Forty-two percent of American adults have circulating 25-OH vitamin D below 20 ng/mL. In Black Americans, the figure is 82 percent. These are people with access to healthcare, in a country where vitamin D testing costs less than a co-pay. The deficiency is epidemic not because the tests are unavailable, but because the standard annual panel does not include them.

The same pattern repeats across B12, ferritin, and the omega-3 index. Not rare deficiencies. Not deficiencies that require exotic testing. Deficiencies that are common, measurable, and consequential, in populations that consider themselves well-nourished, and that the standard annual physical systematically misses, either because the markers are not ordered, or because the ones that are ordered are the wrong measures.

This is the fifth and final article in the blood tests series. The previous four covered metabolic health, cardiovascular risk, hormonal balance, and organ function. The argument across all five has been the same: standard panels detect late-stage dysfunction. They are not built for the earlier, quieter phase where damage accumulates without triggering a flag. Nutritional status is where that gap is probably most visible, because the markers are basic and inexpensive, the deficiencies are widespread, and the consequences extend over decades.

The problem with “checking your levels”

People who do ask about these markers are often told that their levels are “fine” based on reference ranges that have a structural problem: they define sufficiency as the absence of acute deficiency disease, not the level associated with optimal function.

The story repeats across all four markers in this article. The lab reference range for vitamin D was calibrated to prevent osteomalacia, not to support immune regulation or neuromuscular function. The reference range for serum B12 measures total B12, most of which is metabolically inactive, which means functional deficiency can exist with a serum B12 that falls squarely within the normal band. Standard ferritin testing is ordered without its essential companion test, making it nearly impossible to distinguish iron deficiency from iron overload from chronic inflammation. And the omega-3 index is simply not on any standard panel, anywhere.

The result is that a person can complete a comprehensive annual physical, be told their nutritional markers look good, and be substantially deficient in multiple things that affect their cognition, immune function, mitochondrial activity, and long-term cardiovascular risk.

Vitamin D: a hormone, not a vitamin

The name is wrong. Vitamin D3 (cholecalciferol) is not a vitamin in the functional sense; it is the precursor to a steroid hormone. The active form, 1,25-dihydroxyvitamin D (calcitriol), is synthesized from 25-OH vitamin D in the kidney by the enzyme 1-alpha-hydroxylase and operates through nuclear receptors to regulate gene transcription. The vitamin D receptor (VDR) is expressed in nearly every tissue in the body, including immune cells, skeletal muscle, cardiac muscle, brain, gut epithelium, and pancreatic beta cells. This is not the expression pattern of a nutrient needed for one thing. It is the pattern of a hormone involved in the regulation of hundreds of processes.

The precursor that the lab measures, 25-OH vitamin D (also written 25(OH)D), is the correct marker for assessing status. It has a half-life of approximately two to three weeks, making it a stable reflection of recent synthesis and intake. The active form, 1,25-OH D (calcitriol), should not be ordered to assess nutritional status. Calcitriol is tightly regulated by parathyroid hormone (PTH) and by feedback mechanisms that keep it in the normal range even as 25(OH)D falls. A person can have calcitriol in the normal range and 25(OH)D of 12 ng/mL, and the standard interpretation would incorrectly suggest sufficient status.

Synthesis of 25(OH)D begins in the skin, where UVB radiation converts 7-dehydrocholesterol to pre-vitamin D3, which then isomerizes to vitamin D3. This is then hydroxylated in the liver by CYP2R1 to produce 25(OH)D. Melanin competes with 7-dehydrocholesterol for UVB photons, which is a large part of why Black Americans have dramatically higher rates of deficiency: at northern latitudes, darker skin requires substantially more sun exposure to produce the same precursor amount. Sunscreen applied at SPF 30 reduces vitamin D synthesis by approximately 95 percent. Obesity reduces bioavailability because vitamin D is fat-soluble and partitions into adipose tissue, reducing circulating levels independent of synthesis.

The NHANES data showing 42 percent of American adults below 20 ng/mL is not a fringe finding. It has been consistent across multiple rounds of the survey.^[1] The lab reference range typically calls 20-30 ng/mL “sufficient,” but this threshold was set to prevent rickets and osteomalacia, not to support optimal function of the immune system, skeletal muscle, or the other VDR-expressing tissues. Many researchers argue the optimal range for non-skeletal functions is 40-60 ng/mL, based on observational associations with immune function, cancer incidence, autoimmune disease activity, and all-cause mortality.

The intervention data complicates this picture. The VITAL trial, the largest RCT of vitamin D supplementation conducted to date, randomized 25,871 American adults to 2000 IU/day of vitamin D3 or placebo and followed them for a median of 5.3 years. The primary results found no significant reduction in incident cancer or major cardiovascular events in the full supplemented population.^[2] This was a disappointment relative to the observational literature. But the subgroup analyses clarified the picture: the people who were already replete at baseline showed no benefit, which is the expected result of supplementing people who are not deficient. Subgroups with lower baseline 25(OH)D showed larger reductions in cancer mortality and cancer incidence. This is consistent with a threshold effect: supplementation matters when you are deficient, and the threshold matters more than the dose.

A secondary analysis from VITAL found a 28 percent reduction in incident autoimmune disease (rheumatoid arthritis, psoriasis, thyroid disease, polymyalgia rheumatica) in the vitamin D3 arm over five years of follow-up, reaching statistical significance.^[3] This is one of the cleaner signals in the intervention literature.

PTH provides a useful confirmatory marker. When 25(OH)D is low, the kidney has less substrate for calcitriol synthesis. The parathyroid gland responds by secreting more PTH to drive renal 1-alpha-hydroxylase activity upward, compensating to maintain calcitriol at the expense of PTH elevation. Elevated PTH alongside low 25(OH)D confirms functional vitamin D deficiency with secondary hyperparathyroidism. A 25(OH)D that looks borderline becomes more interpretable when PTH is added.

What to order: 25-OH vitamin D (not 1,25-OH D). Add PTH if the result is borderline or if the patient is at high risk. Target: 40-60 ng/mL. Most people achieving this level without sun exposure require supplementation in the range of 2000-5000 IU/day of D3, though individual response varies substantially and testing is necessary to calibrate.

Vitamin B12: what serum levels miss

B12 is not a single molecule. It exists in multiple cobalamin forms, and circulates in blood bound to two carrier proteins with different functional significance. Haptocorrin (HC, also called transcobalamin I) binds approximately 70-80 percent of circulating B12 and delivers it primarily to the liver. This fraction is metabolically inactive for most tissues. Holotranscobalamin (HoloTC, also called active B12 or transcobalamin II-bound B12) carries the remaining 20-30 percent and is the fraction available for cellular uptake via receptor-mediated endocytosis. It is the functionally relevant fraction.

Standard serum B12 measures the total: active plus inactive. A result of 400 pg/mL looks reassuring. But if HoloTC is low, the active fraction available for cellular use is low, and the apparent adequacy of the total reading is misleading. Functional deficiency can exist with serum B12 well within the normal range.

B12 is a cofactor for two enzymes in humans: methionine synthase, which converts homocysteine to methionine, and methylmalonyl-CoA mutase, which converts methylmalonyl-CoA to succinyl-CoA in the mitochondria. When B12 is functionally deficient, both reactions slow. Homocysteine and methylmalonic acid (MMA) accumulate, each of which has independent metabolic consequences. Elevated homocysteine is a known cardiovascular risk factor, addressed in the cardiovascular risk article. MMA elevation is more specific to B12 deficiency, and it rises before serum B12 falls below the lab’s lower limit of normal. MMA is the most sensitive early marker of functional B12 deficiency.

The neurological consequences of B12 deficiency are irreversible. Subacute combined degeneration of the spinal cord, the classic severe presentation, involves demyelination of the posterior and lateral columns and presents with progressive weakness, sensory loss, and cognitive decline. The damage accumulates before serum B12 becomes abnormal. Waiting for the standard marker to fall is therefore a strategy that accepts preventable neurological injury as the acceptable cost of not ordering two additional tests.

The populations most at risk are predictable: strict vegans and vegetarians, because B12 occurs only in animal products with no meaningful plant-based sources; older adults, because gastric acid secretion and intrinsic factor production decline with age, impairing B12 absorption from food even when intake appears adequate; people taking metformin, the first-line medication for type 2 diabetes, which reduces B12 absorption by impairing intrinsic factor-mediated uptake in the terminal ileum, an effect that accumulates over years; and people on long-term proton pump inhibitors, which reduce gastric acid and impair the acid-dependent release of B12 from food proteins.

The metformin interaction deserves specific attention. A 2010 trial following 390 metformin-treated patients found that B12 absorption was reduced in 30 percent of patients at standard doses, and that deficiency developed progressively with duration of treatment.^[4] The standard of care when prescribing metformin does not typically include periodic B12 monitoring unless symptoms develop. This is a gap worth closing proactively.

What to order: serum B12 for baseline. Add methylmalonic acid (MMA) and homocysteine in any borderline result (typically below 400 pg/mL) or in anyone at elevated risk. HoloTC, where available, is the preferred direct marker of B12 status and supersedes total serum B12, but access varies by lab. An MMA above 0.4 µmol/L in the context of low-normal serum B12 indicates functional deficiency regardless of the total reading.

Ferritin: the marker that means two different things

Ferritin is a spherical protein shell that stores iron in a non-toxic form. Each molecule of ferritin can hold up to 4,500 iron atoms. In a healthy person in iron balance, serum ferritin reflects body iron stores: low ferritin means depleted stores, high ferritin means replete or excessive stores. This is the simple version, and it is incomplete in ways that lead to frequent misinterpretation.

Ferritin is also an acute phase reactant. Its synthesis in the liver is upregulated by interleukin-6 and other inflammatory cytokines. In the presence of active inflammation, infection, malignancy, metabolic syndrome, or fatty liver disease, ferritin rises independently of iron stores. A person with chronic low-grade inflammation and genuinely depleted iron stores can have a serum ferritin that looks normal. The iron deficiency is masked by the inflammatory signal.

The reverse problem is equally significant. A ferritin of 400 ng/mL with no known cause could indicate hereditary hemochromatosis (the most common genetic iron overload condition, affecting approximately 1 in 200 people of northern European ancestry), metabolic-associated steatotic liver disease (the non-alcoholic fatty liver spectrum), chronic inflammation, or actual iron overload from frequent transfusions. Most lab reports do not distinguish between these possibilities. A ferritin of 400 ng/mL printed in black ink says nothing about which of these is driving the elevation.

Transferrin saturation is the essential companion test. Transferrin is the primary iron transport protein in blood. Transferrin saturation measures the percentage of iron-binding sites on transferrin that are occupied. In iron deficiency, ferritin is low and transferrin saturation is also low, because there is not enough iron to fill the available binding sites. In iron overload, ferritin is high and transferrin saturation is also elevated, typically above 45 percent, reflecting excess iron flooding the transport system. In inflammation with normal iron stores, ferritin is elevated but transferrin saturation is normal or low, because the inflammation has raised ferritin without increasing circulating iron. This three-pattern framework makes the pairing of ferritin and transferrin saturation essential for interpretation. Neither test alone is adequate.

The consequences of iron deficiency that are most commonly overlooked operate well before anemia develops. Serum ferritin below 30-50 ng/mL represents depleted storage iron. At this level, iron-dependent processes begin to lose efficiency, including: mitochondrial function (iron is a cofactor in the electron transport chain, particularly in complexes I, II, and III); thyroid hormone conversion (the enzyme that converts T4 to T3, iodothyronine deiodinase, is iron-dependent); cognitive function (iron is required for dopamine synthesis, myelination, and general neuronal energy metabolism); and exercise capacity (before hemoglobin falls, reduced iron availability limits oxygen delivery to exercising muscle). These functional consequences are well-documented in women with low-normal ferritin who have no anemia by standard criteria.^[5]

Most labs set their lower reference limit for ferritin at 12-15 ng/mL in women and slightly higher in men. This is the point at which iron stores are essentially zero. A ferritin of 22 ng/mL in a woman who is fatigued, cold, and not improving with sleep will not be flagged. The standard panel has no mechanism to connect the symptom to the deficiency at this level of sensitivity.

Hemochromatosis deserves mention specifically because it is common, underdiagnosed, and addressable with a simple intervention (therapeutic phlebotomy). The HFE gene mutations C282Y and H63D account for the majority of hereditary hemochromatosis in populations of northern and western European ancestry. Most carriers are unaware. Progressive iron deposition in the liver, heart, pancreas, and joints causes cirrhosis, cardiomyopathy, diabetes, and arthropathy. Early detection via ferritin plus transferrin saturation, followed by HFE genotyping if transferrin saturation exceeds 45 percent, prevents all of this. It is not diagnosed on a standard panel.

What to order: ferritin and transferrin saturation together, always. Serum iron alone is nearly useless, reflecting recent dietary intake more than body stores. A fasting transferrin saturation is more reliable than a non-fasting draw. If transferrin saturation is elevated, HFE genotyping should be discussed with the ordering clinician.

Omega-3 index: tissue level, not dietary recall

Omega-3 fatty acids consumed in food or supplements enter cell membranes throughout the body and alter membrane physical properties and the local balance of pro- and anti-inflammatory eicosanoids. The omega-3 index measures EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) as a percentage of total fatty acids in red blood cell (RBC) membranes. Because RBCs have a lifespan of approximately 120 days, the omega-3 index reflects average omega-3 incorporation over the preceding 3-4 months. This is qualitatively different from a serum or plasma omega-3 level, which reflects recent intake (hours to days) rather than tissue saturation. The RBC membrane is the appropriate compartment for assessing functional tissue status.

William Harris, who developed the omega-3 index as a clinical measure, has argued in multiple publications that the RBC-based measurement is the appropriate biomarker for cardiovascular omega-3 risk stratification, rather than dietary recall or plasma levels.^[6] His data from population studies in the United States place the average American omega-3 index around 4-5 percent. Japanese adults, whose fish consumption is substantially higher, average around 8-11 percent. Harris and colleagues identified a range of 8-12 percent as the zone associated with the lowest cardiovascular risk in epidemiological data.

The intervention trial most cited in this space is REDUCE-IT, which randomized 8,179 adults with elevated triglycerides and established cardiovascular disease or diabetes to 4 g/day of icosapentaenoic acid (EPA only, as Vascepa) or mineral oil placebo. The trial found a 25 percent relative risk reduction in major adverse cardiovascular events (MACE) over a median follow-up of 4.9 years.^[7] The result was large and consistent across pre-specified subgroups. It has also been criticized: mineral oil is not an inert placebo; it was associated with small increases in LDL and inflammatory markers in the control arm, which may have artificially inflated the treatment benefit. A second large trial, STRENGTH, used a corn oil control with a combined EPA plus DHA formulation and found no significant cardiovascular benefit, raising further questions about whether the REDUCE-IT result was driven by EPA-specific effects or by the choice of control.^[8]

The earlier JELIS trial, conducted in Japan with 18,645 patients already on statins, found a significant 19 percent reduction in major coronary events with EPA supplementation at 1.8 g/day over five years.^[9] The JELIS population had substantially higher baseline omega-3 indices than Western populations, which matters for interpreting the dose-response relationship. Both trials recruited populations with established cardiovascular disease or high risk; the data for primary prevention in lower-risk populations is less robust.

What is less contested: the omega-3 index correlates with cardiovascular event rates across large observational datasets in a dose-response manner; RBC EPA+DHA below 4 percent is associated with roughly double the cardiovascular event rate compared to levels above 8 percent; DHA is the dominant omega-3 in brain tissue and is required for structural integrity of neuronal membranes; and EPA is the precursor to resolvins and protectins, lipid mediators involved in the resolution of inflammation.

For people taking fish oil supplements, the standard 1-gram fish oil capsule typically contains approximately 300 mg of combined EPA+DHA. At this dose, the expected rise in omega-3 index is modest: approximately 1-2 percentage points over several months. Someone starting at 4 percent will not reach 8 percent on a 1-gram capsule. Higher-dose supplementation, typically 2-4 grams of combined EPA+DHA daily, is needed to move the index substantially. The actual composition of fish oil supplements varies substantially between products; products using re-esterified triglyceride form have higher bioavailability than ethyl ester forms.

The omega-3 index is not included on any standard annual panel. It is ordered separately through labs that specifically measure RBC fatty acid composition, such as OmegaQuant. A standard serum omega-3 or plasma fatty acid panel ordered through a hospital lab is not the same test and does not provide tissue-status information. The distinction matters and is frequently missed.

What to order: omega-3 index (RBC-based), specified explicitly. Not a serum panel. OmegaQuant or equivalent. Target: 8-12 percent. Test after 3-4 months on a stable supplementation protocol.

Four markers and what optimal looks like

Vitamin D

25-OH vitamin D (also written 25(OH)D). This is the storage form and the correct marker for status assessment. Do not order 1,25-OH D to assess nutritional status.

Standard reference ranges vary by lab but typically flag results below 20 ng/mL as deficient and below 30 ng/mL as insufficient. Optimal for most non-skeletal functions: 40-60 ng/mL. Below 30 ng/mL is clearly suboptimal. Above 100 ng/mL is an area where toxicity risk rises, though clinical toxicity from D3 supplementation below 10,000 IU/day is uncommon. Achieve measurement before supplementing to establish baseline and to calibrate dose. Add PTH if borderline.

Vitamin B12

Serum B12 for baseline. Add MMA if B12 is below 400 pg/mL or if the patient is at elevated risk (vegan, vegetarian, older, on metformin or PPI). MMA above 0.4 µmol/L indicates functional deficiency even with serum B12 in the normal range. An MMA that rises on serial testing, even within the reference range, indicates increasing functional inadequacy.

HoloTC, where available, is more direct than serum B12. A HoloTC below 35 pmol/L indicates depleted active B12. Most hospital labs do not offer HoloTC; it may need to be sent to a reference lab.

Ferritin

Always paired with transferrin saturation. Order both or interpret neither. Optimal ferritin for iron-sufficient adults: 50-150 ng/mL in women, 75-200 ng/mL in men. Ferritin below 30 ng/mL in women indicates substantially depleted stores with functional consequences even in the absence of anemia. Ferritin above 200 ng/mL in women or 300 ng/mL in men warrants investigation of the cause; transferrin saturation above 45 percent in a fasting draw triggers a workup for hemochromatosis.

Omega-3 index

RBC EPA+DHA as a percentage of total RBC fatty acids. Ordered through OmegaQuant or equivalent specialty lab, not through standard hospital chemistry panels. Target: 8-12 percent. Reflects 3-4 month average tissue incorporation. A result below 4 percent is associated with substantially elevated cardiovascular risk in observational data. Test after at least 3 months on a stable regimen to get a representative reading.

What to order

None of the four markers in this article appear on a standard annual panel as ordered. Each requires explicit request and, in some cases, ordering through a non-standard lab.

Vitamin D. Order 25-OH vitamin D explicitly. Note that some order forms abbreviate this as “Vitamin D, 25-Hydroxy” or “25-OH-D.” Do not order “1,25-OH vitamin D” or “calcitriol” for status assessment. Add PTH if the result is in the 20-40 ng/mL range to assess secondary hyperparathyroidism.

Vitamin B12. Order serum B12. If the result is below 400 pg/mL, add methylmalonic acid and homocysteine on the same or next draw. In anyone on metformin, any strict vegan or vegetarian, anyone over 60, or anyone with neurological symptoms that cannot be otherwise explained, order MMA proactively. If access permits, order HoloTC directly in place of serum B12 for a more sensitive initial measure.

Ferritin and transferrin saturation. Order both simultaneously. Request a fasting draw for the most reliable transferrin saturation. Most lab order forms allow both to be checked on the same line item. If the ordering form only shows ferritin, add transferrin saturation as a written addition. Serum iron is an inadequate substitute.

Omega-3 index. Order through OmegaQuant (omegaquant.com) or equivalent direct-to-consumer lab, not through a hospital lab panel. Specify “omega-3 index” or “RBC omega-3 fatty acid analysis.” This is typically a finger-stick blood spot test mailed to the lab. It is available without a physician order. Testing makes most sense after at least 3 months on a consistent omega-3 supplement regimen; a baseline before starting supplementation is also useful.

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These are the last four markers in a series of twenty-odd. Across the five articles, the argument has stayed constant: the standard annual blood panel detects disease after it has established itself. It does not detect the processes that are building toward disease. Metabolic dysfunction accumulates silently for a decade. Cardiovascular risk runs on unmeasured particle counts and lipoprotein(a). Hormonal decline proceeds while TSH sits at 3.5 mIU/L. Organ stress shows up years before creatinine crosses into the abnormal range. And nutritional deficiencies, widespread and correctable, are either not tested or tested with markers too blunt to catch the early signal.

The longevity protocol article describes how I’ve structured these measurements into a practical monitoring stack: what I order, how often, and what the targets look like applied to actual results. The protocols in that article are downstream of the framework laid out across this series. Understanding why the markers matter is prior to knowing what to do about the numbers.

Previous: Organ function

References

1. Forrest KY, Stuhldreher WL. (2011). Prevalence and correlates of vitamin D deficiency in US adults. Nutrition Research, 31(1), 48–54. https://pubmed.ncbi.nlm.nih.gov/21310306/
2. Manson JE, Cook NR, Lee IM, et al. (2019). Vitamin D supplements and prevention of cancer and cardiovascular disease. New England Journal of Medicine, 380(1), 33–44. https://pubmed.ncbi.nlm.nih.gov/30419274/
3. Hahn J, Cook NR, Alexander EK, et al. (2022). Vitamin D and marine omega 3 fatty acid supplementation and incident autoimmune disease. BMJ, 376, e066452. https://pubmed.ncbi.nlm.nih.gov/35081349/
4. Calvo Romero JM, Ramiro Lozano JM. (2012). Vitamin B12 in type 2 diabetic patients treated with metformin. Endocrinología y Nutrición, 59(8), 487–490. https://pubmed.ncbi.nlm.nih.gov/22541618/
5. Vaucher P, Druais PL, Waldvogel S, Favrat B. (2012). Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial. Canadian Medical Association Journal, 184(11), 1247–1254. https://pubmed.ncbi.nlm.nih.gov/22777673/
6. Harris WS, Von Schacky C. (2004). The omega-3 index: a new risk factor for death from coronary heart disease? Preventive Medicine, 39(1), 212–220. https://pubmed.ncbi.nlm.nih.gov/15208005/
7. Bhatt DL, Steg PG, Miller M, et al. (2019). Cardiovascular risk reduction with icosapentaenoic acid for hypertriglyceridemia. New England Journal of Medicine, 380(1), 11–22. https://pubmed.ncbi.nlm.nih.gov/30415628/
8. Nicholls SJ, Lincoff AM, Garcia M, et al. (2020). Effect of high-dose omega-3 fatty acids vs corn oil on major adverse cardiovascular events in patients at high cardiovascular risk: the STRENGTH randomized clinical trial. JAMA, 324(22), 2268–2280. https://pubmed.ncbi.nlm.nih.gov/33190147/
9. Yokoyama M, Origasa H, Matsuzaki M, et al. (2007). Effects of eicosapentaenoic acid on major coronary events in hypercholesterolaemic patients (JELIS): a randomised open-label, blinded endpoint trial. Lancet, 369(9567), 1090–1098. https://pubmed.ncbi.nlm.nih.gov/17398308/