Vitamin D is naturally made by the body when skin is exposed to sunlight. It is required by the body for bone formation, for the immune system, for anti-tumor activity and for the reduction of autoimmune disorders. It is used by the intestines to enhance calcium and phosphate absorption, by the kidneys to enhance calcium reabsoption, and by the parathyroid gland to inhibit parathyroid hormone secretion and by the pancreas to stimulate insulin production. A deficiency in vitamin D can result in rickets and osteomalacia, which are conditions in which bone mineralization is impaired.
In the 1930’s, milk was fortified with vitamin D to combat rickets, which was a major public health problem at the time. The addition of vitamin D to milk virtually eliminated this medical condition. In the 1940s, the addition of vitamin D to milk resulted in a decrease in the occurrence of rickets of 85 percent. Products made with milk, such as cheese and yogurt, are generally not fortified. Other foods that are fortified with vitamin D include breakfast cereals, some brands of orange juice, yogurt and margarine. According to the University of Maryland Medical Center, recent studies have found that adequate dietary intake of vitamin D can help reduce the risk of various cancers, arthritis and type 1 diabetes. However, these studies are in the early stages.
Meeting Dietary Requirements
Sunlight exposure on the skin prompts the body to create vitamin D naturally. A light-skinned person can meet their vitamin D requirements with approximately 45 minutes per week of sunlight exposure to the face and arms. A person with dark skin may need as much as three hours of exposure to the face and arms per week to meet their dietary requirements by sunlight exposure. Eggs, cod liver oil, tuna, mackerel, herring and sardines are food sources of vitamin D.
Groups at Risk of Vitamin D Deficiency
Breastfed infants are at risk due to the fact that breast milk does not contain adequate vitamin D and limited exposure to sunlight. Older adults are also at risk for deficiency. The skin of older adults is not as efficient at synthecizing vitamin D and it is not as efficiently converted in the kidneys to its active hormone form. People who have limited exposure to sunlight or dark skin are more likely to develop deficiency since they may not be able to obtain adequate vitamin D from sun exposure. People who have difficulty absorbing fat or people who are using a fat-blocking medication to lose weight are prone to vitamin D deficiency. This vitamin is fat soluble and requires dietary fat for absorption. Obese people are also at higher risk because subcutaneous fat alters the release of vitamin D into circulation.
A portion of the vitamin D3 in milk occurs naturally from the exposure of the cow to sunlight. All lactating animals produce vitamin D phytochemically in the skin. Vitamin D3 that is added to milk is derived from the solvent extraction of 7-ehydrocholesterol from the skins of cows or pigs or sheep lanolin. 7-Dehydrocholesterol is then dissolved in a solvent and exposed to ultraviolet light to create vitamin D3.
Excessive amounts of vitamin D can be toxic. In children, toxicity can occur at 40,000 IU per day in one to four months. In adults, 50,000 IU per day can induce toxicity in several months. Symptoms of vitamin D toxicity include hypercalcemia and kidney disorders. Other symptoms include anorexia, nausea, vomiting, pruritis, or itchiness, excessive thirst and passing large volumes of urine. Vitamin D supplements may interfere with a variety of medications, so talk to your doctor before you take one.