There is probably a supply of vitamin D somewhere in your house; perhaps it sits beside the half-empty bottle of fish oil. Maybe you drag it out of hiding every winter. Or possibly your multivitamin contains vitamin D, but you don’t know how much you are actually taking.

Now for the more difficult question:

Why did you start taking vitamin D in the first place?

If your answer takes longer than finding the bottle, this issue is for you.

Vitamin D is essential.

It supports calcium absorption in the intestine and aids normal bone mineralization. When deficiency becomes severe, children can develop rickets and adults can develop osteomalacia, a condition of poorly mineralized bone that may become painful and weak.

That part of the story has not changed.

What changed was every expectation we began attaching to elevating vitamin D levels.

Over time, lower vitamin D levels were associated with cardiovascular disease, cancer, infections, depression, diabetes, obesity, and higher mortality. Researchers also found vitamin D receptors throughout the body—including immune cells, muscle, nerves, and the gastrointestinal tract. These findings led to the belief that increased vitamin D could have far-reaching positive physiologic effects.

It was an appealing story.

I found this vitamin D receptor argument persuasive for longer than I should have. Physicians are not immune to elegant explanations. Give us a biological mechanism we can draw on a napkin, and we can become rather attached to it.

But finding a receptor on a cell type only tells us that a particular cell is capable of responding to a particular signal. It does not prove that supplying more of that signal improves a human health outcome.

For example, your house may have an electrical outlet in every room. However, plugging 6 lamps into each outlet does not make your house healthier.

This analogy is worth remembering long after our vitamin D conversation is completed.

For years, observational studies consistently found that people with lower vitamin D levels tended to have worse health. That finding directed the medical establishment to recommend pursuing higher vitamin D blood levels.

Then researchers asked: If we raise vitamin D, do target health outcomes improve?

Large randomized trials supplied vitamin D to thousands of people and followed them over time. Across several outcomes that had looked promising in early observational studies, however, routine supplementation in vitamin-D-replete adults did not produce the anticipated benefits. Researchers expected protective signals in: Cancer incidence. Cardiovascular disease. Depression. Weight loss. Respiratory infections. Falls and fractures. That was not the case.

That does not make the earlier research dishonest. It simply changes our understanding of cause and effect with vitamin D.

And this is where my vehicle dashboard analogy becomes useful.

Picture a cold January morning in Northeast Ohio. Your car gauge reads 18°F. Frost covers the windshield. So, you reach under the dashboard, mess with the input, and set the display to 72°F.

Better number.

Same frigid scenario.

Increasing the number does not change the surrounding conditions.

Think about it: who tends to have lower vitamin D?

Someone who spends more time indoors may produce less vitamin D through the skin. A chronically ill person may spend less time outside. Pain can reduce activity and lower vitamin D. Body composition can influence circulating levels because vitamin D is fat-soluble.

For example, imagine an elderly woman whose knees hurt enough that her daily neighborhood walk quietly disappeared two winters ago. Her afternoons are now spent indoors.

As a result, her vitamin D level may be lower.

Her overall health may also worsen because of her inactivity.

But which caused which?

That is exactly the problem observational studies have difficulty resolving.

Sometimes low vitamin D may be part of a medical disorder that genuinely needs evaluation and treatment. Other times, the level is merely reflecting the circumstances surrounding the person’s overall health.

Vitamin D can be an excellent thermometer and, at the same time, inadequate medicine for the condition the thermometer measures.

The standard test is 25-hydroxyvitamin D, written 25(OH)D.

The National Academies/NIH framework used in my Vitamin D Go-To Guide generally considers 20 ng/mL (50 nmol/L) or higher adequate for most healthy individuals. Below 12 ng/mL raises greater concern for deficiency, while 12 to under 20 ng/mL is considered potentially inadequate.

But there is no proven “optimal” target that generally healthy adults need to chase.

The 2024 Endocrine Society prevention guideline did not establish outcome-based target levels for generally healthy adults and advises against routine initial or follow-up testing in that population.

A laboratory reference range and a proven treatment target are not the same thing.

Even measurement has limitations. Different assays can produce meaningfully different results, and circulating vitamin D is largely carried by binding proteins that vary between people.

A lab value is useful information, but you must evaluate it in the context of that individual.

You may have heard large deficiency numbers.

I did too.

In nationally representative U.S. data summarized in the Vitamin D Go-To Guide, about 5% of the population had levels below 12 ng/mL, while about 18% were between 12 and 19 ng/mL.

Some influencers use higher cutoffs that have not been established as outcome-based treatment targets.

That distinction matters because changing the definition changes the size of the “problem.”

It does not necessarily positively impact anyone’s health.

I am not saying that vitamin D testing never matters.

It can matter quite a bit.

Bone pain, proximal muscle weakness, suspected osteomalacia, malabsorption, bariatric surgery, chronic kidney disease, unexplained abnormal calcium, osteoporosis evaluation, and certain disorders or medications can make vitamin D testing part of a legitimate diagnosis or treatment.

The question is not:

“Is vitamin D testing good or bad?”

Ask instead:

What medical decision will my vitamin D test result change?

That one question can clear away a remarkable amount of confusion.

A normal vitamin D result is not a certificate of good health.

It tells you something about vitamin D status. It does not certify your heart, metabolism, immune system, sleep, nutrition, activity, or stress burden. Conversely, a low value does not automatically explain every fatigue, ache, mood change, or vague health symptom.

If something is wrong, look for what is wrong.

Do not let one familiar laboratory number become so important that the rest of your health picture fades from view.

A reader asks: “My vitamin D level is below the ‘optimal’ range I see online. Should I increase my supplement dose until I reach 60 or 80 ng/mL?”

1. Find every source.
Check your vitamin D bottle, multivitamin, bone formula, cod liver oil, protein or nutrition products, and combination supplements.

2. Total your ordinary daily intake.
Add them all together for your total daily dose. Do not change anything yet; just know what you actually take.

3. Write down why you started supplementing with vitamin D.
If the answer is “I’m not sure anymore,” that is useful information to pursue, not a reason to panic.

4. Give every laboratory test a purpose.
Ask what diagnosis, treatment, or medical decision a vitamin D result is intended to change.

5. Look beyond the number.
If you are unwell, let your clinician evaluate the whole picture rather than assuming low vitamin D explains every symptom.

New bone pain, proximal muscle weakness, or a meaningful change in gait deserves medical evaluation rather than simply increasing vitamin D.

Testing and individualized management may also be appropriate with malabsorption, bariatric surgery, chronic kidney disease, unexplained abnormal calcium, osteoporosis or osteomalacia, sarcoidosis, tuberculosis, lymphoma, or medications that affect vitamin D management.

If you are already following a physician-directed vitamin D regimen for a documented problem, do not stop or change it because of this newsletter. Review the reason, dose, monitoring plan, and eventual stopping condition with the clinician managing your care.

Words of wisdom: Let the number inform you without allowing it to become the problem.

© 2026 Aslan Health, LLC. All rights reserved.

This content is educational and is not medical advice. Listening or reading does not create a physician-patient relationship, and nothing here substitutes for individualized care from your own clinician. Do not use it to diagnose or treat a health problem, and do not delay or disregard professional medical advice because of something you read or heard here.

Talk with your physician or pharmacist before starting any supplement — particularly if you are pregnant or nursing, have a diagnosed condition, or take prescription medication. Supplements can interact with medications.

If you think you may have a medical emergency, call 911 or your local emergency number.

These statements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease.