The vitamin is mainly made in the skin by direct sunlight, but many people's dietary intakes of the nutrient fall short. This group includes young children, adults with darker skin and older people who spend more time indoors.

Getting enough vitamin D is important because it helps the body absorb calcium, which builds and maintains healthy bones. It also regulates other cellular functions and has been linked with a number of health outcomes.

Sources Overview

Vitamin D helps your body absorb calcium, a mineral that builds and strengthens bone tissues. Without adequate vitamin D levels, your bones may weaken and break (a condition called osteoporosis in adults, or rickets in children).

Our skin can make vitamin D when it is exposed to sunlight. But many people don't get enough sun to meet their needs, especially in the fall and winter. And people with darker skin have less ability to make vitamin D because their skin's pigment acts like a shade, blocking the sun's effects. [1]

In addition, the skin's ability to make vitamin D decreases with age. Most people over age 65 need a supplement to get adequate amounts of the vitamin. And infants fed only breast milk are at risk for insufficient vitamin D levels unless they're given a daily supplement.

There are two types of supplemental vitamin D, D2 and D3. Your body can more easily absorb D3, which is found in fatty fish such as trout and salmon, cod liver oil, beef liver, and cheese, and is also available as a multivitamin. Talk with your health care provider about whether and how much to take. They might test your blood level of 25(OH)D to determine if you need a supplement.

Health Facts

The body makes vitamin D when bare skin is exposed to sunlight. From May to September, most people can get all the vitamin D they need by spending 15-20 minutes in the sun a few times per week without sunscreen (make sure to wear sunblock on any other parts of the body). Vitamin D obtained from food or supplements is stored in fat cells until needed, and then it's activated in the liver and kidney through a two-step process that turns it into calcitriol. Vitamin D in its active form is the only kind of vitamin D the body uses [3,4].

Age and skin color affect the body's ability to make vitamin D from sunlight. In addition, smog, clouds, and other factors can decrease the amount of vitamin D the skin makes. Those who are institutionalized, those with certain gastrointestinal conditions (like inflammatory bowel disease), those with restricted or limited diets, and those who use anti-convulsant medications may also have lower serum 25(OH)D levels than others.

Research has shown that low levels of vitamin D are associated with various health outcomes, but evidence is inconsistent on whether the vitamin actually affects those outcomes. It does appear to be important for bone health, and many physicians recommend vitamin D supplementation to bring serum levels up to the FNB thresholds.