Can You Make Vitamin D From the Sun in Winter?

Whether you can get vitamin D from the sun in winter depends mostly on how far you live from the equator. In a 1988 study, skin samples set out under clear winter skies in Boston (42.2°N) made no previtamin D3 from November through February, and in Edmonton (52°N) the gap ran from October through March. Farther south, at 34°N and 18°N, winter sunlight still worked. Clouds, the time of day and sunscreen cut the amount further.

Why winter sunlight makes less vitamin D

Your skin makes vitamin D when ultraviolet B (UVB) light, roughly 290 to 320 nanometers, reaches uncovered skin and turns a compound in the skin (7-dehydrocholesterol) into previtamin D3, which then becomes vitamin D3. The NIH Office of Dietary Supplements lists season, time of day, length of day, cloud cover, smog, skin melanin content and sunscreen as factors that change how much UVB reaches the skin and how much vitamin D is made.

In winter the sun stays lower in the sky, and its light takes a longer path through the atmosphere. The 1988 study notes that latitude and season change both the amount and the quality of sunlight reaching the ground, especially in the UVB range, and the 2005 modelling study counts total ozone among the factors that shorten vitamin D production. A winter day can look bright while very little UVB gets through, so brightness is not a reliable sign that your skin is making vitamin D.

What the latitude studies found

The 1988 study by Webb, Kline and Holick, published in the Journal of Clinical Endocrinology and Metabolism, exposed human skin samples and a test compound to sunlight on cloudless days. It measured the skin's ability to start vitamin D production, not blood levels in people. A 2005 modelling study by Engelsen and colleagues in Photochemistry and Photobiology, and a 2016 study by O'Neill and colleagues in Nutrients, estimated the same "vitamin D winter" from UV models.

Latitude	What the study found	Source
18°N and 34°N	Sunlight still started vitamin D production in the middle of winter	Webb, 1988
35°N	No vitamin D winter in the model	O'Neill, 2016
42.2°N (Boston)	No previtamin D3 made on cloudless days from November through February	Webb, 1988
51°N and higher	No production during some periods of the year, even with clear skies	Engelsen, 2005
52°N (Edmonton)	No previtamin D3 made from October through March	Webb, 1988
69°N	A vitamin D winter of about eight months in the model	O'Neill, 2016
70°N	Production can be absent for 5 months	Engelsen, 2005

These figures describe clear-sky conditions. The Engelsen model found that clouds, airborne particles and thick ozone shorten the season of vitamin D production considerably, and can stop it completely even at the equator. The Webb study is also old and small in scope: it tested skin samples in a few places, so it shows the pattern by latitude rather than the exact date your own skin stops.

Making it versus having it

A month with no skin production does not mean your blood level drops to zero that month. The O'Neill study compared its UV model with blood measurements of 25-hydroxyvitamin D, the marker labs use to judge vitamin D status. Irish adults (51°N to 54°N) showed large seasonal swings. Norwegian and Icelandic adults, with far less UVB, showed much smaller swings, and the authors linked that to their much higher vitamin D intakes. A blood marker is a measure of status, not a health outcome. The study shows that intake can steady the level; it does not show what a given level does for any one person.

Other things that cut vitamin D from sunlight

• Glass. UVB does not pass through window glass, so sunshine through a window does not produce vitamin D (NIH Office of Dietary Supplements).
• Skin pigment. More melanin reduces the skin's ability to make vitamin D from sunlight, and the NIH Office of Dietary Supplements says older people make less too.
• Covered skin. Winter clothing leaves little skin exposed. The NIH Office of Dietary Supplements lists homebound people and people whose clothing or work limits sun exposure among those unlikely to get enough vitamin D from sunlight.
• Sunscreen. Sunscreens with an SPF of 8 or more appear to block vitamin D-producing UV rays, according to the NIH Office of Dietary Supplements. The same source says people usually apply too little sunscreen to block all of it, so their skin probably still makes some vitamin D.

The NIH Office of Dietary Supplements also says limiting skin exposure to sunlight is prudent. It calls UV radiation a carcinogen and notes that federal agencies advise sunscreen with an SPF of 15 or higher whenever people are in the sun.

Where winter vitamin D comes from instead

The Food and Nutrition Board set its vitamin D Recommended Dietary Allowances on the assumption that people get minimal sun exposure, as the NIH Office of Dietary Supplements explains. The RDA it lists is 15 mcg (600 IU) a day for ages 19 to 70 and 20 mcg (800 IU) for people older than 70. So the RDA already assumes little help from the sun. A few foods from the NIH Office of Dietary Supplements food table:

• Cod liver oil, 1 tablespoon: 34.0 mcg (1,360 IU)
• Farmed rainbow trout, cooked, 3 ounces: 16.2 mcg (645 IU)
• Sockeye salmon, cooked, 3 ounces: 14.2 mcg (570 IU)
• White mushrooms exposed to UV light, raw, ½ cup: 9.2 mcg (366 IU)
• Vitamin D-fortified 2% milk, 1 cup: 2.9 mcg (120 IU)

On a label, vitamin D may be listed in micrograms, International Units or both. For vitamin D, 1 mcg equals 40 IU, so "25 mcg (1,000 IU)" is one amount written two ways.

Why winter is a good time to add up your sources

The sun is not a vitamin D overdose risk. The NIH Office of Dietary Supplements says experts do not believe too much sun causes vitamin D toxicity, because the skin turns extra previtamin D3 into inactive forms. Supplements are different. The same source says vitamin D toxicity is almost always caused by too much vitamin D from supplements. The Food and Nutrition Board set an adult Tolerable Upper Intake Level (UL) of 100 mcg (4,000 IU) a day, listed by the NIH Office of Dietary Supplements, and noted that even intakes below the UL might have harmful effects over time. The NIH sheet also says taking calcium supplements together with vitamin D supplements may raise the risk of certain adverse effects.

When the days get short, it is easy to start a second product without counting the first. Add the vitamin D in a multivitamin, a calcium supplement, fortified foods and any single vitamin D product together, and compare the total with the UL.

One example: Vitamin K2/D3+

The Vitamin K2/D3+ formula reviewed on this site lists, per one-capsule serving: vitamin D3 (cholecalciferol) 125 mcg (5,000 IU), vitamin K2 (MK-7, as menaquinone) 100 mcg, calcium (as calcium carbonate) 210 mg and BioPerine black pepper extract 5 mg. The label directs one capsule daily, preferably 20-30 minutes before a meal, or as directed by your healthcare professional. One capsule alone gives 125 mcg (5,000 IU) of vitamin D3, which is above the 100 mcg (4,000 IU) adult UL set by the Food and Nutrition Board and listed by the NIH Office of Dietary Supplements. If you already take vitamin D from another supplement, or you are not sure how much is right for you, check with your healthcare provider before use. Any vitamin D from food or another product adds to that 125 mcg. Its 210 mg of calcium also adds to the calcium from food and other supplements. For calcium, the NIH Office of Dietary Supplements lists an adult UL of 2,500 mg a day for ages 19 to 50 and 2,000 mg for ages 51 and older. The same sheet notes that higher intakes of supplemental calcium might increase the risk of kidney stones, and that some people who take calcium supplements might have gas, bloating or constipation.

Its label warning reads: "Do not exceed recommended dose. Pregnant or nursing mothers, children under the age of 18, and individuals with a known medical condition should consult a physician before using this or any dietary supplement. Keep out of reach of children. Do not use if safety seal is damaged or missing. Store in a cool, dry place." If you are pregnant, nursing or trying to conceive, ask your healthcare provider before use.

Medicines matter here too. Vitamin K can affect how blood-thinning medication works: the NIH Office of Dietary Supplements says people taking warfarin (Coumadin) need a consistent vitamin K intake from food and supplements, because sudden changes can raise or lower the drug's effect. If you take an anticoagulant such as warfarin, talk with your healthcare provider before use. The same NIH sheet says antibiotics, bile acid sequestrants and orlistat can have adverse effects on vitamin K levels. The NIH Office of Dietary Supplements also lists vitamin D interactions with orlistat, statins, steroids and thiazide diuretics (thiazides combined with vitamin D might lead to high blood calcium), and calcium interactions with dolutegravir, levothyroxine, lithium and quinolone antibiotics. Its vitamin D and calcium sheets both say that people who take these or other medicines regularly should discuss their vitamin D and calcium status with their healthcare providers.

Frequently asked questions about winter sun and vitamin D

Can you get vitamin D from the sun in winter?

It depends on latitude. A 1988 study found no vitamin D production in skin samples under clear winter skies in Boston (42.2°N) from November through February, while sunlight at 34°N and 18°N still worked in midwinter. Clouds, covered skin and sunscreen reduce it further.

Does sunlight through a window give you vitamin D?

No. The NIH Office of Dietary Supplements says UVB light does not pass through glass, so sitting in a sunny room indoors does not produce vitamin D.

What is a "vitamin D winter"?

It is the part of the year when the sun is too low for enough UVB to reach the ground for skin to make vitamin D. A 2016 European modelling study found it ranged from none at 35°N to about eight months at 69°N.

Can you get too much vitamin D from the sun?

The NIH Office of Dietary Supplements says experts do not believe excess sun causes vitamin D toxicity, because the skin converts extra previtamin D3 into inactive forms. Toxicity is almost always caused by too much vitamin D from supplements, and the adult upper limit is 100 mcg (4,000 IU) a day.

How much vitamin D do adults need when there is little sun?

The Food and Nutrition Board based its RDA on minimal sun exposure: 15 mcg (600 IU) a day for ages 19 to 70 and 20 mcg (800 IU) for people older than 70, as listed by the NIH Office of Dietary Supplements. A healthcare provider can check your own needs.

Related reading

• Vitamin K2/D3+ review
• Vitamin D3 and K2: What Each Does and How to Read the Label
• Optional next step: our supplement audit can help you organize your goals; it does not diagnose anything.

Sources

• Journal of Clinical Endocrinology and Metabolism (Webb AR, Kline L, Holick MF): 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, 1988.
• Photochemistry and Photobiology (Engelsen O, et al.): Daily duration of vitamin D synthesis in human skin with relation to latitude, total ozone, altitude, ground cover, aerosols and cloud thickness, 2005.
• Nutrients (O'Neill CM, et al.): Seasonal Changes in Vitamin D-Effective UVB Availability in Europe and Associations with Population Serum 25-Hydroxyvitamin D, 2016.
• NIH Office of Dietary Supplements: Vitamin D Fact Sheet for Health Professionals, updated 2025.
• NIH Office of Dietary Supplements: Calcium Fact Sheet for Health Professionals, updated 2026.
• NIH Office of Dietary Supplements: Vitamin K Fact Sheet for Health Professionals, updated 2026.

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This article is for education and is not personal medical advice. A qualified healthcare professional can help assess your needs and review the specific formula. Individual results vary.

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