Vitamin D plays an interesting role in may aspects of human health. It plays a role in disease prevention including osteoporosis, some cancers, autoimmune disorders, hypertension, diabetes and has recently been found to effect weight loss.

What is Vitamin D?  It is an oil-soluble (fat-soluble) vitamin that helps in the absorption of calcium and phosphorus in the intestine and suppresses parathyroid hormone (PTH), the hormone that stimulates bone resorption (breakdown). Vitamin D also plays a role in muscle function and in the immune system, but our understanding in these roles are still limited. (1)
Vitamin D can be found in fatty fish, cod-liver oil and eggs. In the United States, cow’s milk is supplemented with Vitamin D and this is often the source from which most people obtain it. Deficiency in Vitamin D often occurs from lack of sun exposure, inadequate intake, surgery to or damage of the intestines ability to absorb, or from kidney or liver damage resulting in problems processing Vitamin D. Recent studies reveal that obesity is a major factor in altering the way the body uses Vitamin D and stimulates PTH.

I find that about 30-40% of my patients are Vitamin D deficient.  Many researches claim this is due to poor sun exposure or the use of sun screens, however, I live in Arizona.  Sun “over exposure” is usually the problem here, yet I still find that 30-40% of my patients are deficient.  My patients should be able to get enough sunlight walking from their cars to the grocery store entrance.  I disagree that “lack of sun exposure is the cause.”  Although our current labs claim vitamin D levels should be above 20 ng/dl, I find people do not get the needed effect until 25 Hydroxy-Vitamin D levels should are greater than 32-35 ng/dl.
Poor Vitamin D intake is usually the problem.

Our bodies convert 25 Hydroxy-Vitamin D into the active molecule 1,25 Dihydroxy-Vitamin D.  Recent studies reveal that higher Body Mass Index (BMI) leads to lower conversion of 25 Hydroxy-Vitamin D to 1,25 Dihydroxy-Vitamin D. (2,3)

Simply adding 25 Hydroxy-Vitamin D as a supplement frequently helps with weight management in many of my patients. Supplementation with 1000-2000 IU is often adequate.  Higher doses should be discussed with your doctor.

References:
1. C P Earthman, L M Beckman, K Masodkar and S D Sibley. The link between obesity and low circulating 25-hydroxyvitamin D concentrations: considerations and implications.International Journal of Obesity (2012) 36, 387–396; doi:10.1038/ijo.2011.119; published online 21 June 2011.
2. Frost M, Abrahamsen B, Nielsen TL, Hagen C, Andersen M, Brixen K. Vitamin D status and PTH in young men: a cross-sectional study on associations with bone mineral density, body composition and glucose metabolism. Clin Endocrinol (Oxf) 2010; 73: 573–580.

3. Konradsen S, Ag H, Lindberg F, Hexeberg S, Jorde R. Serum 1,25-dihydroxy vitamin D is inversely associated with body mass index. Eur J Nutr 2008;47: 87–91.