Vitamin D

What is Vitamin D?

Vitamin D is a unique fat-soluble vitamin that functions more like a hormone in the body. Unlike other vitamins, it can be synthesized by our skin when exposed to ultraviolet B (UVB) radiation from sunlight. There are two main forms: cholecalciferol (vitamin D3), produced in human skin and found in animal products, and ergocalciferol (vitamin D2), derived from plants and fungi. Vitamin D3 is generally considered more effective at raising blood levels of vitamin D.

Often called the “sunshine vitamin,” vitamin D plays crucial roles in calcium absorption, bone health, immune function, and cellular processes throughout the body. Deficiency is surprisingly common, especially in northern latitudes, during winter months, and among people with limited sun exposure.

Health benefits & uses

Bone health (Strong evidence): Vitamin D is essential for calcium absorption in the intestines and maintaining proper calcium and phosphate levels for bone mineralization. Adequate vitamin D prevents rickets in children and osteomalacia in adults, and helps reduce fracture risk in older adults when combined with calcium.

Immune function (Moderate evidence): Research suggests vitamin D supports immune system regulation and may help reduce the risk of respiratory tract infections. Some studies indicate adequate vitamin D status may lower the incidence of colds and flu, particularly in individuals who are deficient.

Muscle function (Moderate evidence): Vitamin D receptors are present in muscle tissue, and adequate levels appear important for muscle strength and balance, potentially reducing fall risk in older adults.

Cardiovascular health (Limited evidence): While observational studies suggest associations between vitamin D deficiency and increased cardiovascular disease risk, clinical trials have not consistently demonstrated that supplementation improves heart health outcomes.

Cancer prevention (Limited evidence): Some observational studies suggest higher vitamin D levels may be associated with reduced risk of certain cancers, particularly colorectal cancer, but results from randomized controlled trials remain inconclusive.

How it works

Vitamin D undergoes a two-step activation process. First, vitamin D3 from skin synthesis or dietary intake is converted to 25-hydroxyvitamin D [25(OH)D] in the liver—this is the storage form measured in blood tests. Next, the kidneys convert 25(OH)D to the active hormone calcitriol [1,25(OH)2D], which regulates calcium absorption and has effects throughout the body.

Calcitriol binds to vitamin D receptors (VDRs) found in many tissues, influencing gene expression and cellular functions. In the intestines, it increases calcium and phosphate absorption. In bones, it helps regulate the balance between bone formation and breakdown. The vitamin D endocrine system also affects immune cells, muscle tissue, and other organs.

Dosage

The Recommended Dietary Allowance (RDA) is 600 IU (15 mcg) daily for adults up to age 70, and 800 IU (20 mcg) for adults over 70. However, many experts suggest these amounts may be insufficient for optimal health, particularly for individuals with limited sun exposure.

Typical supplementation ranges from 1,000-2,000 IU daily for maintaining adequate blood levels, though some individuals may require higher doses. The Tolerable Upper Limit (UL) is 4,000 IU (100 mcg) daily for adults. Optimal blood levels of 25(OH)D are generally considered to be 30-50 ng/mL (75-125 nmol/L).

Dosing should be individualized based on factors including baseline blood levels, sun exposure, skin pigmentation, geographic location, and season. Healthcare providers may recommend higher therapeutic doses for correcting deficiency, sometimes 5,000-10,000 IU daily for limited periods.

Food sources

Natural food sources of vitamin D are limited. The best sources include:

• Fatty fish: salmon, mackerel, sardines, tuna (200-1,000 IU per 3.5 oz serving)
• Cod liver oil (1,360 IU per tablespoon)
• Egg yolks from pasture-raised hens (20-40 IU per yolk)
• Beef liver (15-20 IU per 3.5 oz serving)
• Mushrooms exposed to UV light (400+ IU per cup)

Many foods are fortified with vitamin D, including milk (100-120 IU per cup), breakfast cereals, and some plant-based milk alternatives. However, it’s difficult to obtain adequate vitamin D from food alone, making sun exposure and supplementation important for most people.

Safety & side effects

Vitamin D is generally safe when taken within recommended ranges. Side effects are rare at typical supplementation doses but may occur with excessive intake (typically above 10,000 IU daily for extended periods).

Vitamin D toxicity can cause hypercalcemia, leading to symptoms including nausea, vomiting, weakness, kidney problems, confusion, and heart rhythm abnormalities. However, toxicity is uncommon and typically requires very high doses taken for months.

Some individuals may experience mild gastrointestinal upset when starting supplementation. Taking vitamin D with food, particularly foods containing fat, can improve absorption and reduce stomach irritation.

Interactions

Medications: Thiazide diuretics can increase calcium retention, potentially raising the risk of hypercalcemia when combined with high-dose vitamin D. Glucocorticoids may reduce calcium absorption and increase vitamin D requirements. Some anticonvulsants and weight-loss drugs can decrease vitamin D absorption or increase metabolism.

Supplements: Magnesium is required for vitamin D metabolism, and magnesium deficiency may impair vitamin D function. Calcium absorption is enhanced by vitamin D, but very high calcium intake may interfere with the absorption of other minerals. Vitamin K2 works synergistically with vitamin D for bone and cardiovascular health.

Who should be cautious

Individuals with certain health conditions should consult healthcare providers before supplementing with vitamin D:

• Kidney disease or kidney stones (calcium-based)
• Hyperparathyroidism or hypercalcemia
• Sarcoidosis or other granulomatous diseases
• Lymphoma or other conditions affecting calcium metabolism

People taking medications that affect calcium levels should also seek medical guidance. Pregnant and breastfeeding women should discuss appropriate dosing with their healthcare providers.

Frequently asked questions

What’s the difference between vitamin D2 and D3?

Vitamin D3 (cholecalciferol) is more effective than D2 (ergocalciferol) at raising and maintaining blood levels of vitamin D. D3 is the form produced by human skin and found in animal products, while D2 comes from plant sources. Most experts recommend D3 supplementation when possible.

How much sun exposure do I need for adequate vitamin D?

This varies greatly based on skin tone, geographic location, season, and time of day. Generally, 10-30 minutes of midday sun exposure several times per week may be sufficient for lighter-skinned individuals, while those with darker skin may need longer exposure. Factors like sunscreen use, age, and latitude significantly affect vitamin D synthesis.

Should I get my vitamin D blood level tested?

Testing 25(OH)D levels can be helpful, especially for people at risk of deficiency (limited sun exposure, darker skin, northern latitudes, certain health conditions). Optimal levels are generally considered 30-50 ng/mL (75-125 nmol/L), though some experts suggest higher targets.

Can I take too much vitamin D?

Yes, though toxicity is uncommon with typical supplementation. The tolerable upper limit is 4,000 IU daily for adults. Excessive intake over extended periods can cause hypercalcemia and related complications. Staying within recommended ranges and periodic blood testing can help prevent toxicity.

When is the best time to take vitamin D?

Vitamin D is fat-soluble, so taking it with a meal containing some fat can improve absorption. The timing during the day is less important than consistency. Some people prefer morning dosing to avoid any potential effects on sleep, though this varies among individuals.