By MercyAssistedCare.org Editorial Team
A vitamin D blood result tells you how much of one form of vitamin D, called 25-hydroxyvitamin D or 25(OH)D, was circulating in the sample that was drawn. That number reflects everything the body has taken in or made: food, fortified foods, supplements and sunlight. What it does not tell you is why the number is what it is, whether it is a problem for this particular person, or what, if anything, to change. Those questions belong to the clinician who knows the person's history and medicines.
This guide is for family caregivers and older adults who have a lab report in hand, or who are wondering whether to ask for one. It is general education, not a way to interpret a specific result or choose an amount to take.
What the test actually measures
Vitamin D from every source is changed in the liver into 25(OH)D. According to the National Institutes of Health Office of Dietary Supplements, the blood level of 25(OH)D is the main indicator of vitamin D status because it reflects what comes from sunlight, food and supplements together. MedlinePlus explains that most vitamin D blood tests measure this form, and that results may be reported as “total vitamin D” (D2 and D3 added together) or as the two forms separately.
A report may also include a different test for “active” vitamin D, sometimes labeled calcitriol or 1,25-dihydroxyvitamin D. Both sources say this is generally not used to check whether someone has enough vitamin D. MedlinePlus says it may be used to monitor kidney problems or to look into abnormal blood calcium. If a report lists that test, it is answering a different question.
Four things that get mixed up, side by side
Families often treat what is eaten, what is in a bottle and what the lab measured as the same thing. They are four separate pieces of information. Here is what each one can and cannot tell you.
- Food sources (naturally occurring)
- What it is: Only a few foods naturally contain vitamin D. Fatty fish such as salmon, trout and mackerel are among the best sources, and egg yolks, cheese and beef liver have smaller amounts. For example, the Office of Dietary Supplements lists 3 ounces of cooked sockeye salmon at about 14.2 mcg (570 IU) and one large scrambled egg at about 1.1 mcg (44 IU).
- What it can tell you: Roughly what is coming in from meals.
- What it cannot tell you: What the blood level is. Sunlight, absorption and body changes with age also affect it.
- Evidence limit: The Office of Dietary Supplements notes that getting enough vitamin D from unfortified foods alone is difficult.
- Fortified foods
- What it is: Foods with vitamin D added. In the United States, fortified foods provide most of the vitamin D in the diet. One cup of fortified milk has about 2.9 mcg (120 IU), and fortified soy, almond or oat drinks have about 2.5 to 3.6 mcg (100 to 144 IU) per cup. A serving of a cereal fortified to 10% of the Daily Value has about 2.0 mcg (80 IU).
- What it can tell you: What the Nutrition Facts label says each serving contains.
- What it cannot tell you: How much this person actually eats week to week, or how it adds up with a supplement.
- Evidence limit: Amounts vary by brand, so the label is the source for any specific product.
- Supplements
- What it is: Vitamin D in pills, capsules or drops, as D2 or D3, and sometimes combined with calcium or inside a multivitamin. Labels may use micrograms (mcg) or International Units (IU), and 1 mcg equals 40 IU.
- What it can tell you: What a product label says is in one serving.
- What it cannot tell you: What it does inside one person. The Office of Dietary Supplements says blood levels rise with higher intake, but not in a straight line, and the change depends on the starting level and how long someone takes it.
- Evidence limit: Both D2 and D3 raise blood levels, though most evidence indicates D3 raises and maintains them more.
- A measured blood result (25(OH)D)
- What it is: A snapshot, taken at the time of the blood draw, of the combined effect of all of the above plus sunlight.
- What it can tell you: Whether the level falls in a range that clinicians associate with risk of deficiency, possible inadequacy, sufficiency or, rarely, excess.
- What it cannot tell you: The reason behind the number, what amount would change it, or whether a supplement would prevent a fall or fracture.
- Evidence limit: See the sections below on units, labs and ranges.
A worked example: one number, two opposite meanings
This example is hypothetical. It is built to show what can go wrong, not to describe a real person or suggest what any real result means.
Assumptions we are making:
- A caregiver is looking at a printed lab report for an older parent and sees a vitamin D line that reads simply “28.”
- The caregiver has not noticed which unit the report uses.
- We use the general-population ranges from the National Academies' Food and Nutrition Board, as summarized by the Office of Dietary Supplements. These are not older-adult-specific ranges, and a particular lab's printed ranges may differ.
If the unit is ng/mL: 28 ng/mL equals 70 nmol/L (one ng/mL is 2.5 nmol/L). That is above 20 ng/mL (50 nmol/L), the level the Food and Nutrition Board describes as generally adequate for bone and overall health in healthy people.
If the unit is nmol/L: 28 nmol/L equals about 11 ng/mL (one nmol/L is 0.4 ng/mL). That is below 30 nmol/L (12 ng/mL), the level the Food and Nutrition Board associates with risk of vitamin D deficiency.
The consequence of confusing the two: The same printed number lands in opposite places. A caregiver who assumes the wrong unit could start adding products on top of fortified foods and medicines already in the routine when the level was not low, or could relax about a number that deserved a prompt conversation with the clinician. Neither choice should be made from the number alone. The clinician reads the unit, the lab's own range, the person's history and the current medicine list together.
Evidence limits on this example: The ranges above are reference points, not diagnoses. The Office of Dietary Supplements states that optimal levels for bone and general health have not been established and likely vary by life stage and other factors.
Why a single result has limits
- Different labs, different methods. The Office of Dietary Supplements says the available assays vary considerably, so a finding can come out falsely low or falsely high depending on the method and the laboratory. Comparing numbers from two different labs can mislead.
- No single agreed “ideal” number. The Food and Nutrition Board publishes reference ranges, but the Endocrine Society has not identified levels that mark sufficiency, insufficiency and deficiency, and does not recommend routine testing in healthy people.
- A snapshot in time. The Office of Dietary Supplements reports that 25(OH)D has a circulating half-life of about 15 days. In plain terms, a result reflects a recent stretch of time rather than a lifetime record, so it may look different after a change in sun exposure, diet or products.
- Not proof that a supplement will help. The Office of Dietary Supplements describes the evidence in older adults as inconsistent: vitamin D with calcium produced small bone-density gains in some groups and fewer fractures in institutionalized older people, but results for community-dwelling adults were inconsistent, and a U.S. Preventive Services Task Force review found no reduction in falls. It advises older adults to talk with their health care providers about their needs for vitamin D and calcium as part of an overall bone-health plan.
- Screening evidence is thin. The Office of Dietary Supplements reports that the Task Force found insufficient evidence to assess the benefits and harms of screening adults without symptoms for vitamin D deficiency. MedlinePlus likewise says routine testing is not recommended for everyone.
When a clinician may order one, and why older adults come up
MedlinePlus says a provider may order a vitamin D test when someone has a condition or symptoms that could relate to low vitamin D (such as low bone density, bone pain, muscle weakness or fractures), or when risk is higher. It lists little sun exposure, poor nutrition, weight-loss surgery, conditions that make it hard to absorb nutrients, kidney or liver disease, and medicines that affect vitamin D levels. A test may also be repeated to see whether levels are changing after treatment.
The Office of Dietary Supplements names older adults as a group at increased risk of inadequate vitamin D, in part because skin makes less vitamin D with age, and because older adults tend to spend more time indoors and may take in less of it. That is a reason to have the conversation, not a reason to assume any individual result.
The Office of Dietary Supplements also lists medicines that can interact with vitamin D or affect levels, which is why a full medicine list matters at the appointment:
- Orlistat, a weight-loss medicine, can reduce vitamin D absorption.
- Statins may reduce vitamin D synthesis, and high vitamin D intake from supplements may reduce the potency of some statins.
- Corticosteroids such as prednisone can impair vitamin D metabolism.
- Thiazide diuretics combined with vitamin D supplements may lead to high blood calcium, especially in older adults and people with reduced kidney function.
None of this is a reason to stop or change a medicine. MedlinePlus advises telling the provider about every medicine, vitamin and supplement, because they can affect results, and not stopping any medicine unless the provider says to.
Reference intakes are not a personal dose
The National Academies set Recommended Dietary Allowances for healthy people: 15 mcg (600 IU) a day for ages 51 to 70 and 20 mcg (800 IU) a day for those over 70. The Office of Dietary Supplements notes these are planning values built on the assumption of minimal sun exposure. They are not a prescription, and they are not calculated from anyone's lab result.
An upper limit also exists, set at 100 mcg (4,000 IU) a day for adults. That is a ceiling, not a target. The Office of Dietary Supplements explains that vitamin D toxicity is almost always caused by excessive supplement intake and can raise blood calcium, with nausea, vomiting, muscle weakness and kidney stones among the possible effects. It also reports that the Food and Nutrition Board cautioned that even intakes below the limit might have adverse effects over time. This is one more reason a number on a bottle or a lab report is a question for a clinician or pharmacist.
A vitamin D result is not a memory test
If the reason for blood work is a worry about memory or thinking, a vitamin D level does not answer that worry. Which tests are ordered is the clinician's decision, based on the person's history, symptoms and medicines. Our guide to how dementia is diagnosed explains how clinicians combine history, a medicine review, a thinking test and, when needed, lab work, and why no one test settles the question.
What to bring and ask
The most useful preparation is a written record rather than an interpretation. Our caregiver record for vitamin D products, foods and medicines walks through what to list, including the date of any earlier test. Add the lab report itself, with the unit and the lab's printed range visible. Then consider these questions:
- Which unit is this result in, and what range does this lab use?
- Why was this test ordered, and what question is it meant to answer?
- Does anything on the medicine and supplement list affect vitamin D levels, or the other way around?
- What other sources of vitamin D, such as fortified foods and combination products, should be counted before anything is added?
- Does this result change the plan, and when should it be checked again, if at all?
For the wider picture on supplements and medicine safety, see our supplements and medication safety guide.
When to contact a clinician or pharmacist sooner
Do not wait for a routine visit if an older adult has a fall or near-fall, new or worsening confusion, or new weakness. MedlinePlus also notes that nausea and vomiting, poor appetite, constipation, muscle weakness or weight loss in someone taking vitamin D supplements can be symptoms of too much, and that a provider may order a test in that situation. These are reasons to call, not to guess from a lab number. If someone is unresponsive or has trouble breathing, call emergency services.
Sources
- National Institutes of Health, Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals.
- MedlinePlus, National Library of Medicine. Vitamin D Test.
This article is for general information only. It does not diagnose, treat or replace advice from a doctor, nurse or pharmacist who knows the person's health and medicines.