Factors Affecting D3 Availablility and Conversion in the Body

Insufficient dietary fat - vitamin D is a fat-soluble vitamin and thus requires some dietary fat for conversion. Conversion may also be inhibited by damaged fats (E.g. trans fats).

Certain medical conditions may prevent dietary fat absorption - E.g. pancreatic enzyme deficiency, IBD (such as Crohn's and Whipple's disease), celiac disease, cystic fibrosis, gall bladder or liver disease, partial/total removal of stomach or intestines. Symptoms of fat malabsorption include diarrhea and greasy stools.

Skin Color - A higher melanin content in darker skin reduces D3 production. A tan is the skin's inbuilt protection against producing too much vitamin D.African Americans and other dark-skinned persons who do not get sufficient sun exposure, are at a high risk for vitamin D deficiency.

Parathyroid gland or Kidney Diseases - impair synthesis of CALCITRIOL, the active form of vitamin D.

Aging –production of vitamin D precursors (7-dehydro-cholesterol) and Kidneys'conversion ability diminishes with age. A 70 year old has only 25% of the capacity of a young adult to produce vitamin D3.

MacLaughlin J, Holick MF, Aging decreases the capacity ofhuman skin to produce vitamin D3, J. Clin Invest 1985, 76:1536-1538.

However, the elderly are still able to satisfy their vitamin D needs by going into the sun

 Davie M, et al, 1980; Holick MF, Vitamin D deficiency, N. Eng J Med 2007; 357:266-281

or by utilizing a full-spectrum tanning bed.

Tangpricha V et al, Tanning is associated with optimal vitamin D status and higher bone mineral density, Am J Clin Nutr, 2004

Sunscreen - an SPF of 8 has the capacity to reduce D3 production in the skin by 95%

Matsuoka LY, et al, Sunscreens suppress cutaneous vitamin D3 synthesis, J Clin Endocrinol Metab, Jun;64(6):1165-8, 1987

Obesity - in one study, when exposed to the same amount of natural sunlight, obese subjects produced only about half the amount of vitamin D3 as normal weight subjects.The study concluded that vitamin D insufficiency is likelydue to the decreased bioavailability of vitamin D3 from cutaneousand dietary sources because of its deposition in body fat compartments

Wortsman J, et al, Decreased bioavailability of vitamin D in obesity, Am. J. of Clin. Nutr., 72(32):690-693, 2000