← Vitamin D

The sunshine vitamin

What vitamin D is and why deficiency is so widespread

Vitamin D is a nutrient your body makes when sunlight hits your skin. It works more like a hormone than a typical vitamin, influencing hundreds of processes throughout your body. Despite being free to produce in sunshine, an estimated one billion people worldwide do not get enough of it [1].

Most people living above the 37th parallel (roughly north of Los Angeles or Athens) cannot make sufficient vitamin D from sunlight during winter months. Modern indoor lifestyles, sunscreen use, and darker skin pigmentation all reduce production further [2].

The good news: once you understand why deficiency happens, it is straightforward to fix.

Vitamin D is actually a group of fat-soluble secosteroids. The two main forms are vitamin D2 (ergocalciferol, from plants and fungi) and vitamin D3 (cholecalciferol, produced in human skin). When UVB radiation between 290-315 nm strikes the skin, it converts 7-dehydrocholesterol into previtamin D3, which thermally isomerizes into vitamin D3 [1].

From there, vitamin D3 travels to the liver where it is hydroxylated into 25-hydroxyvitamin D (calcidiol), the main circulating form used to measure status. A second hydroxylation in the kidneys produces the active hormone 1,25-dihydroxyvitamin D (calcitriol), which binds to vitamin D receptors found in nearly every tissue in the body [2].

Several factors converge to make deficiency common:

  • Latitude and season: UVB intensity drops sharply in winter at higher latitudes. At 52 degrees north (London), virtually no cutaneous vitamin D synthesis occurs from October through March [4].
  • Skin pigmentation: Melanin competes with 7-dehydrocholesterol for UVB photons. People with darker skin may need 3-6 times more sun exposure to produce equivalent vitamin D [1].
  • Age: Elderly individuals have roughly 75% less 7-dehydrocholesterol in their skin compared to young adults [3].
  • Sunscreen and clothing: SPF 30 sunscreen reduces vitamin D synthesis by more than 95% [1].
  • Obesity: Vitamin D is sequestered in adipose tissue, lowering bioavailability. Obese individuals often require 2-3 times higher doses to achieve adequate serum levels [3].

A large European meta-analysis found that 40% of Europeans had serum 25(OH)D levels below 50 nmol/L (20 ng/mL), and 13% were below 30 nmol/L (12 ng/mL), meeting criteria for severe deficiency [4].

The 2011 Endocrine Society guideline defines vitamin D deficiency as a serum 25(OH)D below 50 nmol/L (20 ng/mL) and insufficiency as 52.5-72.5 nmol/L (21-29 ng/mL), with a target of at least 75 nmol/L (30 ng/mL) for optimal health [3]. This threshold is higher than the Institute of Medicine's 50 nmol/L cutoff, and the discrepancy remains a point of debate.

Holick's landmark 2007 NEJM review consolidated evidence that vitamin D deficiency was far more prevalent than previously recognized, affecting populations across all age groups and geographies [1]. The review established that brief, sensible sun exposure (roughly 5-30 minutes between 10 AM and 3 PM, twice per week, without sunscreen on arms and legs) can maintain adequate levels in fair-skinned individuals during summer months. However, this recommendation does not apply universally given the variables of latitude, skin type, and UV index.

The Cashman et al. (2016) standardized analysis of 55,844 European individuals across 14 populations used the gold-standard LC-MS/MS assay or VDSP-standardized immunoassays to address inconsistencies in prior prevalence estimates [4]. Their finding that dark-skinned ethnic subgroups had deficiency rates reaching 65-70% underscored the importance of targeted screening and supplementation strategies in at-risk populations.

Dietary sources of vitamin D are limited. Fatty fish (salmon, mackerel, sardines) provide 400-1000 IU per serving, while fortified milk typically contains 100 IU per cup [2]. These amounts are generally insufficient to maintain optimal levels without sun exposure or supplementation, particularly during winter months in temperate climates.

References

  1. Vitamin D deficiencyHolick MF. New England Journal of Medicine, 2007. PubMed 17634462 →
  2. Vitamin D - Fact Sheet for Health ProfessionalsNIH Office of Dietary Supplements. NIH Office of Dietary Supplements, 2024. Source →
  3. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guidelineHolick MF, Binkley NC, Bischoff-Ferrari HA, Gordon CM, Hanley DA, Heaney RP, Murad MH, Weaver CM. Journal of Clinical Endocrinology and Metabolism, 2011. PubMed 21310306 →
  4. Vitamin D deficiency in Europe: pandemic?Cashman KD, Dowling KG, Skrabakova Z, Gonzalez-Gross M, Valtuena J, De Henauw S, Moreno L, Damsgaard CT, Michaelsen KF, Molgaard C, Jorde R, Grimnes G, Moschonis G, Mavrogianni C, Manios Y, Thamm M, Mensink GB, Rabenberg M, Busch MA, Cox L, Meadows S, Sherber S, Kiely M. American Journal of Clinical Nutrition, 2016. PubMed 32252338 →

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