Experts estimate that 40-75% of Americans are deficient in vitamin D. Sometimes called the “sunshine vitamin” because the body produces it when ultraviolet rays from the sun strike the skin and trigger its synthesis, vitamin D plays key roles in calcium consumption and bone health. Deficiency has been shown to lead to an increased risk of Alzheimer’s, cancer and various diseases from cardiovascular to autoimmune disorders. 

However, determining just how much vitamin D you really need can be complicated. While the Institute of Medicine has consistently recommended 600 units per day for adults and children, the Endocrine Society would consider that amount insufficient. “The Endocrine Society recommends 600-1000 units a day for children, and 1500-2000 a day for adults,” Michael F. Holick, PhD, Professor of Medicine, Physiology and Biophysics at Boston University School of Medicine, tells LivingHealthy. To determine how much you should supplement, Holick helped develop a free app called dminder that tells you when your body has made enough vitamin D each day, based on information that you input.

According to Holick, the reason for this widespread deficiency is that even at the height of summer, most people don’t get enough sun to meet vitamin D needs. Unlike our ancestors, who worked outdoors (farming, hunting, gathering, etc.), many of us now spend the majority of our daylight hours inside an office. It doesn’t help that few foods naturally contain vitamin D. Fatty fish (salmon, tuna, swordfish) and canned sardines, as well as egg yolks, are good sources. Milk is also often fortified with vitamin D, as are some breakfast cereals and orange juice. 

Holick notes that he plays tennis and gardens outdoors, but still supplements with 3000-4000 IU of vitamin D daily, year-round. “You cannot become toxic orally with that amount, even if you’re a sun worshipper,” he says. To determine your own vitamin D levels, there are now several at-home vitamin D test kits on the market, including a new one from the Vitamin D Council. This kit requires a few drops of blood on a spot card, which is shipped to a lab to determine your levels. 

Holick believes that these personal tests are not necessary but adds that the kits aren’t harmful. Individuals who are more likely to be D deficient—people who are obese or on medications that impact vitamin D absorption—will know, as their doctors will screen and track their levels, explains Holick. But, he says, “If people want to know their levels and spend money to put their minds at ease, the kits are just fine.”


  1. Vitamin D deficiency
  2. Institute of Medicine: Dietary Reference Intakes for Calcium and Vitamin D
  3. Endocrine: Clinical Practice Guidelines
  4. dminder app
  5. Vitamin D Council test kit