What vitamin D does
Vitamin D is a nutrient your body needs to absorb calcium and phosphorus, which is why its most established role is building and maintaining healthy bones. Your skin makes vitamin D when exposed to sunlight, and you also get smaller amounts from a few foods and from supplements. In the body it is converted to the form measured in blood tests, called 25-hydroxyvitamin D.
The clearest vitamin D benefits are skeletal. According to Mayo Clinic sources, vitamin D supplementation in adults reduces the risk of fractures and falls, and severe deficiency causes bone softening, known as rickets in children and osteomalacia in adults. Vitamin D has been associated with immune function and other outcomes as well, though Mayo Clinic notes that clinical trials of supplements for many non-bone conditions generally have not confirmed a benefit, so those uses remain under study.
What counts as deficient
Deficiency is defined by a blood level, not by symptoms. The NIH Office of Dietary Supplements uses the following general thresholds for 25-hydroxyvitamin D, while noting that a single cutoff does not perfectly fit everyone.
| Blood level (25-OH-D) | What it generally means |
|---|---|
| Below 12 ng/mL (30 nmol/L) | Too low; risk of deficiency and effects on bone |
| 12–20 ng/mL (30–50 nmol/L) | Generally considered inadequate for some people |
| 20 ng/mL (50 nmol/L) or above | Adequate for most people |
| Above 50 ng/mL (125 nmol/L) | May be too high; possible health concerns |
| Above 150 ng/mL (375 nmol/L) | Potentially toxic |
Vitamin D status by 25-hydroxyvitamin D blood level. Source: NIH Office of Dietary Supplements. A clinician should interpret any individual result.
Low vitamin D is common. The NIH notes that in the United States, almost one in four people have vitamin D levels that are inadequate or deficient for bone and overall health. That prevalence is part of why testing is worthwhile for people with risk factors, rather than assuming levels are fine.
Signs of deficiency
Many people with low vitamin D notice nothing at all, which is what makes it easy to miss. When vitamin D deficiency symptoms do appear, they can include fatigue, bone or muscle pain, muscle weakness, and low mood. In severe or prolonged cases, the bone-softening conditions above can develop.
Because these symptoms overlap with so many other causes, they cannot confirm a deficiency on their own. This is general education rather than medical advice, and the reliable way to know is a blood test interpreted alongside your health history by a clinician, which is exactly how Different Health handles vitamin D and nutrient markers in its DH360+ panel.
What causes it
Vitamin D deficiency usually comes down to not making enough and not taking in enough. Since the skin produces vitamin D from sunlight, limited sun exposure is a major driver, whether from indoor lifestyles, covering the skin, northern latitudes, or consistent sunscreen use. Diet contributes too, because relatively few foods naturally contain much vitamin D.
Some groups are at higher risk. Mayo Clinic lists people with obesity, people with darker skin, and adults older than 65, along with those who have limited sun exposure or conditions that impair fat absorption, such as inflammatory bowel disease. Exclusively breastfed infants who do not receive a supplement are also at risk. Knowing whether you fall into one of these groups is useful context for deciding whether to test.
How to fix it
Correcting a deficiency generally means increasing your intake through some combination of supplements, vitamin D-containing or fortified foods, and sensible sun exposure, guided by your clinician. The published reference intakes give a starting frame.
The recommended dietary allowance for vitamin D is 600 IU per day for ages 1 to 70, and 800 IU per day for adults over 70.
Those figures are general population recommendations rather than treatment doses. The NIH notes that a health care provider might recommend higher doses for a period of time to correct a diagnosed deficiency, which is a decision for a clinician rather than guesswork. Dietary sources include fatty fish, egg yolks, and fortified foods like milk and cereal.
One caution matters here. More is not better with vitamin D, because it is fat-soluble and can build up. The NIH warns that very high blood levels, above 150 ng/mL and almost always caused by excessive supplements, can lead to nausea, weakness, confusion, and kidney problems, and the National Academy of Medicine sets an upper limit of 4,000 IU per day for adults. This is a strong reason to base supplementation on a measured level rather than on assumption.
Knowing your own level
Since deficiency is silent and correcting it depends on where you actually start, a blood test is the anchor for the whole process. Testing tells you whether you are low, guides how aggressively to correct it, and lets you confirm afterward that your level has moved into a healthy range without overshooting.
This is the role Different Health plays. Its DH360+ assessment includes a 125-plus biomarker blood panel covering vitamin D and other nutrients, reviewed by an in-house MD, alongside body composition and metabolic markers. Rather than handing you a number, a team of MDs and PhDs interprets it in the context of the rest of your results and builds any nutrition or supplement adjustments into a coached, personalized plan you can retest over time.
Key takeaways
- Defined by blood level: the NIH considers 20 ng/mL or above adequate and below 12 ng/mL deficient for 25-hydroxyvitamin D.
- Bone health is the anchor: vitamin D's clearest benefit is helping absorb calcium and reducing fractures and falls; other benefits remain under study.
- Often silent: deficiency frequently causes no symptoms, so a blood test, not how you feel, confirms it.
- Know your risk factors: obesity, darker skin, age over 65, limited sun, and absorption conditions raise the risk, per Mayo Clinic.
- Correct it carefully: the NIH lists 600 IU/day (ages 1–70) and 800 IU/day (over 70), with higher treatment doses guided by a clinician.
- More isn't better: excess supplements can be toxic, so an upper limit of 4,000 IU/day applies and testing should guide dosing.
Frequently asked questions
What is considered a vitamin D deficiency?
Vitamin D status is measured by a blood test for 25-hydroxyvitamin D. According to the NIH Office of Dietary Supplements, levels of 20 ng/mL (50 nmol/L) or above are adequate for most people, while levels below 12 ng/mL (30 nmol/L) are too low and can affect bone and overall health. Because a single cutoff does not fit everyone, results should be interpreted by a clinician.
What are the symptoms of vitamin D deficiency?
Many people with low vitamin D have no clear symptoms. When present, vitamin D deficiency symptoms can include fatigue, bone or muscle pain, muscle weakness, and low mood, and severe or prolonged deficiency can cause bone softening, known as osteomalacia in adults and rickets in children. Because these signs are non-specific, a blood test is how deficiency is actually confirmed.
What causes vitamin D deficiency?
The main causes are limited sun exposure, low dietary intake, and reduced absorption or production. Mayo Clinic notes higher risk in people with obesity, those with darker skin, adults over 65, people with limited sun exposure, and those with conditions that impair fat absorption such as inflammatory bowel disease. Exclusively breastfed infants without supplementation are also at risk.
How is vitamin D deficiency treated?
It is generally corrected by increasing intake through supplements, diet, and sensible sun exposure, guided by a clinician. The NIH lists a recommended dietary allowance of 600 IU per day for ages 1 to 70 and 800 IU for those over 70, while a provider may recommend higher doses for a period to correct a diagnosed deficiency. Follow-up testing confirms levels have improved.
What are the benefits of vitamin D?
The best-established vitamin D benefits relate to bone health: it helps the body absorb calcium and phosphorus and maintain strong bones, and in adults supplementation reduces the risk of fractures and falls. Vitamin D has also been associated with immune and other functions, but Mayo Clinic notes that clinical trials for many non-bone benefits generally have not confirmed them, so those remain under study.
Can you take too much vitamin D?
Yes. The NIH states that very high blood levels above 150 ng/mL, almost always from excessive supplements rather than sun or food, can cause nausea, weakness, confusion, and kidney problems. The National Academy of Medicine sets an upper limit of 4,000 IU per day for adults. This is why more is not better, and higher doses should be taken only under medical guidance.
References
- NIH Office of Dietary Supplements. Vitamin D — Consumer Fact Sheet.
- NIH Office of Dietary Supplements. Vitamin D — Health Professional Fact Sheet.
- Mayo Clinic. Vitamin D deficiency (Expert Answers).
- Cleveland Clinic. Vitamin D Deficiency: Causes, Symptoms & Treatment.
- Thacher TD, Clarke BL. Vitamin D Insufficiency. Mayo Clinic Proceedings. 2011.