Vitamin D is perhaps best known for its relationship to both calcium and sunshine. Ultraviolet (UV) rays from sunlight trigger vitamin D synthesis in the skin. The liver and kidneys then help convert vitamin D to its active hormone form, calcitriol. Vitamin D helps maintain calcium blood levels by increasing calcium absorption from food and reducing urinary calcium loss. Want to know more? Then take our quiz below.

True or False?
Fortified food, such as milk,is the best source of vitamin D.

Vitamin D may play a key role in the prevention of diabetes and multiple sclerosis (MS).

Vitamin D deficiency is rare today.

The amount of sun exposure for optimal vitamin D synthesis is easy to determine.

1. False. At 1,360 IU per tablespoon, cod-liver oil is far and above the best source of vitamin D available, followed by salmon, mackerel, tuna, and sardines. Fortified milk contains 98 IU per serving.

2. True. A new study suggests that women who get at least 400 IU of vitamin D daily from supplements are 40 percent less likely to develop MS compared with those not taking supplements. A UCLA study showed those with the lowest vitamin D levels had one or more risk factors for type 2 diabetes compared with those with the highest vitamin D levels.

3. False. One in seven adults has been reported to be vitamin D-deficient. Homebound individuals, babies who are exclusively breast-fed, people living in northern latitudes, and others with limited sun exposure are at risk for vitamin D deficiency. The time of day, skin pigmentation, cloud cover, smog, and sunscreen affect UV ray exposure and vitamin D synthesis.

4. False. Although sun exposure is perhaps the most important source of vitamin D, many variables can affect vitamin D synthesis. The National Institutes of Health advises that most people can obtain adequate levels of vitamin D from modest daily exposure of midday summer sun (10 to 15 minutes on bare skin) in the Southwest up through Wyoming and the Southeastern states. For people in other locations, and for everyone during the months of limited sun exposure, consider a combination of increased sunlight exposure, food sources rich in vitamin D, and supplementation.