There are two forms of vitamin D: ergocalciferol, which is found in vegetables and cholecalciferol, which is found in animal products and which the body manufactures when exposed to the sun.
Although vitamin D is classified as a fat-soluble vitamin, it actually functions as a hormone in the body.
The skin contains the vitamin D precursor molecule: 7-dehydrocholesterol. Under exposure to ultraviolet rays the precursor is converted into cholecalciferol (compound with a structure similar to cholesterol). Cholecalciferol is inactive.
Two steps are needed to convert it into the active vitamin D. The first transformation takes place in the liver where the cholecalciferol receives a hydroxyl group (= OH). The second transformation happens in the kidney where the vitamin adds another hydroxyl group and becomes the active form: the calcitriol. For this final step the parathyroid hormone (PTH) is required.
Vitamin D is essential to help us absorb calcium and maintain our bones and teeth health. It promotes the absorption of calcium from the intestines and also reduces calcium loss in the urine.
It regulates calcium and phosphorus metabolism.
This vitamin might help in the treatment of psoriasis and increase our resistance to tuberculosis.
It might also protect against colorectal and breast cancer.
micro grams per day
Who is at risk?
It concerns people who do not have enough sunlight either because they live in cloudy areas or because they do not expose themselves to the sun such as babies, infants, pregnant women, and elderlies.
Elderlies are even more at risk since they have poor dietary intakes in vitamin D. Their skin ability to synthesize it is reduced and for some of them, they take medications that might interfere with vitamin D absorption.
People with kidney, intestine and liver diseases.
In children, severe cases of deficiency can lead to rickets (deformed bones, swollen joints).
In adults, deficiency of vitamin D can lead to osteomalacia (bone formation defect with excessive bone loss). This is different from osteoporosis in which bones are weak and porous.
Vitamin D is found in very few food sources: brewer's yeast, mushrooms and wheat bran, eggs, fish, and fish's oil. For vegetarian the choice is limited but if they expose themselves to sunlight and consume regularly high food source in vitamin D they have nothing to worry about.
Sun exposure is by far our primary source of vitamin D. The darker you skin is the more sun exposure you need. 10 to 15 minutes every day of sun exposure on the face and the hand is sufficient. The vitamin we store during the summer lasts us during the winter. Do not choose your lunchtime to expose you to the sun. At this period of the day the sun is dangerous for your skin. Wait till mid-afternoon. You just need 15 minutes of exposure and not an afternoon lying under the sun. Too much sun without protection can lead to skin cancer.
Vitamin D is considered as a fat-soluble vitamin. It is stored in the body and too much of it might be toxic. It can cause stunted growth, weight loss, calcification of soft tissue, hypercalcemia (high levels of calcium in the blood)...
Be aware that certain medications can block the metabolism of vitamin D (barbiturates, cholesterol-lowering drugs, cortisone).
In some countries you can find fortified milk in vitamin D. Be careful with this product, you might get to much vitamin D.
If you decide to take supplement be sure not to take more than 25 micro grams per day.
Realized by Laurence LIVERNAIS-SAETTEL, Dietitian.
© Copyright L. Livernais-Saettel 2000
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