Guide to Vitamin D

Vitamin D is the sole vitamin that is not found naturally in many foods that we consume. Instead, we typically obtain vitamin D from sunshine on our skin. The exposure of the skin to sunlight triggers a chemical reaction in the body called vitamin D synthesis. While there has been a great deal of media coverage in recent decades about the perils of too much sun exposure, it is essential to get enough sunlight to obtain the recommended daily allowance of vitamin D. Actually, the total time a person needs to spend out in the sun to acquire a sufficient dose of vitamin D is exceedingly small – only a few minutes daily will be adequate, without the risk of adverse effects from ultraviolet light.

Vitamin D and Its Function

The principal activity of vitamin D is to control how much calcium we take in from food. Most of the calcium we absorb is used to develop strong teeth and bones, but it is also required to conduct messages along the nerves and enable muscles, such as the heart, to contract. It is vitamin D that ensures that there is ample calcium in the blood to accomplish these tasks. Other functions requiring vitamin D involve the immune system. Vitamin D may also be a contributing factor in lowering the risk of certain cancers – particularly colon cancer.

Vitamin D3

The form of vitamin D that is made beneath the skin is called vitamin D3, or cholecalciferol. This D vitamin is produced when the sun’s ultraviolet light reacts with a type of cholesterol found naturally under the skin. D3 is changed into a more potent form of vitamin D inside the liver and is then diverted to wherever it is needed most. Some vitamin D stays within the liver and kidneys to assist reabsorption of calcium from the blood. The remaining vitamin D is distributed to the bones to help them hold onto calcium and to the intestines to help with absorption of calcium from food.

Vitamin D in Foods

Although the bulk of vitamin D is synthesized through exposure to sunlight, there are a few foods that do contain a bit of the vitamin naturally. This form of vitamin D is called vitamin D2, or ergocalciferol. D2 is used in the same way as the other types of D vitamins and it is used to make most vitamin D supplements. The few foods that naturally contain vitamin D are tuna, salmon, mackerel, fish liver oils, beef liver, egg yolks, cheese and mushrooms.


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