Your Evidence-Based Guide to Vitamin D

What Is Vitamin D?

Vitamin D is a fat-soluble prohormone with several forms. The two most important in humans are vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). Although often called a vitamin, it acts more like a hormone, influencing calcium and phosphorus balance throughout the body.

Vitamin D3 is naturally found in animal foods and is also produced in your skin when it is exposed to ultraviolet B (UVB) rays from sunlight. Vitamin D2 comes from fungal and plant sources, such as mushrooms, and is also used in fortified foods and supplements.

Sources: University of Washington Department of Laboratory Medicine & Pathology, Cleveland Clinic Journal of Medicine, GoodRx

Why Vitamin D Matters

Your body needs vitamin D to regulate calcium and phosphorus—two minerals that are critical for building and maintaining strong bones. Without enough vitamin D, your bones can become thin, brittle, or misshapen. While these are the most clearly established roles, researchers continue to investigate other potential health effects.

The active form of vitamin D is made through a two-step process: the liver converts both D2 and D3 to 25-hydroxyvitamin D (calcidiol), which is the main circulating form. Then the kidneys and other tissues convert that into 1,25-dihydroxyvitamin D (calcitriol), which is the most metabolically active form.

Sources: University of Washington Department of Laboratory Medicine & Pathology

Key Sources of Vitamin D

Vitamin D is unique because your skin can produce it when exposed to UVB radiation from the sun. For some people, as little as 5 to 30 minutes of sun exposure per day may provide enough, but this varies widely by skin tone, latitude, time of day, and sunscreen use. However, because of the risk of skin cancer, some health organizations recommend obtaining vitamin D from food and supplements rather than intentional sun exposure.

Natural dietary sources are actually few. The richest natural sources are fatty fish and fish liver oils, but even these provide only a fraction of the daily recommended intake when consumed in normal portions. Most people in the US rely on fortified foods like milk and cereals, which contain vitamin D2 or D3.

If you are looking for a convenient comparison of D2 and D3, see the table below.

• Fatty fish (such as salmon, trout, and tuna) and fish liver oils are among the richest natural sources.
• Egg yolks contain small amounts of vitamin D3.
• Mushrooms can provide vitamin D2, especially if they are exposed to UV light.
• Many dairy products, breakfast cereals, and orange juice are fortified with vitamin D.

Sources: GoodRx, Clarity X DNA, Cleveland Clinic Journal of Medicine

Understanding Deficiency

Vitamin D deficiency is common in the United States. An analysis of national survey data from 2011 to 2014 found that almost 20% of the population had blood levels of 25-hydroxyvitamin D (25(OH)D) that put them at risk for inadequacy (30–49 nmol/L or 12–19 ng/mL), and 5% were at risk for deficiency (below 30 nmol/L or 12 ng/mL).

Certain groups are more likely to be deficient. Lack of sun exposure, older age, and obesity are recognized risk factors, as are conditions that affect fat absorption, such as celiac disease or Crohn’s disease. People who take medications like corticosteroids or anti-seizure drugs may also need extra attention to their vitamin D status.

Signs of vitamin D deficiency can be subtle. Some individuals may experience bone pain or back pain, and long-term deficiency can lead to osteoporosis in adults or rickets in children. However, these symptoms are not specific, so a blood test is often needed to confirm.

Sources: Cleveland Clinic Journal of Medicine, Serenity Pregnancy Resource Center, GoodRx

Importance of Testing

How do you know if you are deficient? A simple blood test can measure your vitamin D level. The preferred initial test is the 25-hydroxyvitamin D (25(OH)D) test because it reflects both vitamin D2 and D3 and is the best indicator of total body stores in patients without kidney disease.

Doctors do not recommend routine testing for everyone. Following guidelines from the Endocrine Society and the Choosing Wisely campaign, doctors should order a vitamin D test only when a person has specific risk factors or symptoms. These include obesity, osteoporosis, or long-term treatment with corticosteroids or anti-seizure medications.

It’s important to note that the test for 1,25-dihydroxyvitamin D (the active form) is not useful for routine screening. That test does not reflect your body’s vitamin D reserves and cannot reliably detect toxicity. Your doctor may order it in special situations, but it is not part of standard screening.

Sources: University of Washington Department of Laboratory Medicine & Pathology

Guide to Supplementation

If you cannot get enough vitamin D from sun and food, a supplement can help. Two forms are available: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). While both can raise blood levels, vitamin D3 appears to be more effective at raising and maintaining your 25(OH)D levels. This may be because D3 binds more strongly to vitamin D binding proteins in the blood and is converted more efficiently in the liver.

Both D2 and D3 supplements are available over the counter at common doses like 400, 800, 2,000 IU. Vitamin D2 is also available by prescription, often as high-dose capsules of 50,000 IU, which doctors may use to treat severe deficiency.

For treating a confirmed deficiency, typical replacement therapy is at least 2,000 IU per day, with some doctors recommending up to 5,000 IU. After an initial correction period, a maintenance dose of at least 2,000 IU daily is often used. However, the exact dose should always be guided by your healthcare provider and based on your blood levels.

Taking too much vitamin D can cause harmful side effects, primarily by raising blood calcium to dangerous levels. The risk of toxicity comes from high-dose supplements, not from sun exposure or food. Symptoms of excess vitamin D include nausea, vomiting, weakness, and kidney problems. Never take more than the recommended amount without medical supervision.

Sources: Cleveland Clinic Journal of Medicine, University of Washington Department of Laboratory Medicine & Pathology, GoodRx, MDPI Nutrients

Vitamin D2 vs. D3: A Side-by-Side Comparison

When choosing a supplement, you may wonder about the differences between vitamin D2 and D3. The table below summarizes the key differences, based on current evidence.

Sources: University of Washington Department of Laboratory Medicine & Pathology, Cleveland Clinic Journal of Medicine, GoodRx