Vitamin D Testing and Supplementation: A Science-Based Guide

Vitamin D testing became one of the most ordered blood tests in the United States over the last 15 years, with annual Medicare spending on the test rising more than 80-fold between 2000 and 2014. The boom was driven by observational studies linking low vitamin D to nearly every chronic disease you can name. But the largest randomized trial ever conducted on vitamin D, the VITAL trial published in the New England Journal of Medicine in 2018, did not bear out many of those promises. This guide walks through who actually needs vitamin D testing, what the numbers mean, how much to supplement, and where the science has settled after the dust cleared.

What vitamin D is, and why it is unusual

Vitamin D is technically a hormone, not a vitamin. The body makes it in the skin when ultraviolet B radiation hits 7-dehydrocholesterol, a cholesterol derivative. The liver then converts it to 25-hydroxyvitamin D (25(OH)D), the form measured in blood, and the kidney converts 25(OH)D to the active hormone 1,25-dihydroxyvitamin D. Very few foods naturally contain meaningful amounts. Fatty fish (salmon, mackerel, sardines), cod liver oil, and certain mushrooms exposed to UV light are the main natural sources. In the United States, milk, some juices, and many breakfast cereals are fortified.

The active hormone binds to a nuclear receptor present in nearly every human cell and regulates hundreds of genes. It promotes calcium absorption in the gut, maintains phosphate balance, and supports bone mineralization. Receptors in immune cells, muscle, and the cardiovascular system explain why researchers have studied vitamin D for outcomes far beyond bone health.

Who needs vitamin D testing?

The Endocrine Society's 2011 clinical practice guideline recommends testing for people at risk of deficiency. The list is broad and includes:

• People with dark skin (melanin reduces UVB-mediated vitamin D synthesis)
• Older adults, especially those who are housebound or in long-term care
• People with limited sun exposure, including those who cover their skin for cultural or religious reasons
• People with malabsorption disorders (celiac disease, Crohn's disease, cystic fibrosis, bariatric surgery)
• People with obesity (vitamin D is sequestered in adipose tissue, lowering serum levels)
• People with kidney or liver disease (impaired activation)
• Pregnant and lactating women
• People on medications that alter vitamin D metabolism (anticonvulsants, glucocorticoids, AIDS drugs)

For healthy adults with adequate sun exposure, routine testing is not necessary. The U.S. Preventive Services Task Force concluded in 2021 that evidence is insufficient to assess the balance of benefits and harms of screening for vitamin D deficiency in asymptomatic, non-pregnant adults. Many insurers no longer cover the test without a documented risk factor, partly because of how heavily it was over-ordered in the 2010s.

What vitamin D levels actually mean

The Endocrine Society defines 25(OH)D status as follows:

• Deficient: below 20 ng/mL (50 nmol/L)
• Insufficient: 21 to 29 ng/mL (52.5 to 72.5 nmol/L)
• Sufficient: 30 ng/mL or higher (75 nmol/L or higher)

The National Academy of Medicine (formerly the Institute of Medicine) takes a more conservative view, concluding that 20 ng/mL is adequate for 97.5 percent of the population for bone health. The disagreement between the Endocrine Society and the NAM has fueled years of debate. Most clinicians target somewhere between 30 and 50 ng/mL, which sits comfortably above both thresholds and well below the level at which toxicity becomes a concern.

Levels above 100 ng/mL (250 nmol/L) are considered potentially toxic. Symptoms of toxicity include hypercalcemia, nausea, vomiting, weakness, kidney stones, and in severe cases, vascular calcification and kidney damage. Toxicity almost never occurs from sun exposure or food. It almost always comes from supplements taken at high doses for weeks or months.

The VITAL trial: what 25,871 people told us

The VITAL trial, published in the New England Journal of Medicine in 2018 and 2019, was the definitive test of whether vitamin D supplementation prevents cancer and cardiovascular disease in generally healthy adults. Investigators randomized 25,871 U.S. adults aged 50 and older to 2,000 IU of vitamin D3 per day or placebo, with a median follow-up of 5.3 years.

The headline result was null. Vitamin D supplementation did not significantly lower the incidence of invasive cancer of any type or major cardiovascular events (myocardial infarction, stroke, or cardiovascular death). Subgroup analyses suggested a possible reduction in cancer mortality that emerged after the first two years, but the primary endpoints were negative.

A subsequent analysis published in 2020 in JAMA Network Open looked at autoimmune outcomes and found a 22 percent reduction in incident autoimmune disease (rheumatoid arthritis, polymyalgia rheumatica, psoriasis, and others) with vitamin D supplementation. A separate analysis suggested a small reduction in advanced (metastatic or fatal) cancer, particularly in people with normal weight. These secondary findings are interesting but should be interpreted cautiously. The primary message from VITAL is that routine high-dose vitamin D supplementation does not prevent cancer or heart disease in healthy middle-aged and older adults.

What supplementation does help with

Bone health in older adults

Vitamin D supplementation reduces the risk of falls and fractures in older adults who are deficient, especially when combined with calcium. A 2020 meta-analysis in JAMA Endocrinology by Burt and colleagues found that high-dose vitamin D did not prevent fractures in healthy adults, but repletion in deficient older adults does improve bone density and reduce falls. The Endocrine Society recommends 800 to 1,000 IU per day for adults over 70.

Rickets and osteomalacia

Severe vitamin D deficiency causes rickets in children (soft, deformed bones) and osteomalacia in adults (bone pain and weakness). These conditions are uncommon in fortified countries but still occur in exclusively breastfed infants without supplementation, in people with severe malabsorption, and in some immigrant populations with limited sun exposure.

Pregnancy

The American College of Obstetricians and Gynecologists notes that severe deficiency in pregnancy is associated with preeclampsia, gestational diabetes, and preterm birth, though causality is not firmly established. Most prenatal vitamins contain 400 to 600 IU, which the Endocrine Society considers adequate for most pregnancies, with higher doses reserved for documented deficiency.

How much to supplement, and in what form

The National Academy of Medicine sets the RDA at 600 IU per day for adults up to age 70 and 800 IU per day for adults over 70. The Endocrine Society recommends 1,500 to 2,000 IU per day for adults who need to maintain levels above 30 ng/mL, which is more than the RDA but consistent with what most modern trials have used.

Doses of 1,000 to 2,000 IU per day are safe for almost everyone. Doses above 4,000 IU per day, the NAM tolerable upper intake level, should generally be supervised. People with documented deficiency are sometimes prescribed 50,000 IU weekly for 8 weeks as a repletion protocol, followed by a maintenance dose of 1,000 to 2,000 IU per day.

Vitamin D3 (cholecalciferol) is preferred over vitamin D2 (ergocalciferol). Meta-analyses show D3 is more effective at raising and maintaining serum 25(OH)D levels. D3 is the form the body makes from sunlight. D2 is produced by plants and fungi exposed to UV.

Take vitamin D with a meal that contains some fat. It is fat-soluble, and absorption is meaningfully better with food. Pairing it with vitamin K2 (particularly MK-7) has become popular, and the rationale is plausible (K2 helps direct calcium into bone rather than soft tissue), but the evidence from randomized trials that this combination outperforms vitamin D alone is still thin.

Sunlight, skin, and the seasonal drop

The amount of sun required to make adequate vitamin D depends on latitude, season, time of day, skin type, and how much skin is exposed. A rough rule of thumb from the work of Michael Holick and others: 5 to 30 minutes of midday sun on the face, arms, and legs two to three times per week during summer is enough for most light-skinned people at temperate latitudes. Dark-skinned people need substantially more, sometimes 3 to 6 times longer, because melanin absorbs UVB.

At latitudes above about 37 degrees north (roughly the latitude of San Francisco, St. Louis, and Richmond, Virginia), vitamin D synthesis essentially stops from November through February because the sun's angle is too low. People in those regions who do not supplement or eat fortified foods commonly see 25(OH)D levels drop into the insufficient range by late winter.

Sunscreen blocks UVB and reduces vitamin D synthesis. The standard dermatology advice is to balance brief sun exposure for vitamin D with sun protection for skin cancer prevention. This is a genuine trade-off and there is no universally right answer. People at high risk of skin cancer should rely on food and supplements.

Vitamin D toxicity: rare but real

The main risk of over-supplementation is hypercalcemia. The NAM sets the tolerable upper intake level at 4,000 IU per day for adults. Symptoms of toxicity include nausea, vomiting, weakness, frequent urination, kidney stones, and confusion. Long-term toxicity can cause vascular and soft tissue calcification and kidney damage.

Cases of toxicity in the medical literature almost always involve doses above 10,000 IU per day taken for months, often due to manufacturing errors, mislabeled supplements, or self-prescription. A 2017 case series in the journal Clinical Endocrinology described a cluster of toxicity cases in the UK caused by a poorly formulated supplement delivering hundreds of thousands of IU per capsule. Buy from reputable manufacturers, and if you are taking very high doses, get your level checked.

Testing strategy for an individual

If you are an adult with one or more risk factors listed above, a single 25(OH)D test is reasonable. If the level is below 20 ng/mL, supplement at 2,000 IU per day or follow a repletion protocol if your clinician recommends it, and retest in 3 months. If the level is 20 to 29 ng/mL, supplement at 1,000 to 2,000 IU per day and retest in 3 to 6 months. If the level is 30 ng/mL or higher and you have no new risk factors, routine retesting is unnecessary.

For healthy adults with good sun exposure in summer and a balanced diet, testing is optional. A daily 1,000 IU supplement during winter months is a low-risk, low-cost hedge against the seasonal drop. Calculate your own intake with the Vitamin D Calculator.

The bottom line on vitamin D

The early 2010s enthusiasm for vitamin D as a near-universal preventive therapy has been tempered by VITAL and other large trials. Vitamin D is essential for bone health, deficiency causes real disease, and repletion in deficient older adults reduces falls and fractures. Beyond that, the evidence is much thinner than supplement marketing suggests. Test if you are at risk, supplement at 1,000 to 2,000 IU per day if your level is low or you have limited sun exposure, and resist the temptation to push levels above 60 ng/mL. More is not better, and the cost of over-supplementation is real.