Vitamin D

Vitamin D is a hormone; a chemical substance produced in the body which has a specific regulatory effect on the activity of certain cells or a certain organ or organs. It has various uses within the body:

- It is needed for the absorption and metabolism of calcium and phosphorus, among many other functions, these minerals are necessary for bone health (higher vitamin D levels are associated with greater bone density).

- It regulates the immune system and therefore is beneficial in treating viruses and infections etc.

- It may reduce the risk of multiple sclerosis (I don't know whether or not it treats it though): http://www.telegraph.co.uk/health/healthnews/8444739/Health-benefits-of-vitamin-D.html the figure of 80% seems very high but could be true.

- Reduces cognitive impairment and age-related decline, for example poorer memory or even dementia or Alzheimer's disease.

- Some people take it to reduce the severity of asthma attacks. So it seems to have efficacy in treating this disease.

- It reduces the risk of rheumatoid arthritis in women.

- Many studies have shown that it has a powerful role in reducing incidence of cancer, especially colon cancer: http://www.reuters.com/article/2011/09/01/us-vitamind-idUSTRE78063U20110901 and
http://jnci.oxfordjournals.org/content/98/7/428.full.pdf

- It also increases the recovery rate from tuberculosis.

- Finally, it may prevent heart attacks and increase longevity.

Various uses of vitamin D
http://www.medicalnewstoday.com/articles/161618.php

According to doctor Oz, dietary supplementation of 2,000 to 3,000 IU of vitamin D is completely safe and nowhere near the levels which could cause toxicity. This is supported by evidence suggesting that you can make around 10,000 IU of vitamin D by exposing yourself to direct sunlight for 10 to 15 minutes. Vitamin D is made when exposed to UVB rays from the sun, and consequently using a sun lotion which blocks these rays will compromise your natural ability to produce vitamin D. This reduces total body vitamin D. However, serum (this is essentially blood plasma) vitamin D levels can be reduced by being overweight. This is because vitamin D is fat soluble. In a study comparing individuals who were of normal weight and those who were obese it was found that when both groups were exposed to the same level of ultraviolet radiation, the obese participants had a 55% lower serum level of vitamin D.

Typically, a healthy level of vitamin D is shown by a serum level of more than 30ng/ml, but many experts say that this is still too low and a level of around 60 to 80ng/ml is more appropriate. Some doctors place the safe upper limit at 100 ng/ml but life guards who spend a lot of their time on sunny beaches tend to have serum levels at around 125ng/ml with no signs of toxicity. In fact, serum levels of 200ng/ml may even be safe, but not many people reach this level. For average individuals; every 100 IU of vitamin D that is ingested results in a rise of 1ng/ml of serum vitamin D (this is measured by checking the levels of 25(OH)D). Also, regardless of race, vitamin D serum levels predict bone density (remember its effects on calcium and phosphorus). This is important because people of darker skin are more resistant to UVB rays and hence create lower amounts of vitamin D per amount of ultraviolet radiation. This means that these same people, when living in countries which have fewer hours of sunlight, actually have a very high chance of being vitamin D deficient. In a Boston study it was found that those who tanned once per week in a tanning bed had much higher levels of serum vitamin D (an average of ~48ng/ml) than those who didn't tan (~17-18ng/ml). The people who tanned also had noticeably increased bone density compared to those who didn't.

I can't tell which is more important, D2 or D3, there are conflicting reports, most people tend to think that D3 is more beneficial but I am unsure why and tests on rats (they metabolise many substances in a similar way to humans) indicate that D2 is better for them.