Vitamin B₁₂ Supplementation: Is More Always Better?

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

General comment

This narrative review aims to assess vitamin B₁₂ use in adults without a known deficiency. However, several methodological and content-related issues require clarification and revision.

Major comments

General methodology  - Selection of articles

The authors do not clearly explain how articles involving adults without a known vitamin B₁₂ deficiency were identified. Given the complexity of this criterion, the keywords used, as well as the inclusion and exclusion criteria, should be explicitly defined.

The authors must clarify what the objective is; if it is supplementation in the general population, they must present a literature review with relevant criteria. The article gives the impression that they are conflating systematic supplementation and treatment.

Others comments

Vitamin B₁₂ function (Lines 72–74)

The key aspect here is the transfer of a methyl group from methyltetrahydrofolate for methylation reactions. This point could be stated more clearly to improve scientific accuracy.

Vitamin B₁₂ source (Lines 91–94)

Although the quantities are low, it would be useful to mention certain plant-based sources of vitamin B₁₂ in order to emphasise that this vitamin is derived primarily from foods of animal origin.

Marine algae: Certain varieties (e.g. nori, spirulina, wild algae) may contain measurable amounts of vitamin B₁₂; however, bioavailability varies considerably, and these sources should not be considered reliable.

Wild mushrooms: Some wild mushrooms, particularly those growing in non-contaminated environments, may contain small amounts of vitamin B₁₂, but these quantities are generally insufficient to meet nutritional requirements.

Brewer’s yeast and nutritional yeast: These may contain vitamin B₁₂, but typically in quantities too low to be nutritionally relevant.

Vitamin B₁₂ metabolism -  Intrinsic factor (Line 99)

It should be clearly stated that intrinsic factor is produced by the parietal cells of the stomach.

Vitamin B₁₂ Recommended intakes (Lines 115–117)

The Panel indicates that intakes of 1.5–2 µg/day represent the minimum requirement for maintaining normal haematological status, and are associated with low body stores (approximately 1–2 mg).

In the referenced report, optimal vitamin B₁₂ intake levels for pregnant and breastfeeding women were not established; this limitation should be acknowledged more explicitly.

Medications affecting vitamin B₁₂ status (Lines 186–191)

H₂-receptor antagonists and certain anticonvulsants are not mentioned and should be included in this section.

Causes of vitamin B₁₂ deficiency (Lines 178–195)

Although the population discussed does not have a proven deficiency, the authors begin to describe causes of vitamin B₁₂ deficiency. However, hereditary causes are entirely omitted and should be addressed.

Hereditary disorders and treatment options (Lines 196–219)

No mention is made of hereditary disorders affecting vitamin B₁₂ metabolism. Furthermore, in cases where cyanocobalamin is ineffective, hydroxocobalamin may represent a viable alternative and should be discussed (PMID: 39152755).

Author Response

Manuscript ID: nutrients-4057027

Detailed, itemized response to the comments of Reviewer #1.

This narrative review aims to assess vitamin B₁₂ use in adults without a known deficiency. However, several methodological and content-related issues require clarification and revision.

Response: We thank the Reviewer for their evaluation of our work. Please find a detailed response to the issues raised by the reviewer below:

Major comments

General methodology  - Selection of articles

Comment #1: The authors do not clearly explain how articles involving adults without a known vitamin B₁₂ deficiency were identified. Given the complexity of this criterion, the keywords used, as well as the inclusion and exclusion criteria, should be explicitly defined.
 Response: We thank the Reviewer for raising this important point. As this manuscript is a narrative review, articles were not selected systematically or according to a protocol. Instead, literature selection was guided by the overarching aim of providing a conceptual and clinically oriented overview of vitamin B₁₂ supplementation in the general adult population. Also, no strict inclusion or exclusion criteria were applied for defining “non-deficient” populations, as definitions of vitamin B₁₂ deficiency vary widely across the literature, an issue that is explicitly discussed within the review itself. Applying rigid biochemical thresholds would therefore have risked excluding relevant and informative studies.

In practice, the authors preferred studies conducted in general or asymptomatic adult populations, as well as those that explicitly excluded individuals with known or overt vitamin B₁₂ deficiency or reported baseline vitamin B₁₂ status. We now provide, in the Supplementary Material, a table with representative search terms used to identify relevant literature. This addition is intended to improve clarity regarding the scope of the searches, while remaining consistent with the narrative design of the review.

Supplementary Table 1. Representative core search terms used for literature identification

Search terms were used iteratively and in combination, employing both controlled vocabulary and free-text terms, and adapted across databases.

Finally, to improve transparency, we have clarified in the Methods section that study identification and selection were non-systematic and based on relevance and clinical context rather than predefined criteria. The concerned part of the revised methods section now reads:

“No formal inclusion or exclusion criteria, quality scoring, or risk-of-bias assessment was applied, consistent with the narrative nature of the review” (Lines 77-79)

Comment #2: The authors must clarify what the objective is; if it is supplementation in the general population, they must present a literature review with relevant criteria. The article gives the impression that they are conflating systematic supplementation and treatment.

 Response: As noted by the Reviewer, the objective of this review is to discuss the widespread use of vitamin B₁₂ supplementation in the general population. To make this focus clearer to the reader, we have revised the Abstract to explicitly highlight that our discussion pertains to findings from the general population:

“Vitamin B₁₂ supplementation in the general population has become increasingly common, promoted both as a frequently underdiagnosed deficiency and as a natural energy enhancer.”  (Lines 21-23)

In addition, we have clarified in the Methods section that studies involving non-deficient individuals were prioritized, and we now provide, in Supplementary Table 1, representative examples of the search terms used. The Methods section now reads:

“Studies were preferentially included when they reported baseline vitamin B₁₂ status, excluded individuals with overt deficiency, or examined supplementation effects in general or asymptomatic populations.” (Lines 75-77)

Finally, we have revised several sections of the manuscript to ensure that statements regarding supplementation clearly distinguish between deficient and non-deficient populations, and to specify when conclusions apply to individuals without known disorders of vitamin B₁₂ metabolism. These clarifications have been incorporated throughout the text. For example:

In routes and forms of B12 supplementation:

“To date, none has demonstrated clear superiority over the others in the treatment of vitamin B₁₂ deficiency with respect to clinical outcomes such as normalization of serum B₁₂ concentrations or correction of anemia. These comparisons have been conducted primarily in deficient populations, and evidence comparing the efficacy of different B₁₂ forms in non-deficient individuals is lacking.” (Lines 227-231)

In knowledge gaps:

Regarding supplementation, most evidence on dose, formulation, and route of administration comes from studies in deficient individuals. Data supporting indications beyond overt deficiency are limited, and it remains unclear whether intervention in such contexts provides meaningful clinical benefit.” (Lines 385-388)

And in conclusions:

“Beyond correcting deficiency or the treatment of inherited metabolic defects of intracellular transport, supplementation above sufficiency has not shown consistent gains in energy, mood, or global cognition, and cardiovascular benefits depend on context (e.g., the presence of hyperhomocysteinemia) rather than being universal.” (Lines 417-420)

Others comments

Comment #3:  Vitamin B₁₂ function (Lines 72–74). The key aspect here is the transfer of a methyl group from methyltetrahydrofolate for methylation reactions. This point could be stated more clearly to improve scientific accuracy.

Response: To accommodate the comment of the Reviewer, we have modified the text to be more accurate. The revised version now reads:

“Vitamin B₁₂ (cobalamin) is an essential water-soluble cofactor for two enzymes. In the methionine synthase reaction, methylcobalamin mediates the transfer of a methyl group from methyltetrahydrofolate to homocysteine, forming methionine and regenerating tetra-hydrofolate for one-carbon metabolism and downstream methylation reactions” (Lines 84-87)

Comment #4:  Vitamin B₁₂ source (Lines 91–94). Although the quantities are low, it would be useful to mention certain plant-based sources of vitamin B₁₂ in order to emphasise that this vitamin is derived primarily from foods of animal origin.

Marine algae: Certain varieties (e.g. nori, spirulina, wild algae) may contain measurable amounts of vitamin B₁₂; however, bioavailability varies considerably, and these sources should not be considered reliable.

Wild mushrooms: Some wild mushrooms, particularly those growing in non-contaminated environments, may contain small amounts of vitamin B₁₂, but these quantities are generally insufficient to meet nutritional requirements.

Brewer’s yeast and nutritional yeast: These may contain vitamin B₁₂, but typically in quantities too low to be nutritionally relevant.

Response: in agreement with the Reviewers suggestion, we have added this information to the revised version of the manuscript, which now reads:

“Therefore, humans obtain vitamin B₁₂ from external sources (…). Although trace amounts of vitamin B₁₂ have been detected in certain plant-based foods, including marine algae, wild mushrooms, and brewer’s or nutritional yeast, these foods are not reliable sources of biologically active vitamin B₁₂. The vitamin content is highly variable, often present as inactive analogues, and generally insufficient to meet human nutritional requirements [15]. Consequently, the only dependable plant-based sources of vitamin B₁₂ are foods fortified with synthetic vitamin B₁₂, such as certain cereals and vegetarian meat replacement products, which typically provide 0.5–1.8 µg per 100 g [14]” (Lines 105-112)

Comment #5:  Vitamin B₁₂ metabolism -  Intrinsic factor (Line 99). It should be clearly stated that intrinsic factor is produced by the parietal cells of the stomach.

 Response: We have added this information. The revised manuscript now reads:

“In the duodenum, pancreatic proteases degrade haptocorrin, liberating cobalamin, which then binds to intrinsic factor (IF), produced by the parietal cells of the stomach” (Lines 116-118)

Comment #6:  Vitamin B₁₂ Recommended intakes (Lines 115–117). The Panel indicates that intakes of 1.5–2 µg/day represent the minimum requirement for maintaining normal haematological status, and are associated with low body stores (approximately 1–2 mg). In the referenced report, optimal vitamin B₁₂ intake levels for pregnant and breastfeeding women were not established; this limitation should be acknowledged more explicitly.

Response: As the reviewer correctly notes, the EFSA Panel did not establish optimal vitamin B₁₂ intake levels for pregnant and breastfeeding women based on direct evidence from biomarkers or haematological status. Instead, the Adequate Intake (AI) values for pregnancy (4.5 µg/day) and lactation (5.0 µg/day) were derived from factorial assumptions, including estimates of fetal vitamin B₁₂ accumulation, breast-milk cobalamin concentration, and mean milk transfer. We have clarified this limitation in the revised manuscript, which now reads:

“In Europe, the European Food Safety Authority (EFSA) suggests Adequate Intakes (AIs) of 4.0 µg/day for adults, 4.5 µg/day in pregnancy, and 5.0 µg/day during lactation, anchored to intake–biomarker relationships across multiple B₁₂ status indicators in adults [21]. However, for pregnant and breastfeeding women, these recommendations were not derived from direct biomarker or hematological evidence in these populations, but rather from assumptions regarding fetal vitamin B₁₂ accumulation, breast-milk cobalamin concentrations, and average milk transfer [21]” (Lines 134-140)

Comment #7:  Medications affecting vitamin B₁₂ status (Lines 186–191). H₂-receptor antagonists and certain anticonvulsants are not mentioned and should be included in this section.

Response: Accommodating the comment of the Reviewer, we have added this information to the revised version of the manuscript which now reads:

“Evidence for proton-pump inhibitors and histamine H₂-receptor antagonists is mixed, but, biologically, they can reduce gastric acidity and impair the liberation of food-bound B₁₂ and its transfer from haptocorrin to intrinsic factor, thereby reducing absorption of dietary (though not crystalline) B₁₂ [45,46]. Anticonvulsant drugs have been associated with altered serum B₁₂ concentrations in clinical and observational studies, with agents such as phenobarbital, pregabalin, primidone, and topiramate linked to lower mean B₁₂ levels relative to untreated controls, although evidence is heterogeneous and mechanisms are not fully elucidated [47]” (Lines 210-217)

Comment #8:  Causes of vitamin B₁₂ deficiency (Lines 178–195). Although the population discussed does not have a proven deficiency, the authors begin to describe causes of vitamin B₁₂ deficiency. However, hereditary causes are entirely omitted and should be addressed.

Response: We thank the Reviewer for this comment. Although the focus of the manuscript is on adults without proven deficiency, we agree that hereditary causes of vitamin B₁₂ deficiency should be acknowledged for completeness. We have therefore revised the section to briefly include rare inherited disorders of cobalamin absorption and transport, while clarifying that these conditions are uncommon and typically present early in life, making them less relevant to the general adult population addressed in this review. The revised manuscript now reads:

“The causes of B₁₂ deficiency are linked to inadequate intake or impaired absorption [9,33,38] (…) Rare hereditary causes include congenital intrinsic factor deficiency (GIF), Imerslund–Gräsbeck syndrome due to CUBN or AMN mutations, and transcobalamin deficiency (TCN2), which typically present early in life and are uncommon in the general adult population [48]” (Lines 217-221)

Comment #9:  Hereditary disorders and treatment options (Lines 196–219). No mention is made of hereditary disorders affecting vitamin B₁₂ metabolism. Furthermore, in cases where cyanocobalamin is ineffective, hydroxocobalamin may represent a viable alternative and should be discussed (PMID: 39152755).

Response: In response to the Reviewer’s comment, we now include hereditary disorders as potential causes of vitamin B₁₂ deficiency (see comment #8). We consider that detailed discussion of specific treatment strategies for these hereditary conditions falls outside the scope of the present review.

We have also added a statement in the manuscript noting that hydroxocobalamin is specifically preferred in certain metabolic disorders and have included the reference suggested by the Reviewer. The revised text now reads:

“Hydroxocobalamin shows longer tissue retention when given parenterally, allowing for less frequent dosing [9,47], and is also specifically preferred in certain metabolic disorders [48]” (Lines 231-233)

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript addresses a current and important clinical and nutritional issue: the widespread use of vitamin B₁₂ supplements in populations without confirmed deficiency. The authors attempt a critical review of the common belief that high doses of this vitamin are completely safe, which is a valuable and necessary contribution to the literature.

However, despite its high substantive quality and comprehensive literature review, the work has significant methodological and conceptual limitations typical of narrative reviews, which should be more clearly highlighted and better addressed in the article’s structure.

The authors state that they have conducted a narrative review, which is an acceptable format in Nutrients. However:
there are no clearly defined criteria for the selection and exclusion of studies,
no literature selection scheme (e.g., flow diagram) is presented,
the quality of the included studies and the risk of bias have not been assessed,
the synthesis is purely descriptive.

As a result, the reader cannot assess the extent to which the conclusions are based on all available data or on a selective interpretation of the literature.

The authors should clearly emphasise the limitations of the narrative nature of the review and refrain from making recommendations resembling a clinical consensus.

The authors correctly note the high variability in the prevalence of B₁₂ deficiency depending on the population and the markers used. However:
in some sections, data from high-income countries with populations with very different dietary patterns and access to fortification are combined,
biochemical deficiency is not always clearly distinguished from clinically significant deficiency.

This may lead to excessive relativisation of the problem of deficiency, especially in risk groups (the elderly, patients with gastrointestinal diseases, people on plant-based diets).

The authors consistently demonstrate a lack of clear benefits of B₁₂ supplementation in people without deficiency in terms of energy and fatigue, cognitive function, quality of life, and cardiovascular prevention.

This is consistent with current meta-analyses and RCTs. However, in the section on cognitive function and depression, the argumentation sometimes simplifies complex relationships, such as the role of homocysteine, omega-3 status, or differences in study populations, and it is not always clearly stated that the lack of a population effect does not rule out potential benefits in carefully selected clinical subgroups.

One of the strongest points of the manuscript is the discussion of the potential adverse effects of high doses of B₁₂, including acneiform eruptions and exacerbations of rosacea, hypersensitivity reactions, and possible links to cancer and all-cause mortality.

At the same time, it should be emphasised that:
most of the data come from observational studies,
there is a high risk of reverse causality (high B₁₂ as a marker of disease, not a cause),
the biological mechanisms (microbiome, propionate, infections) are largely hypothetical and experimental.

In several places, the narrative may seem overly alarmist if not read with a full understanding of the limitations of the evidence.

The conclusions are generally cautious and consistent with the data; however, statements suggesting the need to “reassess the widespread use of high doses” could be interpreted as a health policy recommendation. A clearer distinction between the general population and high-risk groups, for whom supplementation remains unquestionably justified, would be useful.

Strengths:
a current and important topic,
a very broad review of the literature,
a reliable emphasis on the lack of benefits of supplementation beyond deficiency,
the courage to raise the issue of potential adverse effects.

Weaknesses:
lack of systematic rigour,
at times overinterpretation of observational data,
insufficient separation of biological hypotheses from clinical evidence,
risk of oversimplification in the message for clinical practice.

Author Response

Manuscript ID: nutrients-4057027

Detailed, itemized response to the comments of Reviewer #2.

The manuscript addresses a current and important clinical and nutritional issue: the widespread use of vitamin B₁₂ supplements in populations without confirmed deficiency. The authors attempt a critical review of the common belief that high doses of this vitamin are completely safe, which is a valuable and necessary contribution to the literature.

However, despite its high substantive quality and comprehensive literature review, the work has significant methodological and conceptual limitations typical of narrative reviews, which should be more clearly highlighted and better addressed in the article’s structure.

Response: We thank the reviewer for the kind appraisal of our work. Please find below a detailed response to the concerns raised by the Reviewer.

Comment #1:  The authors state that they have conducted a narrative review, which is an acceptable format in Nutrients. However: there are no clearly defined criteria for the selection and exclusion of studies, no literature selection scheme (e.g., flow diagram) is presented, the quality of the included studies and the risk of bias have not been assessed, and the synthesis is purely descriptive. As a result, the reader cannot assess the extent to which the conclusions are based on all available data or on a selective interpretation of the literature.

The authors should clearly emphasize the limitations of the narrative nature of the review and refrain from making recommendations resembling a clinical consensus.

Response: The goal of this article was to provide a general, synthesized overview of the discussions surrounding vitamin B₁₂ supplementation in the general population, which we believe is appropriately addressed through a narrative review. We agree with the Reviewer that the absence of elements such as a flow diagram or a formal risk-of-bias assessment may introduce selective emphasis and does not allow readers to formally assess completeness or study quality in the same way as in a systematic review.

To address the Reviewer’s comments and improve transparency, we have strengthened the manuscript in several ways. First, we now explicitly state early in the Methods section that, consistent with the narrative review design, no formal study selection criteria or quality appraisal were applied. The Methods section now reads:

“No formal inclusion or exclusion criteria, quality scoring, or risk-of-bias assessment was applied, consistent with the narrative nature of the review” (Lines 77-79)

Second, we have added a dedicated Limitations paragraph explicitly acknowledging the absence of formal quality appraisal and quantitative synthesis. The Limitations section now states:

“This review has several limitations inherent to its narrative design. Literature identification was not systematic, and no formal inclusion or exclusion criteria, study quality assessment, or risk-of-bias evaluation was performed. As a result, the synthesis may not capture all available evidence, and selective emphasis cannot be fully excluded. Definitions of vitamin B₁₂ deficiency varied widely across studies, limiting direct comparability and precluding quantitative synthesis. Furthermore, conclusions are based primarily on observational data and heterogeneous intervention studies, which restrict causal inference” (Lines 431-438)

Finally, we have revised the Discussion and Conclusions to avoid language resembling clinical consensus or guideline-level recommendations. The Limitations paragraph also clarifies that this review does not intend to provide guideline-level recommendations, stating:

“Accordingly, the findings of this review should be interpreted as descriptive and hypothesis-generating rather than as definitive evidence or clinical consensus, and they should not replace guideline-based decision-making or individualized clinical judgment” (Lines 438-440)

Comment #2:  The authors correctly note the high variability in the prevalence of B₁₂ deficiency depending on the population and the markers used. However: in some sections, data from high-income countries with populations with very different dietary patterns and access to fortification are combined, biochemical deficiency is not always clearly distinguished from clinically significant deficiency.

This may lead to excessive relativisation of the problem of deficiency, especially in risk groups (the elderly, patients with gastrointestinal diseases, people on plant-based diets).

Response: We thank the Reviewer for this recommendation. To accommodate the comment of the Reviewer, we have revised the manuscript to clarify the context in which prevalence estimates of vitamin B₁₂ deficiency were obtained. Specifically, mentions of prevalence throughout the text now explicitly indicate the populations and diagnostic criteria on which these estimates are based. For example, the Abstract now states:

“We evaluate the substantial variability in reported vitamin B₁₂ deficiency prevalence in the general population, which ranges from approximately 2% in high-income countries to up to 46% in settings with limited access to animal-source foods or fortified plant-based alternatives when plasma B₁₂ is used as the biomarker. Prevalence estimates vary further according to the population studied (e.g., high-risk subgroups) and the diagnostic criteria applied, including plasma B₁₂ concentrations, functional biomarkers, or clinical definitions.” (Lines 25-31)

We have also added clarification in the Conclusions to emphasize that prevalence varies further within high-risk groups. The revised text reads:

“The prevalence of vitamin B₁₂ deficiency in the general population is low in fortified, high-income settings but remains substantial in regions with limited access to animal-source foods or fortified plant-based alternatives, or where malabsorption is common. Prevalence estimates can vary further within high-risk groups, such as older adults and individuals following plant-based diets.”  (Lines 409-413)

Comment #3:  The authors consistently demonstrate a lack of clear benefits of B₁₂ supplementation in people without deficiency in terms of energy and fatigue, cognitive function, quality of life, and cardiovascular prevention.

This is consistent with current meta-analyses and RCTs. However, in the section on cognitive function and depression, the argumentation sometimes simplifies complex relationships, such as the role of homocysteine, omega-3 status, or differences in study populations, and it is not always clearly stated that the lack of a population effect does not rule out potential benefits in carefully selected clinical subgroups.

Response: In response to the Reviewer’s comment, we have revised the Benefits section to clarify that, although no clear benefit of vitamin B₁₂ supplementation has been demonstrated in the general population, potential benefits have been reported in selected subgroups (e.g., individuals with hyperhomocysteinemia and mild cognitive impairment, or patients with low-normal vitamin B₁₂ levels and depression). We now explicitly state that the absence of a population-level effect does not preclude clinically relevant effects in these specific subgroups.

We have also revised the mechanistic discussion to clarify that the proposed interaction between homocysteine lowering and omega-3 fatty acid status is based on theoretical hypotheses advanced by the authors of the cited trials, rather than established causal mechanisms. Additional potential explanations suggested in the literature have been incorporated to better reflect the complexity of these relationships.

The revised manuscript now reads:

“For cognition and neurodegeneration, broad meta-analyses concluded that B-vitamin supplementation does not meaningfully alter global cognitive trajectories in unselected populations [61,66]. Importantly, this lack of a population-level effect does not preclude benefits in specific, biologically defined subgroups. Targeted trials in patients with mild cognitive impairment (MCI) showed that high-dose folic acid, B₁₂, and B₆ can slow brain atrophy, especially in participants with elevated homocysteine and sufficient long-chain omega-3 levels. [67–70]. The authors propose several hypotheses to explain this interaction. One possibility is that homocysteine lowering through B₁₂ supplementation restores metabolic pathways that support the incorporation of docosahexaenoic acid (DHA) into neuronal membranes; consequently, structural benefits are most evident when omega-3 substrate availability is sufficient. Additionally, both omega-3 fatty acids and B vitamins may protect against tau hyperphosphorylation, with potential implications for neurofibrillary tangle formation. Both nutrient classes may also attenuate neuroinflammation, further contributing to their potential synergistic effects [69]. Taken together, homocysteine-lowering may translate into structural brain preservation in biomarker-selected sub-groups (e.g., MCI with high homocysteine and good omega-3 status), while routine vitamin B₁₂ supplementation above sufficiency for cognitive enhancement in the general population is not supported [66–70].

Regarding mood, pooled analyses of randomized trials show no consistent short-term antidepressant effect of B-vitamin supplementation in non-deficient populations [71,72]. In contrast, a small randomized trial in patients with low-normal vitamin B₁₂ (190-300 pg/ml) reported higher response rates when injectable B₁₂ was added to standard therapy, suggesting potential benefit in this subgroup [73]. Observational studies further associate low vitamin B₁₂ status with incident depression, although causality remains uncertain [74].”  (Lines 262-286)

Finally, we have incorporated a clarification into the Abstract, which now states:

“Across the literature, supplementation has not shown clear benefit in non-deficient individuals, with no consistent improvements in energy, mood, cognition, or cardiovascular outcomes in the general population. Modest benefits have been reported only in selected subgroups, such as cognitive or cardiovascular outcomes in individuals with hyperhomocysteinemia and antidepressant response in patients with low-normal plasma B₁₂.” (Lines 33-38)

Comment #4: One of the strongest points of the manuscript is the discussion of the potential adverse effects of high doses of B₁₂, including acneiform eruptions and exacerbations of rosacea, hypersensitivity reactions, and possible links to cancer and all-cause mortality. At the same time, it should be emphasised that: most of the data come from observational studies, there is a high risk of reverse causality (high B₁₂ as a marker of disease, not a cause), the biological mechanisms (microbiome, propionate, infections) are largely hypothetical and experimental.

In several places, the narrative may seem overly alarmist if not read with a full understanding of the limitations of the evidence.

Response: In response to the Reviewer’s suggestions, we have made several modifications to Section 6. We now begin this section by explicitly stating that most evidence regarding potential adverse effects of vitamin B₁₂ is derived from observational and experimental studies and should therefore be considered hypothesis-generating. The revised text now reads:

“Nevertheless, rare adverse reactions have been documented. In addition, observational and experimental studies have raised growing interest in the potential long-term effects of excessive vitamin B₁₂ exposure, particularly among non-deficient individuals. Although most of this evidence remains hypothesis-generating.” (Lines 300-304)

We have also revised the text throughout the section to temper the strength of the statements, to clearly indicate when evidence is observational or experimental in nature, and to emphasize that causality remains uncertain. For example, the revised manuscript states:

“Another suggested potential adverse effect is increased cancer risk (…). Given the observational nature of the evidence, causality remains uncertain, and Mende-lian-randomization analyses have not shown consistent effects across studies [112,113]” (Lines 350-352)

And

“Finally, several observational reports link higher circulating B₁₂ to increased all-cause mortality (…) However, reverse causality is possible as elevated circulating B₁₂ may also reflect underlying illness, rather than direct toxicity” (line XX-XX)

Finally, we conclude the section with the following statement:

“These limitations must be considered, especially given the observational nature of these studies.” (Lines 368-370)

Comment #5:  The conclusions are generally cautious and consistent with the data; however, statements suggesting the need to “reassess the widespread use of high doses” could be interpreted as a health policy recommendation. A clearer distinction between the general population and high-risk groups, for whom supplementation remains unquestionably justified, would be useful.

Response: In response to the Reviewer’s comment, we have made several modifications to the manuscript to avoid language that could be interpreted as a health policy or guideline-level recommendation (see comment #2).

Furthermore, we have revised the referenced lines in the Abstract to further temper the wording and to clarify that our conclusions pertain to individuals not belonging to high-risk groups. The revised sentence now reads:

“Overall, our findings suggest that widespread B₁₂ use in individuals not belonging to high-risk groups warrants re-evaluation” (Lines 42-43)

Comment #6:  Strengths: a current and important topic, a very broad review of the literature, a reliable emphasis on the lack of benefits of supplementation beyond deficiency, the courage to raise the issue of potential adverse effects.

Weaknesses:  lack of systematic rigor, at times overinterpretation of observational data, insufficient separation of biological hypotheses from clinical evidence, risk of oversimplification in the message for clinical practice.

Response: We thank the reviewer for recognizing the strengths of our work. We hope that the revisions made in response to the Reviewer’s suggestions have adequately addressed the identified weaknesses.

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript treats the assigned topic in a very general manner. The level of substantive content is too low for Nutriens in its current form. However, if the main sections of this manuscript were expanded (especially points 5 and 6), the prospects are promising.

My suggestions:
Expand points 5, 6, and 7—these are the main sections of this manuscript. Provide more details about the cited studies. Add diagrams and tables.

Author Response

We thank the reviewer for the feedback. Please see the attachment for a detailed response. The line numbers refer to the clean version of the revised manuscript.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

This review is based on version 3 of the manuscript.

While I understand this was not a systematic review and instead a narrative review, it still begs the question of why the authors didn’t follow standardized guidelines for conducting the review (e.g., SANRA).  This really would have strengthened the work and helped to follow a transparent and structured practice to help avoid bias. Perhaps it is not too late to go back and try and try to adhere to some sort of guidelines.

Line 65-66: The NIH dietary supplement fact sheets are not to be considered guidelines. See Disclaimer below each Fact Sheet:

Disclaimer

This fact sheet by the National Institutes of Health (NIH) Office of Dietary Supplements (ODS) provides information that should not take the place of medical advice. We encourage you to talk to your health care providers (doctor, registered dietitian, pharmacist, etc.) about your interest in, questions about, or use of dietary supplements and what may be best for your overall health. Any mention in this publication of a specific product or service, or recommendation from an organization or professional society, does not represent an endorsement by ODS of that product, service, or expert advice.

Line 227-229: Is there any research to cite for this statement?

Line 342-343: This statement is very strong considering that this review did not follow any systematic method – “Observational cohorts and registry studies repeatedly associate elevated plasma B₁₂ with incident cancer within 1–12 months [106–108]”. The authors should describe the studies and be careful with statements that may generalize the full literature (which was not reviewed).  

Line 378: One of the primary purposes of a narrative review is to identify knowledge gaps and methodological challenges. This section would benefit from greater detail and a more comprehensive synthesis that integrates findings across the different sections of the paper.

Is reference 9 cited correctly?

Author Response

We thank the reviewer for the feedback. Please see the attachment for a detailed response. The line numbers refer to the clean version of the revised manuscript.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

I would like to thank the authors for their contribution to improving this manuscript. I have one comment: Figure 1 was generated using AI. Was this reported to the editors, and was the data contained therein thoroughly verified? Furthermore, it contains certain conclusions, problems, and guidelines that stem from some source data. Therefore, the source of the data that formed the basis for this graphic should be indicated.

Author Response

We thank the Reviewer for the suggestion. Please find a detailed response in the attachment.

Author Response File: Author Response.pdf