Vitamin D in Winter: Why Levels Drop and What You Can Do

Vitamin D in winter is harder to come by for a simple reason: your skin makes vitamin D from the sun's UVB rays, and from late fall through winter much of the US gets too little UVB for that to happen. In a classic study, sunlight in Boston produced no vitamin D precursor in skin samples from November through February.^[2] Food provides relatively little, so many people look to a daily supplement such as Orzax Vitamin D3 2,000 IU, a dose within the adult upper limit.^[1]

Below, we explain why levels drop, who is most likely to run low, how much vitamin D you need, and how to take it well during the darker months.

How your skin makes vitamin D

When UVB light with a wavelength of about 290 to 320 nanometers reaches uncovered skin, it converts a cholesterol-like compound (7-dehydrocholesterol) into previtamin D3, which then becomes vitamin D3.^[1] Your liver and kidneys then convert it into its active forms.

That process depends entirely on UVB reaching your skin. Anything that reduces UVB, or reduces your skin's ability to use it, reduces how much vitamin D you make.

Why vitamin D levels drop in fall and winter

Latitude and the "vitamin D winter"

In winter the sun sits lower in the sky, so UVB travels through more atmosphere and much of it never reaches the ground. Researchers exposed human skin and a vitamin D precursor to sunlight on cloudless days at different latitudes.^[2]

• Boston (42.2°N): no previtamin D3 was produced from November through February.^[2]
• Edmonton, Canada (52°N): the ineffective period stretched from October through March.^[2]
• 34°N and 18°N: sunlight still produced previtamin D3 in the middle of winter.^[2]

The authors called this seasonal gap the "vitamin D winter." For reference, 34°N runs near Los Angeles, while Chicago, New York, Seattle, and Minneapolis all sit at or north of Boston's latitude.

Other factors that lower winter vitamin D

Season, time of day, day length, cloud cover, smog, skin melanin, and sunscreen all affect vitamin D synthesis.^[1] In fall and winter, several of these stack up at once.

• Indoor life and clothing: shorter days and colder weather mean less skin exposed to less sun.
• Skin tone: in a small study of 5 volunteers, a standard UV dose raised blood vitamin D up to 60-fold in lightly pigmented volunteers but did not significantly change it in heavily pigmented volunteers.^[4]
• Age: skin from people aged 77 and 82 produced less than half the previtamin D3 of skin from 8- and 18-year-olds.^[3]
• Sunscreen: it's listed among the factors that affect synthesis, but it remains important for skin protection.^[1]

Why a sunny summer doesn't carry you through

Blood 25-hydroxyvitamin D, or 25(OH)D, the main marker of vitamin D status, has a circulating half-life of about 15 days.^[1] Summer sun helps, but without ongoing sources from food, sun, or supplements, levels can drift down over a long winter.

Who is more likely to be low on vitamin D

According to the NIH Office of Dietary Supplements, these groups are at higher risk of inadequate vitamin D:^[1]

• Breastfed infants, since breast milk is low in vitamin D
• Older adults, whose skin makes less vitamin D and who often spend more time indoors
• People with limited sun exposure, such as those who are homebound or cover most of their skin
• People with darker skin
• People with conditions that limit fat absorption
• People with obesity (BMI 30 or higher)
• People who have had gastric bypass surgery

Diet alone rarely closes the gap. In NHANES 2015–2016, 92% of men and more than 97% of women got less than 400 IU a day from food and beverages.^[1] In NHANES 2011–2014, 18% of people were at risk of inadequacy and 5% at risk of deficiency based on blood levels.^[1]

How much vitamin D per day?

The Recommended Dietary Allowance (RDA) and the Tolerable Upper Intake Level (UL) come from the Food and Nutrition Board. The UL is the maximum daily intake unlikely to cause harm for most people, not a target.^[1]

†Adequate Intake. Source: NIH Office of Dietary Supplements.^[1]

Is vitamin D3 2,000 IU a lot?

A 2,000 IU (50 mcg) daily dose is more than the 600–800 IU RDA but half of the 4,000 IU adult upper limit.^[1] It's a common choice for adults in fall and winter, when sun contributes little. Count vitamin D from all sources, including multivitamins and fortified foods.

Higher-dose products, such as 10,000 IU softgels, exceed the adult upper limit and are intended for use only under a healthcare provider's guidance, typically after a blood test.

Testing with your clinician

The best way to know your status is a blood test for 25(OH)D.^[1] The NIH uses these general cutoffs:^[1]

• Below 12 ng/mL (30 nmol/L): deficient
• 12 to under 20 ng/mL (30–50 nmol/L): inadequate for bone and overall health in some people
• 20 ng/mL (50 nmol/L) or higher: adequate for most people
• Above 50 ng/mL (125 nmol/L): potential for adverse effects

Food sources of vitamin D

Few foods are naturally rich in vitamin D. Fatty fish and fortified foods are the main dietary sources.^[1]

Source: NIH Office of Dietary Supplements.^[1]

Vitamin D vs. D3: which form?

"Vitamin D" on a label can mean D2 (ergocalciferol, made from yeast) or D3 (cholecalciferol, the form your skin makes, usually sourced from lanolin or lichen).^[1] Both are well absorbed.^[1]

A meta-analysis of randomized trials found vitamin D3 raised blood 25(OH)D more effectively than D2. The difference was clearest with large, infrequent doses and was not significant with daily dosing.^[5] Most daily supplements, including ours, use D3.

What vitamin D does in the body

• Bone: vitamin D helps the body absorb calcium and maintain normal calcium and phosphate levels for bone mineralization.*^[1]
• Muscle: it's needed for normal development and growth of muscle fibers and supports normal muscle function.*^[1]
• Immune system: immune cells carry vitamin D receptors, and vitamin D helps modulate both innate and adaptive immune responses.*^[7] Vitamin D supports normal immune function.*

Magnesium also plays a supporting role: the enzymes that process vitamin D appear to require it.*^[8] Learn more in our guide to magnesium complex benefits, or start with the basics in Vitamin D 101.

Best time to take vitamin D3

There's no strong evidence for a best time of day. What matters more is taking it with food. Fat in the gut enhances vitamin D absorption, although some is absorbed without it.^[1]

In a randomized study of 50 healthy older adults, peak blood vitamin D3 was 32% higher when a single 50,000 IU dose was taken with a fat-containing meal than with a fat-free meal.^[6] Practical tips:

1. Take vitamin D3 with your largest meal or any meal that includes some fat, such as eggs, yogurt, nuts, avocado, or fish.
2. Pick a consistent time so it becomes a habit.
3. Start in early fall, as days shorten, rather than waiting for midwinter.

How Orzax vitamin D3 fits

Orzax Vitamin D3 2,000 IU comes in small softgels with no artificial colors, preservatives, sweeteners, or flavors. At 2,000 IU, it stays within the 4,000 IU adult upper limit.^[1] Vitamin D supports normal immune function and healthy bones and muscles.*

If you'd like to pair it with black seed oil, the Daily Immune Support Duo combines Vitamin D3 2,000 IU softgels with Black Seed Oil Liquid (5,000 mg cold-pressed black seed oil per 5 mL serving, 1% thymoquinone). You can read what research says about black seed oil in our black seed oil benefits guide.

Safety: who should talk to a clinician first

• Upper limits: don't exceed 4,000 IU a day from all sources (teens and adults) unless your clinician recommends it. Children have lower limits (see the table above).^[1]
• Blood levels: 25(OH)D above 50 ng/mL (125 nmol/L) is associated with potential adverse effects.^[1]
• Medications: the weight-loss drug orlistat can reduce vitamin D absorption, and statins may interact with vitamin D. If you take medication regularly, discuss your vitamin D intake with your healthcare provider.^[1]
• Pregnancy and nursing: the RDA is 600 IU and the UL is 4,000 IU. Talk with your provider before starting a supplement.^[1]
• Health conditions: if you have kidney or liver problems or a condition that affects fat absorption, get personalized advice.
• Children: give vitamin D supplements to children only as directed by a pediatrician.

Frequently Asked Questions

Do you need more vitamin D in winter?

Your daily requirement doesn't change, but your skin makes little or none in winter at many US latitudes.^[2] That means more of your vitamin D has to come from food or supplements during those months.

How much vitamin D3 should I take per day?

The RDA is 600 IU for ages 1–70 and 800 IU for adults over 70. Adults shouldn't exceed 4,000 IU a day unless a clinician advises it.^[1] Your ideal amount depends on your diet, sun exposure, and blood level.

Is 2,000 IU of vitamin D3 a lot?

No. It's above the RDA but half of the 4,000 IU upper limit for adults.^[1] Just count vitamin D from all your supplements and fortified foods.

What's the best time of day to take vitamin D3?

Any time that fits your routine, ideally with a meal containing fat, which improves absorption.^[1][6]

Can you get vitamin D from the sun in winter?

It depends on where you live. At about 42°N and farther north, winter sunlight produced no previtamin D3 in skin samples for several months, while at 34°N and further south it still did.^[2]

References

1. National Institutes of Health, Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals. NIH ODS
2. 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. J Clin Endocrinol Metab. 1988;67(2):373-378. PubMed
3. MacLaughlin J, Holick MF. Aging decreases the capacity of human skin to produce vitamin D3. J Clin Invest. 1985;76(4):1536-1538. PubMed
4. Clemens TL, Adams JS, Henderson SL, Holick MF. Increased skin pigment reduces the capacity of skin to synthesise vitamin D3. Lancet. 1982;1(8263):74-76. PubMed
5. Tripkovic L, Lambert H, Hart K, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012;95(6):1357-1364. PubMed
6. Dawson-Hughes B, Harris SS, Lichtenstein AH, Dolnikowski G, Palermo NJ, Rasmussen H. Dietary fat increases vitamin D-3 absorption. J Acad Nutr Diet. 2015;115(2):225-230. PubMed
7. Aranow C. Vitamin D and the immune system. J Investig Med. 2011;59(6):881-886. PubMed
8. Uwitonze AM, Razzaque MS. Role of Magnesium in Vitamin D Activation and Function. J Am Osteopath Assoc. 2018;118(3):181-189. PubMed

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. This article is for informational purposes only and is not a substitute for professional medical advice. Consult your healthcare provider before starting any supplement, especially if you are pregnant, nursing, taking medication, or have a medical condition.