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Vitamin D is the new kid on the block. Long neglected andthought of only as a bone vitamin that was needed to regulate calcium metabolism, it is now known to be involved in many more biological reactions. The most significant finding in recent years was that vitamin D3, or calciferol, was not the active form of vitamin D. That honor belongs to the molecule known by the chemical name 1 alpha, 25 dihydroxyvitamin D3.
Vitamin D is derived from the cholesterol molecule. In the skin, cholesterol is converted to a new form called 7-dehydrocholesterol, which reacts with ultraviolet B (UVB) light at wavelengths between 270–300 nm (peak 295–297 nm). These wavelengths are found in sunlight when the UV index is greater than 8–9<sup>a</sup>. In temperate regions during the spring and summer, adequate amounts of vitamin D3 as the provitamin D can be made in the skin with only 10–15 minutes of sun exposure at least twice a week. Although not recommended, exposing the face, arms, hands or back without sunscreen makes enough vitamin D to meet the daily requirement of the body. If you make more vitamin D by being in the sun for longer periods, the body will simply destroy it. You can see the cholesterol molecule in Figure 1.
The only active form of vitamin D is the 1 α, 25, dihydroxy D3. There are two starting molecules from which active vitamin D can be synthesized. One form is called cholecalciferol, which is formed directly from 7-dehydrocholesterol and is derived from cholesterol. See Figure 1 for a clear illustration. The second starting material is ergocalciferol, called vitamin D2, which is formed from ergosterol, a compound that is very similar to cholesterol that occurs in plants and fungi.
In the skin, newly made cholecalciferol is sent to the liver where it is converted to 25 hydroxy vitamin D3 by enzymes in the mitochondria, which is the first essential step in making active vitamin D. When you see or use the word “vitamin D” it will mean 1 α, 25, dihydroxy D3. Now it must have one more hydroxy group (OH) attached at the number one carbon in the alpha position. This process occurs in the kidney in the proximal tubule. At this stage, it is the real McCoy—vitamin D. Here are the steps in sequence so you can remember them:
From cholesterol to 7-dehydrocholesterol to cholecalciferol to 25 hydroxy D3 (in liver) to 1 α, 25, dihydroxy D3 (in kidneys). Really, it’s very easy.