Vitamin D: benefits, deficiency, food sources, supplements, dosage & risks (complete guide)

Vitamin D is a fat-soluble vitamin essential for many body functions. What makes it unique is that our skin can produce it when exposed to sunlight — hence the nickname “the sunshine vitamin.” There are two main forms: vitamin D3 (cholecalciferol), made in the skin under UV exposure and found in animal-based foods, and vitamin D2 (ergocalciferol), which comes from plants such as UV-exposed mushrooms and fortified foods. Both are active, but D3 is usually considered more effective at raising blood vitamin D levels, per the NIH Office of Dietary Supplements.

Once in the body, vitamin D is converted by the liver and kidneys into calcitriol, its active form, which acts more like a hormone than a typical vitamin. Yet, many people worldwide still don’t get enough vitamin D, especially during winter or in regions with little sunlight. That’s where food and supplements come into play. But before taking a supplement, it’s important to understand what vitamin D does, how much you really need, and the risks of too little — or too much.

Why vitamin D matters so much for your health

Vitamin D is best known for its role in bone health. It helps the intestines absorb calcium and phosphorus and prevents rickets or osteomalacia when levels are adequate.

For older adults, vitamin D combined with calcium helps lower the risk of osteoporosis and fractures. Its influence doesn’t stop at bones: vitamin D receptors appear in many tissues, suggesting roles in muscle function, immune modulation, and inflammation control, as summarized by the Cleveland Clinic.

Some studies suggest adequate vitamin D may lower the risk of infections and autoimmune diseases, though research is ongoing. In short, vitamin D is a quiet but powerful ally for overall health.

What happens when your vitamin D levels are too low

Deficiency is usually defined as a blood level of 25(OH)D under 12 ng/mL (30 nmol/L), while insufficiency is between 12 and 20 ng/mL. In the US, about 1 in 4 people fall below adequate levels, with higher rates in older adults, people with darker skin, or those who spend little time outdoors.

Risk factors include limited sun exposure, living in northern latitudes, aging skin (which produces less D3), obesity (vitamin D is stored in fat tissue), digestive issues that impair absorption (like celiac disease or Crohn’s), and certain medications (anticonvulsants, steroids, weight-loss drugs). Babies who are exclusively breastfed also need extra vitamin D since breast milk is naturally low in it.

The consequences? Weaker bones, increased fracture risk, rickets in children, osteomalacia in adults, and possibly reduced immune resilience. This is why screening and supplementation are often recommended for at-risk groups.

How much vitamin D do you really need every day?

The Institute of Medicine (now the National Academies) sets the RDA at about 600 IU (15 µg) for most children and adults, and 800 IU (20 µg) for people over 70. Babies under 12 months need 400 IU (10 µg).

In practice, it’s hard to reach these numbers through diet alone. Average intake from food is often below 200 IU per day in many populations. Sun exposure can cover part of the needs — around 10–15 minutes of midday sun on the face and arms may be enough in summer — but in winter or in cloudy regions, the skin simply doesn’t make enough. That’s when supplements become useful.

The best food sources of vitamin D

Few foods naturally contain vitamin D, but there are some reliable options:

• Fatty fish such as salmon, mackerel, sardines, tuna, and trout deliver 200–600 IU per serving, and cod liver oil exceeds 1,000 IU per tablespoon, according to the NIH ODS consumer fact sheet.
• Egg yolks provide small amounts (about 40 IU each).
• Liver (like beef liver or cod liver) contains vitamin D, though it’s not commonly eaten today.
• Mushrooms exposed to UV light can supply significant amounts of vitamin D2.
• Fortified foods: in the US and Canada, milk is usually fortified (about 120 IU per cup), as well as some plant-based milks, breakfast cereals, juices, and margarines.

Because natural sources are limited, fortified foods play an important role, especially for those who don’t eat fish.

Supplements: vitamin D2 vs. vitamin D3

Most supplements contain vitamin D3, usually derived from lanolin (sheep’s wool fat) or fish oil. D2 supplements, sourced from yeast or mushrooms, are also available and suitable for vegans. Both work, but D3 is more effective at raising and maintaining blood levels.

You’ll find supplements in drops, capsules, tablets, or even high-dose ampoules prescribed by doctors. Over-the-counter doses are usually 400–1000 IU per serving. In some medical cases, doctors prescribe higher doses like 50,000 IU once a month.

For infants, daily drops of 400 IU are recommended to prevent rickets. Older adults, people with little sun exposure, or those with darker skin may also benefit from daily supplements, especially in winter. Still, before starting, it’s wise to ask a healthcare professional. Blood tests can help determine the right dose.

Can you take too much vitamin D?

Yes — and that’s where caution is needed. Vitamin D toxicity, though rare, usually happens from taking excessive supplements over time, not from sun exposure. Too much vitamin D raises blood calcium (hypercalcemia), leading to nausea, vomiting, weakness, frequent urination, kidney stones, or even kidney damage if unchecked, warns MedlinePlus.

The safe upper intake level is set at 4,000 IU (100 µg) per day for adults. Short-term medical treatments may involve higher doses, but those should only be done under supervision. Toxicity generally occurs after sustained intakes of 10,000 IU or more per day.

So the rule of thumb? Aim for sufficiency, but don’t overdo it. The balance lies in meeting daily needs through a mix of food, moderate sun, and supplements if necessary.

Key takeaway

Vitamin D is much more than a bone vitamin. It supports muscles, immunity, and overall well-being. Yet, deficiency is common worldwide due to modern lifestyles and limited sun exposure. The good news is that with the right mix of sun, diet, and — if needed — supplements, most people can keep their levels within a healthy range.

Moderation matters: not too little, not too much. Just enough to let the “sunshine vitamin” quietly do its job.

Sources:

• NIH Office of Dietary Supplements – https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
• MedlinePlus – https://medlineplus.gov/vitamind.html
• Cleveland Clinic – https://my.clevelandclinic.org/health/articles/15050-vitamin-d--vitamin-d-deficiency
• National Academies of Medicine (Dietary Reference Intakes) – https://www.nationalacademies.org/hmd/Activities/Nutrition/SummaryDRIs/DRI-Tables.aspx