Taking vitamin D without this one blood test is like taking blood pressure medication without ever measuring your blood pressure. Most people do it anyway. Most doctors let them.
So I went pretty deep on this one because I was about to make the same mistake myself.
Around 25% of Americans are below the clinical deficiency threshold on a standard blood test. Most have no idea because nobody ran the test. The USPSTF decided there wasn't enough evidence to recommend routine screening for people without symptoms, which basically gave primary care doctors permission to skip it. So they do.
Here is what that actually costs you. Not mortality stats, those are murky with vitamin D. The stuff that hits closer to home for anyone over 50: muscle loss accelerates when vitamin D is low because the receptors are sitting right in muscle tissue. Bone density drops. A 2025 study found deficiency more than doubled sarcopenia risk in older adults. And sarcopenia feeds fall risk, falls feed fractures, and hip fractures in older adults are genuinely one of the worst outcomes in medicine. That chain is real and it's modifiable.
The immune piece is more solid than people give it credit for. Martineau 2017, BMJ, 25 RCTs, 11,000 people. Respiratory infection protection was real, strongest by a wide margin in people who were actually deficient going in. Same story every time with this one. Works when you need it. Does almost nothing when you don't.
And this is where the whole debate goes sideways. VITAL. ViDA. D-HEALTH. The big trials showed nothing, people cite them constantly. What they don't say is that VITAL participants came in averaging around 29 ng/mL. That's already above the NAM deficiency cutoff. You gave vitamin D to people who weren't deficient and it didn't do much. Correct. That's not the same question as what happens when you fix a real deficiency.
The test is called a serum 25-hydroxyvitamin D, written as 25(OH)D. Not the active form, the storage form. You can walk into your next physical and ask for it by name. Out of pocket it runs maybe $30-60 if insurance doesn't cover it. The target most functional medicine practitioners and the Endocrine Society land on is 30 to 50 ng/mL. Below 20 and you have a real problem. 2,000 IU of D3 daily is a well-supported starting point for most people. Retest in 3 months.
K2 was one I didn't see coming. When D3 raises calcium absorption, that calcium needs somewhere to go. K2, specifically MK-7, activates the proteins that push it into bone and keep it out of arterial walls. A 2023 RCT in JACC Advances looked at combined K2 MK-7 and D3 in 389 older men. Primary outcome was null across the full group, but the subgroup with higher baseline coronary calcium scores showed slowing of progression. Early data, not definitive, but the mechanism makes sense and the downside is essentially zero.
D3 not D2 by the way. Most prescription vitamin D is D2, which is significantly less effective at raising your actual levels. Worth knowing.
Have you ever actually tested your level? If you take it every day and have never checked, you genuinely don't know if it's doing anything. If you tested, found a real deficiency, and corrected it, I want to hear what changed. Drop it in the comments.
Full breakdown with all sources at link in comment.