Vitamin D is a fat-soluble vitamin that plays a role in the formation and maintenance of strong, healthy bones. Without adequate vitamin D, bones can become thin, brittle, or misshapen. Research also suggests that this important vitamin may help protect us from certain cancers and autoimmune disorders.

The National Academies of Sciences now recommends a daily intake of 200 international units (IU) for children and adults up to 50 years of age, and 400 to 600 IU for adults older than age 50. However, researchers are questioning if that's enough for optimal health.

There's still debate about how much is enough. Most researchers agree that a daily intake of 800 to 1000 IU would benefit many people, especially those at increased risk of vitamin D deficiency, including: babies who are exclusively breast-fed, older adults, people with fat malabsorption, such as those with celiac disease, people with limited sun exposure, and people with darker skin.

Many foods are fortified with vitamin D, but even so it can be difficult to obtain adequate amounts of this nutrient with food alone. Sun exposure is one of the most important sources of vitamin D. UV rays from the sun trigger vitamin D synthesis in the skin. Season, geographic latitude, time of day, cloud cover, smog and sunscreen alter the effectiveness of UV rays and vitamin D synthesis.

Although dermatologists disagree, many experts believe we should have 10 to 15 minutes of sun exposure without sunscreen twice weekly. Sunscreen with an SPF of at least 15 should be used at all other times.

There are only a few commonly consumed foods that are good sources of vitamin D. Some of the best food sources of this important nutrient include cod liver oil, salmon, mackerel, tuna, sardines, as well as vitamin D-fortified milk.

If you don't get adequate levels of vitamin D from your diet or sunshine, talk to your doctor about an appropriate supplement.