1. Many people do not get enough sun exposure to create their own vitamin D.

2. Some bodies produce more Vitamin D for the same amount of sun exposure as other bodies. For example, darker skin produces less vitamin D.

3. Some bodies convert Vitamin D into other useful compounds less efficiently than others. This is commonly done in the liver and kidneys.

4. Those with Vitamin D deficiencies have problems with their muscles, bones, and immune systems. We do not fully understand the impact of Vitamin D, but it also seems to have an impact on mental health, and is a key part of SAD.

All vitamins are essential, that's pretty much where the name comes from: "vital amine" (not to be confused with must see Japanese cartoons).

There are diseases associated with shortage of vitamins, such as scurvy for vitamin C, or rickets for D.

Contrary to popular belief vitamin D is a hormone. Hormones are your body's chemical messengers. Having a deficiency in a hormone can have a lot of negative effects.

There are two sources of vitamin D: food & supplements and synthesis by the body in the skin on exposure to ultraviolet rays from the sun. Since a lot of us spend a lot of time indoor or behind glass (which blocks the necessary wavelengths) it's important to monitor it.

People with darker skin also need more sun exposure to get the required healthy levels. People living in areas where winters are very dark and very long are also at risk of deficiency.

Um… search pubmed for vitamin D. It plays an essential role in arming and activating the immune system. Low vitamin d is associated with increased risk of cancer heart disease and overall mortality.

Vitamin D is a precursor to a wide range of essential human hormones including both estrogen and testosterone.