Vitamin D in Winter: Do You Need a Supplement? Sun, Foods, RDA and the Evidence

If you live in roughly the northern half of the United States, winter sun probably won't give you much vitamin D from November through February. A classic study found that sunlight on clear days in Boston (42°N) made no vitamin D precursor in skin samples from November through February. At 34°N, about the latitude of Los Angeles, winter sun still worked. That doesn't automatically mean you need a supplement. Your body stores vitamin D from summer, fortified foods help, and national survey data suggest most Americans have adequate blood levels.

Here's the short version from official US sources. Adults up to age 70 need 600 IU (15 mcg) a day, and adults over 70 need 800 IU (20 mcg). The upper limit for adults is 4,000 IU (100 mcg) a day. Supplements make the most sense when food intake is low and winter sun is out of reach, or when you're in a higher-risk group. Large trials in generally healthy adults have not found that extra vitamin D prevents fractures, cancer or heart disease, or improves athletic performance.

Can you make vitamin D from winter sun in the US?

Your skin makes vitamin D when ultraviolet B (UVB) rays hit it. In winter, the sun sits lower in the sky, and its light passes through more atmosphere. Much less UVB reaches the ground, especially as you go farther north.

The best-known measurements come from a 1988 study by Webb, Kline and Holick. They exposed skin samples to sunlight on cloudless days:

• Boston, 42.2°N: no previtamin D3 formed from November through February.
• Edmonton, Canada, 52°N: the "vitamin D winter" ran longer, from October through March.
• 34°N and 18°N: sunlight still produced previtamin D3 in midwinter.

Boston's latitude is close to that of Chicago, Cleveland and Detroit, and New York City is only slightly farther south. Much of the southern US, including Southern California, southern Arizona, most of Texas, the Gulf Coast and Florida, lies near or south of 34°N, so midday winter sun there can still make some vitamin D.

The study measured conversion in a lab setup, not blood levels in people. Real-world output also depends on time of day, clouds, smog, altitude, skin tone and how much skin is bare. A northern winter walk with only your face uncovered won't add much. According to the NIH Office of Dietary Supplements (ODS), UVB doesn't pass through glass either, so a sunny window doesn't help.

Your body stores some from summer

Winter isn't a cliff edge. The body keeps some vitamin D in fat tissue, and the main circulating form, 25-hydroxyvitamin D [25(OH)D], has a half-life of about 15 days. ODS cites NHANES 2011–2014 blood data for Americans aged 1 and older:

• 5% were at risk of deficiency (below 12 ng/mL).
• 18% were at risk of inadequacy (12 to under 20 ng/mL).

Deficiency risk peaked at 7.6% in adults aged 20 to 39, and it was 17.5% among non-Hispanic Black Americans.

Food alone usually falls short. In NHANES 2015–2016, men averaged 204 IU a day from food and drinks, and women averaged 168 IU, against a 600 IU RDA. Sun and supplements make up most of the difference, and in winter the sun part shrinks.

How much vitamin D do you need? RDA and upper limit

The Food and Nutrition Board of the National Academies set these values in 2010, and ODS still uses them. The RDAs assume minimal sun exposure, so they're meant to cover a sunless winter. One microgram (mcg) equals 40 IU.

*Adequate Intake for infants; there is no RDA under 12 months.

The Daily Value (%DV) on US food and supplement labels is 20 mcg (800 IU) for adults and children 4 and older. So "100% DV" on a label is 800 IU, a little more than the RDA for most adults.

Best food sources of vitamin D

Few foods are naturally rich in vitamin D, and most of what Americans get from food comes from fortified products. The amounts below are from the ODS table, which draws on USDA FoodData Central. A spot check of FoodData Central (16.7 mcg per 100 g of cooked sockeye salmon) matches the 3-ounce figure.

• Fatty fish is the standout. A 3-ounce portion of salmon or trout covers most of a day's RDA. If you already eat fish for omega-3s, as covered in our guide to omega-3 fatty acids and rich foods, you're getting vitamin D too.
• Fortification varies. Almost all US milk is fortified with about 120 IU per cup. Cheese and ice cream usually aren't. Cereals, plant milks, juice and yogurt vary by brand, so check the Nutrition Facts label.
• Mushrooms vary widely. UV-treated mushrooms can be high in vitamin D2. Ordinary portabellas in the ODS table have almost none.
• It adds up slowly. Two cups of fortified milk and an egg come to about 284 IU, roughly half the adult RDA.

For how vitamin D fits with other vitamins, see our overview of vitamin types and sources.

Who is at higher risk of low vitamin D in winter?

ODS lists these groups as more likely to have inadequate vitamin D:

• Older adults. Skin makes less vitamin D with age, and older people tend to spend more time indoors.
• People with darker skin. More melanin reduces vitamin D production. ODS notes it isn't clear the lower blood levels cause the same harm. Black Americans, for example, have lower fracture and osteoporosis rates than White Americans.
• People with limited sun exposure. This includes people who are homebound, work indoors or wear clothing that covers most of the skin.
• People with obesity or after gastric bypass.
• People with conditions that limit fat absorption. Examples include Crohn's disease, ulcerative colitis, celiac disease and cystic fibrosis.
• People who avoid milk. This includes allergy, lactose intolerance and vegan diets, unless fortified alternatives fill the gap.
• Breastfed infants. The American Academy of Pediatrics recommends 400 IU a day for them.

Indoor athletes show up in the sports research too. A 2015 meta-analysis of 23 studies (2,313 athletes) found that 56% were below 32 ng/mL, a stricter cutoff than the 20 ng/mL the Food and Nutrition Board uses. Risk was higher in winter and spring and in indoor sports. These were mostly small observational studies, so the numbers are rough.

Medicines matter too. ODS notes that steroids such as prednisone can lower vitamin D, and orlistat reduces its absorption. Thiazide diuretics taken with vitamin D can raise calcium too high.

Should you get your vitamin D level tested?

For healthy adults without symptoms, US guidance doesn't support routine testing. The USPSTF (2021) found the evidence insufficient to weigh the benefits and harms of screening asymptomatic adults. That's an "I statement": the evidence is lacking, but it doesn't say testing is harmful. The Endocrine Society's 2024 guideline suggests against routine 25(OH)D testing, including for people with obesity or darker skin, because no trial evidence shows it improves outcomes.

Testing makes more sense when there's a medical reason, such as osteoporosis, malabsorption or certain medications. Results can vary between labs. For reference, the Food and Nutrition Board considers 20 ng/mL (50 nmol/L) or more adequate for most people. Below 12 ng/mL (30 nmol/L) is linked to deficiency, and above 50 ng/mL (125 nmol/L) is linked to potential harm.

What the evidence says about supplements for bones, muscle and performance

Vitamin D helps you absorb calcium, and severe deficiency softens bones (osteomalacia) and can weaken muscles. The open question is whether extra vitamin D helps people who aren't deficient. Large trials mostly say no.

VITAL: 2,000 IU a day in healthy older adults

The US VITAL trial randomized 25,871 men 50 and older and women 55 and older to 2,000 IU of vitamin D3 a day or placebo for a median of 5.3 years. Vitamin D didn't lower invasive cancer (hazard ratio 0.96) or major cardiovascular events (0.97). A 2022 analysis found no reduction in total, nonvertebral or hip fractures, and results didn't differ by age, sex, race, BMI or starting vitamin D level. The main limitation is that only 2.4% of participants started below 12 ng/mL, so VITAL says little about people who are truly deficient.

More isn't better for bones

A three-year Calgary trial gave 311 healthy adults aged 55 to 70 either 400, 4,000 or 10,000 IU a day. Wrist bone density fell slightly more at 4,000 and 10,000 IU than at 400 IU. Bone strength didn't differ.

Muscle and athletic performance

• DO-HEALTH (2020): in 2,157 adults aged 70 and older, 2,000 IU a day for three years didn't improve physical performance or reduce nonvertebral fractures.
• Athletes, 2017 meta-analysis (13 trials, 532 athletes): 3,000–5,000 IU a day raised low levels, even in winter above 45°N. None of the seven trials that tested performance found a significant gain over about 12 weeks.
• Athletes, 2019 meta-analysis (8 trials, 284 athletes): no effect on overall strength or jump power. A subgroup analysis suggested a possible benefit for lower-body strength and indoor athletes. The studies were small and measured strength differently, so this is a hint, not proof.

Stress fractures in training

Among 5,201 female Navy recruits, those given 2,000 mg of calcium plus 800 IU of vitamin D a day through eight weeks of basic training had 20% fewer stress fractures than those on placebo (5.3% vs 6.6%). Because the two were given together, the trial can't separate their effects, and recruits in basic training aren't typical gym-goers. For the calcium side, see our guide to calcium and calcium-rich foods.

What the Endocrine Society recommends

Its 2024 guideline suggests vitamin D beyond the RDA, without testing, for four groups: people aged 1 to 18, adults 75 and older, pregnant people, and adults with high-risk prediabetes. For healthy adults under 75, it suggests against going above the RDA to prevent disease. In other words, the RDA is the target for most active adults, and a supplement is one way to reach it when sun and diet fall short.

Vitamin D2 vs D3: which form?

Supplements contain vitamin D2 (ergocalciferol, made from yeast) or vitamin D3 (cholecalciferol, usually from sheep's wool lanolin, with vegan versions made from lichen). Both are well absorbed and both raise blood levels. ODS says most evidence shows D3 raises 25(OH)D more and keeps it up longer. A 2012 meta-analysis agreed, but the advantage was clear mainly with large, infrequent doses and was lost with daily dosing. The Endocrine Society suggests daily doses rather than large intermittent ones for adults over 50. Fat in a meal helps absorption, according to ODS, though some is absorbed without it.

Can you take too much vitamin D?

Yes, and it almost always comes from supplements. ODS says sun exposure doesn't cause toxicity, although frequent tanning-bed use can push levels very high. Too much vitamin D raises blood calcium. That can cause nausea, vomiting, muscle weakness, confusion, excessive thirst and kidney stones, and in extreme cases kidney failure and heart rhythm problems.

• The 4,000 IU adult UL isn't a toxicity line. The Food and Nutrition Board said symptoms are unlikely below 10,000 IU a day. It also warned that intakes under the UL might cause harm over time, and it advised avoiding blood levels above about 50–60 ng/mL.
• Count everything. A multivitamin, a calcium-plus-D product and a separate D supplement can stack up. Check the %DV on each label.
• Watch calcium too. In the Women's Health Initiative, 1,000 mg of calcium plus 400 IU of vitamin D daily raised kidney stone risk by 17% over seven years.
• Check your medications. High intakes may interact with thiazide diuretics and some statins.

A practical winter checklist

1. Know your sun. At Boston's latitude (about 42°N) and farther north, assume winter sun adds little from November through February.
2. Add up your food. Fatty fish once or twice a week plus fortified milk or plant milk goes a long way.
3. Close the gap, don't overshoot it. If food falls short, a supplement that brings your total to around 600–800 IU a day matches the RDA and Daily Value, far below the 4,000 IU UL. Go higher only on a clinician's advice.
4. Ask a clinician if you're in a higher-risk group. That includes being over 75 or pregnant, a malabsorption condition, bariatric surgery, regular steroid use or bone problems.
5. Keep the rest of the plan. Vitamin D is one piece of bone and muscle health. Strength and weight-bearing exercise, enough calcium and protein, and sleep are the rest. Our guide to keeping workouts and sleep on track after the clocks change has tips for darker evenings.

FAQ

Sources

• NIH Office of Dietary Supplements: Vitamin D Fact Sheet for Health Professionals (updated June 27, 2025): RDAs, ULs, food table, risk groups, NHANES status data, D2 vs D3, toxicity and medication interactions.
• NIH Office of Dietary Supplements: Vitamin D Fact Sheet for Consumers.
• USDA FoodData Central: SR Legacy entry 173692, salmon, sockeye, cooked, dry heat (16.7 mcg vitamin D per 100 g).
• Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3. J Clin Endocrinol Metab 1988.
• Demay MB et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab 2024.
• US Preventive Services Task Force: Vitamin D Deficiency in Adults: Screening (2021), published as Krist AH et al., JAMA 2021.
• Manson JE et al. Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease (VITAL). N Engl J Med 2019.
• LeBoff MS et al. Supplemental Vitamin D and Incident Fractures in Midlife and Older Adults. N Engl J Med 2022.
• Burt LA et al. Effect of High-Dose Vitamin D Supplementation on Volumetric Bone Density and Bone Strength. JAMA 2019.
• Bischoff-Ferrari HA et al. DO-HEALTH Randomized Clinical Trial. JAMA 2020.
• Farrokhyar F et al. Prevalence of vitamin D inadequacy in athletes. Sports Med 2015.
• Farrokhyar F et al. Effects of Vitamin D Supplementation on Serum 25(OH)D and Physical Performance in Athletes. Sports Med 2017.
• Zhang L, Quan M, Cao ZB. Vitamin D supplementation and muscle strength and power in athletes: a meta-analysis. PLoS One 2019.
• Lappe J et al. Calcium and vitamin D supplementation decreases incidence of stress fractures in female Navy recruits. J Bone Miner Res 2008.
• Tripkovic L et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25(OH)D. Am J Clin Nutr 2012.