If you have osteoporosis, or you are trying to prevent it, vitamin D has probably come up at every visit. Most of us have been told to take it. Many of us have been told a number. Far fewer of us have been told what it actually does, why the dose is the dose, or whether the bottle on the kitchen counter is really helping.
I want to walk through what vitamin D is, how your body uses it, why measuring it sometimes makes sense, and where the evidence is honestly weaker than the supplement aisle would have you believe.
A Plain-Language Tour of How Vitamin D Works
Vitamin D is not really a vitamin in the classic sense. Your skin makes it when ultraviolet B (UVB) light from the sun hits it, and from there it goes on a two-stop journey before it can do anything useful.
The first stop is your liver, which converts it into the form measured on your blood test, called 25-hydroxyvitamin D, or 25(OH)D. That is the storage form. It is the number on the lab slip.
The second stop is your kidneys, which add another small chemical change to turn it into the active hormone, called 1,25-dihydroxyvitamin D (or calcitriol). Only this active form can tell your gut to absorb more calcium and your bones to use it (per the NIH Office of Dietary Supplements).
This matters for one quiet reason that often gets missed. If your kidneys are not working well, that final activation step suffers. People with chronic kidney disease can have a perfectly fine 25(OH)D level on paper and still be functionally short on the active hormone their bones need. This is true no matter the source. Sun-made vitamin D goes through exactly the same two stops as anything you swallow, so sunshine is not a workaround for kidneys that are not working well. When kidney function is significantly impaired, physicians often address this with specific prescription medications that provide a pre-activated form of vitamin D. If you have any degree of renal impairment, your bone-health plan deserves a more nuanced conversation than "take 1,000 IU and we are done."
Should You Measure It?
This is where the guidelines have shifted in a way most patients have not heard about yet.
For years, routine vitamin D testing was common. In 2024, the Endocrine Society reviewed the evidence and concluded that for most healthy adults between 19 and 74, routine screening with a 25(OH)D level does not improve outcomes (Endocrine Society Clinical Practice Guideline summary). They also stopped endorsing the old target of 30 ng/mL, because the evidence for that specific threshold turned out to be thinner than people assumed.
That guidance is for the general population. If you have osteoporosis, malabsorption, kidney disease, are on long-term glucocorticoids, have had bariatric surgery, or are starting a bone-active medication like a bisphosphonate or Prolia, measuring your level is reasonable and often important. Many bone medications work less well when vitamin D is low, and some can cause a meaningful drop in blood calcium if you start them while deficient.
So the question is not "should everyone get tested." It is "do I have a reason to know my number." For most readers of this blog, the answer is yes.
What "Deficient" Really Means
The National Academies define deficiency as a 25(OH)D level below 12 ng/mL (30 nmol/L), and inadequacy as 12 to 20 ng/mL (NIH ODS fact sheet). Levels at or above 20 ng/mL are considered adequate for bone health in healthy adults. Many bone specialists still prefer to see osteoporosis patients somewhere in the 30 to 50 ng/mL range, even though the Endocrine Society no longer formally endorses that target.
What is clear from large U.S. surveys is that real deficiency is not rare. In NHANES data, roughly 5% of Americans are deficient and another 18% are in the inadequate range (NIH ODS). Those numbers climb higher in older adults, in people with darker skin, in people who cover for cultural or sun-safety reasons, and in anyone who spends most daylight hours indoors. That is most of modern adult life.
Food First, Even Though Food Is Hard
Food is the most physiologically reasonable way to get vitamin D, because it arrives with the other nutrients your bones use, including protein, calcium, magnesium, and vitamin K. The problem is that very few foods contain meaningful amounts.
The honest list is short:
- Fatty fish like salmon, sardines, mackerel, and trout
- Cod liver oil, if you can stomach it
- Egg yolks, in modest amounts
- Mushrooms exposed to UV light during growing
- Fortified foods like milk, plant milks, some yogurts, and some cereals
A typical 3-ounce serving of cooked salmon delivers roughly 400 to 600 IU. A cup of fortified milk adds about 100 IU. A couple of egg yolks add maybe 80. You can see the math problem. Eating your way to 800 IU every day is possible if you happen to love fatty fish, and a stretch otherwise.
This is why food alone usually does not get most adults where they need to be, and why so many of us end up needing some combination of sun and supplements to fill the gap.
Sunshine, Cautiously
When UVB light hits bare skin, you make vitamin D quickly and efficiently. A fair-skinned adult in summer midday sun can produce several thousand IU in 10 to 20 minutes of casual exposure to arms and legs. The catch is that the same UVB that makes vitamin D is the part of sunlight that causes skin cancer, and the trade-off is real.
A few practical realities are worth knowing:
- North of roughly the 37th parallel (a line that runs through the middle of the United States), winter sun is too weak to make vitamin D for several months of the year, no matter how long you stand in it.
- Sunscreen, clothing, glass windows, and darker skin all reduce production.
- After about age 65, the skin's ability to make vitamin D drops significantly.
I am not asking anyone to skip sunscreen. I am suggesting that a short walk in shorts and a short-sleeve shirt on a warm afternoon, before you reach for the sunblock, is not a reckless act for most people. It is one piece of a sensible plan, along with food and, for many of us, a supplement.
There is also a piece of this that does not show up on any lab report. Time outside in the sun, especially on a walk or sharing a meal with people you love on a patio, is one of the more reliable mood boosters we know of. Bone health is a long project, and the things that keep you doing it are not just the milligrams and the IUs. They are the afternoons you actually enjoyed. Sunshine, gentle movement, and company tend to arrive in the same package, and your bones are not the only part of you that benefits.
Supplements: Form, Dose, and Prescription versus Over the Counter
Two forms are widely sold. Vitamin D3 (cholecalciferol) is the form your skin makes, and it raises blood levels more efficiently than the alternative. Vitamin D2 (ergocalciferol) is plant-derived, requires a prescription at the high doses commonly used, and is what you will get if your doctor sends in a script for 50,000 IU weekly to correct a deficiency.
Most over-the-counter D3 supplements come in 1,000 to 5,000 IU tablets or soft gels. For maintenance in an adult with osteoporosis, somewhere in the range of 800 to 2,000 IU daily is what most clinical trials supporting fracture care have used (UK pharmacy clinical summary, 2026).
A few notes that matter:
- D3 is fat-soluble, so take it with a meal that has some fat in it.
- The 2024 Endocrine Society guideline prefers daily, lower-dose D over big weekly or monthly boluses in adults over 50, because the daily approach gave more consistent results in trials.
- The safe upper limit without medical supervision is 4,000 IU daily. Higher doses can be appropriate for true deficiency, but those should be guided by labs and a clinician, not chosen off a shelf.
If your level is truly low, your physician may prescribe a loading regimen, often something like 50,000 IU once a week for 6 to 10 weeks, then a transition to a daily maintenance dose. This is one of the genuinely useful places for the prescription form.
The Part I Want You to Hear Clearly
Here is where I want to be honest with you, because the supplement industry will not be.
For people who are already vitamin D sufficient, the evidence that taking more vitamin D meaningfully reduces fractures is weak. A large 2026 systematic review in The BMJ, pooling data from 36 trials and over 92,000 participants, concluded that routine vitamin D supplementation in older adults did not produce a clinically meaningful reduction in fractures (BMJ systematic review, May 2026, summarized in SciTechDaily). Megadosing has not been shown to do better than reasonable doses, and very high doses have actually been linked to more falls in some studies.
What the same body of evidence does support is this: correcting actual deficiency matters. People who are genuinely low benefit from getting back into the sufficient range, especially when they are also taking a bone-active medication that needs vitamin D to work properly. Beyond that point, more is not better.
So the practical message is not "vitamin D is useless." It is also not "everyone should be on 5,000 IU." It is something more nuanced and more useful: know whether you are deficient, fix it if you are, maintain a reasonable intake once you are not, and stop expecting a pill to do what only the full package of food, movement, sunlight, and medication when indicated can do together.
What to Take Home
- Vitamin D needs your kidneys to do its real job, so kidney problems change the conversation.
- Most healthy adults do not need routine testing, but most osteoporosis patients reasonably do.
- Real deficiency is common in the U.S. and worth correcting.
- Food helps but rarely gets you there alone. A short, sensible amount of sun helps too.
- For maintenance, 800 to 2,000 IU of D3 daily, with food, is a reasonable range for most adults with osteoporosis. Higher doses should be guided by labs.
- More vitamin D is not the same as better bones. Correcting deficiency is what matters.
If you take nothing else from this post, take this: your vitamin D level is one number in a much larger picture, and the goal is not to chase it as high as possible. The goal is to make sure it is not standing in the way of everything else you are doing for your bones.