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, 1985

However, the elderly are still able to satisfy their vitamin D needs by going into the sun  Davie, 1980 or by utilizing a full-spectrum tanning bed. Tangpricha V, 2004

Sunscreen.   An SPF of 8 has the capacity to reduce D3 production in the skin by 95%.  Matsuoka, 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, 2000

References

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

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

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

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

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