Vitamin D in Winter: D3 vs D2, How Much to Take, and When

By mid-November, many of us living at northern latitudes are getting very little vitamin D from the sun, even on bright days. That's why so many people start asking the same questions this time of year: vitamin D3 vs D2, which one? How much vitamin D in winter is enough? And is there a best time to take it? This guide answers each question and tells you how strong the evidence is behind each answer. The short version: supplements reliably raise blood levels in winter (strong evidence). D3 looks like the better choice (moderate and growing evidence). The bigger promises about immunity, energy and fitness are modest at best, or not shown in trials.

Why vitamin D levels drop in winter

Your skin makes vitamin D when it's exposed to UVB light from the sun. How much you make depends on the season, the time of day, the length of the day, cloud cover, smog, how much melanin is in your skin, and sunscreen use [2]. In late fall and winter the sun sits lower in the sky, days are shorter and we cover up. At many northern latitudes, the UVB reaching your skin becomes too weak to make much vitamin D at all.

The body stores some vitamin D from summer, but those stores run down. A small 2026 study in Scientific Reports showed this happening in real time. Researchers in Italy followed 45 healthy adults, 23 amateur runners and 22 non-runners, from October to March. Non-runners who didn't take a supplement saw their average blood vitamin D (measured as 25-hydroxyvitamin D, or 25(OH)D) fall from about 26 ng/mL to about 18 ng/mL over the study [1]. Runners who trained outdoors without a supplement held fairly steady, which the authors put down to time spent outside [1].

That's the problem a winter supplement is meant to solve.

Who is most likely to run low

Anyone can get low on vitamin D in winter, but some groups are at higher risk. Here's who, based on the NIH Office of Dietary Supplements (ODS) [2]:

• People with darker skin. More melanin means the skin makes less vitamin D from the same amount of sunlight. In winter, when UVB is already weak, this matters even more.
• Older adults. Skin makes vitamin D less efficiently with age. Older people also tend to spend more time indoors. The recommended intake also rises to 800 IU after age 70.
• People living farther from the equator. At higher latitudes there's a longer winter stretch when the skin makes little or no vitamin D. Even so, latitude alone doesn't reliably predict a population's average vitamin D level, partly because people build up stores during the sunnier months.
• People who stay mostly indoors. This includes people who are housebound, live in nursing homes, work night shifts, or cover most of their skin outdoors for cultural or religious reasons.
• People with obesity. Body fat holds on to more vitamin D, so people with a BMI of 30 or more tend to have lower blood levels and may need more to reach the same level. The same applies after gastric bypass surgery.
• People with fat malabsorption. Vitamin D is fat-soluble. Conditions that affect fat absorption can reduce how much you take in, including Crohn's disease, celiac disease, cystic fibrosis and some liver diseases.
• Breastfed babies. Breast milk alone doesn't give babies enough vitamin D, so breastfed infants are usually advised to take a supplement. Babies have their own doses, so always follow your pediatrician's advice.

Signs of low vitamin D, and why a blood test beats guessing

People often search for "vitamin D deficiency symptoms winter" hoping for a clear checklist. In truth, mild low vitamin D usually causes no obvious symptoms.

When deficiency is severe and lasts a long time, it affects the bones. In children it can cause rickets. In adults it can cause osteomalacia (softening of the bones), with bone pain and muscle weakness [2]. Tiredness, low mood and frequent colds are common in winter for many reasons. They aren't reliable signs of low vitamin D on their own.

That's why a blood test beats guessing. Serum 25(OH)D is the standard measure of vitamin D status [2]. The NIH ODS gives these general reference points [2]:

• Below 30 nmol/L (12 ng/mL): deficiency
• 30–50 nmol/L (12–20 ng/mL): possibly too low for bone and overall health in some people
• 50 nmol/L (20 ng/mL) or higher: adequate for most people
• Above 125 nmol/L (50 ng/mL): linked to possible harm. Higher is not better.

Labs report results in either ng/mL or nmol/L. To convert, 1 ng/mL equals 2.5 nmol/L. Check which unit your result uses before comparing it with anything you read online.

If you're in a higher-risk group, have bone or muscle symptoms, or are thinking about taking more than a standard dose, ask your healthcare provider whether a test makes sense.

Vitamin D3 vs D2: what the newest research shows

There are two main supplement forms:

• Vitamin D3 (cholecalciferol) is the form your skin makes. Most D3 supplements are made from lanolin (sheep's wool), and an animal-free D3 made from lichen is also available [2].
• Vitamin D2 (ergocalciferol) comes from plants and fungi. It's made by exposing yeast or mushrooms to UV light. You'll find it in some prescriptions, fortified foods and vegan products.

Evidence grade: moderate and growing in favor of D3.

The two forms were long treated as interchangeable. But most trials, though not all, find that D3 raises blood 25(OH)D more and keeps it higher for longer than D2 [2][4][7].

A 2012 systematic review and meta-analysis in the American Journal of Clinical Nutrition pooled randomized trials comparing the two forms. It concluded that D3 raises blood 25(OH)D more effectively than D2 [4]. The advantage was clearest when vitamin D was given as large, infrequent doses. With daily doses, the difference was not statistically significant in that analysis [4]. A 2024 meta-analysis in Advances in Nutrition then looked specifically at daily dosing. Across 12 daily-dose comparisons, the rise in total 25(OH)D was about 10 nmol/L (roughly 4 ng/mL) smaller with D2 than with D3, although the gap was less clear in people with a BMI above 25 [7].

The newer finding: D2 may lower your D3. A systematic review and meta-analysis published in Nutrition Reviews in September 2025 asked a different question: what does taking D2 do to the D3 already in your blood? [3]

• The researchers searched studies published between 1975 and 2023. Of the 20 randomized trials in the review, 18 reported a fall in 25(OH)D3 after D2 supplements [3].
• When 11 of the trials (655 participants in total) were pooled, people taking D2 ended their trials with 25(OH)D3 levels about 18 nmol/L (roughly 7 ng/mL) lower than control groups. Measured as change over the course of the trials, the gap was about 9 nmol/L (roughly 4 ng/mL) [3].
• The authors suggest that D2 may speed up how quickly the body clears 25(OH)D3 [3].

There are important caveats. The pooled trials were small, and the review could not say whether a lower D3 level matters for health when total vitamin D is adequate [3]. Taking D2 still raises your total vitamin D level, so it is not "anti-vitamin D."

What this means in practice: For most people, D3 is the sensible default. If you're vegan or vegetarian, D2 still works, and plant-based D3 made from lichen is another option [2]. If your prescription is for D2, don't switch without talking to your prescriber.

How much vitamin D in winter? What the trials and guidelines say

Evidence grade: strong that supplements raise blood levels in fall and winter.

The clearest recent example is the 2026 Scientific Reports study [1]. Healthy adults took 2,000 IU of vitamin D3 a day for 8 weeks, from October to December:

• In runners, average 25(OH)D rose from about 30 to 35 ng/mL [1].
• In non-runners, it rose from about 25 to 30 ng/mL. That's roughly 62 to 75 nmol/L [1].
• Over the study, non-runners who didn't take a supplement fell from about 26 to 18 ng/mL [1].

So on average, a moderate daily dose kept levels up while unsupplemented levels slid. Group levels stayed within the adequate range and well below the 50 ng/mL (125 nmol/L) mark linked with possible harm. The study was small (45 people) and short, but it fits a large body of research showing that vitamin D supplements reliably raise blood levels [2].

Official intakes and upper limits

The amounts below come from the NIH ODS. They cover vitamin D from all sources combined (food, fortified drinks and supplements) [2]. One microgram (mcg) equals 40 IU.

*Adequate Intake (AI) for infants.

Recommendations vary somewhat from country to country. European regulators set the same adult upper limit, 100 mcg (4,000 IU) a day [8].

A calm, practical takeaway for adults: 600–800 IU covers the official recommendation. Many people choose 1,000–2,000 IU a day during the darker months, and the 2026 trial showed 2,000 IU raised late-fall blood levels in healthy adults [1]. Both are well below the upper limit.

The upper limit is a ceiling, not a target. Toxicity usually comes from taking very large supplement doses, far above the upper limit, for a long time. But the NIH ODS also cautions that long-term intakes above the upper limit might have harmful effects over time, even before outright toxicity [2]. Doses above 4,000 IU a day should only be taken on medical advice, for example to correct a deficiency confirmed by a blood test.

Does vitamin D help immunity, mood or energy? An honest look at the evidence

Evidence grade: modest or mixed.

Immunity

"Vitamin D and immunity" is probably the biggest reason people start supplementing in winter. Here the evidence is real but small, and it has weakened recently.

The largest analysis is a 2021 meta-analysis in The Lancet Diabetes & Endocrinology. The researchers identified 46 eligible randomized trials with 75,541 participants. Outcome data were available from 43 of them, and the main comparison against placebo drew on 37 trials [5]:

• Overall, vitamin D slightly reduced the odds of having at least one acute respiratory infection, such as a cold or chest infection, by about 8 percent [5].
• In subgroup analyses, the protective effect showed up in trials that used daily doses of 400–1,000 IU. But the differences between dosing schedules and dose sizes were not statistically significant, so the analysis did not prove that daily dosing, or any particular dose, works better [5].
• Vitamin D did not increase serious adverse events [5].

An update by the same research group, published in 2025 with three more trials added, came to a weaker conclusion. The pooled effect was similar in size but no longer statistically significant, and there was no evidence that age, baseline vitamin D level, dose size or dosing frequency changed the result [6].

The honest summary: if a sensible daily dose reduces your chance of catching a respiratory infection at all, the effect is small, and the most recent analysis could not rule out no effect. Higher doses haven't been shown to add protection. Vitamin D is not a substitute for vaccines, sleep, handwashing, or medical care when you're unwell.

Physical performance and energy

The 2026 trial specifically tested whether winter vitamin D improves fitness. Supplementation raised blood levels, but it made no difference to maximal oxygen uptake, leg strength or jump power, in either runners or non-runners [1]. The authors saw only a weak correlation between vitamin D levels and aerobic fitness, not an effect of the supplement itself [1].

If you're tired in winter, vitamin D may be worth checking. But many other things can cause fatigue, including low iron, thyroid problems, poor sleep and depression. Don't assume a supplement will fix it.

Mood

Low winter mood is common, and it's tempting to link it to low winter vitamin D. The best evidence comes from a large US trial (VITAL-DEP) in which 18,353 adults aged 50 and over took 2,000 IU a day or a placebo for a median of 5.3 years. Vitamin D did not reduce the risk of depression or improve mood scores [2][9].

If your mood dips every winter, talk to your doctor. Seasonal affective disorder has its own effective treatments, including light therapy and talk therapy.

Food sources and how far they get you

Very few foods are naturally rich in vitamin D. Here are approximate amounts per serving, based on the NIH ODS food table [2]:

• Cod liver oil, 1 tablespoon: about 1,360 IU. One caution: a tablespoon can also contain more preformed vitamin A than the adult daily upper limit of 3,000 mcg, and the amount varies by product [10]. Check the label, count all your vitamin A sources, and don't combine it with other vitamin A supplements.
• Rainbow trout (farmed, cooked), 3 oz / 85 g: about 645 IU
• Sockeye salmon (cooked), 3 oz: about 570 IU
• UV-exposed mushrooms, ½ cup: about 360 IU. This is vitamin D2.
• Fortified milk or plant milks, 1 cup: roughly 100–150 IU, depending on the brand
• Fortified breakfast cereal, 1 serving: about 80 IU. Check the label.
• Egg, 1 large: about 44 IU
• Canned sardines, 2 fish: about 46 IU
• Canned light tuna, 3 oz: about 40 IU

You can see the challenge. A portion of fatty fish two or three times a week helps a lot, but most people's everyday diets supply only a few hundred IU at best. That's why winter supplements are commonly recommended for people who get little sun.

Food, vegan options and vitamin D3 vs D2: UV-exposed mushrooms and many fortified plant milks contain D2. If you follow a plant-based diet and want D3, look for supplements labeled as lichen-derived.

Best time to take vitamin D and what to take it with

Evidence grade: practical advice, limited trial evidence.

Vitamin D is fat-soluble [2]. Taking it with a meal or snack that contains some fat is a sensible, low-effort habit, such as breakfast with eggs or yogurt, or dinner with fish, olive oil or nuts.

As for the time of day, there's no good trial evidence that morning works better than evening. The best time to take vitamin D is whenever you'll remember it consistently. Linking it to a daily meal helps.

Other practical points:

• A steady daily dose is the simplest approach. Most of the trials behind the respiratory-infection findings used daily doses, and large monthly or yearly doses haven't shown an advantage [5][6]. Small, steady doses also make it easier to stay well within the upper limit.
• Pick a format you'll actually take. Vitamin D comes as capsules, tablets, drops, gummies and dissolving films. The 2026 trial used a film that dissolves in the mouth [1]. Check the dose per serving, whichever you choose.
• Check all your sources. Add up the vitamin D in your multivitamin, calcium-plus-D tablet, fortified foods and any standalone supplement. Gummies in particular are easy to overeat.
• Choose third-party-tested products where possible. Supplement quality varies, and independent testing helps confirm the label is accurate.

How to try this: a simple winter vitamin D plan

Here's a calm, middle-of-the-road approach for healthy adults:

1. Start in the fall. October or November is ideal, as UVB weakens. Continue through to around March or April. In the 2026 study, blood levels fell again by March in people who had stopped supplementing in December, so a short course doesn't carry you through the whole winter [1].
2. Pick vitamin D3 unless you have a reason to use D2, such as diet or a prescription [2][3][4][7].
3. Choose a modest daily dose: 600–800 IU to meet the official recommendation, or 1,000–2,000 IU if you get little sun or are in a higher-risk group. Stay at or below 4,000 IU a day unless your clinician advises otherwise [2].
4. Take it with food that contains some fat, at whatever time of day you'll remember.
5. Eat vitamin D-rich foods when you can: fatty fish, eggs, fortified foods and UV-exposed mushrooms.
6. Consider a blood test if you're at higher risk, have symptoms, take medications that affect vitamin D, or have been told to take higher doses. Levels take time to settle after a change in dose, so ask your clinician when to retest.
7. Keep your expectations realistic. The main goal is to prevent winter deficiency and support bone health. Any effect on colds is small at best, and the latest analysis could not confirm one. A large trial found no mood benefit, and the 2026 trial found no fitness benefit [1][5][6][9].

If you'd like help applying this to your own diet, medications or health goals, you can ask AskMN, Sylvia or NutriGen on askmn.ai about your situation.

When to be careful: upper limits, medication interactions and who should check first

Too much vitamin D

Vitamin D toxicity almost always comes from supplements, not sunlight or food. It raises blood calcium levels (hypercalcemia). This can cause nausea, vomiting, muscle weakness, confusion, pain, loss of appetite, dehydration, excessive thirst and urination, and kidney stones. In severe cases it can cause kidney failure or irregular heart rhythms [2].

Toxicity usually follows very large doses taken for a long time, but long-term intakes above the upper limit may carry risks well before that point [2]. This is the main reason to avoid megadoses "just in case."

Medication interactions

The NIH ODS highlights these interactions [2]:

• Orlistat (a weight-loss drug, sold as Xenical or Alli) reduces vitamin D absorption, because vitamin D is absorbed with dietary fat.
• Statins (e.g. atorvastatin, simvastatin): vitamin D may affect how these drugs are processed, and statins may affect vitamin D. Mention your supplement to your prescriber.
• Steroid medicines such as prednisone can lower calcium absorption and interfere with vitamin D metabolism. People on long-term steroids may need their vitamin D and bone health monitored.
• Thiazide diuretics (e.g. hydrochlorothiazide) reduce how much calcium leaves the body in urine. Combined with vitamin D supplements, especially in older adults or people with kidney problems, they can lead to high calcium levels.

Who should talk to a clinician before supplementing

• People who are pregnant or breastfeeding. Standard intakes are generally appropriate, but dosing should be guided by your care team.
• Parents of babies and children. Their upper limits are much lower (see the table).
• Anyone with kidney disease or kidney stones, hyperparathyroidism or other parathyroid problems, high blood calcium (hypercalcemia), or sarcoidosis, tuberculosis or another granulomatous disease. In these conditions, even ordinary doses of vitamin D can push calcium too high.
• People with malabsorption conditions or a history of bariatric surgery, who may need tailored doses and monitoring.
• Anyone taking the medicines listed above, or digoxin or other heart-rhythm medicines, where high calcium can be risky.
• Anyone who has been prescribed high-dose vitamin D. Follow your prescriber's plan rather than adding extra on your own.

Taken together: a modest daily dose of D3 through the darker months is a well-supported, low-risk habit for most adults. Anything beyond that should be guided by a blood test and a conversation with your clinician.

If you're pregnant, breastfeeding, on medication or managing a health condition, talk to a qualified healthcare provider before starting vitamin D.

Sources

1. Gervasi M, Fernández-Peña E, Donati Zeppa S, et al. (2026). Effects of vitamin D supplementation during autumn and winter on blood biomarkers and physical performance in runners and non runners. Scientific Reports, 16:10094. https://www.nature.com/articles/s41598-026-38528-1 (full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC13022445/)
2. NIH Office of Dietary Supplements (accessed 2026). Vitamin D: Fact Sheet for Health Professionals. National Institutes of Health. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
3. Brown EIG, Darling AL, Robertson TM, et al. (2025). Effect of Vitamin D2 Supplementation on 25-Hydroxyvitamin D3 Status: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Nutrition Reviews (published online September 2025). https://doi.org/10.1093/nutrit/nuaf166 (PubMed: https://pubmed.ncbi.nlm.nih.gov/40973107/)
4. 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. American Journal of Clinical Nutrition, 95(6):1357–1364. https://pubmed.ncbi.nlm.nih.gov/22552031/
5. Jolliffe DA, Camargo CA Jr, Sluyter JD, et al. (2021). Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials. The Lancet Diabetes & Endocrinology, 9(5):276–292. https://pubmed.ncbi.nlm.nih.gov/33798465/
6. Jolliffe DA, Camargo CA Jr, Sluyter JD, et al. (2025). Vitamin D supplementation to prevent acute respiratory infections: systematic review and meta-analysis of stratified aggregate data. The Lancet Diabetes & Endocrinology, 13(4):307–320. https://www.sciencedirect.com/science/article/pii/S2213858724003486 (PubMed: https://pubmed.ncbi.nlm.nih.gov/39993397/)
7. van den Heuvel EGHM, Lips P, Schoonmade LJ, et al. (2024). Comparison of the effect of daily vitamin D2 and vitamin D3 supplementation on serum 25-hydroxyvitamin D concentration (total 25(OH)D, 25(OH)D2, and 25(OH)D3) and importance of body mass index: a systematic review and meta-analysis. Advances in Nutrition. https://pubmed.ncbi.nlm.nih.gov/37865222/
8. EFSA Panel on Nutrition, Novel Foods and Food Allergens (2023). Scientific opinion on the tolerable upper intake level for vitamin D, including the derivation of a conversion factor for calcidiol monohydrate. EFSA Journal, 21(8):e08145. https://pmc.ncbi.nlm.nih.gov/articles/PMC10407748/
9. 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. https://jamanetwork.com/journals/jama/fullarticle/2768978
10. NIH Office of Dietary Supplements (accessed 2026). Vitamin A and Carotenoids: Fact Sheet for Health Professionals. National Institutes of Health. https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/

Educational information only, not medical advice. Talk to a qualified healthcare provider before starting any supplement or herb, especially if you are pregnant, breastfeeding, taking medication or managing a condition.