Supplementation is required to prevent a vitamin D deficiency for most people, since dietary sources of vitamin D tend to be inadequate,[1] and the amount of vitamin D produced in the skin through sun exposure can vary with geographical location[2] and individual skin pigmentation.[3]
Vitamin D insufficiency, defined as blood levels less than or equal to 20 ng/mL, may affect as much as 50% of the population worldwide,[4][5] including 41.6% of Americans. Unsurprisingly, low levels are especially common in Americans with darker skin; Vitamin D insufficiency affects 82.1% of the African American/Black population, and 69.2% of the Hispanic population.[6]
More surprisingly, it appears that even people who live in countries with sunlight year round often don’t produce enough vitamin D.[7] This is probably due, at least in part, to most people now working indoors, leaving little skin exposed when venturing outside, and sometimes also using sunscreen.
However, the only way to know for sure if your vitamin D levels are sufficient is to undergo a 25-hydroxyvitamin D blood test. Assessing your vitamin D levels twice in a year — in midsummer and midwinter, when there is the most and least sunlight — is an efficient way of estimating what your levels are around the year.
In Canada and the United States, the Recommended Daily Allowance (RDA) for vitamin D falls between 400 and 800 IU (10–20 μg) (International Units).[8] These amounts, which some researchers criticize as inadequate,[9][10] can be obtained naturally from only a few food sources, notably fatty fish (such as salmon, tuna, and sardines). However, in Canada and the United States, milk is often fortified with vitamin D.
Since few foods are rich in vitamin D, supplementation is a valid option. Should your blood test show that your vitamin D levels are low, start supplementing with 2,000 IU (50 μg) a day (a number that, based on the available evidence, should provide the best balance of efficacy and safety), then get tested again after a couple of months.
To summarize: It is estimated that as much as half the world, including more than 40% of Americans, have low levels of vitamin D (≤20 ng/mL). People with darker skin and people who seldom go unprotected under the sun are especially at risk. The only sure way to test for vitamin D deficiency is a 25-hydroxyvitamin D blood test; if levels are low, supplement.