Almost every claim about how many Americans are deficient in something is a claim about where a line was drawn. Move the line and the number moves with it, without a single blood level having changed.
This page shows the survey figures with their thresholds welded on. Each card names the exact cutoff, the body that set it, the survey it came from, the years it covers and the population it describes. A percentage never appears here without all five.
It also separates four different things that get called deficiency, because they are not the same claim and only one of them can ever carry a letter grade.
Last reviewed: August 2026
How this page relates to our grades
Nothing on this page carries a letter. A prevalence figure describes how common a blood level or a dietary intake is in a population. It is not evidence that correcting it changes an outcome, and only human randomized trials are admissible as grade evidence.
Where a graded intervention exists, the grade lives on its own record and rests on its own trials. A gap being common is not a reason for a letter, and a gap being rare is not a reason against one.
Four things called deficiency
1. Intake gap
Usual dietary intake sits below the Estimated Average Requirement. This is measured by asking people what they ate. It says nothing about their blood.
2. Status gap
A biochemical measurement falls below a chosen cutoff. Threshold-dependent by construction, and the cutoff is often contested.
3. Clinical deficiency
A named deficiency disease with signs a clinician can observe. Far rarer than either of the above, and not interchangeable with them.
4. Responsive deficiency
Correcting the gap changed a clinical outcome in a randomized trial. This is the only rung that can ever sit under a letter, and it is the rung the other three are routinely mistaken for.
The distance between rung one and rung four is where most supplement marketing lives. A survey finds half the country below the intake requirement, and the number is then presented as though half the country would benefit from a pill. Those are different claims resting on different evidence, and the second one usually has none.
The figures, with their thresholds
Ordered by how much can be established, least first. Magnesium comes before vitamin D because for magnesium the argument is not about where to draw the line, it is that the common test cannot locate the line at all.
RBC Magnesium
Intake gap
Measured by: Serum magnesium is the common test. It is a poor index of total body magnesium.
approximately 50% (not stated as an interval in the source)
dietary intake gap, not a blood measurement, defined as usual intake below the Estimated Average Requirement
Threshold set by
Estimated Average Requirement, National Academies Dietary Reference Intakes; characterization of magnesium as a shortfall nutrient from the 2015 Dietary Guidelines Advisory Committee
Survey
dietary intake assessment cited in review, not a blood-based survey estimate, reported 2016, citing the 2015 Dietary Guidelines Advisory Committee
Population
United States, general population, varies by age group
There is no simple, rapid, accurate laboratory test for whole-body magnesium status. Serum below 0.75 mmol/L is useful for severe deficiency. Between 0.75 and 0.85 mmol/L a loading test can identify deficient subjects, and the loading test is the closest thing to a gold standard, but it is unsuitable where kidney or intestinal function is disturbed. Cellular measurements frequently disagree with one another. The current serum reference interval of 0.75 to 0.95 mmol/L was derived from the NHANES I distribution in 1974, that is, from how the population's values were spread, not from which values predict an outcome.
Arnaud 2008, Br J Nutr, review of magnesium status assessment; Costello 2016, Adv Nutr, on the serum reference interval · PMID 18598586, PMID 28140318
Note
This record's own EBL reference value is stated for RBC magnesium rather than serum. RBC magnesium avoids some of the objections to serum, but it has not been validated against clinical outcomes either, and the cellular measures are known to disagree.
Why this carries no grade
Magnesium does not merely lack an answer, it lacks a measurement. Without a status assay that tracks body stores, biochemical prevalence cannot be estimated at all, and no trial can enroll people on the basis of a status this test cannot establish.
Vitamin B12
Status gap
Measured by: Serum vitamin B12, or plasma methylmalonic acid as a functional marker
3% to 26% (range across commonly used cutoffs, not a confidence intervalThe range the true effect probably falls in. A wide interval means the estimate is uncertain.)
low B12 status by serum, defined as varies by cutoff chosen for serum B12
Threshold set by
no single body; the paper compares cutoffs in common clinical use
This is the clearest demonstration on the site that a prevalence number means nothing without its threshold and its assay. One population, one set of blood draws, and the answer moves nearly ninefold depending on where the line is drawn and which marker is measured.
Why this carries no grade
The prevalence range below measures how many people fall under a chosen line, not whether raising them above it changes anything. No letter follows from it.
Vitamin D (25-OH)
Status gap
Measured by: Serum 25-hydroxyvitamin D
5.0% (95% CI 4.1 to 6.2)
at risk of deficiency, defined as below 30 nmol/L (12 ng/mL)
Threshold set by
National Academies of Sciences (formerly Institute of Medicine), Dietary Reference Intakes
There is no single agreed cutoff, and the most-quoted one has been withdrawn. The National Academies treat 50 nmol/L (20 ng/mL) as sufficient for most people. The Endocrine Society's 2011 guideline used 75 nmol/L (30 ng/mL), and that is the number still circulating in most clinical and consumer material. Its 2024 guideline replaced the 2011 one and declined to set concentration thresholds defining sufficiency, insufficiency or deficiency for generally healthy people, on the grounds that trials have not established outcome-specific thresholds. The prevalence figures above are computed against the National Academies cutpoints. Against the withdrawn 2011 target a far larger share of the population falls below the line, without a single blood level having changed.
Holick 2011, Endocrine Society guideline, superseded; Demay 2024, Endocrine Society guideline; Shah 2024, supporting systematic reviewA review that searches for every study on a question using a method set in advance, then assesses them together. · PMID 21646368, PMID 38828931, PMID 38828942
Note
The survey language is 'at risk of deficiency', not 'deficient'. NHANES measures a blood level in a population sample; it does not diagnose individuals.
Why this carries no grade
A prevalence figure describes how common a blood level is. It is not trial evidence that correcting that level changes an outcome, so it cannot carry a letter. The grade for supplementing vitamin D sits on the vitamin D3 record and rests on its own trials.
What this page does not do
It does not tell you whether to test, and it does not tell you what to do with a result. A population prevalence is not a statement about any individual, and NHANES does not diagnose anyone. The survey language is deliberately "at risk of deficiency" rather than "deficient", and that wording is preserved here.
Three nutrients are covered so far. The remainder are being sourced the same way, one threshold and one survey cycle at a time, and none will appear here until they are.