Vitamin D first of all isn't technically a vitamin since the body can produce it internally from cholesterol as long as it has access to sunlight. However, this ability is reliant on pigment and most white people have a shortage of Vitamin D, especially if you life in a place that doesn't have much sun. I live in Southern California, drive a convertible and generally spend quite a lot of time in the sun and I had a pretty severe shortage when I tested myself during my last check-up about a year and a half ago. My doctor suggested I would start eating supplements and I did.
Having low levels of Vitamin D can lead to any number of serious health problems (Source Wikipedia).
- Bone Health
- Multiple sclerosis
- Immune Deficiency
I can just from a personal note say that I have not had a serious cold or flu since I started taking Vitamin D supplements and I usually get sick at least once a year normally (Usually in the early spring). It's also ironic that something that improves by tanning is also good to prevent cancer, but so seem to be the case.
As always with anything medical if you want to test this out, check with your doctor. Too high levels of Vitamin D can also be harmful so take a blood test, my doctor told me that this was just recently added as something they normally test for during a routine check up here in the US, but I would ask about it just to be sure.