Key Takeaways
Vitamin D
Vitamin D is a fat-soluble nutrient that your body produces when your skin is exposed to ultraviolet B (UVB) sunlight. It plays a central role in helping your body absorb calcium, supporting bone strength, and keeping your immune system functioning properly. Unlike most vitamins, it acts more like a hormone once it's activated inside your body.
Vitamin D exists in two main dietary forms: D2 (ergocalciferol), found mostly in plants and fungi, and D3 (cholecalciferol), found in animal-based foods and produced by skin exposure to UVB radiation. D3 is generally considered more effective at raising blood levels of the vitamin.
Why Vitamin D Is Different From Other Vitamins
Most vitamins must come entirely from the food you eat. Vitamin D is the exception. Your skin manufactures it when exposed to UVB rays from sunlight — which is why it earned the nickname "the sunshine vitamin." Once made in the skin, it travels to the liver and kidneys, where it is converted into an active form that the body can use.
This dual origin — partly from sun, partly from food — makes vitamin D unique among micronutrients. It also means that changes in your lifestyle, location, or environment can quietly reduce how much your body produces, often without any obvious warning signs at first.
To understand where vitamin D fits into your overall nutritional picture, our guide to vitamins and minerals offers a helpful starting point.
Sun Protection and Vitamin D: A Nuanced Balance
Using sunscreen is important for reducing skin cancer risk, and health authorities continue to recommend it for prolonged outdoor exposure. Some concern exists that consistent, full-body sunscreen use may reduce vitamin D synthesis. In practice, most people don't apply sunscreen perfectly or in sufficient quantities to block all UVB rays, and brief periods of unprotected incidental sun exposure still occur. Talk to your healthcare provider about how to balance sun safety with your vitamin D needs.
What Vitamin D Actually Does in Your Body
Vitamin D's best-known job is helping the intestines absorb calcium — the mineral that keeps bones and teeth strong. Without enough vitamin D, calcium from food passes through largely unused, regardless of how much dairy or leafy greens you eat. Over time, this can reduce bone density and raise the risk of fractures.
Beyond bone health, vitamin D plays roles in muscle function, immune response, and cell growth regulation. Research continues to explore its possible connections to mood regulation and cardiovascular health, though these links are still being studied and are not yet considered established facts.
~35%
U.S. adults estimated to have insufficient vitamin D levels
According to data from the National Health and Nutrition Examination Survey (NHANES), a substantial portion of American adults have blood levels considered insufficient or deficient.
600–800 IU
Daily vitamin D intake recommended by the National Academies
The National Academies of Sciences, Engineering, and Medicine set the Recommended Dietary Allowance at 600 IU for adults up to age 70, and 800 IU for those over 70.
Few dozen
Foods that naturally contain vitamin D
The USDA dietary guidelines note that vitamin D is found naturally in very few foods, making fortified products and sun exposure especially important for most people.
Where to Get Vitamin D From Food
Food sources of vitamin D are limited compared to most other nutrients. The best natural sources include:
- Fatty fish — salmon, mackerel, sardines, and tuna provide some of the highest amounts per serving.
- Egg yolks — a modest but accessible source, especially from pasture-raised hens.
- Beef liver — contains vitamin D alongside other nutrients such as iron and vitamin A.
- Fortified foods — cow's milk, many plant-based milks, orange juice, and breakfast cereals are commonly fortified with vitamin D in the United States.
Because so few foods contain it naturally, fortified products make up a significant portion of most Americans' dietary vitamin D intake. A practical guide to essential vitamins can help you see how vitamin D compares to other key nutrients.
Who Is Most Likely to Be Deficient
Despite its nickname, vitamin D deficiency is widespread — even in regions with abundant sunshine. Several factors increase risk:
- Limited sun exposure: Working indoors, using high-SPF sunscreen consistently, or living at northern latitudes during winter all reduce skin production.
- Skin tone: Melanin, the pigment that gives skin its colour, reduces the skin's ability to produce vitamin D from UVB rays. People with darker skin tones may need significantly more sun exposure to produce equivalent amounts.
- Age: As we get older, the skin becomes less efficient at producing vitamin D, and the kidneys become less effective at converting it to its active form.
- Body weight: Vitamin D is fat-soluble, meaning it can become sequestered in fatty tissue and less available in the bloodstream.
- Digestive conditions: Conditions that impair fat absorption — such as Crohn's disease, celiac disease, or short bowel syndrome — reduce the amount of dietary vitamin D absorbed.
Men are among those who frequently fall short. Our article on nutrients most men don't get enough of explores this pattern in more depth.
Get Your Levels Checked First
Before purchasing any vitamin D supplement, ask your healthcare provider for a simple blood test called a 25-hydroxyvitamin D test. This tells you whether your levels are actually low and, if so, by how much. Supplementing without knowing your baseline can lead to unnecessary expense or, in rare cases, taking more than your body needs.
Vitamin D and Bone Health: A Two-Nutrient Story
Bone strength depends on both calcium and vitamin D working together — neither is sufficient on its own. This pairing matters especially for women, where low levels of both nutrients over time can contribute to osteoporosis. Our article why women need both calcium and vitamin D explains this relationship clearly.
For a deeper look at how these two nutrients interact at the absorption level, see how calcium and vitamin D work as a team.
This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. If you have concerns about your vitamin D levels or are considering supplementation, please consult a qualified healthcare provider.