There’s no doubt that vitamin D is important. It promotes calcium absorption and it is necessary for bone growth, notes a National Institutes of Health fact sheet. “Without sufficient vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults,” it adds. “Together with calcium, vitamin D also helps protect older adults from osteoporosis.”

Human skin can create the vitamin — which is found in foods like fish, eggs, fortified milk, and cod liver oil — when directly exposure to sunlight, and “as little as 10 minutes of exposure is thought to prevent deficiency,” according to the Mayo Clinic.

But a broad misunderstanding abounds about how much vitamin D is necessary, and what to do when one lacks sufficient amounts of it.

“This preoccupation with vitamin D status has led to an enormous amount of testing, which is expensive and not as reliable or consistent as we might like,” Barbara Gilchrest, a Mass. General Hospital dermatologist, said at the American Academy of Dermatology’s annual meeting in March. “If everybody just took one supplement a day, [the tests] would be unnecessary.” (The conference was covered by Medscape.)

The Institute of Medicine recommends are that people have a vitamin D level of at least 20 ng/mL. “In my practice, and in most, it is not uncommon to see a vitamin D level less than 20,” writes physician Monique Tello on the Harvard Health Blog. “When that happens, we tell the patient that they are deficient and recommend fairly aggressive replenishment, as well as ongoing supplementation.”

But as Dr. Tello notes, a 2011 Endocrine Society guideline recommends levels about 30 ng/mL, and the society added, “to guarantee sufficiency, we recommend between 40 and 60 ng/mL for both children and adults.” A November 2016 New England Journal of Medicine article “Vitamin D Deficiency — Is There Really a Pandemic?” moved in the opposite direction, and suggested that the original level of 20 ng/mL was never even meant as a cut-off line below which patients are vitamin D deficient. Real deficiency, they wrote, is well below 20 ng/mL.

“What that means is that if you have a level of 20 ng/mL, you have a 97.5 percent chance that you’re getting all the vitamin D you need,” Dr. Gilchrest said, as Medscape reports. “For many people, 16 or 12 ng/mL is adequate. Half the population is believed to have totally adequate vitamin D at a level of 16 ng/mL.”

The Medscape article also quotes dermatologist Susan Roper, who says that many of her mostly elderly patients suffer from misleading tests that show their vitamin D levels are too low. Elderly patients, whose skin is thinning, already have low vitamin D levels, according to Dr. Roper, and when they fast before taking the test, that adds to misleading data and overdosing.

“They’re put on toxic levels of vitamin D, sometimes 4000 to 10,000 IU a day,” she told Medscape. “A lot of my patients already have arthritis, and that high level of vitamin D makes it worse.”