Vitamin D has been called a cure for almost everything, and vitamin D blood tests have become a routine add-on for many people. The research of the last decade has brought this back down to earth. Vitamin D matters for bones, but for most healthy adults, more testing and higher doses have not delivered the benefits many hoped for.

What vitamin D does and where it comes from

Vitamin D helps the body absorb calcium and keep bones strong. Severe deficiency in adults causes osteomalacia, with bone pain and muscle weakness. The body makes vitamin D when bare skin is exposed to the sun, though not through a window. Natural food sources are limited: fatty fish such as salmon, trout and mackerel are the best, with small amounts in egg yolks, cheese and mushrooms, and many milks and cereals are fortified.

The NIH Office of Dietary Supplements lists the recommended daily amounts for adults as 600 IU (15 mcg) through age 70 and 800 IU (20 mcg) from age 71. Sunshine is not a safe substitute for diet or supplements, since the same ultraviolet light raises skin cancer risk, something we write about in skin cancer checks in Florida.

Who should be tested

For adults without symptoms, routine testing is not recommended by the major groups. The U.S. Preventive Services Task Force concluded in 2021 that the evidence is insufficient to recommend for or against screening. In 2024, the Endocrine Society went further and suggested against routine vitamin D blood tests in the general population, including people with obesity or darker skin, because trials have not shown that testing improves outcomes or defined the ideal level.

Testing still makes sense when there is a medical reason. The NIH lists groups more likely to run low: older adults, people who rarely get sun on their skin, people with darker skin, people with Crohn’s disease, celiac disease or ulcerative colitis, and people with obesity or a history of gastric bypass. Osteoporosis, kidney disease, some medicines and unexplained bone pain or low calcium are other common reasons a doctor orders the test.

If you are tested, the NIH offers this guide to the 25-hydroxyvitamin D result: 20 ng/mL or above is adequate for most people, below 12 ng/mL is too low, and above 50 ng/mL is too high and may cause problems.

Who benefits from supplements

The 2024 Endocrine Society guideline suggests extra vitamin D, without testing, for a few groups: children and teens, adults 75 and older, people who are pregnant, and people with high-risk prediabetes. For people over 50 who take it, the panel suggested a modest daily dose rather than large doses taken weekly or monthly. It suggested against taking more than the standard daily amounts to prevent disease in healthy adults under 75.

That advice rests on large trials. In VITAL, more than 25,000 adults took 2,000 IU of vitamin D a day or a placebo for about five years. Vitamin D did not lower the rate of cancer or of heart attacks, strokes and cardiovascular deaths. For bones, vitamin D works best as part of a bigger plan; our article on bone density screening covers the rest.

How much is too much

More is not better. The NIH sets the upper limit for adults at 4,000 IU (100 mcg) a day from all sources, unless a doctor prescribes more for a time to treat a deficiency. Very high blood levels can cause nausea, vomiting, muscle weakness, confusion, dehydration and kidney stones, and in extreme cases kidney failure and heart rhythm problems. These almost always come from supplements, not sun or food.

Check for interactions too. The NIH notes that high-dose vitamin D may make some statins work less well, that thiazide water pills plus vitamin D can raise calcium too high, and that steroids such as prednisone can lower vitamin D. Many multivitamins already contain vitamin D, so add up every source; see multivitamins: who needs them.

For people with kidney disease, vitamin D and calcium are handled differently, and decisions should go through your doctor. Dr. Shtern is board-certified in Internal Medicine and Nephrology and can help you decide whether a test or a supplement makes sense for you. Our labs and tests page explains how testing works at Legacy Medical, and the first visit is a complimentary meet and greet.

This article is general information, not advice for your situation. Members can call the office with questions about their own care.