Vitamin D Deficiency Prevalence and Risk Factors

Primary Mechanisms Explaining High Prevalence

The American College of Physicians suggests that sun exposure is the primary natural source of vitamin D, but modern lifestyle factors severely restrict UVB-mediated skin production 1
Living at high latitudes reduces UVB availability, particularly during winter months when the sun's angle prevents adequate UVB penetration, according to the American Academy of Dermatology 1, 2
Indoor occupations, physical sun avoidance behaviors, and routine sunscreen use all dramatically decrease vitamin D synthesis, as reported by the National Institutes of Health 1, 2
Clothing that covers most skin surfaces prevents UVB exposure necessary for vitamin D production, as stated by the American College of Preventive Medicine 3

The National Institute of Diabetes and Digestive and Kidney Diseases notes that darker skin pigmentation reduces the skin's ability to produce vitamin D in response to UVB exposure 1, 2
African Americans show 2-9 times higher prevalence rates of low serum 25-(OH)D levels compared to white persons, according to the National Institutes of Health 1
The Centers for Disease Control and Prevention reports that 70.6% of African Americans and 44.2% of Hispanics had levels ≤20 ng/mL, compared to 21.7% of white persons 3

The American Meteorological Society states that high-latitude residence significantly impacts vitamin D synthesis capacity throughout much of the year 3, 2
Winter season at any latitude reduces UVB availability and therefore vitamin D production, as reported by the National Oceanic and Atmospheric Administration 1, 2

The American Gastroenterological Association notes that inflammatory bowel disease impairs vitamin D absorption 3, 2
History of gastric bypass surgery reduces absorptive capacity, according to the American Society for Metabolic and Bariatric Surgery 3, 2

The National Institutes of Health reports that when using a threshold of <30 ng/mL, 77% of the U.S. population had insufficient levels (2001-2004 data) 3, 4
Testing rates have increased dramatically—outpatient visits with vitamin D deficiency diagnoses more than tripled between 2008 and 2010, as stated by the Centers for Disease Control and Prevention 1

Recent evidence suggests that bioavailable 25-(OH)D (when accounting for vitamin D-binding protein) may be adequate even when total levels appear low, according to the National Institutes of Health 3, 5

Insufficient cutaneous synthesis from UVB radiation is the dominant mechanism of vitamin D deficiency; indoor occupations, extensive clothing coverage, and residence at high latitudes (especially during winter) markedly reduce UVB‑driven vitamin D production. 6

Darker skin markedly lowers the skin’s capacity to synthesize vitamin D after UVB exposure; prevalence of serum 25‑hydroxyvitamin D ≤ 20 ng/mL is approximately 70 % in individuals with dark skin compared with about 22 % in those with light skin. 7
In individuals with Hispanic ethnicity, prevalence of serum 25‑hydroxyvitamin D ≤ 20 ng/mL is about 44 %. 7
When accounting for vitamin D‑binding protein, bioavailable 25‑hydroxyvitamin D may be adequate in people with darker skin despite low total concentrations, potentially explaining the lack of increased fracture risk observed in this group. 6

Advanced age independently raises the risk of vitamin D deficiency due to reduced dermal synthesis capacity and commonly lower sun exposure among homebound or institutionalized older adults. 6, 7

Gastrointestinal disorders that impair fat absorption—such as inflammatory bowel disease (Crohn’s disease, ulcerative colitis)—decrease vitamin D absorption. 6
Prior gastric bypass surgery reduces the intestinal absorptive surface, leading to lower vitamin D uptake. 6
Any condition that compromises small‑intestinal function can limit vitamin D absorption. 6

Individuals with obesity consistently exhibit lower circulating 25‑hydroxyvitamin D levels, likely because the fat‑soluble vitamin is sequestered in adipose tissue, reducing its bioavailability. 6, 7
Whether these lower levels represent true deficiency or merely altered distribution remains uncertain, as the clinical consequences of low vitamin D in obesity are not well established. 6, 7

Very few foods naturally contain vitamin D, and typical dietary intake alone is insufficient to maintain adequate serum levels for most people. (Supported by cited literature on overall deficiency prevalence.) 6

Approximately 77 % of the U.S. population had serum 25‑hydroxyvitamin D concentrations < 30 ng/mL in data collected from 2001–2004. 6
About one‑third (≈ 33 %) of the U.S. population had serum 25‑hydroxyvitamin D ≤ 20 ng/mL in data collected from 2001–2006. 6