UV index and sunlight’s interaction with the skin, the eyes, and the brain. Why it matters.

The UV index is a sunburn meter, not a mood. The World Health Organization sets it at 40 times the sunlight left after the McKinlay–Diffey erythema curve. Index 1 is 25 mW/m². Index 3 is 75 mW/m². That 3 is where the UVB that opens 7-dehydrocholesterol usually gets too weak to fill a vitamin D reservoir. Below it, the yield gets small.

Dawn and dusk do not supply that yield. Webb, Kline, and Holick measured it in 1988. On cloudless days, Boston at 42.2° N made no previtamin D3 from November through February. Edmonton at 52° N made none from October through March. At 34° N and at 18° N, midwinter sun still did.

A fair-skinned body, nearly uncovered, given 1 faint-pink dose of midday UVB, can match a single oral dose on the order of 10,000 to 25,000 IU. Then the same skin refuses to keep going.

Continued UVB converts leftover previtamin D3 into lumisterol and tachysterol, side products that are not vitamin D3. Holick’s 2007 review in the New England Journal of Medicine is the map of that brake. Cutaneous production therefore does not run away the way a megadose capsule can. You can burn. You cannot, by sun alone, push this pathway into the toxicity of an oral overdose. Index 3 is not a switch that flips at 2.99. A gazing window at sunrise or sunset is not this pathway: the eye is reading visible light, and the skin is not writing the hormone. The rest of the essay is the route after the brake, the surveys, the trials, and the separate job the eye does for the brain.

UVB, about 290–315 nm, is absorbed in the epidermis by 7-dehydrocholesterol.

Do not look at the disk outside the 1st 45 minutes after sunrise or the last 45 minutes before sunset.

The pathway

UVB, about 290–315 nm, is absorbed in the epidermis by 7-dehydrocholesterol. The energy opens the B ring and makes previtamin D3, which body heat rearranges into vitamin D3 over the next 1 to 2 days. After a large UV dose, circulating vitamin D3 typically peaks at 24–48 hours. Vitamin D3 binds vitamin D–binding protein, goes to the liver, and is hydroxylated to 25-hydroxyvitamin D, the storage form a blood test measures. Holick gives that storage form a circulating half-life of about 2 weeks. When calcium or phosphate is low, parathyroid hormone tells the kidney to add a 2nd hydroxyl and release 1,25-dihydroxyvitamin D, calcitriol, the active hormone.

Calcitriol is the hormone. Vitamin D3 from sun or food is the precursor. The kidney ships calcitriol into the blood. Calcitriol binds the vitamin D receptor, a nuclear receptor in the same family as the receptors for estrogen, testosterone, and cortisol, and changes transcription. Many cells outside the kidney also carry 1α-hydroxylase and can make a little calcitriol on the spot from circulating 25-hydroxyvitamin D. That is why effects show up outside bone. Calling every form “the hormone” is imprecise. The circulating hormone is calcitriol. What is unusual is that human skin can make the precursor from UVB.

Liver 25-hydroxylation is largely driven by how much precursor arrives. Renal 1-hydroxylation is gated by parathyroid hormone, phosphate, FGF23, and calcitriol itself. Yesterday’s sun does not set today’s hormone in a straight line.

Index 3 is a sunburn scale

The weighting curve published by McKinlay and Diffey in 1987 follows erythema. It is high through the UVB and falls hard across the UVA. Previtamin D3 uses a related band, crowded between about 290 and 315 nm, not the whole erythema curve. Ozone removes the short end first. When the sun is low, the vitamin D wavelengths disappear while some longer ultraviolet, and all of the visible light, is still there. Index 3 became the public mark because that is where cutaneous production is usually no longer worth counting. Production does not snap off at 2.99. It thins as the zenith angle grows.

Ordinary window glass blocks the UVB that starts the reaction. A bright room behind glass is not a dose. Holick states that limit in the same 2007 review. Age cuts the supply from the other end. MacLaughlin and Holick, in the Journal of Clinical Investigation in 1985, measured surgically obtained skin from ages 8 to 92. Epidermal 7-dehydrocholesterol fell with age. The epidermis made more than 80% of the previtamin D3. Skin from people aged 77 and 82 made previtamin D3 at less than 1/2 the rate of skin from people aged 8 and 18. The older skin still worked. It worked less.

How common a low reservoir is

Forrest and Stuhldreher analyzed NHANES 2005–2006, 4,495 adults, and defined deficiency as 25-hydroxyvitamin D at or below 20 ng/mL, which is 50 nmol/L. The overall rate was 41.6%. It was 82.1% in non-Hispanic Black adults, 69.2% in Hispanic adults, and 30.9% in non-Hispanic White adults. Those 3 rates sit inside 1 survey and 1 cutoff. They are not a theory about pigment. They are the table.

A higher line, under 30 ng/mL, is often called insufficiency. Ginde, Liu, and Camargo, using NHANES, reported that the share of U.S. residents at or above 30 ng/mL fell from 45% in 1988–1994 to 23% in 2001–2004. In that later wave, levels under 30 ng/mL were nearly universal in non-Hispanic Black participants, 97%, and in Mexican-American participants, 90%. The share of non-Hispanic Black participants at or above 30 ng/mL was 3%. The assays used across those survey waves were later shown to be imperfectly aligned, so the size of the national drop is not a pure change in bodies (Schleicher and colleagues, American Journal of Clinical Nutrition, 2016). The gap inside 1 assay year is not that problem.

Melanin in the epidermis absorbs UVB before it reaches 7-dehydrocholesterol. Clemens and colleagues, Lancet 1982, gave the same ultraviolet dose to lightly and heavily pigmented volunteers. Circulating vitamin D3 rose in fair skin and barely moved in heavily pigmented skin. In that small series, 1 heavily pigmented volunteer needed about 6 times the exposure for a similar rise. The sample is too small to be a dosing chart. The direction is the one larger surveys keep finding. The pathway is the same. The photons that start it are not equally available.

What a low level does, and what raising it does not

Circulating 25-hydroxyvitamin D is the reservoir that tissues outside the kidney draw on to make local calcitriol. Immune cells, muscle, gut, and endothelium express the receptor and, often, the activating enzyme. When the reservoir is empty, intestinal absorption of calcium and phosphate falls, osteoid fails to mineralize, proximal muscle weakens, and the antimicrobial peptide cathelicidin is under-supplied. Liu and colleagues showed that last step in human macrophages in Science in 2006: the cell makes calcitriol locally, then makes the peptide. Filling a frankly low reservoir is a repair. Pushing a person who is already sufficient much higher mostly plateaus.

The proven job is mineral. Severe deficiency causes rickets in a child and osteomalacia in an adult. Combined calcium plus vitamin D can modestly reduce fracture risk in older people who start low. Large trials in people who are already adequate usually show little extra bone or fall benefit. The opposite error is a huge intermittent dose. Sanders and colleagues, JAMA 2010, gave 2,256 women aged 70 or older a single oral 500,000 IU once a year. Falls rose (rate ratio 1.15) and fractures rose (rate ratio 1.26) against placebo. The extra falls clustered in the 1st 3 months after the dose. A daily amount in the hundreds of IU is not that experiment. A once-a-year capsule is.

The strongest extra-skeletal trial signal is respiratory. Martineau and colleagues pooled individual data from 25 randomized trials, 11,321 people, in The BMJ in 2017. Vitamin D reduced acute respiratory infection (adjusted odds ratio 0.88, 95% confidence interval 0.81 to 0.96). Daily or weekly dosing, without extra bolus doses, carried the result (0.81). Bolus dosing did not (0.97). Among people on daily or weekly vitamin D, the reduction was larger when the starting 25-hydroxyvitamin D was under 25 nmol/L, which is 10 ng/mL (adjusted odds ratio 0.30, 0.17 to 0.53). That subgroup is the people who started lowest, and the interval is wide. It is not a reason to take a megadose.

For multiple sclerosis the cleanest human evidence is not a treatment trial. Mokry and colleagues, PLoS Medicine 2015, used Mendelian randomization in up to 14,498 cases and 24,091 controls. Each genetically determined 1-standard-deviation decrease in log-transformed 25-hydroxyvitamin D was associated with a 2.0-fold increase in the odds of MS (95% confidence interval 1.7 to 2.5). A sensitivity analysis that dropped variants more open to pleiotropy still gave an odds ratio of 1.7 (1.3 to 2.2). That is evidence about susceptibility, in people of European ancestry in that genetics consortium. It is not evidence that a large dose treats MS that is already established.

Cancer and the heart do not get the same sentence. The VITAL trial, Manson and colleagues, New England Journal of Medicine 2019, randomized 25,871 adults to 2,000 IU of vitamin D3 a day or placebo for a median of 5.3 years. Invasive cancer was not reduced (hazard ratio 0.96, 0.88 to 1.06). Major cardiovascular events were not reduced (0.97, 0.85 to 1.12). Death from cancer, a secondary count, was 0.83 (0.67 to 1.02), and that interval crosses 1. Mood, diabetes, obesity, and most cognition outcomes likewise fail once the trials are limited to people who were already sufficient. Risk, where it shows, sits at the bottom of the range.

How much skin actually makes

The 10,000 to 25,000 IU figure is a whole-body 1 minimal erythemal dose in young adults, set next to a single oral dose in Holick’s comparison studies. It is not what 20 minutes of arms and face delivers. Dowdy, Sayre, and Holick, in 2010, discuss the working rule that about 1/4 of a minimal erythemal dose on about 1/4 of the body is on the order of 1,000 IU by mouth. Arms and legs are about 20–25% of body surface. That rule is a bench translation, not a personal assay.

Webb and colleagues modeled UK lunchtime sun, all-weather, in Nutrients in 2018. White Caucasian adults needed 9 minutes a day at lunchtime from March to September for winter 25-hydroxyvitamin D to stay at or above 25 nmol/L (10 ng/mL), if forearms and lower legs were bare in June through August and a smaller area was bare in the cooler months of that span. Hands and face alone did not meet that target. The companion paper on Fitzpatrick type V found about 25 minutes under the same kind of lunchtime plan, from March to September, again with forearms and lower legs bare in high summer. Both papers are climatology plus exposure rules. They are not a prescription, and 25 nmol/L is the UK floor they used, not a claim that 9 minutes fills every lab’s “sufficient” line.

Approximate midday times at UV index about 6, arms and legs bare, aiming near 1,000 IU and staying short of a burn. Latitude, season, age, cloud, and clothing move them. They are working estimates scaled from minimal erythema, not 1 trial.

• Type I, very fair, always burns: about 5–10 min
• Type II, fair, burns easily: about 8–15 min
• Type III, medium, tans: about 12–20 min
• Type IV, olive or light brown: about 20–30 min
• Type V, brown: about 25–40 min, with the UK lunchtime model near 25 min against about 9 min for white skin
• Type VI, dark brown or Black: about 40–60 min or longer. Older photobiology found up to about 6–10 times the UV of type II for the same previtamin D3

Stop before pink. Above index 3, expose arms and legs rather than the face if the goal is the vitamin. Cover, or use sunscreen, once the short session is done. The brake described above is why sun does not overdose the vitamin. It is not why sun cannot injure skin.

The eye is a different band

Ultraviolet and the clock are not the same photons. In the adult eye the cornea takes most UVB and the lens takes most UVA. Boettner and Wolter mapped that transmission in 1962. Very little ultraviolet reaches an adult retina. The circadian signal is visible light, weighted toward the blue, and it enters through cells that are not the rods and cones.

Those cells are intrinsically photosensitive retinal ganglion cells. They contain melanopsin, peak near 460–480 nm, and still fire when rods and cones are silenced (Berson, Dunn, and Takao, Science 2002; Hattar and colleagues, Science 2002). Their axons run the retinohypothalamic tract to the suprachiasmatic nucleus. From there, a sympathetic chain either permits or clamps norepinephrine on the pineal. Darkness lets melatonin synthesis run. Light, especially short-wavelength visible light, stops it. Brainard and colleagues, Journal of Neuroscience 2001, measured the action spectrum for melatonin suppression in humans. It matches melanopsin, not UVB. Human pinealocytes do not see the sky. The retina is the light meter.

Eyles and colleagues, Journal of Chemical Neuroanatomy 2005, mapped the vitamin D receptor and 1α-hydroxylase in human brain, in neurons and glia, strongest in the hypothalamus and in large neurons of the substantia nigra. That is anatomy. It shows the brain can respond to calcitriol and, in places, make it locally. It does not show that a sunbath treats depression, and the trials summarized above do not support that leap.

The pineal, mineral, and timed light

Pinealocytes make melatonin at night from serotonin, through AANAT and then ASMT. Circulating melatonin is the chemical night signal. It times sleep propensity and peripheral clocks. Acute light at night suppresses it within minutes. First-light visible exposure and midday vitamin D are different photons, different organs, and different jobs.

Two minerals in the gland must not be collapsed into 1 story. Corpora arenacea, brain sand, are hydroxyapatite concretions. Radiologists use a calcified pineal as a midline landmark. Separately, Baconnier, Lang, and colleagues, Bioelectromagnetics 2002, described calcite microcrystals, typically under 20 µm, distinct from the hydroxyapatite. Electron diffraction and Raman spectroscopy identified calcite. A crystal of that sort can, in principle, show piezoelectric behavior, as otoliths can. A job in human sensation — electromechanical, magnetic, or otherwise — has not been shown. It is a reason to measure. It is not a radio in the skull.

Calcification itself is common. Tan, Xu, Zhou, and Reiter, Molecules 2018, review how often the gland mineralizes. Degree of calcification has been correlated with lower urinary 6-sulfatoxymelatonin, a marker of pineal output (Kunz and colleagues, Neuropsychopharmacology 1999). Heavier calcification has been linked, in some series, with more fragmented sleep and a flatter circadian amplitude, and it is reported more often in some Alzheimer’s and psychiatric samples. The associations are not all causal. Some studies find no simple link to a sleep complaint. The careful reading is narrower: as secretory cells are replaced by mineral, the gland’s capacity to write night into the blood can decline. That is a medical question. It is not a product claim about “decalcifying” the gland.

Timed light is already a treatment, which is why the circuit is not optional. Bright light for seasonal affective disorder is among the better-replicated non-drug interventions in psychiatry (Golden and colleagues, American Journal of Psychiatry 2005). Morning light advances circadian phase. Urban night light, shift work, and indoor days without a real morning are the inverse experiment. They blur the edge the suprachiasmatic nucleus and the pineal are built to read.

The split, once: UV index at or above 3, on uncovered skin, is the vitamin D pathway, epidermis then liver then kidney. Visible daylight into the open eye, especially the blue-weighted part, is the circadian pathway, ganglion cell then clock then pineal. Neither is a reason to look at the sun outside the 1st 45 minutes after sunrise or the last 45 minutes before sunset. The eye is neural tissue. The pineal is how that tissue timestamps night. The skin writes a precursor only when the short ultraviolet is actually there.

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