The source matters because there are two common types of vitamin D: D2 and D3. Sunlight lets you produce D3, while supplements are typically D2. In food that's

The source matters because there are two common types of vitamin D: D2 and D3. Sunlight lets you produce D3, while supplements are typically D2. In food that's been fortified, it will probably have D2, while food that has natural vitamin D (like fatty fish) will probably have D3. Although the two forms are metabolized through different mechanisms, the difference is negligible for most people. However, for some people (depending on genetics, diet, or lifestyle), supplemental Vitamin D2 will not be an adequate replacement for Vitamin D3. [0] http://ajcn.nutrition.org/content/84/4/694.full http://ajcn.nutrition.org/content/84/4/694.full