Vitamin D

Vitamin D is a fat-soluble vitamin that helps calcium absorption and bone health; deficiency is often defined as serum 25-hydroxyvitamin D [25(OH)D] below 20 ng/mL. It is made in the skin after sunlight exposure and also comes from foods and supplements. The most important clinical point is that vitamin D status is assessed with a blood 25(OH)D test, not by symptoms alone, because low levels can be silent for a long time.

What it is

Vitamin D, also called calciferol, is a nutrient and hormone precursor needed for normal calcium and phosphate balance, bone mineralization, and muscle function. In foods and supplements, it mainly appears as vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D3 is the form made in human skin after ultraviolet B exposure, and NIH notes that D3 generally raises and maintains blood 25(OH)D levels better than D2.

Natural food sources are limited. Fatty fish, fish liver oils, egg yolks, and some mushrooms provide vitamin D, while many countries also use fortified foods. In India, low vitamin D status is common despite abundant sunlight, likely because of indoor work, air pollution, darker skin pigmentation, clothing coverage, and low intake of fortified foods in some diets. This matters because deficiency can contribute to rickets in children, osteomalacia in adults, and, over time, poorer bone health.

A simple comparison is below:

Form	Common name	Main source	Key point
Vitamin D3	Cholecalciferol	Skin synthesis, animal foods, supplements	Usually raises 25(OH)D more effectively
Vitamin D2	Ergocalciferol	Some mushrooms, supplements	Effective, but may be less potent for maintaining levels
25(OH)D	Calcidiol	Blood test form	Best marker of vitamin D status
1,25(OH)2D	Calcitriol	Active hormone form	Not the routine test for deficiency

How it works

Vitamin D from sunlight, food, or supplements is biologically inactive at first. The liver converts it to 25(OH)D, the main circulating form measured in blood. The kidneys then convert part of it to 1,25-dihydroxyvitamin D, the active hormone form.

Its best-established role is to increase intestinal absorption of calcium and phosphorus. This supports bone formation and remodeling and helps prevent low blood calcium, which can cause muscle cramps or spasms. Vitamin D receptors are also present in many tissues, so researchers have studied possible effects on immune function, inflammation, muscle performance, and cell growth. Those broader effects are biologically plausible, but not all have translated into clear clinical benefits in trials.

Evidence and uses

The strongest evidence for vitamin D is in skeletal health. Adequate vitamin D helps prevent nutritional rickets in children and osteomalacia in adults. In older adults, vitamin D, especially when calcium intake is also adequate, can support bone health and may reduce fracture risk in some groups with low intake or deficiency.

Common evidence-based uses include:

1. Prevention and treatment of deficiency: This is the clearest indication.
2. Bone health support: Particularly in people at risk of osteoporosis, low sun exposure, or malabsorption.
3. Rickets and osteomalacia management: Usually as part of a broader plan that includes calcium assessment.

Evidence is much less certain for non-bone outcomes. Studies have examined vitamin D for cardiovascular disease, cancer, diabetes, depression, infections, and autoimmune conditions. Results are mixed, and routine supplementation for these purposes is not supported for the general population without deficiency. Evidence in humans is limited or inconsistent for many of these claims.

Recommended intake depends on age and life stage. The NIH Office of Dietary Supplements lists a Recommended Dietary Allowance of 600 IU (15 mcg) daily for most people ages 1 to 70 years and 800 IU (20 mcg) daily for adults older than 70 years. Needs can be higher in some people with obesity, malabsorption, very low baseline levels, or certain medications, but treatment should be guided by a clinician.

Safety and interactions

Vitamin D is essential, but more is not always better. The NIH lists a tolerable upper intake level of 4,000 IU (100 mcg) per day for most adults from all sources unless a clinician prescribes otherwise. Excess intake can cause vitamin D toxicity, leading to hypercalcemia. Symptoms can include nausea, vomiting, constipation, weakness, confusion, kidney stones, and, in severe cases, kidney injury.

Important interaction and safety points include:

Issue	Why it matters
Thiazide diuretics	Can increase risk of high calcium when combined with high vitamin D intake
Orlistat	Can reduce absorption of vitamin D
Glucocorticoids	Can impair vitamin D metabolism and bone health
Some anticonvulsants	Can increase vitamin D breakdown
Digoxin	High calcium from excess vitamin D can raise arrhythmia risk

People with granulomatous diseases, some lymphomas, primary hyperparathyroidism, kidney disease, or a history of kidney stones may need closer monitoring. Because vitamin D is fat-soluble, it can accumulate over time. If you are taking a supplement regularly, especially high-dose sachets, capsules, or injections, it is sensible to review the dose with a clinician or pharmacist.

Diagnosis / how it's measured

Vitamin D status is usually assessed with a blood test for serum 25(OH)D. This is the correct routine test because it reflects vitamin D from sun exposure, food, and supplements. The active hormone, 1,25(OH)2D, is not the standard screening test for deficiency and can be misleading.

Common cutoffs vary somewhat by guideline, but many clinical references use:

• Below 20 ng/mL (50 nmol/L): deficiency
• 20 to 29 ng/mL (50 to 74 nmol/L): insufficiency in some frameworks
• 30 ng/mL and above: adequate in some clinical practices

Not everyone needs screening. Testing is usually considered for people with osteoporosis, recurrent fractures, malabsorption, chronic kidney or liver disease, obesity, dark skin with low sun exposure, older age, anticonvulsant use, or symptoms such as bone pain and proximal muscle weakness.

When to see a clinician

See a clinician if you have bone pain, muscle weakness, recurrent falls, fragility fractures, or risk factors for deficiency. Infants, pregnant people, older adults, and people with limited sun exposure or restrictive diets may need special attention. In India, strict vegetarian diets with low fortified food intake can make dietary vitamin D intake especially low.

Also seek advice before starting high-dose supplements if you have kidney disease, sarcoidosis, hyperparathyroidism, a history of kidney stones, or take interacting medicines. Children with delayed growth, bowed legs, or suspected rickets need prompt medical evaluation.

Limitations and open questions

Vitamin D research is complicated by differences in baseline deficiency, body weight, sun exposure, assay methods, and whether calcium intake is adequate. That is one reason observational studies often suggest broad benefits while randomized trials show smaller or no effects outside bone health.

There is also ongoing debate about the ideal target blood level for the general population and whether population-wide screening is useful. Evidence is stronger for correcting deficiency than for using vitamin D as a general wellness supplement. For most people, the practical approach is to avoid deficiency, use evidence-based doses, and not assume that high-dose vitamin D prevents major chronic diseases.