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Reference topic 19: The Largest Organ

Vitamin D from sun vs supplements, and what skin evolved for

ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.79 sources7 graded recommendationsTopic hub: Sleep and light

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In 1903, workers improving the drainage at Gough's Cave in Cheddar Gorge, Somerset, uncovered a nearly complete human skeleton. Radiocarbon dating later put the man at about 10,000 years old, the oldest almost complete Homo sapiens skeleton ever found in Britain. Like everyone in Europe at the time, he could not digest milk as an adult.1

For more than a century Cheddar Man had bones but no complexion. Then scientists recovered his DNA from the petrous bone of his inner ear, the densest bone in the body.12 A forensic system that predicts appearance from genes gave its most probable answer: "blue/green eyes, dark brown possibly black hair, and dark or dark to black skin."2 Dutch model makers then built a face on a copy of his skull.13 It showed the oldest near-complete Briton with pale eyes and dark skin.3

Mismatch card

The system

Skin: pigment shield, vitamin D factory, barrier and habitat.

What it expects

Sun matched to its pigment, changing gradually with the seasons, mostly outdoors and often in shade.

What it gets

Pale skin under subtropical sun, dark skin at high latitudes, indoor lives and sudden, burning doses.

What happens in the gap

Skin cancer where sun outruns pigment; low vitamin D, and at the extreme rickets, where pigment outruns sun.

The handrail

Regular unburnt sun, protection for long exposure, vitamin D where sun falls short, clean hands (●●○)

Strength of evidence

●●○ Geography, ancient DNA, cohorts and one sunscreen trial support the picture, but vitamin D pills have not prevented major disease in adults who mostly had enough.

What this topic covers

  • ·A Compromise Written in Melanin
  • ·Pale Is Recent
  • ·The Indoor Winter
  • ·A Pill Is Not the Sun
  • ·Pale Skin Under a Southern Sun
  • ·A Habitat, Not a Wrapper

The practical version is in the book: Chapter 7, Light and Dark.

The handrail

Each recommendation carries the strength of the evidence behind it. How the grades work.

  1. Get short, regular, unburnt sun

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    When the UV index is 3 or above, a few minutes outdoors on most days keeps most people's vitamin D adequate; when it falls below 3, as in a southern Australian winter, spend time outside at midday with some skin uncovered.76 At high latitudes in winter the sun makes little or no vitamin D (in Britain, not enough from October to early March).2612 Darker skin needs longer.1034 Stop well before pink. With past skin cancer, many moles, drugs that make skin sensitive to light, or an organ transplant, follow your clinician's advice.

  2. Never burn, and protect children and teenagers most

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    48

  3. For long exposure, use shade, clothing and a hat, then sunscreen

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    Evidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism. for squamous cell cancer·Evidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials. for melanoma

    When the UV index is 3 or above, Cancer Council Australia advises SPF 50 or 50+, broad-spectrum and water resistant, as "the last line of defence."77 Use it to prevent burning, not to stay out longer.71 Keep babies under six months in shade and clothing, and ask a pediatrician about sunscreen.

  4. Skip sunbeds

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    First use before 35 is associated with 87% higher melanoma risk.78

  5. Take vitamin D when the sun cannot supply it

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Evidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials. against deficiency·Evidence grade: Emerging. Early, small, animal-only, or actively contested. against cancer, heart disease or depression

    Britain's National Health Service (NHS) suggests 10 micrograms (400 IU) a day in autumn and winter, all year with dark skin or little time outdoors, and no more than 100 micrograms (4,000 IU) a day over age 10 (50 micrograms at ages one to 10) unless a doctor advises otherwise.12 The US recommended intake is 600 IU to age 70, then 800 IU.28 Prefer daily doses to big monthly ones, which cut no deaths in D-Health.7940 With kidney disease, sarcoidosis or high blood calcium, ask a doctor first.

  6. Test with a reason

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    The Endocrine Society found no trial evidence for routine screening, even for people with dark skin, and suggests against it.79 Ask about a test if you have a condition such as osteoporosis, malabsorption or kidney disease.

  7. Wash as often as suits you, and wash your hands always

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    Evidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism. for handwashing·Evidence grade: Emerging. Early, small, animal-only, or actively contested. that washing less helps skin

    Plain soap is enough.64 With eczema, seal each bath with moisturizer straight away: in a small trial in children, twice-daily baths sealed this way beat twice-weekly ones (●●○).80

The skeptic's corner

The case against this topic, from the research itself.

Vitamin D is this chapter's clearest case of an association outrunning its trials. Low levels travel with obesity, poor health and less education, any of which could explain a link to disease.29 The Institute of Medicine judged the evidence for cancer, heart disease, diabetes and autoimmunity "inconsistent, inconclusive as to causality."28 A genetic workaround, Mendelian randomization, treats inborn gene variants that nudge vitamin D as a natural experiment. One such study found deaths climbing steeply below about 20 ng/mL. Its method was later questioned, and the authors' own reanalysis weakened the case.42 Testing boomed anyway: US Medicare vitamin D tests rose 547% in under a decade.65 What would change the picture is a large trial in people who are truly deficient.

The skin-color model has its own soft spot. Its folate half rests on thin human evidence: in a 24-person trial, ultraviolet A (UVA) light from sunbeds did not lower blood folate.66 Rival ideas have their advocates. Skin cancer, which kills many people with albinism young at low latitudes, may have driven the first darkening instead.67 So may drought and UVB stressing the skin's water barrier.68

The claim that sunscreen is toxic misreads a real finding. In trials run by the US Food and Drug Administration (FDA), heavy, repeated use put several chemical filters into the blood above the level that prompts further safety testing. Yet the first report said the results did not mean people should stop using sunscreen,69 and the FDA has not concluded those ingredients are unsafe.70 A better-documented hazard is how people use it. In a blinded trial, young Europeans given SPF 30 rather than SPF 10 spent about a quarter more time in the sun on holiday, and burned just as often.71

Sun-avoidance studies need caution too. Swedish women who reported avoiding the sun died at about twice the rate of the most sun-exposed over 20 years,72 but ill people may stay indoors. In UK Biobank, a large British health study, even sunbed use, a recognized cause of cancer, was linked to lower mortality.7374 The authors saw a benefit of UV; more simply, healthier people may seek it out. And Australia's counts include overdiagnosis: cancers that would never have harmed anyone. One modelling study put that at about three-quarters of melanomas in situ (confined to the skin's top layer) and a third of thin invasive ones.75

Terms this topic defines

Related topics

Sources cited above

  1. 1.Lotzof, K. (2018, 7 February). Cheddar Man: Mesolithic Britain's blue-eyed boy. Natural History Museum. https://www.nhm.ac.uk/discover/cheddar-man-mesolithic-britain-blue-eyed-boy.html (The reconstruction by Kennis & Kennis Reconstructions featured in Channel 4's documentary The First Brit: Secrets of the 10,000 Year Old Man.) (He stood about 166 cm tall and died in his twenties; his diet would have included red deer, aurochs, freshwater fish, seeds and nuts. The face, by Adrie and Alfons Kennis, was unveiled in February 2018.)
  2. 2.Brace, S., Diekmann, Y., Booth, T.J., van Dorp, L., et al. (2019). "Ancient genomes indicate population replacement in Early Neolithic Britain." Nature Ecology & Evolution, 3(5), 765-771. https://doi.org/10.1038/s41559-019-0871-9 (author correction to figure colors, conclusions unaffected: https://doi.org/10.1038/s41559-019-0912-4). Cheddar Man's genome was sequenced at about 2.3-fold coverage. (The team, from the Natural History Museum and University College London, included Selina Brace, Tom Booth, Ian Barnes and Chris Stringer.)
  3. 3.Gibbens, S. (2018, 7 February). "Britain's dark-skinned, blue-eyed ancestor explained." National Geographic. https://www.nationalgeographic.com/history/article/ancient-face-cheddar-man-reconstructed-dna-spd
  4. 4.Perretti, S., Santos, P., Vizzari, M.T., Tassani, E., et al. (2025). "Inference of human pigmentation from ancient DNA by genotype likelihoods." PNAS, 122(29), e2502158122. https://doi.org/10.1073/pnas.2502158122 (348 genomes spanning 45,000 to 1,700 years ago; the authors call their skin-color categories "admittedly somewhat arbitrary.")
  5. 29.Forrest, K.Y., and Stuhldreher, W.L. (2011). "Prevalence and correlates of vitamin D deficiency in US adults." Nutrition Research, 31(1), 48-54. https://doi.org/10.1016/j.nutres.2010.12.001 (NHANES 2005-2006, 4,495 adults). (At or below 20 ng/mL in 2005-2006: 41.6% of adults; 82.1% of Black, 69.2% of Hispanic and 30.0% of white adults.)
  6. 28.Ross, A.C., Manson, J.E., Abrams, S.A., Aloia, J.F., et al. (2011). "The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine: what clinicians need to know." Journal of Clinical Endocrinology and Metabolism, 96(1), 53-58. https://doi.org/10.1210/jc.2010-2704 (the recommended intakes correspond to a blood level of at least 20 ng/mL, or 50 nmol/L).
  7. 42.Sutherland, J.P., Zhou, A., and Hyppönen, E. (2022). "Vitamin D deficiency increases mortality risk in the UK Biobank: a nonlinear Mendelian randomization study." Annals of Internal Medicine, 175(11), 1552-1559. https://doi.org/10.7326/M21-3324; reanalysis: Hyppönen, E., Sutherland, J.P., and Zhou, A. (2024). "Vitamin D deficiency and mortality: a reanalysis of a prior Mendelian randomization study." Annals of Internal Medicine, 177(12), 1743-1746. https://doi.org/10.7326/ANNALS-24-02796 (a correction to the reanalysis appeared in 2025: https://doi.org/10.7326/ANNALS-25-02471). (The threshold was about 50 nmol/L; the reanalysis used a newer statistical method.)
  8. 65.Szabo, L. (2018, 20 August). "The man who sold America on vitamin D, and profited in the process." KFF Health News. https://kffhealthnews.org/aging/how-michael-holick-sold-america-on-vitamin-d-and-profited/ (investigative reporting: more than 10 million tests for Medicare patients in 2016, at a cost of $365 million). (A 547% rise from 2007 to 2016.)
  9. 66.Gambichler, T., Bader, A., Sauermann, K., Altmeyer, P., et al. (2001). "Serum folate levels after UVA exposure: a two-group parallel randomised controlled trial." BMC Dermatology, 1, 8. https://doi.org/10.1186/1471-5945-1-8
  10. 67.Greaves, M. (2014). "Was skin cancer a selective force for black pigmentation in early hominin evolution?" Proceedings of the Royal Society B, 281(1781), 20132955. https://doi.org/10.1098/rspb.2013.2955 (Jablonski and Chaplin replied the same year: https://doi.org/10.1098/rspb.2014.0517).
  11. 68.Elias, P.M., Menon, G., Wetzel, B.K., and Williams, J. (2010). "Barrier requirements as the evolutionary 'driver' of epidermal pigmentation in humans." American Journal of Human Biology, 22(4), 526-537. https://doi.org/10.1002/ajhb.21043
  12. 69.Matta, M.K., Zusterzeel, R., Pilli, N.R., Patel, V., et al. (2019). "Effect of sunscreen application under maximal use conditions on plasma concentration of sunscreen active ingredients: a randomized clinical trial." JAMA, 321(21), 2082-2091. https://doi.org/10.1001/jama.2019.5586; Matta, M.K., Florian, J., Zusterzeel, R., Pilli, N.R., et al. (2020). "Effect of sunscreen application on plasma concentration of sunscreen active ingredients: a randomized clinical trial." JAMA, 323(3), 256-267. https://doi.org/10.1001/jama.2019.20747 (the 2019 trial applied sunscreen to 75% of the body four times a day for four days). (The first report: "these results do not indicate that individuals should refrain from the use of sunscreen.")
  13. 70.U.S. Food and Drug Administration (2026). Questions and answers: FDA's regulatory actions on over-the-counter sunscreen. https://www.fda.gov/drugs/understanding-over-counter-medicines/questions-and-answers-fdas-regulatory-actions-over-counter-sunscreen
  14. 71.Autier, P., Doré, J.F., Négrier, S., Liénard, D., et al. (1999). "Sunscreen use and duration of sun exposure: a double-blind, randomized trial." Journal of the National Cancer Institute, 91(15), 1304-1309. https://doi.org/10.1093/jnci/91.15.1304 (86 young Europeans; 72.6 vs 58.2 hours.)
  15. 72.Lindqvist, P.G., Epstein, E., Landin-Olsson, M., Ingvar, C., et al. (2014). "Avoidance of sun exposure is a risk factor for all-cause mortality: results from the Melanoma in Southern Sweden cohort." Journal of Internal Medicine, 276(1), 77-86. https://doi.org/10.1111/joim.12251 (29,518 women; exposure self-reported).
  16. 73.Stevenson, A.C., Clemens, T., Pairo-Castineira, E., Webb, D.J., et al. (2024). "Higher ultraviolet light exposure is associated with lower mortality: an analysis of data from the UK Biobank cohort study." Health & Place, 89, 103328. https://doi.org/10.1016/j.healthplace.2024.103328 (the authors conclude that "the benefits of UV exposure may outweigh the risks in low-sunlight countries").
  17. 74.El Ghissassi, F., Baan, R., Straif, K., Grosse, Y., et al. (2009). "A review of human carcinogens, part D: radiation." Lancet Oncology, 10(8), 751-752. https://doi.org/10.1016/S1470-2045(09)70213-X (UV-emitting tanning devices classified in IARC Group 1, carcinogenic to humans).
  18. 75.Lindsay, D., Bell, K.J.L., Olsen, C.M., Whiteman, D.C., et al. (2024). "Estimating the magnitude and healthcare costs of melanoma in situ and thin invasive melanoma overdiagnosis in Australia." British Journal of Dermatology, 191(6), 906-913. https://doi.org/10.1093/bjd/ljae296 (a modelling study). (For 2021: 71 to 76% of melanomas in situ and 28 to 34% of thin invasive melanomas.)
  19. 76.Cancer Council Australia (n.d.). Vitamin D. https://www.cancer.org.au/cancer-information/causes-and-prevention/sun-safety/vitamin-d
  20. 26.Webb, A.R., Kline, L., and Holick, M.F. (1988). "Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin." Journal of Clinical Endocrinology and Metabolism, 67(2), 373-378. https://doi.org/10.1210/jcem-67-2-373 (Boston lies at 42°N, Edmonton at 52°N.)
  21. 12.NHS (2020). Vitamin D (page last reviewed 3 August 2020). https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/ ("You cannot overdose on vitamin D through exposure to sunlight.")
  22. 10.Clemens, T.L., Adams, J.S., Henderson, S.L., and Holick, M.F. (1982). "Increased skin pigment reduces the capacity of skin to synthesise vitamin D3." The Lancet, 319(8263), 74-76. https://doi.org/10.1016/s0140-6736(82)90214-8
  23. 34.Farrar, M.D., Kift, R., Felton, S.J., Berry, J.L., et al. (2011). "Recommended summer sunlight exposure amounts fail to produce sufficient vitamin D status in UK adults of South Asian origin." American Journal of Clinical Nutrition, 94(5), 1219-1224. https://doi.org/10.3945/ajcn.111.019976 (mean rise 4.3 vs 10.5 ng/mL in the white comparison group).
  24. 48.Wu, S., Han, J., Laden, F., and Qureshi, A.A. (2014). "Long-term ultraviolet flux, other potential risk factors, and skin cancer risk: a cohort study." Cancer Epidemiology, Biomarkers & Prevention, 23(6), 1080-1089. https://doi.org/10.1158/1055-9965.EPI-13-0821 (Nurses' Health Study II, 1989-2009; cumulative UV was estimated from ambient UV at each participant's home address, not from personal sun habits). (108,916 nurses; basal cell carcinoma 2.35 times and squamous cell carcinoma 2.53 times the risk in the highest UV group.)
  25. 77.Cancer Council Australia (n.d.). About sunscreen. https://www.cancer.org.au/cancer-information/causes-and-prevention/sun-safety/about-sunscreen
  26. 78.Boniol, M., Autier, P., Boyle, P., and Gandini, S. (2012). "Cutaneous melanoma attributable to sunbed use: systematic review and meta-analysis." BMJ, 345, e4757. https://doi.org/10.1136/bmj.e4757 (relative risk 1.87, 95% CI 1.41-2.48, for first use before age 35).
  27. 79.Demay, M.B., Pittas, A.G., Bikle, D.D., Diab, D.L., et al. (2024). "Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline." Journal of Clinical Endocrinology and Metabolism, 109(8), 1907-1947. https://doi.org/10.1210/clinem/dgae290 (for people over 50 who take vitamin D, the panel suggests daily doses rather than intermittent high doses).
  28. 40.Neale, R.E., Baxter, C., Romero, B.D., McLeod, D.S.A., et al. (2022). "The D-Health Trial: a randomised controlled trial of the effect of vitamin D on mortality." Lancet Diabetes & Endocrinology, 10(2), 120-128. https://doi.org/10.1016/S2213-8587(21)00345-4 (all-cause mortality HR 1.04, 95% CI 0.93-1.18; cancer mortality excluding the first two years HR 1.24, 1.01-1.54; the authors suggest this dosing "might not be appropriate in people who are vitamin D-replete"). (21,315 participants.)
  29. 64.Aiello, A.E., Coulborn, R.M., Perez, V., and Larson, E.L. (2008). "Effect of hand hygiene on infectious disease risk in the community setting: a meta-analysis." American Journal of Public Health, 98(8), 1372-1381. https://doi.org/10.2105/AJPH.2007.124610 (Gastrointestinal illness 31% lower, respiratory illness 21% lower.)
  30. 80.Cardona, I.D., Kempe, E.E., Lary, C., Ginder, J.H., et al. (2020). "Frequent versus infrequent bathing in pediatric atopic dermatitis: a randomized clinical trial." Journal of Allergy and Clinical Immunology: In Practice, 8(3), 1014-1021. https://doi.org/10.1016/j.jaip.2019.10.042 (42 children with moderate-to-severe eczema in a crossover trial; both arms applied moisturizer straight after each bath).

This topic carries 79 notes in the research manuscript; the 30 above are those behind the material on this page. Full source register.