Vitamin D is fat-soluble vitamin, part of a group of 13 essential vitamins that are vital for the body to function properly. It is present in only a handful of foods but can also be produced naturally in the body through exposure to ultraviolet B sunlight. Maintaining adequate levels of vitamin D is important, as vitamin D deficiency can lead to a variety of health problems, the most prevalent being hypocalcemia.
Functions of Vitamin D
Vitamin D obtained from foods, sunlight exposure and supplements is biologically inactive and must undergo two conversion stages within the body. The first stage occurs in the liver and converts vitamin D to calcidiol. The second occurs in the kidney, where calcidiol is converted into the biologically active calcitriol. This active form of vitamin D is essential for promoting calcium absorption in the gut and maintaining calcium and phosphate levels in the blood. It is also important in the regulation of neuromuscular and immune function, as well as in the reduction of inflammation.
Sources of Vitamin D
Foods that contain vitamin D include oily fish, such as salmon, mackerel, sardines and tuna. It is also present in smaller amounts in eggs, beef liver and cheese. Yet the biggest dietary source of vitamin D is from fortified foods, such as fortified milk and cereals. Nearly the entire U.S. milk supply is fortified with vitamin D. Some fruit juices also contain added vitamin D. Despite this, most people are able to synthesis enough vitamin D through exposure to sunlight.
Vitamin D Deficiency
Vitamin D deficiency can be caused by a number of factors, including inadequate diet coupled with limited sun exposure, disorders that impair vitamin D adsorption and conditions that impair the conversion of vitamin D into its biologically active form, such as liver and kidney disease. A lack of active vitamin D leads to impaired calcium intake, which is a major cause of bone-softening diseases.
Hypocalcemia Caused by Vitamin D Deficiency
Hypocalcemia, when the levels of calcium in the blood are dangerously low, is the primary effect of vitamin D deficiency. It causes the demineralization of bone, leading to the childhood disease rickets and the adult diseases osteomalacia and osteoporosis. Rickets is characterised by stunted bone growth and skeletal deformity. Osteomalacia and osteoporosis lead to increased bone fragility.
Groups at Risk of Vitamin D Deficiency
Certain groups are particularly vulnerable to vitamin D deficiency. Breast-fed infants are at risk as breast milk is a poor source of vitamin D and breast-feeding mothers are encouraged to supplement their child's diet. Obesity is another risk factor as fat is able to seize vitamin D, altering its release into the circulation. The elderly are also vulnerable as the skin's capacity to produce vitamin D and the body's ability to convert it to its active form declines with age. Those with an improper diet, limited sun exposure and individuals with darker skin are also susceptible to vitamin D deficiency.
References
- National Institutes of Health: Dietary Supplement Factsheet: Vitamin D.
- "The American Journal of Clinical Nutrition"; Overview of general physiological feature and functions of vitamin D; HF DeLuca; 2004
- "Alternative Medicine Review"; Benefits and requirements of vitamin D for optimal health: a review; WB Grant and MF Hollick; 2005
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