What are the reasons for poor vitamin D absorption, particularly in older adults with limited sun exposure and gastrointestinal disorders such as celiac disease or Crohn's disease?

Reasons for Poor Vitamin D Absorption

Poor vitamin D absorption primarily results from gastrointestinal malabsorptive disorders (inflammatory bowel disease, celiac disease, Crohn's disease, history of gastric bypass), inadequate sun exposure, and older age—all of which significantly impair vitamin D status and increase fracture risk. 1, 2

Primary Mechanisms of Poor Absorption

Gastrointestinal Malabsorptive Conditions

• Inflammatory bowel disease (IBD) directly impairs intestinal vitamin D absorption through mucosal inflammation and damage to the absorptive surface 1, 3
• Celiac disease causes malabsorption of fat-soluble vitamins, including vitamin D, due to villous atrophy and reduced absorptive capacity 4
• Crohn's disease patients have multiple contributing factors: inadequate dietary intake, impaired conversion to active metabolite, increased catabolism, increased urinary excretion, and genetic variants affecting vitamin D hydroxylation and transport 5
• History of gastric bypass surgery permanently reduces absorptive capacity by bypassing portions of the small intestine where vitamin D is absorbed 1, 3

Urinary Losses in Nephrotic Syndrome

• Patients with nephrotic-range proteinuria lose vitamin D-binding protein (DBP) and 25-hydroxyvitamin D in urine, creating increased vitamin D requirements 1
• This mechanism is particularly relevant in chronic kidney disease patients with significant proteinuria 1

Contributing Factors Beyond Absorption

Limited Sun Exposure (Especially in Older Adults)

• Reduced outdoor activity and physical sun avoidance dramatically decrease UVB-mediated skin production of vitamin D 2, 3
• Older adults (>60 years) have reduced endogenous synthesis capacity even with identical sun exposure compared to younger individuals 1
• Living at high latitudes or during winter months prevents adequate UVB penetration necessary for vitamin D synthesis 1, 3
• Homebound or institutionalized individuals have minimal sun exposure, placing them at particularly high risk 1

Darker Skin Pigmentation

• Increased melanin content reduces the skin's ability to produce vitamin D in response to UVB exposure 1
• African Americans show 2-9 times higher prevalence of low 25-(OH)D levels compared to white persons 1, 3

Obesity

• Greater sequestration of vitamin D into adipose tissue reduces bioavailability, though the clinical significance remains debated 1, 2

Drug Interactions Affecting Absorption

• Mineral oil interferes with absorption of fat-soluble vitamins, including vitamin D preparations 6, 7
• This is a commonly overlooked but easily preventable cause of poor absorption 6

Clinical Implications in High-Risk Populations

Chronic Kidney Disease Patients

• Multiple mechanisms converge: reduced sun exposure due to inactivity, lower dietary intake of vitamin D-rich foods (fish, dairy), reduced skin synthesis capacity, and urinary losses with proteinuria 1
• Even in dialysis patients, 25-(OH)D levels below 15 ng/mL worsen secondary hyperparathyroidism 1

Inflammatory Bowel Disease

• Vitamin D deficiency prevalence is substantially higher than the general population due to disease-specific factors including inflammation and malabsorption 4
• Inadequate sun exposure compounds the malabsorption problem in these patients 5

Critical Pitfalls to Avoid

• Do not assume dietary supplementation alone will correct deficiency in malabsorptive conditions—higher doses are typically required 4
• Monitor for concurrent calcium deficiency, as adequate dietary calcium is necessary for clinical response to vitamin D therapy 6, 7
• In elderly patients, absorption of oral vitamin D may be attenuated, requiring dose adjustments 6, 7
• Avoid relying solely on total serum 25-(OH)D measurements in certain populations, as bioavailable vitamin D (accounting for binding protein) may differ from total levels 1