Vitamin D is actually not classified as a vitamin. It is considered a prohormone and its active form functions as a hormone. In the earlier years Vitamin D was always associated directly with milk. As the progression of research has shown, Vitamin D is prominent in many foods. The most vitamin D rich foods are milk, margarine, fortified cereals, fortified orange juice, canned tuna and other fish such as mackerel and salmon, and some whole grain products. A natural source of vitamin D is sunlight. Because people get varied amounts of sunlight it is difficult to set dietary recommendations. Someone who works indoors may need more supplemental Vitamin D than someone who works outdoors.
Vitamin D is absorbed through the small intestine. If not absorbed properly or in insufficient amounts there is a risk of diseases such as celiac disease, Crohn’s disease, cystic fibrosis, and pancreatic insufficiency.
The active hormone form of vitamin D has the chemical name calcitrol. It is produced by the combined action of the skin, liver and kidneys. The skin absorbs ultraviolet rays from the sun and is converted into vitamin D. It is then transported to the liver where it is converted to 25-hydroxycholecalcalciferal. It then moves on to the kidneys. In the kidneys the active hormone vitamin D is completed.
Vitamin D is key in the transport of calcium and phosphorous in the small intestine. It also promotes bone mineralization and maintains blood calcium levels. Some things that can hinder the proper levels and absorption of vitamin D in the body are limited hours in the sun and use of sunscreen, lower milk intake, and excessive body weight.
Vitamin D is used in the body for more than just bone health. The muscles need it to move, the nerves need it to carry messages from the brain to the rest of the body, and the immune system needs it to help fight off invading viruses and bacteria.
References: Schlenker, Roth. (2011). Williams ' Essentials of Nutrition and Diet Therapy, 10th Edition
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