Vitamin D: Benefits, Sources, Deficiency & What You Should Know About Supplementation

Vitamin D is unusual among nutrients: your body can technically produce it on its own, using nothing but sunlight and cholesterol in the skin. In theory, that should make deficiency rare. In practice, low vitamin D status is one of the most common nutrient gaps identified in blood testing across many populations — a gap driven by indoor lifestyles, sunscreen use, climate, skin tone, and the fact that very few foods naturally contain meaningful amounts of it.

This article covers what vitamin D actually does, how the body gets it (from both sun and food), who’s most likely to fall short, and what to know if you’re considering a supplement.

What Does Vitamin D Do in the Body?

Vitamin D’s best-established role is regulating calcium and phosphate absorption in the gut, which is why it’s so closely tied to bone health — without enough vitamin D, the body can’t effectively use the calcium you eat, regardless of how much calcium is in your diet. Beyond bone metabolism, vitamin D receptors are found in tissue throughout the body, and it’s involved in:

• Calcium absorption and bone mineralization
• Muscle function
• Immune system regulation
• Cell growth modulation

Research has explored associations between vitamin D status and cardiovascular health, mood, and immune function, but it’s important to be precise here: association is not the same as proven benefit from supplementation. Large randomized trials on vitamin D supplementation for outcomes like depression, diabetes prevention, or cardiovascular disease have generally shown limited or no benefit in people who weren’t deficient to begin with. The clearest, most consistently supported role for vitamin D remains bone health and calcium regulation.

How the Body Gets Vitamin D

There are three routes:

• Sun exposure — UVB rays trigger a chemical reaction in skin that produces vitamin D3. This is the primary source for many people historically, but it’s inconsistent: it depends on latitude, season, time of day, skin pigmentation, sunscreen use, age, and how much skin is exposed. People with darker skin produce less vitamin D per unit of sun exposure due to higher melanin content, which naturally filters UVB.
• Food — very few foods contain substantial vitamin D naturally.
• Supplements — used to fill the gap when sun exposure and diet aren’t sufficient.

Because reliable sun exposure isn’t practical (or advisable, given skin cancer risk) as a primary strategy, most health authorities emphasize food and supplementation as the more controllable sources.

Food Sources of Vitamin D

Natural dietary sources are limited, which is a big part of why deficiency is so widespread:

• Fatty fish — salmon, mackerel, sardines, trout
• Egg yolks
• Beef liver (smaller amounts)
• UV-exposed mushrooms (some varieties are treated with UV light specifically to boost vitamin D content)
• Fortified foods — milk, some plant-based milk alternatives, some breakfast cereals, and orange juice in certain markets

Because so few whole foods are rich in vitamin D, fortified foods carry a disproportionate share of dietary intake in many countries.

Who’s Most at Risk of Vitamin D Deficiency?

• People with limited sun exposure — those who work indoors, live in northern latitudes, or cover most of their skin
• People with darker skin tones, due to reduced UVB-driven synthesis
• Older adults — skin becomes less efficient at producing vitamin D with age
• People who are obese — vitamin D is fat-soluble and can be sequestered in fat tissue, lowering circulating levels
• People with conditions affecting fat absorption — Crohn’s disease, celiac disease, cystic fibrosis, and those who’ve had gastric bypass surgery
• Exclusively breastfed infants, since breast milk is typically low in vitamin D (this is why pediatric vitamin D drops are commonly recommended)

Signs and Symptoms of Low Vitamin D

Mild to moderate deficiency is often asymptomatic, which is part of why it’s usually identified through blood testing rather than symptoms alone. When symptoms do appear, they can include bone pain, muscle weakness, and fatigue. In children, severe long-term deficiency can cause rickets (soft, weakened bones); in adults, the equivalent condition is osteomalacia. These are relatively uncommon in developed countries today but not extinct, particularly in high-risk groups.

How Much Vitamin D Do You Need?

The NIH Office of Dietary Supplements lists the following Recommended Dietary Allowances (RDA), measured in International Units (IU):

The tolerable upper intake level for adults is generally set at 4,000 IU/day from supplements and food combined, though healthcare providers sometimes recommend higher short-term doses to correct a diagnosed deficiency — this should be done under medical supervision with blood testing, not self-directed.

It’s worth noting that these RDA figures assume minimal sun exposure. They’re intentionally conservative baseline numbers rather than optimal targets for every individual, which is one reason blood testing (25-hydroxyvitamin D) is the most reliable way to know where you personally stand, rather than assuming intake alone tells the full story.

D2 vs. D3: What’s the Difference?

Vitamin D supplements come in two forms:

• Vitamin D2 (ergocalciferol) — derived from plant/fungal sources (yeast exposed to UV light)
• Vitamin D3 (cholecalciferol) — derived from animal sources (like lanolin from sheep’s wool) or, increasingly, from lichen for a vegan-friendly D3 option

Because D3 tends to be more effective at raising and maintaining vitamin D status in the blood, it’s the more commonly recommended form in over-the-counter supplements, though D2 remains in use, particularly in some prescription formulations.

Vitamin D and Bone Health

This is the area with the strongest, most consistent evidence. Vitamin D works alongside calcium — without sufficient vitamin D, the intestines absorb only a fraction of dietary calcium, regardless of how much calcium you consume. Long-term inadequate vitamin D (combined with low calcium intake) is linked to reduced bone mineral density and increased fracture risk, particularly in older adults. This is why vitamin D and calcium are so often discussed together, and why many bone-health supplement formulations pair the two — along with vitamin K2, which plays a role in directing calcium toward bone rather than soft tissue.

Safety and Considerations

Vitamin D is fat-soluble, meaning excess amounts are stored in the body rather than excreted, which makes very high-dose, long-term supplementation without monitoring a legitimate safety concern (unlike water-soluble vitamins, which are more easily cleared). Signs of vitamin D toxicity are almost always related to excessive supplement use, not food or sun exposure — the skin has a natural mechanism that prevents overproduction from sunlight, and food sources rarely supply excessive amounts on their own. Symptoms of toxicity can include nausea, weakness, and in severe cases, elevated blood calcium levels.

People taking certain medications — including some steroids, weight-loss drugs that reduce fat absorption, and specific epilepsy medications — should talk to a healthcare provider before supplementing, since these can interact with vitamin D metabolism.

Key Takeaways

• Vitamin D’s best-established role is regulating calcium absorption and supporting bone health.
• Very few foods naturally contain significant vitamin D; fatty fish, egg yolks, and fortified foods are the main sources.
• Risk factors for deficiency include limited sun exposure, darker skin tone, older age, obesity, and certain digestive conditions.
• Adult RDA is generally 600–800 IU/day, with an upper limit of 4,000 IU/day from supplements and food combined.
• D3 is generally more effective than D2 at raising and maintaining blood vitamin D levels.
• Because deficiency is often symptomless, blood testing is the most reliable way to know your status.

Sources & Further Reading

• National Institutes of Health, Office of Dietary Supplements — Vitamin D: Fact Sheet for Health Professionals
• National Institutes of Health, Office of Dietary Supplements — Vitamin D: Fact Sheet for Consumers

*This article is for general educational purposes and is not a substitute for individualized medical advice. Speak with a healthcare provider before starting any new supplement, particularly if you have an existing health condition or take medication.*