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Apr. 26, 2010

How does the sun give me vitamin D?

by Molly Nickerson

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Vitamin D is a critical nutrient for our bodies. It is responsible for the maintenance of normal blood levels of calcium and phosphorus. By aiding in the absorption of calcium, vitamin D plays a vital role in the maintenance of bone strength. The diseases that caused by a lack of vitamin D are rickets in children (skeletal deformities) and osteomalacia in adults (weak bones and muscles). Recently, vitamin D has been found to protect against diseases such as osteoporosis, hypertension, cancer, and some autoimmune conditions.

Vitamin D is found in foods such as fatty fishes, eggs, beef liver, and UV light-exposed mushrooms. So….why is it that people head out into the sun and say, “I need to go and get some vitamin D?”

It is because UVB exposure above the UV index level of 3 is necessary to activate the vitamin D. There are several forms, or vitamers, of vitamin D and they are not all equal. The foods mentioned above contain a pro-form (7-dehydrocholesterol) that becomes active (cholecalciferol) in the skin following UV exposure. The cholecalciferol then travels from the skin to the liver, where it is hydroxylated (calcifediol/25-hydroxyvitamin D3). Finally, the modified vitamin moves to the kidney, where it undergoes another round of hydroxylation to become calcitriol (1,25-dihydroxyvitamin D3), the most active form of vitamin D3. The vitamer of vitamin D that is supplemented in foods is cholecalciferol- it has already been irradiated and is therefore active.

It is possible to ingest too much vitamin D, which makes sense since its role is to maintain normal blood calcium levels. Too much vitamin D results in hypercalcaemia, a condition that can lead to increased blood pressure, weight loss, nausea, vomiting, and even kidney damage. For pregnant women, hypercalcaemia can cause very serious fetal damage. Excessive vitamin D levels occur when people take too many supplements- not from getting too much sun exposure.

About Molly Nickerson

The views expressed are those of the author and are not necessarily those of Science Friday.

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