Vitamin D is produced endogenously when UV rays from sunlight strike the skin, and it is also added to some foods and available as a dietary supplement. Vitamin D supplements can contain either vitamin D2 (ergocalciferol) or D3 (cholecalciferol).
Sun exposure is an important source of vitamin D, but too much time in the sun without sunscreen increases your risk for skin cancer. Foods are also good sources of vitamin D, including fatty fish, liver and eggs, and fortified foods.
Sunlight Benefits
Sunlight provides the primary source of vitamin D, which helps maintain healthy levels in the blood. It also plays a role in regulating the nervous and musculoskeletal systems.
The skin makes vitamin D from UVB rays that reach the skin and trigger a specific protein to be converted to 7-dehydrocholesterol (cholecalciferol), the form of the vitamin that circulates in the body. Vitamin D deficiency is common worldwide and occurs when people don't get enough sunlight or food sources of vitamin D. It affects bones, muscles and the immune system.
Light-skinned individuals can make enough vitamin D by exposing skin to the sun without sunscreen for 10-30 minutes three times per week. Darker-skinned individuals need more time in the sun to produce vitamin D, and those who live farther from the equator may struggle to produce sufficient amounts of vitamin D in their skin. Sunscreen with SPFs of 8 or higher significantly reduce vitamin D production in the skin.
Vitamin D is naturally present in a few foods, including egg yolks from free-range chickens and some mushrooms. It is fortified in many foods, and dietary supplements are available. Supplements are recommended for those who can't get enough vitamin D through their diet, especially people who use sunscreen regularly or live far from the equator, and seniors.
Dietary Sources
Many foods contain vitamin D, including fatty fish (especially salmon and tuna), beef liver, egg yolks, and fortified milk. Some researchers have linked low vitamin D levels to disease, and the Food and Nutrition Board (FNB) has established ULs for the nutrient.
The FNB states that signs and symptoms of toxicity are unlikely with daily intakes below 250 mcg (10,000 IU). However, it has been found that low vitamin D levels can lead to an increased risk of cancer at certain sites, and may increase the frequency of falls and fractures in older adults.
Vitamin D regulates the amount of calcium and phosphate in the body, helping to keep bones and teeth strong. A deficiency of the nutrient can cause bone deformities such as rickets in children and osteomalacia in adults, with symptoms including bone pain and weakness.
Most people can obtain adequate amounts of vitamin D from sunlight and a few foods, but some might need to consider taking dietary supplements. Supplements can be taken as vitamin D2 or D3. The supplemental form of D2 is typically made with UV irradiation of the sterol ergosterol in yeast, while D3 is produced by irradiating 7-dehydrocholesterol from lanolin extracted from the wool of sheep. Vegetarians and vegans can find animal-free D3 supplements derived from lichen.
Expert Guide
Vitamin D plays an important role in bone health. It helps regulate calcium and phosphate in the body, which keep bones strong. Vitamin D deficiency can cause rickets in children, osteomalacia (soft bones) in adults and can lead to heart problems.
From late March/early April to September in the UK most people can make all the vitamin D they need from sunlight on their skin. From October to March, however, they may need to take a supplement. Those at highest risk of vitamin D deficiency, including pregnant and breastfeeding women, babies up to 1 year, and older people should consider taking a supplement all year round.
Other good sources of vitamin D include oily fish such as salmon and trout. One cup of cooked wild mushrooms (cremini) provides 136 IU of vitamin D, or 17% of the DV, and free-range egg yolks also contain small amounts of this vitamin.
Herring, usually eaten smoked or pickled, is another good source of vitamin D. A 3.5-ounce serving of fresh Atlantic herring provides 193 IU of vitamin D, or 24% of the DV. However, be aware that pickled herring can be high in sodium.