On cloudless winter days in Boston, samples of human skin were set out in the sun. From November through February they made no vitamin D at all. Further north, in Edmonton, the blank stretch ran from October to March.
So the "sunshine vitamin" has an off season, and for months of every year the sky above us has none to give. Vitamin D3 is the form our skin makes and the form animal foods carry. When the sun steps back, it has to come from somewhere else, and only a handful of foods hold much of it.
Why vitamin D3 matters
The name is arguably a mistake. Our skin makes most of what we have, so it behaves less like a vitamin from food and more like a hormone we build from sunlight.
Vitamin D helps the body manage calcium and phosphate, the raw materials of bones, teeth and muscles. Without enough of it, children can develop the bone deformities of rickets, and adults a deep bone pain called osteomalacia.
Rickets haunted London in the Industrial Revolution, when fog and factory smog blocked the sun. In German, Dutch, Hungarian and Swedish it is still called "the English disease".
Extra is another story. Among 25,871 generally healthy adults, five years of daily vitamin D made no difference to the number of broken bones. Nobody had been picked for being low, which is the point: enough is enough.
It has a hand in our defences too. Across 25 tests and 11,321 people, taking vitamin D lowered the odds of a cold, flu or chest infection by about 12%.
Mood is where the promise runs ahead of the evidence. Low levels go with low mood, yet when 18,353 adults aged 50 and over took vitamin D or a dummy pill for five years, their mood scores ended up the same. Having enough matters. More than enough buys us nothing extra.
The seven sources
1. Sunlight
This is the one our bodies were built around. When the sun's UVB rays, the part of sunlight that also burns, reach bare skin, the skin makes vitamin D3 on the spot.
The proof came in the winter of 1918–19, when Kurt Huldschinsky, in Germany, showed that rickets could be treated with ultraviolet lamps.
It takes less time than we tend to think. When 120 white adults in Manchester were given the equivalent of thirteen minutes of summer sun around the middle of the day, three times a week, in a T-shirt and shorts, nine in ten reached a sufficient level within six weeks.
Three things change the sum. Glass blocks almost all UVB, so a sunny window warms us and gives us nothing. Darker skin needs longer in the sun to make the same amount, and so does tanned skin, because the same pigment blocks UVB. And skin in its seventies and eighties makes less than half of what young skin does.
Skin tone has a history here. As people settled further north, where the sun is weak for part of the year, lighter skin spread because it makes vitamin D more easily.
Body fat is a fourth. After the same amount of ultraviolet light, vitamin D in the blood rose 57% less in adults with obesity than in lean adults. Their skin made just as much. It seems to have been held in body fat.
Where we live sets the season. In the Boston test, skin set out at the latitude of Los Angeles, and further south, went on making vitamin D through the middle of winter.
Sunscreen is the next question. Under a lamp, it sharply cuts the vitamin D the skin makes. In the two real-life tests of daily sunscreen, vitamin D levels were no different, though the sunscreens used were of moderate strength.
A sunbed is no shortcut. It gives out mostly UVA, the rays that tan and age the skin, and only 4 to 10% UVB, the rays that make vitamin D.
Sunlight never gives us too much vitamin D. The skin holds only so much of the raw material, so its output tops out. Overdoses come from bottles, and some on record came from products holding far more than the label said.
The sun itself is another matter, so keep it short: a little while with arms and legs bare, then shade and a hat for the rest of the day. A wide-brimmed covers the face, ears and neck in one go.
Take your few minutes when the sun is high, and move into the shade well before your skin turns pink.
2. Oily fish
Salmon, mackerel, sardines, herring and trout are the richest foods we have for vitamin D3, and they bring omega-3 fats along with it.
Vitamin D is older than bones. Plankton drifting in the ocean make it in sunlight to this day, with no skeleton to spend it on.
Where the fish grew up matters. When salmon were tested, the farmed fish held about a quarter of the vitamin D of the wild ones.
Tinned fish counts. Sardines, salmon, pilchards and mackerel from a tin are all oily fish, and the cheapest way onto this list. A tin of is an easy start. Tuna, fresh or tinned, is the one that falls short.
Put oily fish on the table at least twice a week, and choose wild when you can.
3. Cod liver oil
Long before anyone knew what a vitamin was, northern Europe swore by it. By the 1800s a teaspoon a day was a household habit, foul taste and all.
Its best story comes from London Zoo. In 1899 the litters of lion cubs there were dying with rickets. John Bland-Sutton had the lionesses and their cubs moved from lean horse meat to goat, bones included, with cod liver oil, and that solved it.
It also gave the vitamin its letter. In 1922 Elmer McCollum destroyed the vitamin A in cod liver oil and found the oil still cured rickets in dogs. Something else was doing it. It was the fourth vitamin to be named, so it got the fourth letter.
The oil is rich in vitamin D3, and just as rich in vitamin A, which is the catch. Too much vitamin A over many years may weaken the bones. If you eat liver every week, leave the cod liver oil on the shelf. In pregnancy, leave it there too: too much vitamin A can harm a baby's development. is checked by an independent lab for purity and freshness.
Choose a purified oil, in liquid or capsules, and remember it brings vitamin A with it.
4. Egg yolks
The vitamin D in an egg lives in the yolk, so eat the whole egg.
How the hen lived shows up in the egg. Hens kept indoors, hens with the run of the outdoors, and hens with a choice of both were compared over four weeks. The yolks from the birds that saw the sun held three to four times as much vitamin D3. One warning came with it: free-range eggs bought at random from supermarkets were low, because a door to the outside only counts if the hens use it.
Buy from a farm where you can see the hens outside.
5. Liver
Beef liver may not be everyone's favourite, and it's one of the most nutrient-dense foods we have: vitamin D3 along with vitamin A, iron and the B vitamins.
That richness is the reason to keep it to once a week. Liver is so high in vitamin A that more than that can add up to too much, and it's a food to avoid in pregnancy.
If the taste is the problem, try it as pâté, or blend a little into minced beef.
6. Red meat
The quiet one on the list. Red meat carries vitamin D, and it shows in the blood. Among 2,107 British adults, the meat eaters had the highest vitamin D levels and the vegans the lowest. The gap between them was 20% in summer and 38% in winter, just when the sun leaves the job to our plates.
In the dark months, let meat, fish and eggs take turns at dinner.
7. Supplements
A capsule is the fallback when the sun and the plate fall short, and in winter that is often the case. The official advice in Britain is that everyone should consider one through autumn and winter. People who are rarely outdoors, who cover most of their skin, or who have dark skin may need one all year.
Babies and small children are a case of their own. In Britain a daily supplement all year is advised for breastfed babies and for children aged one to four. Infant formula already has vitamin D added.
Here is the surprise on the list. Most vitamin D3 starts as lanolin, the grease in sheep's wool (the word is Latin lana, wool, plus oleum, oil), which is purified and put under ultraviolet light, the same trick our skin performs. A vegan version is made from lichen.
Take it with food. When 50 older adults took vitamin D with breakfast, 32% more reached the blood with a meal containing fat than with a fat-free one. And more is no kinder to us: too much from a bottle, over a long time, builds up calcium and can damage the kidneys and heart.
It has a partner as well. Every enzyme that activates vitamin D seems to need magnesium, so a capsule does less for someone who is short of that mineral.
If you suspect you're low, a blood test settles it. The one to ask for is 25-hydroxyvitamin D, the form that circulates in the blood.
The one plant source
Mushrooms are the exception to the animal rule. Given a spell in sunlight, common mushrooms make vitamin D2, and they can end up holding more than most vitamin D foods.
D2 is the weaker cousin, though. When the two forms were compared head to head, D3 did better at raising blood levels.
What about fortified foods?
The idea is a century old: in 1923 Harry Steenbock showed that shining ultraviolet light on food raised its vitamin D. It was used most memorably on milk, and by 1945 rickets had all but vanished from the United States. Today some cereals, spreads and plant milks have vitamin D added. They count. They also tend to arrive with added sugar and a long ingredient list, so read the label before you lean on them.
How running low feels
A shortage builds slowly, and its signs are easy to blame on age or a hard week: an ache in the back, ribs, hips or legs, muscles too weak to make light work of the stairs or of rising from a chair, cramps and twitches, and bones that break more easily than they should. In children the tell-tale sign is bowed legs.
None of these proves anything alone. The blood test does.
A plan for the dark months
That Boston skin made nothing for four months of the year, and ours is no different. Summer looks after itself with a few short spells of sun. Winter is the part to plan for. South of the equator, it's the other half of the year.
Summer's store runs out, too. The form of vitamin D that circulates in the blood falls by half every two to four weeks without a top-up, so what we build up by late summer drains away through autumn. By the end of winter, the level in our blood can be a third to a half lower than at the end of summer.
It takes one small habit to carry us to the day the sun is high enough to take over again. Put oily fish on the menu twice this week.
Wondering whether you're low in anything else? Nutrient Deficiencies: Who's at Risk and Which Test to Get — who runs short of what, and the test that shows it.
Curious what the sun does to skin over the years? Key Factors That Speed Up Aging (And How to Slow Them Down) — the sun, sleep, smoking and the rest, each with what slows it.
Know someone who wilts when the days get short? Send them this before the dark months arrive.
Disclaimer: This article is general information. For advice about your own health, talk to a qualified health professional. If your mood has been low for weeks or you're having thoughts of harming yourself, reach out for help today.
Affiliate Disclosure: This article contains affiliate links. As an Amazon Associate, All For Life earns from qualifying purchases, at no extra cost to you. We only link to products we would recommend without the commission.
Sources
Webb AR, Kline L, Holick MF (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. The Journal of Clinical Endocrinology & Metabolism 67(2):373–378. doi.org/10.1210/jcem-67-2-373
Wikipedia. Vitamin D. en.wikipedia.org
NHS. Vitamin D. nhs.uk
Wikipedia. Rickets. en.wikipedia.org
LeBoff MS, Chou SH, Ratliff KA, et al. (2022). Supplemental vitamin D and incident fractures in midlife and older adults. New England Journal of Medicine 387(4):299–309. doi.org/10.1056/NEJMoa2202106
Martineau AR, Jolliffe DA, Hooper RL, et al. (2017). Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ 356:i6583. doi.org/10.1136/bmj.i6583
Okereke OI, Reynolds CF, Mischoulon D, et al. (2020). Effect of long-term vitamin D3 supplementation vs placebo on risk of depression or clinically relevant depressive symptoms and on change in mood scores: a randomized clinical trial. JAMA 324(5):471–480. doi.org/10.1001/jama.2020.10224
Rhodes LE, Webb AR, Fraser HI, et al. (2010). Recommended summer sunlight exposure levels can produce sufficient (≥20 ng ml−1) but not the proposed optimal (≥32 ng ml−1) 25(OH)D levels at UK latitudes. Journal of Investigative Dermatology 130(5):1411–1418. doi.org/10.1038/jid.2009.417
MacLaughlin J, Holick MF (1985). Aging decreases the capacity of human skin to produce vitamin D3. Journal of Clinical Investigation 76(4):1536–1538. doi.org/10.1172/JCI112134
Wortsman J, Matsuoka LY, Chen TC, Lu Z, Holick MF (2000). Decreased bioavailability of vitamin D in obesity. The American Journal of Clinical Nutrition 72(3):690–693. doi.org/10.1093/ajcn/72.3.690
Neale RE, Khan SR, Lucas RM, et al. (2019). The effect of sunscreen on vitamin D: a review. British Journal of Dermatology 181(5):907–915. doi.org/10.1111/bjd.17980
NHS. Fish and shellfish. nhs.uk
Lu Z, Chen TC, Zhang A, et al. (2007). An evaluation of the vitamin D3 content in fish: is the vitamin D content adequate to satisfy the dietary requirement for vitamin D? The Journal of Steroid Biochemistry and Molecular Biology 103(3–5):642–644. doi.org/10.1016/j.jsbmb.2006.12.010
NHS. Vitamin A. nhs.uk
NHS. Vitamins, minerals and supplements in pregnancy. nhs.uk
Kühn J, Schutkowski A, Kluge H, Hirche F, Stangl GI (2014). Free-range farming: a natural alternative to produce vitamin D-enriched eggs. Nutrition 30(4):481–484. doi.org/10.1016/j.nut.2013.10.002
Crowe FL, Steur M, Allen NE, et al. (2011). Plasma concentrations of 25-hydroxyvitamin D in meat eaters, fish eaters, vegetarians and vegans: results from the EPIC–Oxford study. Public Health Nutrition 14(2):340–346. doi.org/10.1017/S1368980010002454
Wikipedia. Cholecalciferol. en.wikipedia.org
Wiktionary. lanolin. en.wiktionary.org
Dawson-Hughes B, Harris SS, Lichtenstein AH, et al. (2015). Dietary fat increases vitamin D-3 absorption. Journal of the Academy of Nutrition and Dietetics 115(2):225–230. doi.org/10.1016/j.jand.2014.09.014
Uwitonze AM, Razzaque MS (2018). Role of magnesium in vitamin D activation and function. The Journal of the American Osteopathic Association 118(3):181–189. doi.org/10.7556/jaoa.2018.037
Cardwell G, Bornman JF, James AP, Black LJ (2018). A review of mushrooms as a potential source of dietary vitamin D. Nutrients 10(10):1498. doi.org/10.3390/nu10101498
Tripkovic L, Lambert H, Hart K, et al. (2012). Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. The American Journal of Clinical Nutrition 95(6):1357–1364. doi.org/10.3945/ajcn.111.031070
NHS. Rickets and osteomalacia. nhs.uk
Wikipedia. Calcifediol. en.wikipedia.org