The latest issue of the Journal of Nutrition published a study done on 1,800 people in Norway. They measured vitamin D levels and found that the more obese the lower the levels of vitamin D.

Obesity associated with lower vitamin D levels made sense since vitamin D is a fat-soluble vitamin and much of the vitamin produced in the skin or ingested is distributed in fat tissue. The study suggested that people who are obese may be less able to convert vitamin D into its hormonally active form.
Studies are ongoing regarding the recommended dosage for disease prevention. The Institute of Medicine (IOM) warned that intake of vitamin D over 4,000 IU (international units) per day increases the risk for harm to the body.

Along with calcium, vitamin D has long been recognized as important for bone health. Recently, studies have suggested that the vitamin plays a part in a certain diseases, including heart disease, diabetes, autoimmune diseases and some cancers. Obesity is a known risk factor for many of these diseases.