Vitamin D is best known for keeping bones strong and healthy. It helps the body absorb calcium, which helps prevents rickets in children and osteoporosis in adults. Muscles need vitamin D to move, and the immune system needs the nutrient to fight off bacteria and viruses.

Research suggests vitamin D may also help prevent certain types of cancer, diabetes, heart disease and multiple sclerosis. In addition to weak bones, pain and fatigue, a study found those with a vitamin D deficiency were more likely to develop Alzheimer’s or another form of dementia.

Nicknamed the “sunshine nutrient,” vitamin D is produced in our bodies when skin is exposed to sunlight. According to the National Institute of Health, 10-15 minutes of sunshine three times a week on your face, arms, back or legs (without sunscreen) should be adequate. However, certain populations are at higher risk for a vitamin D deficiency.  

• Older adults. As people age the skin’s ability to produce vitamin D decreases.
• People with dark skin. Darker skin absorbs less sunlight than lighter skin.
• People in the northern part of the U.S. The farther one is from the equator, the less direct sunlight they receive, especially in the winter.
• People who spend a lot of time indoors during the day. Whether someone is homebound, stuck in the office or works nights, people who are rarely outside tend not to get enough vitamin D from sunlight. It’s important to note, sunlight coming through a window doesn’t produce vitamin D.
• People who are overweight. It’s believed fat holds onto the vitamin D and hinders its release into the blood stream.

Additionally, certain medications and health conditions can decrease a person’s vitamin D levels.

Very few foods naturally contain vitamin D. Fatty fish such as salmon, tuna and mackerel are some of the best sources of vitamin D. Eggs, beef liver, mushrooms and cheese have small amounts of the nutrient. Most vitamin D in America is in fortified foods such as milk, yogurt, cereal and orange juice.

However, experts say it’s hard to get enough vitamin D from foods alone. Special vitamin D lamps and supplements are available, but seek medical guidance first. Your physician can perform a simple blood test to see if your vitamin D levels are low and if supplements or light therapy is necessary.

Remember, the best source of vitamin D is sunshine. So make an effort to get outside to help prevent a “d-ficiency.”

Editor’s Note: This article was originally published Jan. 28, 2016. It has been revamped and updated for accuracy and comprehensiveness.