Vitamin D - Benefits, Deficiency Symptoms And Food Sources
Alternative name :: Sunshine vitamin
Vitamin D was first recognized by McCollum as the component of 'good fats' that cured rickets. Chemically, compounds with vitamin D activity are sterols. The two forms of the vitamin, which are important, are vitamin D2 (ergocalciferol) of plant origin and vitamin D3 (cholecalciferol) of animal origin. Vitamin D2 is formed when ergosterol (found in plants) is exposed to ultraviolet rays. Vitamin D3 is the chief form occurring in animal cells and develops in the skin on exposure of 7 -dehydrocholesterol to ultraviolet rays from sunshine.
Dietary vitamin D is absorbed along with food fats from the intestine; bile salts are essential for effective absorption. Disease conditions in which fat absorption is affected also hinder vitamin D absorption. Vitamin D made in the skin enters the blood where it circulates attached to a specific protein.
Vitamin D itself is an inactive, storage form of the vitamin concentrated in the liver. It is rapidly hydroxylated (addition of-OH group) to 25 - hydroxy vitamin D3(calcidiol) in the liver. Calcidiol is further hydroxylated to 1, 25 -dihydroxy : vitamin D3 (calcitriol) in the kidney. Calcitriol is considered the active form of the vitamin, which acts as a hormone in calcium metabolism. As vitamin D is necessary for proper bone calcification, it is also called calciferol (to signify an alcohol promoting calcification).
Vitamin D is soluble in fats and organic solvents but insoluble in water. It is stable to heat and is not easily oxidized.
Functions of vitamin D
Vitamin D plays an essential role in the metabolism of calcium and phosphorus. Calcium is required for the development of bones and teeth. Blood calcium levels have to be very tightly maintained for normal functioning of the nervous system. The prohormone vitamin D, which gives rise to the hormone calcitriol, regulates blood calcium levels within a narrow range.
Vitamin D plays a special role in bone growth by making calcium and phosphorus available in the blood. When the blood calcium level becomes too low - for example, when calcium intake from diet is inadequate - the parathyroid gland secretes parathyroid hormone (PTH). PTH promotes the activation of vitamin D to calcitriol, the active form of the vitamin. Calcitriol restores normal blood calcium in three ways - by facilitating absorption of dietary calcium from the intestine, increasing the mobilization of calcium from the bone into the blood, and increasing the re-absorption and retention of calcium by the kidneys. These actions increase the levels of calcium and phosphorus in the blood. Normal calcium levels permit deposition of calcium in bones and teeth. It is also important for normal nervous function and muscle contraction.
Scientists have discovered other roles for vitamin D. Vitamin D has been found in many other tissues such as the brain, nervous system, pancreas, skin, muscles, reproductive organs, liver, kidneys, hormone secreting glands, etc. Vitamin D may have some important role in these tissues as well.
Benefits of vitamin D
Vitamin D is necessary for growth, and is especially important for the normal growth and development of bones and teeth in children. Specifically, vitamin D may help to:
Daily allowances of vitamin D
The minimum Recommended Dosage Allowance of Vitamin D are :-
Rich sources of vitamin D
Fish liver oils, fatty saltwater fish, dairy products, and eggs all contain vitamin D. It is found in butter, cod liver oil, dandelion greens, egg yolks, halibut, liver, milk, oatmeal, salmon, sardines, sweet potatoes, tuna, and vegetable oils.
Vitamin D from the sun
Most of the world's population relies on natural exposure to sunlight to maintain adequate vitamin D nutrition. The sun imposes no risk to vitamin D toxicity. For most people, exposing hands, face, and arms on a clear summer day for 10 to 15 minutes, at least 3 times a week, should be sufficient to maintain vitamin D nutrition. Dark-skinned people require longer sunlight exposure than others, to derive the same amount of vitamin D from sunlight exposure. The ultraviolet rays of the sun, which promote vitamin D synthesis, can be blocked by heavy clouds, smoke, or fog. People who are unable to go outdoors frequently or dark-skinned people who live in cloudy or smoggy cities should consume foods fortified with vitamin D or therapeutic supplements to prevent its deficiency.
Deficiency of vitamin D
A shortage can lead to softening of the bone and muscle twitching and convulsions, and in children it causes rickets - resulting in bent legs. In adults, the shortage causes loss of minerals from the bones, (osteomalacia) where the bones are sore, tender, and weak muscles with the possibility of deafness developing. In older people, osteoporosis may appear when protein is also lost from the bone. Vitamin D in short supply is also linked to having a burning sensation in the mouth and throat, diarrhea, insomnia and visual problems.
Vitamin D toxicity
Vitamin D toxicity is also called hypervitaminosis D. Vitamin D toxicity does not result from sun exposure. The liver's storage capacity is smaller for vitamin D than for vitamin A. Toxicity results when people take vitamin D doses of 10,000 to 50,000 IU/day over many years.
Excessive amount of vitamin D results in excessive absorption of calcium from the intestine and the elevation of blood calcium levels. Symptoms of toxicity include loss of appetite, nausea, vomiting, diarrhea, excessive thirst, weight loss, polyuria (excessive urination), severe itching, muscular weakness and joint pain. Excessive blood calcium, deposits in the soft tissues such as kidneys leads to formation of stones. Calcium may deposit in the arteries making them hard. This could be dangerous if it occurs in the arteries of the heart and lungs, leading to death.
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