The research team, led by Ewa Karakulska-Prystupiuk and colleagues in Warsaw, analyzed 341 patients who received allogeneic stem cell transplants between 2014 and 2023. For each patient, the investigators recorded the lowest vitamin B12 and folate levels measured within two years after the transplant, applying deficiency thresholds derived from the NHANES population surveys. The results were striking: 49 percent of patients showed reduced vitamin B12 levels below 300 pg/mL, with 17 percent falling below 200 pg/mL, and folate deficiency, defined as a level under 3 ng/mL, affected 46 percent of the cohort.

Graft-versus-host disease occurs when donor immune cells recognize the recipient’s tissues as foreign and mount an attack. When this process becomes chronic and involves the gastrointestinal tract, the lining of the gut—the very tissue responsible for absorbing nutrients—is inflamed and damaged. The Polish team hypothesized that this mucosal injury, combined with reduced dietary intake and the effects of immunosuppressive drugs, would translate into measurable vitamin depletion, particularly for B12 and folate, both of which are absorbed through intestinal mechanisms vulnerable to inflammatory damage.

To test this, the investigators carved out a subgroup of 68 patients with confirmed chronic graft-versus-host disease, whose median age was 45.5 years. Among them, 42 had gastrointestinal involvement. When the researchers compared median vitamin levels between the two groups, the difference was unequivocal. Patients with gastrointestinal chronic graft-versus-host disease had a median vitamin B12 level of 258 pg/mL, versus 442 pg/mL in patients whose chronic disease spared the gut. Deficiency was documented in 88.6 percent of the gastrointestinal group, compared with just 11.4 percent of the others, a difference that reached high statistical significance with a p value below 0.001.

Perhaps even more telling was the dose-response relationship. The researchers found an inverse correlation between the severity of gastrointestinal graft-versus-host disease and vitamin B12 concentrations: the worse the gut involvement, the lower the circulating B12. This gradient supports a mechanistic interpretation in which progressive mucosal destruction progressively impairs the absorption machinery—including the ileal uptake pathway that normally captures vitamin B12 bound to intrinsic factor—rather than the deficiency arising from a single binary event.

Folate, the other B vitamin central to red blood cell production and DNA synthesis, followed a similar directional pattern. Levels were lower in the gastrointestinal group, consistent with the idea that an inflamed gut absorbs less of this nutrient as well, but the difference did not reach statistical significance. The authors suggest this may reflect folate’s more diffuse absorption along the small intestine, or the confounding influence of supplementation practices, though the precise explanation remains a question for future work.

The study then turned to anemia, one of the most persistent quality-of-life problems after transplantation. At 12 months post-transplant, hemoglobin concentrations were significantly lower in patients with gastrointestinal chronic graft-versus-host disease, 12.2 g/dL versus 13.25 g/dL in those without gut involvement, with a p value of 0.006. Since both vitamin B12 and folate deficiencies classically cause macrocytic anemia—oversized, underperforming red cells—one might expect the vitamin deficits to explain the anemia. They did not. Macrocytic anemia predominated in both patient groups, but it was not associated with the vitamin deficiencies, indicating that the red cell failure after transplantation stems from multiple converging causes rather than a single nutrient shortfall.

This dissociation is arguably the study’s most instructive result. Post-transplant anemia can arise from ongoing inflammatory suppression of the bone marrow, iron dysregulation, renal dysfunction, medications, and the residual effects of the conditioning regimen itself. The Warsaw data suggest that clinicians cannot assume that correcting B12 or folate levels will resolve anemia in these patients, and conversely, that a normal blood film does not rule out dangerous vitamin depletion. The two problems run on separate tracks, and each demands its own surveillance.

The practical implications are considerable. Vitamin B12 deficiency is not a benign laboratory curiosity; untreated, it can produce neurological injury, cognitive impairment, and worsening fatigue in a population already burdened by treatment toxicities. Given that roughly nine in ten patients with gastrointestinal chronic graft-versus-host disease in this cohort were deficient, the authors’ findings argue for routine, periodic measurement of B12 and folate in this subgroup, with a low threshold for supplementation even in the absence of anemia. Whether prophylactic supplementation should extend to all transplant recipients remains an open question the retrospective design cannot answer.

The study does carry the inherent limits of a single-center retrospective analysis, and vitamin levels were defined by the lowest recorded value rather than serial trends, which may capture transient dips. Still, the size of the cohort, the uniform threshold definitions, and the striking statistical separation between patient groups give the findings weight. As survival after allogeneic stem cell transplantation continues to improve, attention is steadily shifting from survival alone to the long-term functional health of survivors. Identifying a preventable, treatable deficiency affecting nearly half of all recipients—and nearly all of those with gut-directed graft-versus-host disease—is a concrete step in that direction, one that could translate into simple blood tests and inexpensive supplements protecting patients from an entirely avoidable secondary burden.

Subject of Research: Vitamin B12 and folate deficiency after allogeneic hematopoietic stem cell transplantation and its link to gastrointestinal chronic graft-versus-host disease and anemia.

Article Title: Vitamin B12 and folate deficiencies after allo-HSCT: association with gastrointestinal chronic graft-versus-host disease and anemia

Article References: Karakulska-Prystupiuk, E., Kumorek, A., Tomaszewska, A., Kacprzyk, P., Biliński, J., Jędrzejczak, W. W., & Basak, G. W. (2026). Vitamin B12 and folate deficiencies after allo-HSCT: association with gastrointestinal chronic graft-versus-host disease and anemia. Annals of Hematology. https://doi.org/10.1007/s00277-026-07276-7

Image Credits: AI Generated

DOI: 10.1007/s00277-026-07276-7

Keywords: allogeneic stem cell transplantation, vitamin B12 deficiency, folate deficiency, graft-versus-host disease, gastrointestinal GvHD, anemia, macrocytic anemia, hematology, bone marrow transplant, post-transplant care, malnutrition, Annals of Hematology

The study, published in JAMA Neurology, examined whether previously unrecognized autoantibodies could help explain cases of myelopathy that do not fit established infectious, inflammatory, structural, metabolic, or genetic diagnoses. Myelopathy can produce weakness, sensory changes, impaired coordination, stiffness, gait difficulties, and disturbances in bladder or bowel function. Although clinicians routinely investigate vitamin B12 deficiency in such patients, conventional blood tests do not always reveal whether the nervous system is receiving enough of the vitamin to maintain normal cellular metabolism.

Vitamin B12, also known as cobalamin, is essential for several biochemical reactions that support the nervous system. It functions as a cofactor for methionine synthase, an enzyme involved in methylation reactions and the production of molecules needed for DNA synthesis and myelin maintenance. It is also required by methylmalonyl-CoA mutase, which helps process certain fatty acids and amino acids. When B12-dependent pathways fail, abnormal metabolites can accumulate, methylation can become impaired, and the protective myelin surrounding nerve fibers may deteriorate. In the spinal cord, this damage can preferentially affect the posterior and lateral columns, producing the characteristic pattern known as subacute combined degeneration.

For B12 to reach the interior of a cell, it must pass through a specialized transport pathway. After B12 circulates in the bloodstream bound to carrier proteins, cells internalize the vitamin through receptors and intracellular trafficking mechanisms. CD320, also called the transcobalamin receptor, recognizes transcobalamin-bound B12 and helps bring the vitamin into cells through receptor-mediated uptake. Once internalized, B12 is processed and delivered to the cellular compartments where its cofactor functions are required. Antibodies that recognize CD320 could theoretically interfere with this process, although the study’s findings do not by themselves establish that the antibodies directly cause neurological injury.

The investigators used a case-control design to compare individuals with idiopathic myelopathy with control participants and searched for antibodies directed against CD320. The study detected these autoantibodies in a portion of patients with unexplained spinal cord disease. The association appeared particularly relevant among individuals whose clinical presentation resembled subacute combined degeneration, raising the possibility that a functional B12 deficiency may exist within the central nervous system even when routine systemic measurements do not provide a clear explanation.

A central nervous system-restricted deficiency would be difficult to detect with standard screening alone. Blood concentrations of B12 may be influenced by dietary intake, supplementation, binding proteins, liver function, kidney function, and other medical conditions. In addition, total circulating B12 does not necessarily indicate how efficiently the vitamin is transported into particular tissues. More sensitive metabolic tests can provide indirect evidence that B12-dependent reactions are failing. Measurements of methylmalonic acid and homocysteine, for example, may reveal impaired B12 metabolism, although their interpretation requires clinical context and can be affected by other conditions.

The study therefore points toward a possible two-stage diagnostic approach for selected patients with unexplained myelopathy. First, clinicians could screen for antibodies against CD320 when the presentation suggests impaired B12-dependent spinal cord metabolism. A positive antibody result could then be followed by metabolic testing designed to determine whether the central nervous system is functionally deficient in B12. This strategy could be particularly relevant when patients have neurological findings or magnetic resonance imaging patterns suggestive of subacute combined degeneration but lack a straightforward explanation based on serum B12 levels or more common causes of myelopathy.

The findings may also help explain why some patients with idiopathic myelopathy remain undiagnosed despite extensive testing. Autoimmune diseases do not always arise from antibodies that attack a tissue directly. In some disorders, antibodies disrupt receptors, transporters, enzymes, or other components of cellular communication. An antibody directed at CD320 could represent such a mechanism by interfering with nutrient delivery rather than causing immediate, visible destruction of neurons. However, the presence of an antibody is not proof of pathogenicity. Further work will be needed to determine whether anti-CD320 antibodies block receptor activity, accelerate receptor removal, alter intracellular B12 trafficking, or simply mark another immune process associated with the disease.

The results also raise questions about treatment. If anti-CD320 antibodies are shown to impair B12 transport, conventional vitamin supplementation might not be sufficient for every affected patient, depending on the severity and location of the transport defect. Physicians would need evidence from functional studies and clinical trials before considering immune-directed therapies, intensified B12 replacement, or combinations of both. At present, the study supports additional diagnostic investigation rather than a definitive treatment protocol. Patients with suspected B12-related neurological disease should be evaluated by clinicians who can integrate symptoms, imaging, nutritional history, metabolic markers, antibody testing, and alternative diagnoses.

The researchers emphasize that the findings apply to a subset of people with idiopathic myelopathy, not to all cases of spinal cord disease. Myelopathy can result from compression, inflammation, infection, toxic exposure, vascular injury, inherited disorders, malignancy, nutritional deficiency, or other autoimmune conditions. Anti-CD320 testing will require validation in larger and more diverse populations before its accuracy, clinical thresholds, and predictive value are known. Nevertheless, the discovery offers a new biological link between autoimmunity and B12-dependent nerve metabolism and suggests that some apparently unexplained spinal cord disorders may reflect a hidden failure of vitamin transport inside the nervous system.

Subject of Research: Autoantibodies against the transcobalamin receptor CD320 in idiopathic myelopathy and central nervous system-restricted vitamin B12 deficiency.

Web References: https://doi.org/10.1001/jamaneurol.2026.2778

References: JAMA Neurology study identified by DOI 10.1001/jamaneurol.2026.2778.

Keywords: idiopathic myelopathy, CD320, transcobalamin receptor, vitamin B12, cobalamin deficiency, subacute combined degeneration, autoantibodies, autoimmune neurology, spinal cord disease, central nervous system metabolism