Causes of Vitamin D Deficiency in Urban Populations

10/31/20252 min read

Overview

Vitamin D deficiency in cities is common. It results from reduced cutaneous synthesis of vitamin D₃ from ultraviolet-B (UVB) light plus low dietary intake and medical factors that impair absorption or metabolism. Multiple environmental and behavioral features of urban life converge to lower serum 25-hydroxyvitamin D [25(OH)D] levels. Office of Dietary Supplements+1

Environmental and Built-Environment Drivers

  • Air pollution attenuates UVB. Particulate matter (PM), nitrogen dioxide, and other pollutants scatter and absorb UVB, reducing the skin’s capacity to make vitamin D. Observational and ecological studies associate higher pollution with lower 25(OH)D. PMC+3Nature+3PMC+3

  • “Urban canyon” shading. High-rise density and narrow streets limit direct sun angles and exposure windows, decreasing ambient UVB at street level and indoors. (Inference from built-environment physics aligned with UVB requirements for cutaneous synthesis.) Office of Dietary Supplements

  • Indoor, climate-controlled living. Greater time spent inside offices, schools, transit, malls, and gyms reduces sun exposure compared with outdoor rural work. NCBI

Behavioral Factors

  • Work and lifestyle patterns. Office and shift workers often miss midday UVB. Commuting in enclosed vehicles and screen-time leisure further reduce incidental sun. NCBI

  • Sun avoidance practices. Regular sunscreen use, sun-protective clothing, and deliberate shade seeking lower skin UVB dose; these behaviors are more prevalent in urban settings. NCBI

Dietary and Policy Factors

  • Low intake of vitamin D–rich foods. Typical urban diets provide limited vitamin D unless fatty fish, egg yolks, or cod liver oil are frequent. Office of Dietary Supplements

  • Insufficient food fortification. Where fortification policies are limited or inconsistently implemented, dietary vitamin D remains low, increasing reliance on sun exposure. Office of Dietary Supplements

Physiologic and Demographic Factors that Interact with Urban Living

  • Darker skin pigmentation. Higher melanin reduces cutaneous vitamin D synthesis for a given UVB dose; in urban contexts with already low UVB exposure, deficiency risk rises. MedlinePlus

  • Aging. Older adults synthesize less vitamin D in skin and may have reduced renal activation; many live largely indoors in cities. MedlinePlus

  • Obesity. Vitamin D is sequestered in adipose tissue, lowering circulating 25(OH)D; obesity prevalence is often higher in urban areas. Office of Dietary Supplements

  • Pregnancy and lactation; exclusively breastfed infants. Increased maternal demand and low vitamin D in human milk elevate risk without supplementation. Urban indoor living compounds this. World Health Organization+1

Medical and Pharmacologic Causes

  • Malabsorption disorders. Celiac disease, inflammatory bowel disease, cystic fibrosis, and bariatric surgery reduce vitamin D absorption. Office of Dietary Supplements

  • Chronic liver or kidney disease. Impairs hepatic 25-hydroxylation or renal 1-α-hydroxylation, lowering active vitamin D. NCBI

  • Medications. Enzyme-inducing anticonvulsants, glucocorticoids, antiretrovirals, and some antifungals can increase catabolism or reduce activation of vitamin D. NCBI

Seasonal and Geographic Modifiers

  • Latitude and season. At higher latitudes or during winter, urban residents receive less UVB. Pollution and shading magnify this seasonal deficit. Office of Dietary Supplements

Systems View: Why Cities Drive Risk

Urban environments decrease UVB dose (pollution, shading), human behavior lowers exposure time (indoor work, transport, avoidance), and diet often fails to compensate (limited fortification). Physiologic factors then determine who becomes deficient within this low-UVB, low-intake context. Clinicians should consider these layered risks when deciding whom to test and treat. Endocrine Society+1

Key Sources

NIH Office of Dietary Supplements fact sheet; StatPearls clinical review; Endocrine Society guideline; studies on air pollution and UVB/vitamin D. clinmedkaz.org+5Office of Dietary Supplements+5NCBI+5

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