studies, noting the established link between vitamin D deficiency and a host of common, flu-like symptoms. Even before COVID, when patients complained of fatigue, muscle aching or cramping, vitamin D “was often at play when the bloodwork came back,” she says. Turley, who has spent the better part of the last ten months prescribing customized immune-boosting tinctures and supplement regimens for her clients, finally created a catch all product—her first—to keep up with the rising demand caused by the pandemic. Adaptivir, which just launched and which includes (among many other plant-derived ingredients) isatis for its antiviral properties and astragalus to encourage immunity and reduce inflammation, is a two part tincture and capsule system with 1,500 IUs of vitamin D. “With the two-step program, you’re providing optimal immune support by promoting the body’s natural defenses,” she says—which is a good prophylactic for everyone this winter, she suggests.