Vitamin D is a fat-soluble vitamin that can be absorbed from foods and also synthesized in the skin. Vitamin D is synthesized from ultraviolet-B radiation, originating from sunlight. While many vitamins and minerals require enzymes to be absorbed, vitamin D does not.
Vitamin D From Sunlight
The easiest and most common way to absorb vitamin D is by exposure to UVB radiation. Enzymes are not needed to synthesize vitamin D within the skin. However, it is absorbed as a biologically inert form. Vitamin D is then stored in the epidermis and dermis. People who have long exposure times to sunlight will have higher levels of vitamin D, but at very high concentrations the body will degrade the excess vitamin D.
Foods with Vitamin D
People living in northern latitudes have little exposure to UVB radiation, as do people with darker pigmented skin and the elderly. Many people also avoid sunlight for fears of skin cancer or premature aging. Consequently, for such individuals, vitamin D should be acquired through the diet. This is difficult because it is not found naturally in many foods. Fatty fish, such as mackerel, salmon and sardines, are high in the vitamin, as are fish liver oils and eggs from hens fed vitamin D. Commonly fortified foods include milk, orange juice, cereals and breads.
Vitamin D Absorption From Foods
Enzymes do not play a part in vitamin D ingestion. But as a fat-soluble vitamin, it helps when lipid-containing foods are consumed in conjunction with the vitamin, which facilitates absorption. When the vitamin is absorbed in the intestine, the fat-soluble compound is incorporated into the chylomicron fraction, which is a large globule made out of lipids and protein. From there, it is absorbed into the lymph and enters circulation.
Vitamin D Activation
While enzymes do not help the absorption of this vitamin, they are necessary to activate it. The first necessary hydroxylation occurs in the liver, and the second occurs in the kidney. These hydroxylations, or additions of a molecule of oxygen and hydrogen, are what enable vitamin D to perform its varied functions. Since enzymes are what enables the hydroxylation to occur, they are still crucial for vitamin D function, although they do not help in its absorption.
References
- "Biochemical, Physiological, and Molecular Aspects of Human Nutrition"; Martha H. Stipanuk; 2006
- "Modern Nutrition in Health and Disease"; Maurice E. Shils; 2006
- Linus Pauling Institute: Vitamin D
- Medline Plus: Rickets
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