More on Vitamin D Deficiency
 

Further, most of us have been dutifully eating less fat, especially from things like lard and eggs, and hence deficiencies tend to be widespread.  Also, the utilization of D in the body seems to be lower when we consume lots of polyunsaturated (from many vegetable oils) and monounsaturated (from olive and canola oils) fatty acids, like many of us do these days.  Trans-fatty acids in addition seem to interfere in some way with the enzymes the body needs to use vitamin D, and trans-fatty acids are practically impossible to avoid unless you don't eat out or eat any packaged foods.

Deficiency seems to result in soft teeth and brittle bones, nearsightedness, psoriasis, insomnia, nosebleeds and fast heartbeat.  A more severe deficiency causes rickets in children, which creates symptoms like weak, soft leg bones and flaring ankles and wrists.  There is also mounting evidence that a lack of vitamin D in the diet may contribute to cancer, as well as osteoporosis.

According to an article on Online Science News, the rates of MS and other autoimmune diseases are rare in people who live near the equator, where the UV-B rays from the sun are intense and people produce lots of vitamin D year round.  Low blood concentrations of D were also linked to gum disease, this from a federally sponsored National Health and Nutrition Examination Survey of over 10,000 people.
 

Absorption of Vitamin D
 

Because vitamin D is absorbed in the jejunum and the ileum of the small intestine, it depends on enough bile salts in order to be properly absorbed.  As a fat soluble vitamin, D also needs dietary fat in order to be properly absorbed.  Hence anyone who has a hard time absorbing fats, like those with sprue or celiac disease, Cystic Fibrosis (CF), pancreatic enzyme deficiency, Crohn's disease or some types of liver disease, might have a hard time getting enough vitamin D from their foods.  (Some symptoms of fat malabsorption are having oily stools and diarrhea.)  Obesity or other surgery, where they remove part of the stomach or intestines or staple it shut often keeps you from absorbing enough of many different nutrients, including fats.   For these people, exposure to sun at the right time is especially important.

Vitamin D Supplementation
D Toxicity

Adams, Hollis, "Vitamin D:  Syntheiss, Metabolism and Clinical Measurement."  In Coe, F.L. Favus, M.J. Disorders of Bone and Mineral Metabolism:  2^nd Edition, Baltimore, MD:  Lippincott Willimas and Wilkins, 2002. (Review)