Risk factors for low serum vitamin D levels
| RISK FACTORS | REASON |
|---|---|
| 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 skin | For various religious or cultural reasons |
| Inadequate dietary intake | Limited 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 |
| Obesity | Irreversible sequestration of VTD in the fat pool, especially if body mass index is >30 and person does little outdoor activity94 |
| Exclusive breastfeeding | Breast milk is low in VTD10,95; supplementing with 4000 IU of VTD has been shown to achieve adequate levels in both mother and child96 |
| Pregnancy | Adequate maternal VTD levels are required to ensure fetal bone health and general health of mother and child97–100 |
| Age | |
| • 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’s101–102; 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 conditions | Malabsorption 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, thiazides104–106; 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 VTD | Some 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.