Every November, the same advice starts circulating: “Take your vitamin D.” Sometimes it’s good advice. Sometimes it turns into a guess based on a friend’s dose or a headline. The better question isn’t whether vitamin D is important (it is). It’s whether your food, your sunlight, and your health situation already cover what you need this time of year.
Why Winter Changes the Vitamin D Picture
Your skin makes vitamin D when bare skin is exposed to the sun’s UVB rays. In much of the U.S., that system mostly stops working in winter. In a classic study, skin samples exposed to cloudless winter sunlight in Boston made no previtamin D from November through February. Farther north, in Edmonton, that “vitamin D winter” ran from October through March. Locations farther south, such as Los Angeles, still produced vitamin D in midwinter.
Other things cut skin production too: clouds, smog, older age, darker skin, sunscreen, and staying indoors. Sunlight through a window doesn’t help either, because glass blocks UVB, according to the NIH Office of Dietary Supplements.
None of this means everyone becomes deficient by February. The body stores some vitamin D, and many people get meaningful amounts from food. It does mean winter is a fair time to check whether your diet is doing its share.
How Much Vitamin D Do Adults Need?
The NIH lists daily recommended amounts of 15 mcg (600 IU) for adults 19 to 70, and 20 mcg (800 IU) for adults 71 and older. Pregnant and breastfeeding women are also at 600 IU.
The upper limit for adults is 100 mcg (4,000 IU) a day from all sources combined, including food, fortified drinks, multivitamins and single-nutrient supplements. A clinician may prescribe more for a short time to treat a diagnosed deficiency, but that’s a supervised treatment plan, not a daily habit to copy.
Start With Food
Very few foods naturally contain much vitamin D, so fortified foods do a lot of the work in American diets. Useful sources include:
- Fatty fish such as trout, salmon, tuna and mackerel
- Milk, which in the U.S. is almost always fortified with about 120 IU per cup, and many soy, almond and oat milks
- Fortified breakfast cereals, some orange juices and yogurts
- Egg yolks, beef liver and cheese (small amounts)
- Mushrooms, especially those exposed to UV light
Check the Nutrition Facts label, since fortification varies by brand. If you rarely eat fish or fortified foods, your winter intake may be lower than you think. Our guide to improving mineral intake without guesswork walks through a simple food audit you can use for vitamins too.
Who May Need to Pay Closer Attention
The NIH notes that some groups have a harder time getting enough vitamin D:
- Older adults, because aging skin makes less vitamin D
- People who rarely go outside, or who keep most of their skin covered
- People with darker skin
- People with Crohn’s disease, celiac disease, ulcerative colitis or other conditions that limit fat absorption
- People with obesity or who have had gastric bypass surgery
- Breastfed infants, who need a 400 IU daily supplement
The Endocrine Society’s 2024 guideline goes a step further for some groups. It suggests routine vitamin D supplementation for children and teens, adults 75 and older, pregnant people, and adults with high-risk prediabetes. For healthy adults under 75, it advises against going above the standard recommended amounts to prevent disease. For people 75 and older, it prefers a daily, lower dose over occasional large doses.
What Vitamin D Can and Can’t Do
This is where the marketing usually gets ahead of the research.
What’s well established: vitamin D helps your body absorb calcium, and a long-term shortage can cause soft, painful bones (osteomalacia in adults, rickets in children). Getting recommended amounts of vitamin D and calcium helps maintain healthy bones. If bone health is your main concern, our article on calcium versus magnesium supplements and our piece on bone and joint nutrition cover the rest of the picture.
What’s less certain: according to the NIH summary of clinical trials, vitamin D supplements haven’t been shown to prevent heart disease, ease depression, improve blood sugar in most people, or help with weight loss. Supplements don’t appear to lower the chance of getting cancer, though they may slightly reduce the risk of dying from it. Even for falls and fractures in older adults, the evidence is mixed.
So vitamin D is essential, but it isn’t a cure-all. Low levels are worth fixing. Extra beyond what you need is unlikely to add benefits and can add risk.
Should You Get Tested?
Many people assume a blood test is the first step. For generally healthy people, the Endocrine Society suggests against routine vitamin D testing, because research hasn’t defined a target level that prevents disease. Testing still makes sense when there’s a medical reason, such as bone disease, low calcium, malabsorption, or a medication that affects vitamin D. Your clinician can decide whether it applies to you.
If you’re tested, the NIH considers 20 ng/mL (50 nmol/L) or above adequate for most people, below 12 ng/mL too low, and above 50 ng/mL possibly too high.
Choosing a Supplement Sensibly
If you and your clinician decide a supplement makes sense, keep it simple:
- Check what you already take. Many multivitamins already contain vitamin D. Our guides on how to choose a multivitamin and what multivitamins can really do can help you read the label.
- D3 or D2? Both raise blood levels, but D3 may raise them higher and for longer.
- Take it with food. Vitamin D is fat-soluble, so it’s absorbed best with a meal or snack that has some fat. More on that in our article on vitamin absorption factors.
- Stay under the upper limit. Add up every source, including fortified drinks, so you stay at or below 4,000 IU a day unless your clinician advises otherwise.
Medication Interactions and Safety
Too much vitamin D can be harmful. Very high blood levels can cause nausea, vomiting, muscle weakness, confusion, excessive thirst and kidney stones, and in extreme cases kidney failure or an irregular heartbeat. This almost always comes from supplements, not sunlight.
The NIH also flags several medication interactions:
- Orlistat (Xenical, alli) can reduce vitamin D absorption.
- Statins such as atorvastatin, lovastatin and simvastatin may work less well with high-dose vitamin D.
- Steroids such as prednisone can lower vitamin D levels.
- Thiazide diuretics combined with vitamin D could push blood calcium too high.
If you have kidney disease, a history of kidney stones, high calcium, or a condition such as sarcoidosis, talk with your clinician before supplementing. A pharmacist can quickly check your full list of medicines and supplements. Our article on whether vitamins can interact with medicines explains why that list matters.
The Bottom Line
Winter sunlight in much of the U.S. does little for vitamin D, so this is a good season to look at your food, your habits and your risk factors. Many adults can meet their needs with fortified foods, fatty fish and a modest supplement when there’s a real gap. Older adults, pregnant women and people with absorption problems deserve extra attention, and anyone on the medicines above should check with a pharmacist first. Aim for enough, not more.
Shopping note: Info Health News has a commercial relationship with Youngevity and may earn a commission on purchases. If you’ve decided a vitamin D product fits your plan, you can browse options on our Youngevity products page. Product pages aren’t independent evidence for health claims, so compare the vitamin D dose per serving with your total intake before you buy.
This article is for educational purposes only and isn’t medical advice. Talk with your doctor or pharmacist before starting a supplement, especially if you take prescription medication or have a health condition. See our medical disclaimer.
