Vitamin D is a fat soluble vitamin important for good health. It can be obtained from natural food sources, sunlight exposure or as a supplement. Vitamin D is needed for the absorption of calcium and phosphorous, bone mineralisation, bone growth and bone remodeling, neuromuscular and immune function, and reducing inflammation. Along with calcium, vitamin D helps to prevent osteoporosis in older adults.
Food Sources of Vitamin D
Vitamin D is found naturally in few foods. Good sources include cod liver oil and other fish liver oils, and fatty fish such as salmon, tuna, mackerel and sardines. Vitamin D is also found in egg yolks, beef liver, cheese and some mushrooms. Some foods are fortified with vitamin D, such as milk and milk products, margarine, bread, breakfast cereals and orange juice.
Vitamin D is also available as a supplement or as an injection. The Food and Nutrition Board suggests a daily intake of 5 mcg (200 IU) of vitamin D is adequate for adults aged 19 to 50 years, if there is no exposure to sunlight. This is increased to 10 mcg (400 IU) for adults aged 51 to 70, and15 mcg (600 IU) for adults aged 71 and over.
Sun Exposure and Vitamin D Synthesis
The most important source of vitamin D is sunlight exposure. Uncovered skin absorbs ultraviolet B (UVB) rays, which enable the body to synthesise vitamin D, but it is difficult to say exactly how much sun exposure is needed by each person. The intensity of UVB rays is affected by factors such as the time of year, time of day, latitude, cloud cover, smog, shade, and the amount of melanin in the skin.
Cancer Research UK advises only a short time in the sun is needed for the body to synthesise enough vitamin D. Continued sun exposure that allows for tanning or getting sunburned is unnecessary to meet vitamin D requirements, and increases the risk of skin cancer. People who live in countries of northern latitude can build up their vitamin D levels during the summer months in order to prevent vitamin D deficiency during the winter.
Risk Factors and Symptoms of Vitamin D Deficiency
Vitamin D deficiency can lead to bone softening due to poor bone mineralisation. This is known as osteomalacia (bone pains, weak muscles and fractures) in adults, and rickets (deformity of the skeleton, bow legs, aches and pains) in children. Long term, a vitamin D deficiency and poor calcium absorption may result in osteoporosis.
A vitamin D deficiency can be caused by not eating enough vitamin D-containing foods, getting too little sun exposure, or poor absorption of vitamin D. The following groups of people are particularly vulnerable:
- People aged 50 and over - Older people cannot synthesise vitamin D as efficiently as they used to.
- People with limited exposure to sunlight - People who spend most of their time indoors, such as elderly people who are housebound or living in residential homes, are at risk as UVB rays cannot penetrate glass.
- People who cover their skin when outside - Fully covering skin with clothing when outside prevents UVB rays penetrating the skin.
- Darker skinned people - High levels of melanin filter out UVB rays, so more exposure to sunlight is needed to manufacture sufficient vitamin D.
- People who absorb vitamin D poorly - Liver disease, Crohn’s disease, cystic fibrosis and gastric bypass surgery may result in poor absorption of vitamin D.
- People who are very overweight - Obesity increases the requirement for vitamin D.
If regular exposure to sunlight is not possible, more foods should be eaten that contain vitamin D. Supplementation may also be necessary to avoid symptoms of vitamin D deficiency, although high levels of vitamin D may be toxic. For more information, read Vitamin D Facts: Vitamin D Supplements, Toxicity and Hypercalcaemia.
For information about the other fat soluble vitamins (A, E and K), read What are the Fat Soluble Vitamins?
Sources:
- Netdoctor. “Osteomalacia and Rickets (Vitamin D Deficiency).” (Accessed 19th September 2009
- Office of Dietary Supplements. “Dietary Supplement Fact Sheet: Vitamin D.” (Accessed 19th September 2009)
- Cancer Research UK. “Vitamin D, Sun Exposure and Cancer.” (Accessed 19th September 2009)
Sharon Kirby - Sharon is a freelance health writer and contributes to various print and online publications. Writing credits include Nursing Times, Good ...
