Vitamin B12 and Folate in Athletes: Beyond Anemia Prevention
Table of Contents
Introduction
In my experience, athletic patients are often well-informed about nutrition and vitamins. They are typically interested in tracking their biomarkers, and quite often they are particularly concerned about things like iron levels. What tends to come as a surprise to many, however, is vitamin B12 deficiency. In the broader context of B12 and folate in athletes, this gap in awareness is clinically significant. While it may not be as common as iron deficiency, I still see it relatively frequently in practice—especially in patients following a vegetarian or plant-based diet.
A 2024 systematic review and meta-analysis of 19 studies confirmed that unsupplemented adult vegans show significantly lower serum B12, elevated homocysteine, and elevated MMA compared to omnivores—demonstrating both circulating and functional deficiency[1]. An earlier review using functional markers such as MMA and holotranscobalamin found deficiency rates of 62% in pregnant women, 25–86% in children, and 21–41% in adolescents across vegetarian and vegan populations[2]. What is less frequently discussed is that normal serum B12 does not exclude functional deficiency. In recreational endurance athletes, data suggest that circulating B12 markers may not fully reflect intracellular vitamin B12 status, and that altered B12 metabolism may be present despite apparently normal serum levels[3].
In this article, I explore the mechanistic roles of B12 and folate in athletes’ performance, explain why standard serum testing can miss a significant proportion of deficient athletes, and outline a practical approach to testing and supplementation—particularly for plant-based and high-volume training populations.
What B12 and Folate Do in Athletes
Vitamin B12 (cobalamin) functions as a cofactor for two critical enzymatic reactions. The first, via methylcobalamin, drives methionine synthase—the enzyme responsible for converting homocysteine to methionine. This reaction is also the gateway to S-adenosylmethionine (SAM), the universal methyl donor involved in DNA methylation, neurotransmitter synthesis, myelin maintenance, and over 100 other substrate reactions[4]. The second, via adenosylcobalamin, drives methylmalonyl-CoA mutase—a mitochondrial enzyme essential for metabolizing odd-chain fatty acids and certain amino acids into succinyl-CoA for entry into the tricarboxylic acid (TCA) cycle[4].
From a performance standpoint, these are not peripheral functions. B12 is required for normal myelination of peripheral and central nerve tissue, healthy erythropoiesis in bone marrow, and efficient fat and amino acid catabolism during sustained exercise [4]. When B12 becomes limiting, all three pathways degrade simultaneously.
Folate (vitamin B9) operates in tight biochemical partnership with B12. Its primary athletic relevance is threefold: it is essential for de novo DNA synthesis (critical during the accelerated cell turnover caused by heavy training), it participates alongside B12 in homocysteine remethylation, and it supports the production of new erythrocytes and leukocytes in bone marrow [5]. Folate deficiency and B12 deficiency produce overlapping hematological pictures—both cause megaloblastic anemia characterized by large, immature red blood cells with impaired oxygen-carrying capacity [5].
The importance of folate in DNA synthesis is also clearly demonstrated in fetal development. Folate deficiency is strongly associated with congenital malformations, particularly neural tube defects. For this reason, in clinical practice we routinely recommend higher folate intake during pregnancy—and often already during the preconception period, as supplementation should ideally begin months before conception. As a water-soluble vitamin, folate has a wide safety margin, and the risk of toxicity with supplementation is low when used within recommended clinical ranges.
The practical implication is that these two vitamins cannot be assessed in isolation. Elevated homocysteine can reflect deficiency of either, while elevated MMA is specific to B12 alone.
In my experience, clinicians are generally aware of this, which is why these markers are often assessed together in our laboratory panels as part of the standard workup for anemia—particularly when megaloblastic anemia is suspected or has been identified.
That said, the underlying cause often depends on the patient population. In younger individuals—and especially in athletes—the cause is often dietary. In older patients, however, it is more commonly related to conditions such as pernicious anemia or intrinsic factor deficiency, and sometimes malabsorption disorders such as celiac disease, where the issue is impaired absorption rather than dietary intake.
The Role of B12 and Folate in Athletes’ Performance and Recovery
The intersection of high metabolic demand and dietary restriction creates a convergent risk for B12 and folate insufficiency in athletes. Vitamin B12 is found in significant concentrations only in animal-derived foods. For plant-based athletes who do not actively supplement, deficiency becomes highly likely over time. A systematic review and meta-analysis of 19 studies found that unsupplemented vegans demonstrated significantly lower serum B12 (p=0.01), elevated total homocysteine (p<0.001), and elevated MMA compared to omnivores—confirming functional as well as circulating deficiency[1].
Even among omnivorous athletes, there is evidence of an altered B12 metabolic pattern that standard serum testing fails to capture. In a study of 72 recreational endurance athletes compared to 46 inactive controls, serum B12 and folate were comparable between groups—but athletes showed significantly higher MMA concentrations (242 versus 175 nmol/L) despite comparable serum levels [3]. This pattern raises the possibility that training may alter B12 metabolism at the cellular level, though the study does not by itself confirm clinically important tissue deficiency; further investigation using the full functional marker panel is needed to characterize this pattern in individual athletes.
A six-year study of 243 elite track and field athletes found that hemoglobin rose with B12 concentrations up to approximately 400 pg/mL and did not significantly change from 700 pg/mL onward. Across all 1,131 samples collected, 18.1% fell below 400 pg/mL, with low values significantly more common in strength than endurance athletes—leading the authors to recommend active monitoring and supplementation for any athlete at or below that level [3].
In practice, we also often differentiate between different types of vegetarians. Many patients who identify as vegetarians follow a lacto-ovo vegetarian diet, meaning they avoid meat but still consume eggs and/or dairy products. In these cases, B12 status is often less of a concern, although intake can still be borderline depending on overall diet quality. Some may also include fish in their diet.
Others, however, follow a fully plant-based (vegan) diet, and in these individuals, adequate B12 intake almost always requires oral supplementation, as dietary sources alone are insufficient and deficiency will develop over time without it.
The Serum B12 Problem
Serum B12 measurement has been the default clinical test for decades, but its limitations are now well-established in the literature. The core issue is that total serum B12 includes a substantial haptocorrin-bound fraction that is not the main fraction used for cellular delivery. Only holotranscobalamin (holoTC) — the fraction bound to transcobalamin — is biologically active and available for cellular uptake via specific receptors on all cells [12]. Because the inactive haptocorrin-bound fraction dominates the total B12 signal, serum B12 can appear adequate while the functionally active fraction is depleted.
This means that a person with apparently adequate serum B12 may still be functionally deficient if the biologically active fraction is insufficient. Clinical guidelines acknowledge this limitation, noting that MMA or homocysteine can help clarify suspected functional deficiency when serum B12 is inconclusive[6].
Methylmalonic acid (MMA) accumulates specifically when B12 is insufficient at the tissue level to drive the methylmalonyl-CoA mutase reaction. Critically, MMA begins to rise before serum B12 falls below conventional deficiency thresholds—making it an early and specific indicator of functional deficiency [6]. When MMA is combined with homocysteine measurement, the pair achieves 99.8% sensitivity for functional B12 deficiency at the tissue level [7]. For athletes with normal serum B12 but persistent unexplained fatigue, mood disturbance, or fine motor impairment, MMA is the test that changes clinical management.
One important caveat: MMA can be mildly elevated in renal insufficiency independent of B12 status, so interpretation should always be paired with kidney function assessment and clinical context [6].
In Finland, we almost always use B12-holotranscobalamin (B12-TC2) rather than total serum B12. In most cases, this serves as a sufficiently accurate screening tool at the primary care level.
In more detailed anemia workups, we may also include methylmalonic acid (MMA), but this is typically done in more specialized settings—such as hematology or gastroenterology—particularly when investigating suspected malabsorption. At least in Finland, this is the usual approach.
What Deficiency Actually Feels Like
The neurological and performance consequences of B12 deficiency are frequently underappreciated because they precede macrocytic anemia by months to years. The pathway is straightforward: impaired methionine synthesis disrupts myelin maintenance, reducing nerve conduction velocity and efficiency of neuromuscular signaling. Impaired adenosylcobalamin function limits mitochondrial energy generation from fatty acid catabolism.
In clinical presentation, approximately 30–50% of B12-deficient patients show some degree of neurological involvement [6]. For athletes, the functionally significant early symptoms include persistent fatigue not explained by training load, difficulty concentrating during technical training or competition, mood changes, and subtle proprioceptive or coordination deficits. These are the symptoms most commonly missed because they resemble overtraining syndrome, relative energy deficiency in sport (RED-S), or simply high-intensity fatigue.
The difficulty is compounded by the absence of anemia in early deficiency. Research in pediatric populations found that fatigue and concentration difficulty occurred as presenting symptoms without accompanying anemia [8]. For adult athletes, the clinical implication is that waiting for a macrocytic anemia signal means waiting too long.
In athletes, I often see presentations that overlap with general fatigue or even overtraining-like states, which can make the clinical picture less clear. In these cases, iron deficiency and thyroid dysfunction are usually the primary suspicions. In comparison, B12 or folate deficiency is often considered secondary in the overall assessment, which can make the diagnosis more challenging.
In practice, key diagnostic clues often come from the patient’s diet, as well as from gastrointestinal symptoms or a known history of conditions such as celiac disease.
Folate: The Testing Gap
Folate testing presents its own interpretive challenge for athletes. Serum folate reflects intake over the preceding 24–48 hours—a single fortified meal or multivitamin will temporarily normalize a serum result that would otherwise indicate chronic insufficiency [5][9].
Red blood cell (RBC) folate is the superior marker for athlete assessment. Because folate is incorporated into erythrocytes during their formation in bone marrow, RBC folate reflects tissue folate status over the preceding 2–3 months—the full lifespan of a circulating red cell [9]. It is not susceptible to transient dietary variation and maps more accurately to bone marrow folate reserves, where DNA synthesis for new blood cell production occurs.
“StatPearls and NCBI clinical data indicate that RBC folate is a sensitive marker for longer-term folate deficiency, while serum folate is more influenced by recent dietary intake[5]. A serum folate below 2 ng/mL indicates deficiency, and 2–4 ng/mL is considered borderline[5]. For plant-based athletes, measuring both serum and RBC folate can provide complementary information: serum folate reflects recent intake, whereas RBC folate offers a better picture of longer-term folate status.
A critical clinical point is that elevated homocysteine with normal MMA is more consistent with folate deficiency, whereas elevated MMA alongside elevated homocysteine is more suggestive of vitamin B12 deficiency[5]. Measuring serum B12, MMA, folate, and homocysteine together can help reduce diagnostic uncertainty and improve differentiation between the two.
In my experience, folate testing is also often limited by practical factors, as it is typically performed as a fasting test. In everyday clinical practice, when a patient presents with fatigue and a standard “fatigue panel” is ordered, folate is often not included for this reason. It is not as easy to obtain on the spot during a routine visit, but usually requires a separate appointment under fasting conditions.
As a result, folate testing is sometimes underutilized. In many cases, clinicians rely on indirect markers—such as normal hemoglobin, absence of macrocytosis, and adequate ferritin—as reassurance. Additionally, since folate deficiency is relatively uncommon in many populations, it may not be prioritized in the initial workup.
Interestingly, folate deficiency is sometimes detected incidentally when patients undergo other fasting laboratory tests—such as glucose or lipid panels—where additional markers may be included. However, even in these settings, folate is often not part of standard panels, including many routine occupational health check-ups.
This creates a potential diagnostic gap, where folate deficiency may go unrecognized in certain patients.
Supplementation: Oral, Sublingual, or Injection?
The traditional assumption has been that intramuscular (IM) injection is the only reliable route for correcting significant B12 deficiency. Current evidence does not support this hierarchy for most athletes.
A 2025 systematic review and meta-analysis found no statistically significant difference between oral, sublingual, and IM routes of B12 administration for either raising serum cobalamin (p=0.270) or reducing homocysteine (p=0.485) [10]. Across 17 studies, all routes produced a pooled mean serum B12 increase of +402.6 pg/mL [10]. A separate network meta-analysis of 13 studies including 4,275 patients reached the same conclusion: all three routes are comparably effective, with sublingual preferred over IM on grounds of tolerability and absence of injection risk [11].
The mechanistic basis is well-established: at high oral or sublingual doses (500–2,000 µg), about 1.2% of B12 is absorbed through passive diffusion independent of intrinsic factor—sufficient to meet daily requirements and gradually replete stores even in individuals with impaired intrinsic-factor-mediated absorption [11].
In clinical practice, we typically start with oral treatment in milder cases. Historically, intramuscular (IM) therapy was considered the gold standard—especially in malabsorptive conditions—but there has been a shift back toward oral dosing at sufficiently high doses in recent years.
In most cases, when the deficiency is dietary, long-term IM treatment is rarely required. Injections may be used initially if a more rapid correction is needed, but maintenance can usually be managed with oral supplementation.
That said, IM treatment has not disappeared from practice. It is still common to see patients who have been on long-term injection therapy—often for years—such as regular hydroxocobalamin (e.g., Cohemin) listed in their medication records. In some cases, this reflects historical practice, and a transition to oral therapy could be considered. However, if the treatment has been stable and effective for years, it is often continued rather than changed.
Conclusion
Vitamin B12 and folate deficiency in athletes is not simply a question of anemia, but a broader issue of metabolic function, neurological performance, and recovery. In my experience, the main challenge is not the treatment—but recognizing when these deficiencies should be suspected in the first place. Symptoms such as fatigue, reduced performance, and impaired concentration are often attributed to training load, iron deficiency, or thyroid dysfunction, while B12 and folate are considered only later in the diagnostic process.
The limitations of standard testing, combined with practical barriers—particularly in folate assessment—mean that clinically relevant deficiencies can be overlooked even in otherwise well-monitored athletes. At the same time, modern diagnostics such as holotranscobalamin and functional markers like MMA have improved our ability to detect deficiency earlier, when interpreted in the right clinical context.
In practice, the most reliable approach remains simple: integrate laboratory findings with diet, symptoms, and underlying risk factors. In plant-based athletes, B12 deficiency is largely preventable with appropriate supplementation, while in other patients, gastrointestinal conditions and malabsorption must be considered.
Treatment is rarely complicated. In most cases, oral supplementation is sufficient—even in situations previously thought to require injections—although intramuscular therapy still has a role in more severe or complex presentations.
Ultimately, improving outcomes requires a shift in perspective: from viewing B12 and folate solely through the lens of anemia, to recognizing their central role in energy metabolism, neuromuscular function, and athletic performance.
References
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