Vitamin D in winter: who may need more and how to choose a supplement

Last updated: November 10, 2026

Vitamin D is a fat-soluble vitamin that helps your body absorb calcium, and calcium and vitamin D together help support healthy bones.† Unlike most vitamins, it has two sources: the food and supplements you take in, and the vitamin D your own skin makes when it is exposed to sunlight. That second source is why the season matters so much, and why vitamin D is one of the supplements people think about most once the clocks fall back.

Why winter changes the picture

Your skin needs ultraviolet B (UVB) light to make vitamin D, and the amount of UVB that reaches the ground depends on the angle of the sun. In fall and winter, the sun stays lower in the sky, the days are shorter, and more of the UVB is filtered out by the atmosphere. A classic study in Boston found that skin made almost no vitamin D from November through February, and much of the northern half of the country is in a similar position.

Other everyday factors reduce production at any time of year: time spent indoors, clothing that covers most of your skin, sunscreen, cloud cover and air pollution. Window glass blocks UVB, so sitting in a sunny room does not count. Skin with more melanin needs more sun exposure to make the same amount, and skin makes less vitamin D as we get older. None of this is a reason to skip sun protection, which health experts recommend; it is one more reason food and supplements matter.

Who may need more vitamin D

According to the NIH Office of Dietary Supplements, these groups are more likely to get too little vitamin D:

• Breastfed infants, because breast milk contains little vitamin D. Pediatric guidance commonly suggests a 400 IU daily supplement for breastfed and partially breastfed infants; ask your pediatrician.
• Older adults, whose skin makes vitamin D less efficiently and who often spend more time indoors.
• People with limited sun exposure, such as those who are homebound or who cover most of their skin.
• People with darker skin.
• People with conditions that reduce fat absorption, since vitamin D needs dietary fat to be absorbed.
• People with obesity or who have had gastric bypass surgery.

Food sources of vitamin D

Few foods contain vitamin D naturally. Fatty fish and UV-exposed mushrooms are the richest sources, and many U.S. milks, plant milks and cereals are fortified. Approximate amounts from the NIH Office of Dietary Supplements:

Fortification varies by brand, so check the Nutrition Facts label for the amount per serving.

How much do you need?

The Recommended Dietary Allowance (RDA) is the daily amount that meets the needs of nearly all healthy people. The Tolerable Upper Intake Level (UL) is the highest daily amount from all sources that is unlikely to cause harm. Both come from the NIH Office of Dietary Supplements:

*Adequate Intake for infants. Labels now list vitamin D in micrograms (mcg), often with IU in parentheses. To convert, 1 mcg equals 40 IU, so 25 mcg is 1,000 IU and 50 mcg is 2,000 IU. Our glossary entry on IU explains the older unit.

Vitamin D2 or D3?

Supplements contain one of two forms. Vitamin D2 (ergocalciferol) is made from plant or yeast sources. Vitamin D3 (cholecalciferol) is the form your skin makes; in supplements it usually comes from lanolin (sheep wool), and some products use lichen as a plant-based source. Both raise vitamin D levels in the blood, and most studies suggest D3 raises them more and keeps them up longer. If you avoid animal products, check whether a D3 product states its source, or choose D2.

How to choose a vitamin D supplement

• Check the form and amount. Look for D2 or D3 and the amount per serving in mcg and IU. Many products contain 1,000–2,000 IU; higher-strength products are often meant to be used on a doctor's advice.
• Add up every source. Your multivitamin, calcium supplement, cod liver oil and fortified foods may all contain vitamin D. Keep the total under the UL unless your doctor advises otherwise.
• Pick a format you will take. Softgels usually contain oil, which helps absorption. Drops suit infants and children; use the dropper exactly as directed. Gummies often contain added sugar.
• Take it with a meal. Vitamin D is fat-soluble, so many people take it with a meal that contains some fat.
• Read the other ingredients. Check the allergen statement and the oil used in softgels.
• Look for quality signals. A clear best-by date and lot number, manufacturer contact details and, where a label states it, an independent seal. Our guide What USP, NSF and other seals really mean explains what seals cover.

Our How to read a supplement label page walks through the rest of the Supplement Facts panel.

Should you get a blood test?

The usual test measures 25-hydroxyvitamin D, written as 25(OH)D. The NIH Office of Dietary Supplements summarizes these ranges: levels below 30 nmol/L (12 ng/mL) are associated with a risk of deficiency; 50 nmol/L (20 ng/mL) or higher is adequate for bone and overall health in most people; and levels above 125 nmol/L (50 ng/mL) may be linked to adverse effects. U.S. labs usually report ng/mL. A test is not routinely needed for everyone, so ask your doctor whether it makes sense for you, especially if you are in one of the groups above.

Too much vitamin D

Vitamin D from sunlight does not build up to harmful levels, but supplements can. Very high intakes over time can raise blood calcium, which can cause nausea, vomiting, muscle weakness, confusion, loss of appetite, excessive thirst and urination, and kidney stones. Vitamin D can also interact with some medications, including orlistat, statins, steroids such as prednisone, and thiazide diuretics. Talk to your doctor or pharmacist before starting a supplement, especially if you're pregnant or nursing, take medication, or have a medical condition.

A simple winter checklist

• Eat vitamin D foods a few times a week: fatty fish, UV-exposed mushrooms, fortified milk or plant milk.
• Get outdoors in daylight when you can, while following sensible sun protection.
• If you choose a supplement, stay within the Upper Intake Level and count every source.
• Talk to your doctor about testing if you are in a higher-need group.
• Store supplements in a cool, dry place and check the best-by date.

Related

Browse Vitamins, Immune Support, Omega-3 & Fish Oil (cod liver oil also provides vitamin D) and Women's, Men's & Kids' Health. Before you start, read Supplement safety and How to read a supplement label.

This guide is for general education and isn't a substitute for advice from your doctor, pharmacist or registered dietitian. Talk to your doctor or pharmacist before starting a supplement, especially if you're pregnant or nursing, take medication, or have a medical condition.

†These statements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease.

Sources

• NIH Office of Dietary Supplements: Vitamin D Fact Sheet for Health Professionals
• NIH Office of Dietary Supplements: Vitamin D Fact Sheet for Consumers
• MedlinePlus: Vitamin D
• Harvard T.H. Chan School of Public Health, The Nutrition Source: Vitamin D
• Dietary Guidelines for Americans
• NIH Office of Dietary Supplements: Dietary Supplements, What You Need to Know