Your Vitamin D Blood Test Measures the Precursor, Not the Hormone

She is thirty-eight, and Crohn’s disease has cost her most of her small bowel across two operations. Her vitamin D blood test came back at 47 nmol/L, flagged low normal, and filed. Nobody asked why a woman who works outdoors and takes a supplement was sitting there at all.

That report was not wrong, exactly. It was answering a different question than the one her body was asking. The video for Chapter 10 of The Angry Gut, by Dr. Gurpreet Singh Padda, MD, MBA, MHP, and Ami Michelle Grimes, argues that vitamin D is a hormone. This article is about the lab slip: what it measures, what moves it, and what a regenerative practice does with it.

What a vitamin D blood test actually measures

The standard test reports 25-hydroxyvitamin D. That is the liver’s storage product, the substrate. The active hormone, 1,25-dihydroxyvitamin D, is made from it in the kidney and, locally, inside immune cells and the gut lining.

The two differ enormously in strength. In human cell work, the precursor reached half its gene-regulating effect at an average of 322 nM, while the hormone did so at 0.48 nM. Below 250 nM, the precursor barely touched gene expression.

The fair objection: those experiments used circulating blood cells, where the activating enzyme is barely present, so they show what the precursor cannot do alone rather than what gut tissue does with it. You cannot make the hormone without substrate, and the substrate number is not the tissue’s signal. Treat the report as a fuel gauge, not a speedometer.

Same blood, different answer

Before interpreting a level, ask how it was produced. When survey assays were restandardized, the share of Americans below 50 nmol/L rose from 22% to 31%, while a German survey’s share below 30 nmol/L fell from 28% to 13%. The same methods paper calculated that one person’s apparent level could land anywhere from 20 to 35 ng/mL depending on the laboratory method, and its authors called for suspending pooled reviews built on unstandardized results.

Mass spectrometry is the reference method. In one national survey, the original immunoassay reported 6.5% of people below the deficiency line, standardization projected 11.4%, and re-running every stored sample by mass spectrometry confirmed 11.2%.

Cholesterol quietly bends the result

This is the detail that should change how metabolic patients read their labs. Lipids interfere with the common immunoassay. Compared against mass spectrometry, the immunoassay under-read by 5.5 nmol/L overall. In people with lower non-HDL cholesterol the gap was 4.0 nmol/L; above the cutoff it widened to 10.2. Standardizing the assay did not remove the interference.

Follow that forward. The insulin-resistant patient with high cholesterol gets a falsely low vitamin D, and the published link between high cholesterol and low vitamin D partly existed only on that assay. The metabolically healthy patient gets reassurance. If your lipids are elevated, the method used for your test is not a trivia question.

Whose blood built the reference range

A reference interval describes a crowd. More than 40 international guidelines exist, and their recommended levels span 10 to 40 ng/mL. One committee treats 30 nmol/L as the risk line and 50 nmol/L as enough for 97.5% of people; another starts deficiency at 50 and aims above 75. Globally, 15.7% of people sit below 30 nmol/L and 76.6% below 75. A problem that affects one person in seven or three in four, depending on the committee, is a definitional argument, not a diagnosis.

The crowd is also not neutral. Across 55,844 Europeans, 17.7% fell below 30 nmol/L in the October-to-March months against 8.3% in summer. Within one representative Finnish sample, 50.4% of Kurdish immigrants and 28.0% of Somali immigrants were below that line, against 4.5% of Russian-speaking immigrants and 0.4% of native Finnish adults. Same country, same winter. The intake tables that follow from these ranges were built mostly on light-skinned volunteers, living indoor lives, and then handed to everyone.

The terrain that drains the number

A damaged bowel lowers the level, not the other way around. Genetically predicted vitamin D, which illness cannot move, gave hazard ratios of 0.98 for Crohn’s disease and 1.01 for ulcerative colitis in the Copenhagen and UK Biobank cohorts. Low vitamin D did not cause her disease.

But deficiency tracks how badly the gut is working. Across inflammatory bowel disease studies, the odds of deficiency were 1.85 with active disease, 1.61 after prior surgery, 1.78 with biologic exposure and 1.38 for Crohn’s over colitis. Being non-Caucasian carried 3.79, larger than any disease variable. Two drivers compound here: a fat-soluble precursor that a resected or inflamed bowel cannot take up, and skin that makes less in weak northern light. Add a third that is social: work, housing and screens that keep people inside.

Treatment evidence in the bowel is thin and short. Across 22 trials and 1,874 patients, one outcome had enough data: relapse fell, risk ratio 0.57, at low certainty. The 2024 Endocrine Society guideline suggests against routine testing because no trial of a screening strategy exists, and it was written for people with no established indication. A missing small bowel is an indication.

The gut barrier behind this problem was the subject of the chapter on a gut short of oxygen, and the next step, what crosses that barrier and reaches the spine, is in the disc microbiome. It also fits the wider pattern of the gut driving joint pain.

Repairing the deficit without chasing a number

Our position: when someone has a reason to be deficient, we test, we replace daily rather than in large boluses, and we correct and stop instead of pushing the level ever higher. No trial has set a vitamin D target for malabsorbing, post-surgical or post-bariatric patients, so the target rests on mechanism and on the harm seen with excess, applied to the patient in the room.

The dose is individual. To keep 97.5% of dark-skinned people at high latitude above 25 nmol/L through winter took about 956 IU a day; the same analysis warned that at intakes covering everyone to the highest target, the top 2.5% could reach 158 nmol/L or more.

The repair work sits upstream of the pill. Cool the bowel so it can absorb. Get regular daylight on skin, which makes the precursor on a steady schedule the body can regulate. Reduce excess body fat, which holds the molecule out of circulation. This is the same logic behind why metaflammation decides whether treatment works.

When you next see your result, ask three things: was it immunoassay or mass spectrometry, are my lipids high enough to bend it, and which committee’s range is on this report? The full study list, with every number and what each study cannot show, is in the Chapter 10 Deep Dive.

Frequently asked questions

What is a normal vitamin D level?

It depends on which committee you ask. One widely used framework treats 30 nmol/L as the risk threshold and 50 nmol/L as adequate for nearly everyone’s bones, while another calls anything under 50 deficient and targets above 75. Guidelines worldwide span 10 to 40 ng/mL. A normal flag tells you where you sit in a population, not whether your tissue has enough. We explain how the result matters for repair in vitamin D status and musculoskeletal healing.

Is the vitamin D blood test accurate?

Mass spectrometry is the reference method, and common immunoassays drift from it. In one Canadian survey the immunoassay read 5.5 nmol/L low on average, and the gap more than doubled in people with higher non-HDL cholesterol. Restandardization has also moved national deficiency rates by several percentage points. Ask which method your lab used. A broader panel for judging whether the body can heal is described in the metabolic audit.

Why is vitamin D called a hormone instead of a vitamin?

Because the body makes it and regulates it. Skin produces it from cholesterol in sunlight, the liver and kidney convert it, and it acts through a nuclear receptor in the same family as the receptors for estrogen, cortisol and thyroid hormone. Parathyroid hormone and fibroblast growth factor 23 then adjust how much becomes active. Its receptor mostly steers immune genes, which connects to how the immune system learns what to tolerate.

Should everyone get a vitamin D test?

Not routinely. The 2024 Endocrine Society guideline suggests against testing in the general population because no trial has tested a screening strategy. That reasoning does not cover people with a clear reason to run low, such as bowel disease, prior intestinal surgery, darker skin at northern latitudes or significant excess weight. For them the test answers a real question. Inflammation markers deserve the same targeted logic, as in hsCRP and inflammation biomarkers.

Can gut problems cause low vitamin D?

Yes. Vitamin D is fat-soluble, so a bowel that is inflamed, shortened by surgery or poorly absorbing lets less of it in. In inflammatory bowel disease, active disease, prior surgery and biologic exposure all raised the odds of deficiency, while genetic studies show low vitamin D did not cause the bowel disease itself. The deficit is a consequence that renews daily. The barrier at the center of it is covered in the leaky gut wall.

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Medically reviewed by Dr. Gurpreet Singh Padda, MD, MBA, MHP, board certified in anesthesiology, pain medicine, interventional pain management, addiction medicine and obesity medicine. Adapted from Chapter 10 of The Angry Gut by Dr. Padda and Ami Michelle Grimes. Last reviewed September 2026.