Biological and medical causes
Absorption of vitamin D from food is variable because vitamin D is a fat soluble nutrient that depends on normal digestion and transport. Michael Holick of Boston University School of Medicine has written about the need for bile salts to form micelles that carry vitamin D into intestinal cells and then into chylomicrons for lymphatic transport. Conditions that impair fat digestion such as pancreatic insufficiency, bile salt deficiency, celiac disease and Crohn disease reduce that micelle formation and therefore lower absorption. Surgical changes to the gut after procedures like Roux en Y gastric bypass bypass portions of the small intestine where absorption normally occurs and are well described by clinicians in endocrine and gastroenterology literature.
Genetic and physiological modifiers
Genetic differences also matter. Variants in the vitamin D binding protein and the vitamin D receptor alter how much vitamin D is carried in the blood and how cells respond to it, which helps explain why two people who eat the same food may show different blood levels. Robert P. Heaney of Creighton University emphasized that body composition affects bioavailability because vitamin D is sequestered in adipose tissue, so obesity can lower circulating concentrations even when dietary intake is similar. Age reduces efficiency too; older skin and intestine are less effective in handling vitamin D and its metabolites, a point underscored in clinical guidance from the Endocrine Society.
Medications, diet and cultural context
Certain medications such as anticonvulsants and glucocorticoids accelerate vitamin D breakdown and reduce blood levels, and diets very low in fat or lacking fortified foods reduce intake opportunities. JoAnn Manson of Harvard T H Chan School of Public Health has examined how dietary patterns and fortification policies shape population vitamin D status, noting that cultural practices around sun exposure and clothing influence total vitamin D status because dietary sources provide only part of the body pool. In communities with limited sun exposure and low dietary fortification, inefficient absorption has greater practical impact.
Consequences and clinical relevance
Reduced absorption raises the risk of vitamin D deficiency with consequences for bone health including osteomalacia in adults and rickets in children and for increased fracture risk over time. It can also influence immune function and metabolic health in ways that are still under study, which is why expert groups and clinicians assess risk factors such as malabsorption, medications, obesity and genetics when testing and treating. Addressing underlying digestive disorders, adjusting dietary fat and using targeted supplementation under medical guidance often restores adequate vitamin D status when absorption from food is insufficient.