What are the primary causes of vitamin D deficiency in adults, particularly those with limited sun exposure, inadequate diet, or underlying medical conditions such as celiac disease or Crohn's disease?

Primary Causes of Vitamin D Deficiency in Adults

The primary causes of vitamin D deficiency are inadequate sun exposure, insufficient dietary intake, and impaired intestinal absorption—with sun exposure being the dominant factor since most humans depend on UVB-mediated skin production for their vitamin D requirements. 1, 2, 3

Major Causative Factors

Inadequate Sun Exposure (Most Common)

• Limited UVB exposure is the leading cause because sun exposure is the primary natural source of vitamin D for most humans, and modern lifestyle factors severely restrict UVB-mediated skin production. 3
• Living at high latitudes reduces UVB availability, particularly during winter months when the sun's angle prevents adequate UVB penetration. 2, 3
• Indoor occupations, physical sun avoidance behaviors, and routine sunscreen use all dramatically decrease vitamin D synthesis. 3
• Homebound or institutionalized individuals have minimal sun exposure, placing them at particularly high risk. 2
• Time of day, season, and cloud cover significantly affect the length of UVB exposure needed to attain sufficient vitamin D levels. 4

Insufficient Dietary Intake

• Very few foods naturally contain vitamin D, and fortified foods are often inadequate to satisfy vitamin D requirements. 1
• Daily dietary vitamin D intake of 600 IU in adults aged 18-70 years and 800 IU in adults older than 70 years should meet the needs of 97.5% of the adult population, but average intake in the population is often below these levels. 4

Impaired Intestinal Absorption

• Inflammatory bowel disease (including Crohn's disease and celiac disease) directly impairs intestinal vitamin D absorption through mucosal inflammation and damage to the absorptive surface. 2
• History of gastric bypass surgery permanently reduces absorptive capacity by bypassing portions of the small intestine where vitamin D is absorbed. 2
• Other malabsorptive conditions reduce vitamin D absorption capacity. 1, 2

Population-Specific Risk Factors

Darker Skin Pigmentation

• Increased melanin content reduces the skin's ability to produce vitamin D in response to UVB exposure. 2
• African Americans show 2-9 times higher prevalence rates of low serum 25-(OH)D levels compared to white persons, and Hispanics show 2-3 times higher rates. 4, 2, 3
• Important caveat: Despite lower total serum 25-(OH)D levels in African Americans, bioavailable 25-(OH)D (when accounting for vitamin D-binding protein) may be adequate, and fracture risk in African Americans is actually half that of white persons. 4, 3

Older Age

• Older adults (>60 years) have reduced endogenous synthesis capacity even with identical sun exposure compared to younger individuals. 2
• Aging reduces the skin's ability to produce vitamin D in response to UVB exposure. 4

Obesity

• Greater sequestration of vitamin D into adipose tissue reduces bioavailability, though this vitamin D may still be bioavailable. 4, 2

Special Considerations for Underlying Medical Conditions

Nephrotic Syndrome

• Patients with nephrotic-range proteinuria lose vitamin D-binding protein and 25-hydroxyvitamin D in urine, creating increased vitamin D requirements. 2

Chronic Kidney Disease

• Even in dialysis patients, 25-(OH)D levels below 15 ng/mL worsen secondary hyperparathyroidism. 2

Inflammatory Conditions

• 25-(OH)D may act as a negative acute-phase reactant, and its levels may decrease in response to inflammation, which can complicate interpretation. 4

Clinical Pitfalls to Avoid

• Do not rely solely on total serum 25-(OH)D measurements in certain populations (particularly African Americans), as bioavailable vitamin D may differ from total levels. 2, 3
• Sun exposure to prevent vitamin D deficiency is not generally recommended because it increases the risk for skin cancer associated with UVB radiation. 4
• Variability between assay methods and between laboratories using the same methods may range from 10% to 20%, and classification of samples as "deficient" or "nondeficient" may vary by 4% to 32% depending on which assay is used. 4