SOURCE OF VITAMIN D

SOURCE OF VITAMIN D. With enough sunshine, animal and human bodies can make their own active vitamin D. Few foods naturally contain vitamin D. Wild cod often have large quantities of vitamin D in their livers. So, cod liver oil is one of the few food sources of vitamin D. Egg yolks have a little vitamin D, as does butter. Some plants make a diff erent form of vitamin D from ultraviolet light. It is called vitamin D2, or ergocalciferol. Both vitamin D2 and D3 can be converted by human bodies into active vitamin D, but D2 is not absorbed as efficiently as D3. It takes three times more vitamin D2 over a longer period of time to get adequate levels in the blood than it does for D3. Oft en, the vitamin D added to milk, breakfast cereals, and cheese or provided in multivitamin supplements is ergocalciferol.

In theory, the best kind of vitamin D is D3, made from the sun, but modern people just do not get enough sun exposure to avoid deficiency. Our original human ancestors evolved in Africa, wore little or no clothing, and were exposed to warm, direct sunlight most of the year. They acquired ample vitamin D from sunshine. Today, on the other hand, people live in northern latitudes, cover up with clothing, inhabit smoggy cities, and spend much time indoors. In addition, light-skinned people absorb UVB radiation more easily than dark-skinned people.

About 20 to 30 minutes of summertime exposure is enough for a fair-skinned person (with face, arms, legs, and part of the torso bare), but someone with darker skin may need up to three hours to make enough vitamin D for good health. Older people also absorb UVB radiation less easily than younger people. People who wear sunscreen to protect against skin cancer block UVB absorption. Even glass windows prevent the synthesis of active vitamin D. Choosing shade over direct sunlight reduces vitamin D absorption by 60%. In winter, the angle of the sun prevents absorption for everyone in temperate climates.

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