Table 5

Risk factors for low serum vitamin D levels

Inadequate exposure to the sun
 • Skin type- Dark skin requires up to 5 times the length of exposure because of melanin content
 • Season, latitude, angle of the sun- People living at latitudes higher than the 37th parallel cannot get adequate amounts of UVB from the sun during winter months
 • Use of sunscreen22,90- Continuous use of sunscreen with greater than factor 8 UVB protection22; controversial because of risk of skin cancer, but UVB decreases risk of internal cancer
 • Time of day- Ultraviolet B is at its maximum from 10:00 AM to 2:00 PM91; exposure to 1 minimal erythemal dose* in a swimsuit can provide the equivalent of 10 000 IU of VTD92
 • Covering the skinFor various religious or cultural reasons
Inadequate dietary intakeLimited intake of foods rich in VTD, such as oily fish and fish-liver oil, low intake of fortified foods or no use of supplements; strict vegans and non-milk drinkers are at higher risk93
ObesityIrreversible sequestration of VTD in the fat pool, especially if body mass index is >30 and person does little outdoor activity94
Exclusive breastfeedingBreast milk is low in VTD10,95; supplementing with 4000 IU of VTD has been shown to achieve adequate levels in both mother and child96
PregnancyAdequate maternal VTD levels are required to ensure fetal bone health and general health of mother and child97100
 • Decreased production of VTD through the skin- A 70-year-old person’s skin can synthesize only 25% as much VTD as a young person’s101102; conversion of 7-dehydrocholesterol in aging skin is considerably lessened103
 • Age-related lactose intolerance- Reduced intake of fortified milk
 • Immobility- More time housebound or in hospital; many are institutionalized
 • Aging kidneys- Decreased renal conversion of VTD
Comorbid conditionsMalabsorption syndromes, such as Crohn disease, Whipple disease, cystic fibrosis, and sprue, as well as severe liver disease23
Drug interactions
 • Drugs that impair VTD activation or increase its clearance- Phenytoin, carbamazepine, rifampin, cimetidine, thiazides104106; lithium raises parathyroid hormone levels and lowers levels of the active hormone 1,25-dihydroxyvitamin D107
 • Drugs that impair VTD absorption- Mineral oil laxatives or fat substitutes, such as Olestra24; obesity management medications, such as orlistat108; or bile-acid sequestrants, such as cholestyramine and colestipol109
Variations in metabolism of VTDSome Indo-Asians have increased 24-hydroxylase activity that results in low serum levels of 25-hydroxyvitamin D110
  • IU—international units, UVB—ultraviolet B, VTD—vitamin D.

  • * The amount of sunlight to which a person can be exposed before the skin begins to turn slightly red. Minimal erythemal dose varies from person to person depending on skin type.