Perspectives and factors associated with informal human milk sharing: a mixed-methods systematic review protocol

Background The practice of informal human milk sharing is a relatively new phenomenon and poses significant questions in the domain of infant feeding. Informal human milk sharing is a means of donating human milk from another lactating individual who is not the child’s biological parent, in a casual manner, that is, without the involvement of health professionals or agencies. The advent of digital technology has facilitated the donation and receipt of human milk through digital online platforms and thus has amplified this modern practice. This research aims to comprehensively examine and synthesize evidence about the motivations, barriers, facilitators and experiences of individuals who both donate (donors) and the primary care givers of the infants who receive (recipients) human milk informally to provide to infants. Methods A mixed-methods systematic review will be undertaken. This review will consider qualitative, quantitative and primary mixed-methods studies which report on the factors associated with informal milk sharing, and on donors’ and recipients’ experiences of the practice. Primary mixed-method studies will be included if the individual qualitative and quantitative components can be extracted. Five databases will be searched for studies on informal human milk sharing published from inception of the database. Study quality will be evaluated using the standardized JBI critical appraisal tools, selected based on the methodology in each individual study. Data extraction will be conducted using the JBI mixed methods data extraction form followed by data transformation, synthesis and integration. This mixed-methods systematic review will follow a convergent integrated approach in accordance with JBI guidance. Discussion Informal human milk sharing is a novel practice in the domain of infant feeding. This review will enable a thorough understanding of this practice from both the donors and recipients’ perspective and will have implications for healthcare professionals, policy and future clinical decision-making. Protocol registration number PROSPERO CRD42023405653


Introduction
Breastfeeding is one of the most beneficial public health endeavours to optimize child health and survival (North et al., 2022).The World Health Organization advocates the early initiation of breastfeeding at birth, exclusive breastfeeding until 6 months and the continuation of breastfeeding along with complementary foods until 2 years of age and beyond (World Health Organization, 2021).Breastfeeding is an integral component of many of the Sustainable Development Goals (SDGs) which are espoused by all member states of the United Nations (UN).The SDGs comprise 17 interlinked global goals intended to create a healthier and more sustainable world.Breastfeeding is crucial to achieving many of the SDGs which aspire to end poverty, protect the planet and ensure peace and prosperity for all (Holden et al., 2017;United Nations, 2022).Breastfeeding has numerous recognised benefits which span individual, societal, environmental and economical domains.Human milk itself is a complex, individual-specific bodily fluid which is dynamic.It is characterised by vast individual variability pertaining to nutritional composition and bioactive constituents (Mosca & Giannì, 2017).Correspondingly, scientific research has confirmed that human milk is the perfect, unparalleled source of nutrition for infants (Lessen & Kavanagh, 2015).Therefore, the need to position breastfeeding as a global public health priority is acknowledged.Globally, only 44% of infants aged 0-6 months are exclusively breastfeed (World Health Organization, 2021), which is suboptimal.Notably, there is often a disparity between the intention to breastfeed and the actuality as often the barriers to breastfeeding transcend beyond individuals and encompass social, cultural and political tiers (Brown, 2017).
It is widely acknowledged that there is no one universally accepted definition or explicit meaning of the term 'breastfeeding'.Breastfeeding is a complex process and comprises a multitude of meanings and dimensions.Notably, there is a lack of accuracy and consistency in uniformly defining breastfeeding (Labbok & Krasovec, 1990).However, the term 'breastfeeding' is typically recognised as being direct feeding of an infant at the breast (Rosenbaum, 2022).Nevertheless, this term can also pertain to a variety of other practices which can include: combination feeding which is also recognised as non-exclusive feeding where an infant receives both breastmilk and a commercial milk formula (CMF), and exclusive breast milk pumping whereby the infant receives only breastmilk expressed from their mother (Rasmussen et al., 2017).More than three and a half decades have elapsed since it was acknowledged that the term 'breastfeeding' alone was inadequate to precisely depict the multitude of breastfeeding behaviours that are recognized within the domain of 'breastfeeding' (Labbok & Krasovec, 1990).In the early 1990's, The Interagency Group for Action on Breastfeeding devised specific categories where standardised and consistent terminology relating to outlining and categorising breastfeeding (Labbok & Krasovec, 1990).More recently, the development of a widely accessible online lactation glossary to support consistent research and care provision in the domain of lactation has been devised (Boss et al., 2020).
When breastfeeding is unsuccessful or cannot be undertaken, parents may choose to exclusively pump or provide CMFs (Rosenbaum, 2022).Some infants in paediatric or neonatal intensive care units (PICU/ NICU) may receive donated human milk which is medically prescribed and provided by a milk bank when a mother's own milk (MOM) is not available.Human milk banks (HMB) play a fundamental role in the provision of donor human milk (DHM) to infants who cannot otherwise receive MOM (Haiden & Ziegler, 2016).DHM is predominately reserved for infants born preterm where it has been widely affirmed that DHM as a substitute for MOM has instrumental benefits including improving clinical outcomes and reduction in the two central causes of mortality in this vulnerable cohort, namely sepsis and necrotising enterocolitis (Cortez et al., 2018;Miller et al., 2018;Patel & Kim, 2018;Tran et al., 2020).Human milk (HM) is beneficial for the health of infants as clinical practices have demonstrated that numerous beneficial components are preserved following pasteurization of the human milk (Peila et al., 2016).What is more, the provision of human milk to infants reduces the risks associated with CMF feeding such as otitis media, asthma, types 1 and 2 diabetes, atopic dermatitis, and infant hospitalization secondary to lower respiratory tract diseases (McNiel et al., 2010;Victora et al., 2016).There is global agreement that MOM is the preferred source of nutrition for infants and if not available pasteurized donor human milk (DHM) from an established milk bank is the preferable choice (Committee on nutrition et al., 2017;ESPGHAN Committee on Nutrition et al., 2013;Pound et al., 2020;World Health Organization, 2011).A novel means of sharing expressed milk which is emerging in recent times is the practice of informal human milk sharing (IHMS) which comprises receiving DHM via informal means without the use of a milk bank (Rosenbaum, 2022).
The practice of human milk sharing is not a new entity and has origins in antiquity.Accordingly, this practice has evolved

Amendments from Version 1
We would like to thank Dr O'Sullivan, Ms Drouin, Prof Gonzalez de Oliveira and Dr Abdulahi sincerely for their invaluable feedback on the protocol for our mixed-methods systematic review.Their expertise and thorough review have greatly contributed to the enhancement of this protocol and we are appreciative of the time and effort afforded by them to this process.In the methods section, we have modified this to reflect that studies that explore milk banks, wet nursing, milk sharing for profit alone, will not be included, however studies which include both may be included.In the aim and review questions, we have amended the 'perspectives' question to be more specific.The term 'perspectives' has been modified to ' experiences of'.We have added the justification for incorporating the JBI methodological guidance for mixed-methods systematic reviews.We have provided a rationale for searching three databases at the preliminary search and have now expanded this to five databases for the second systematic search.We have chosen the terms 'recipient' and ' donor' in the manuscript and have removed variations which were presented in the original manuscript such as 'receiver'.We have incorporated all the useful suggestions within your reviewer report including adding citations, abbreviations and terminology appropriate to the study protocol.
Any further responses from the reviewers can be found at the end of the article from an alternative feeding practice for 'need', whereby this practice was essential for infant survival to contemporary times where substitutes are available, such as CMFs and donated non-maternal human milk (Stevens et al., 2009.The sharing of milk derived from the practice of 'wet nursing' whereby infants were breastfed by a lactating woman who was not the biological mother of the infant (Stevens et al., 2009.This ancient practice and alternative means of feeding has evolved considerably from being a necessity (2000BC) for infant survival, to a substitute of choice (950BC to 1800AD) (Stevens et al., 2009).This phenomenon of human milk sharing has progressed further and diffused from the practice of wet nursing to the sharing and receiving of pumped breastmilk via milk banking, and further again to involve the practice of informal, 'peer-to-peer' human milk sharing (Wagg et al., 2022).The interchangeable terms for human milk sharing are noted as 'peer to peer milk sharing' and 'informal human milk sharing' as this practice is dependent on the donors and recipients to collaborate and agree on the specific terms of the exchange (Palmquist et al., 2019).Human milk sharing is described in the literature as the voluntary, altruistic and commerce-free trading of human milk for infant feeding, outside of the milk banking system (Peregoy et al., 2022).In recent years, 'for-profit' human milk banks have been established (UNICEF, 2021).These private entities operate a business where breastmilk is purchased from a lactating woman and then processed and sold to hospitals (UNICEF, 2021).Additionally, various online platforms and internet sites facilitate the commercial sale of human milk by individuals who are expressing milk and can advertise specific details such as images and the cost of human milk per volume (Steele et al., 2015).
The advent of digital technology and the advancement of social media has had an explicit influence on informal milk sharing among people in online communities, where more information relating to this practice has become available in recent years (Akma Jamil et al., 2021).Milk sharing websites are actively used platforms for the exchange of expressed human milk (Keim et al., 2013).Additionally, parallel advancements in relation to refrigeration and high-quality, double electric breast pumps have also facilitated the increased impetus in this modern practice (Peregoy et al., 2022).Consequently, over the past decade, research into the practice of online breastmilk sharing has become more prevalent corresponding with the advancement in digital and online networks, internet enabled smartphones and social networking apps (Akma Jamil et al., 2021;Noyes, 2020).
In recent years, there has been an increasing number of studies exploring various areas relating to the practice of IHMS, but at present, there is no existing or ongoing mixedmethod or individual systematic review on the subject.Preliminary searches of CINAHL (EBSCOhost), Scopus (Elsevier), and MEDLINE have been undertaken and no systematic reviews on the topic were identified.There have been two recently published scoping reviews which have mapped the existing literature on the practice of IHMS in the United States of America and globally, respectively (Akma Jamil et al., 2021;Kullmann et al., 2022).These prominent reviews in the field of IHMS have provided the impetus to enrich the breadth and depth of understanding of this contemporary practice.Thus, there is a justification for undertaking a mixed methods systematic review (MMSR) on this subject to efficiently integrate the existing evidence with the intention of providing explicit information including whether findings are consistent within the literature and can be generalized across settings and populations, as well as informing healthcare decision making in the future (Büchter et al., 2020) (Mulrow, 1994).This MMSR will provide a comprehensive and methodological synthesis of the broader topic of IHMS and more specifically include the experiences of donors and recipients and the factors associated with the practice.This review may also guide future research on the topic of IHMS.

Study aim and review questions
A mixed-methods systematic review (MMSR) is proposed to evaluate the quantitative, qualitative and primary mixed-method studies pertaining to the practice of informal human milk sharing.The following questions aligned to the review question are: • What are the factors (motivations, barriers and facilitators) associated with the practice of IHMS?
• What are the experiences of recipients and donors IHMS?
The findings of this review will be of value to researchers, healthcare professionals and policy makers.It may develop a richer understanding of the factors which are associated with this practice and it will enrich our understanding of the subjective experiences of both donors and recipients of IHMS.

Inclusion criteria
This MMSR protocol has incorporated a PICo framework (Population, phenomenon of Interest, COntext) as recommended by JBI for (MMSR) (Stern et al., 2020), and this is visually represented below.The PICo framework (Table 1) facilitated the identification of the defining characteristics for the inclusion criteria for the MMSR.

Population:
The review will consider studies that include donors and/or recipients of IHMS.While the recipient of the human milk will be an infant, for the purposes of this MMSR the term recipient refers to the individual who sourced the human milk for the infant and in most studies this will be a mother, father, primary caregiver, or biological/ non biological parent.Studies which include donors and/or recipients of human milk via milk banks alone will not be included.Additionally, studies which only incorporate other means of milk sharing, specifically: formal and or/informal milk sharing with financial compensation will not be included as well as studies which include other means of milk sharing such as wet nursing.If studies contain IHMS and other means of milk sharing as outlined above, they will be included, only if data relating to IHMS is presented separately and/or can be extracted independently.Studies that look at clinical outcomes of infants who are provided with human milk either from milk banks or by an informal means will not be included.Additionally, research which documents an exploration of experiences from individuals other than recipients or donors of human milk will not be included.

Phenomena of interest:
This review will consider studies which report on the concept of IHMS, specifically on donors and recipients of human milk via informal means and the factors associated with this practice, including the motivations, facilitators, barriers and experiences of this practice.
Context: This review will consider studies from any geographic location globally.
Type of studies: This MMSR will consider qualitative, quantitative, and primary mixed-methods study designs for inclusion.Primary mixed-methods studies will only be taken into consideration if it is possible to explicitly extract the data from the quantitative or qualitative components (Lizarondo et al., 2022).The justification for this is founded on the JBI methodological guidance for MMSR which proposes that the disaggregation of quantitative data will need to be completed in primary-mixed methods studies (Stern et al., 2020).In this instance, the disaggregated quantitative data will then be qualitized in order for the integration to occur.If primarymixed methods studies are presented already in an integrated format and disaggregation cannot be conducted, these studies will be excluded.Included studies will comprise peer-reviewed, published, full text articles and grey literature sources.This MMSR will consider all languages and year of publication.The reference lists of articles attained will be manually searched for additional, pertinent papers.
The justification for conducting a MMSR is to develop contextual understanding of the topic and to enhance credibility through the amalgamation of diverse categories of evidence (Hong et al., 2018).Additionally, conducting a MMSR will enable the enhancement, depth and richness of comprehension to facilitate an inclusive understanding of the evidence (Hong et al., 2018).It will enable a comprehensive insight into the practice as well as ascertain if the findings and evidence from the literature confirm, supplement or contradict one another, thus facilitating a rich and broad perspective on the subject of IHMS from a global perspective.
This proposed MMSR will be conducted in accordance with the JBI methodology guidance for conducting MMSRs as outlined in Chapter 8: Mixed methods systematic reviews in the JBI Manual for Evidence Synthesis (Aromataris & Munn, 2020).MMSRs are widely regarded as the most complex and least developed of all systematic reviews (Stern et al., 2021).
The recently updated JBI methodology guidance serves as a crucial resource in this continually evolving methodology (Stern et al., 2021).Thoroughly documenting and extensively disclosing the methods utilized in the synthesis is an essential necessity and a defining characteristic of a well-conducted systematic review (Stern et al., 2021).Therefore, the JBI methodological guidance has been selected as a means of enabling a rigorous and methodical approach to conducting this imminent MMSR.The protocol has been submitted and accepted for registration on International Prospective Register of Systematic Review PROSPERO CRD42023405653.This MMSR will employ a convergent integrated approach as articulated in JBI guidance which includes data transformation and enables reviewers to merge quantitative and qualitative data (Stern et al., 2020).This is the preferable approach, as the review question can be addressed by both research paradigms.This MMSR will follow the recommendations from JBI for reviewers following a convergent integrated approach which is the qualitisation of quantitative data, where quantitative data will be transformed and converted into textual descriptions (Aromataris & Munn, 2020).This will enable the converted data to be compiled together and thus facilitate integration (Stern et al., 2020).The mixing of data by the means of integration is a distinguishing feature of a MMSR and the appropriate integration is an instrumental defining characteristic of this type of systematic review (Stern et al., 2020).

Search strategy
A comprehensive search strategy will aim to discover both published and unpublished studies.A three-phase process will be utilized for the search strategy as recommended by JBI.First, a preliminary search of CINAHL (EBSCOhost), Scopus (Elsevier), and MEDLINE will beundertaken incorporating key terms The JBI methodological guidance recommends an initial search of at least two initial databases in this preliminary step (Lizarondo et al., 2020).These three databases were selected due to their dependability and availability across an extensive range of subject areas.Following this, an analysis of keywords/terms in the titles, abstract and index terms used to describe the articles obtained in initial preliminary search will enable a more comprehensive search strategy to be developed.The following databases will be searched from inception to date: CINAHL (EBSCOhost), Scopus (Elsevier), MEDLINE, Embase (OvidSP) and Web of Science.Subsequently, the reference lists for all studies selected will be manually examined for additional studies which may be included.Boolean operators and controlled vocabulary terms will be used as required for each database search.An additional search of grey literature will be completed based on the key terms used for the database search.This will include searching Google Scholar, Open Grey, and Dissertation Abstracts International.It has been acknowledged that the inclusion of grey literature is vital to ensure a fully comprehensive systematic review.

Study selection
Following the search, all identified citations will be loaded into Zotero Citation Management System version 6/2022.These will be imported to Covidence.JBI recommends the incorporation of Covidence (reviewed in the October 2018 issue of the Journal of the Medical Library Association) for the screening process and then uploading the final studies into SUMARI for the continuation of the review process.Titles and abstracts will then be reviewed in Covidence by two independent reviewers for assessment against the inclusion criteria for the review.The full text of selected citations will be assessed in detail against the inclusion criteria by two independent reviewers.Each full text study which is excluded will be documented and reported in the systematic review.
Any discrepancies that develop between the reviewers will be settled through discussions or with the help of a third independent reviewer where this cannot be resolved (Aromataris & Munn, 2020).The results of the search will be reported in full in the final review and presented in a Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) flow diagram (Page et al., 2021).
The contemporary 27-item PRISMA checklist updated in 2020 and published in 2021 will be used (Page et al., 2021).

Assessment of methodological quality
The final studies included will be appraised and evaluated for methodological quality.Quantitative and qualitative studies and the relevant components of mixed-methods studies (qualitative and quantitative components) which are selected for retrieval will be assessed by two independent reviewers for methodological quality using the standardised critical appraisal instruments from JBI SUMARI, dependent on study design (Aromataris & Munn, 2020).Using the appropriate standardized critical appraisal tools from JBI SUMARI, the methodological quality of mixed-methods studies will be evaluated in connection to the qualitative components and quantitative components.If required, relevant authors of papers will be contacted to seek clarification or to request any missing data (Aromataris & Munn, 2020).Any disagreements that may transpire between the reviewers will be resolved through discussion and consultation with a third reviewer.Following the critical appraisal, the results will be documented in descriptive format and visually presented in table format.Data extraction and synthesis will be performed on all studies included in the final review regardless of the results of their methodological quality (Aromataris & Munn, 2020).Grey literature will be appraised and evaluated by using the AACODS checklist (Authority, Accuracy, Coverage, Objectivity, Date, Significance) which was developed specifically for use with grey literature resources (Tyndall, 2010).

Data extraction
Quantitative and qualitative data will be extracted from all studies included in the review by two independent reviewers using the standardised JBI data extraction form for MMSR following a convergent integrated approach (Aromataris & Munn, 2020).Specific information about the populations, research techniques, phenomena of interest, context, and results relevant to the review questions will all be included in the data that is extracted.The following information will also be included in the data extraction process: study design, methods, sampling technique, inclusion and exclusion criteria, study setting, geographic location, and significant findings aligned with the research questions.The quantitative data will be the results of statistical tests undertaken, which will be either descriptive and/or inferential, depending on data (Aromataris & Munn, 2020).The degree of credibility of qualitative data will be determined by how closely the findings match the themes and/or subthemes and the corresponding illustrations (Aromataris & Munn, 2020).JBI guidance recommends three levels of credibility: unequivocal, credible and not supported (Aromataris & Munn, 2020).The foundation for evaluating, exploring, summarising, and elucidating the body of evidence is foremost initiated by data extraction forms, which are the vital link between primary original research and systematic reviews (Büchter et al., 2020).As a result, their creation, trial testing, and use are fundamental steps in the process of conducting systematic reviews.

Data transformation
Following extraction, quantitative data will then be transformed into 'qualitized' data.This process will involve the transformation of quantitative data into textual descriptors or narrative interpretation of the quantitative results, so as to respond directly to the review questions.This process is recommended by JBI as it is less error-prone than assigning numerical values to qualitative data, termed quantitization (Stern et al., 2020).

Data synthesis and integration
This review will follow a convergent integrated approach in accordance with the JBI methodology for MMSRs (Aromataris & Munn, 2020).This will comprise the assembling of the transformed, qualitized data with the qualitative data.Following this, the assembled data are then categorised and combined based on similarities in meaning to create a set of integrated findings in the form of line of action statements.

Study status
This MMSR protocol has been developed and registered on PROSPERO (CRD42023405653).We are currently completing the systematic search which will be followed by screening, data extraction, data synthesis and integration as outlined above.

Discussion
IHMS is a novel practice which is rapidly growing and evolving in recent times.This MMSR will facilitate a comprehensive understanding of this practice from both the donors and recipients of human milk carried out in an informal manner.This MMSR will help us gain a deeper understanding of the factors that contribute to this practice as well as the experiences of both donors and recipients of IHMS.It has been documented that MMSRs can augment the capability of findings and contribute to policy and clinical decision making (Stern et al., 2020).It is anticipated that the findings from this MMSR will be beneficial to healthcare professionals, researchers and policy makers, as well as provide a robust and comprehensive understanding of this practice from various perspectives. sharing.
I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard.

Introduction
Informal human milk sharing is used several times in the texts.Once it is abbreviated in the introduction section, better to use the abbreviated one after that.

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Page3, paragraph 2 -The sentence "Nevertheless, this term can also pertain to a variety of other practices which can include: combination feeding which is also recognised as nonexclusive feeding where an infant receives both breastmilk and a commercial milk formula (CMF), and exclusive breast milk pumping whereby the infant receives only breastmilk expressed from their mother."needs citation.

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Page 3 paragraph 3 -does "MOM" stand for mother's own milk or mother's own human milk?Be consistent.

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Page 3 paragraph 3 -does HM stand for human milk?If so, it has to be written in full when it first appears in the text.
○ WHO appears only once.Thus it is better to avoid the abbreviation.

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Page 3 paragraph 4 -The sentence "Accordingly, this practice has evolved from an alternative feeding practice for 'need', whereby this practice was essential for infant survival to contemporary times where substitutes are available, such as CMFs and donated nonmaternal human milk."needs citation.

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Page 3 paragraph 4 -The sentence "The sharing of milk derived from the practice of 'wet nursing' whereby infants were breastfed by a lactating woman who was not the biological mother of the infant."needs citation.

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Page 3 paragraph 4 -The sentence "This phenomenon of human milk sharing has progressed further and diffused from the practice of wet nursing to the sharing and receiving of pumped breastmilk via milk banking, and further again to involve the practice of informal, 'peer-to-peer' human milk sharing."needs citation.

Mariana González de Oliveira
Neonatal Medicine, University of Health Sciences of Porto Alegre -UFCSPA, Porto Alegre, Rio Grande do Sul, Brazil I greatly appreciate the chance to engage with this thoughtfully written paper, presenting a welltimed and captivating overview.The paper outlines a meticulous protocol for an upcoming mixedmethods systematic review, aimed at comprehensively capturing and consolidating insights into the diverse populations, interests, and contextual aspects surrounding the informal sharing of human milk within existing scholarly works.Undoubtedly, this research subject holds significant promise in enriching our comprehension of this dynamic and enduring phenomenon.
I have some minor observations that may be useful for the authors.

Introduction:
Although breastfeeding terminology may not be consistently used in literature, there are some references to refer to, such as Boss's et al. 1 and CDC's glossary.
In the second sentence, please change "complimentary" to "complementary".
I suggest looking at Cesar Victora's papers which have published pioneer and extensive work on breastfeeding benefits for infants.
I suggest changing the last sentence to "may", instead of "will": it may give an idea that the authors already know the results of a review that is about to be done.The same may apply to the statements under "study aim and review questions".

Methods:
Although the authors have chosen to exclude wet nursing, maybe you should consider including this practice in your revision.
As made very clear, this proposed MMSR will be conducted in accordance with the JBI methodology, please consider justifying why.
In the search strategy, consider explaining why you did not include Embase in your preliminary search.
Consider including "perspectives" about the practice at your phenomena of interest, as you included it as a review question.

Discussion:
The authors bring the term "receivers", but in other places, "recipients" is used.Personally, I consider "receivers" more appropriate, but I suggest you choose one term and be consistent about it.Overall: JBI's guidance and methods appear to be central to the work described in this paper.While this may be somewhat implicit, please consider adding a short description addressing why.

References
Throughout, the authors use variations of the phrases "It has been affirmed", "It is characterized," "It has been stated," "it has been documented", "it has been articulated" when summarizing a cited source.Since it is implicit that a source stated/documented the content for which they are being cited, consider removing these statements for brevity. Introduction: Paragraph 1 Please add citation(s) to support the sentence "Correspondingly, scientific research has confirmed that human milk is the perfect, unparalleled source of nutrition for infants."

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In the second-to-last sentence, consider removing "which is suboptimal given the multiplicity of benefits derived from breastfeeding."This is already stated in other ways elsewhere in the paragraph.

Paragraph 2
The following points are re-stated here multiple times: 1) breastfeeding can have several meanings, 2) there has been a lack of consistency in the use of the term, and 3) using the term to define all types of human milk provision to an infant is inadequate.Consider consolidating the content on these points.

Methods: Search Strategy
The author states three databases (CINAHL, Scopus, and MEDLINE) are used for the preliminary search using key terms.Then, after a more comprehensive search strategy is developed, four databases (CINAHL, Scopus, MEDLINE and Embase) are used.Consider describing why Embase was used in the second search and not the first.

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Page 5, "Study selection" PRISMA is misspelled as "PRIMSA" in the second to last sentence ○ Page 5, "Assessment of methodological quality" Consider the following rewording to the second sentence for clarity: "Relevant components of the quantitative, qualitative and mixed-methods studies selected for retrieval will be assessed by two independent reviewers for methodological quality using standardized critical appraisal instruments from JBI SUMARI, dependent on study design (Aromataris & Munn, 2020)."

Data Extraction
The authors state twice in this paragraph that the form represents the connection between the research studies and the SR.Consider removing one.

Data synthesis and integration
Consider removing "and integrated together" from the last sentence, since integration is mentioned twice here.Thank you for the opportunity to review this very interesting paper.This paper clearly describes a proposed protocol for a mixed-methods systematic review aiming to understand the factors associated with informal milk sharing and the perspectives of those involved in informal milk sharing.This is an understudied topic internationally, and also in the Irish context.The results of the proposed research will have public health and clinical relevance.
I have some minor comments on the paper for the authors that they may find helpful: Throughout the entire paper, consider using an alternative word to "donating" and "donor" when describing informal milk sharing as I believe that most would consider "donating" to be associated with the milk bank.In terms of your review, using both words (and other variations) to search makes sense but in your description of the behaviour, I think distinguishing informal sharing from milk banking is important.Consider "providing" or "provider" perhaps.So, this would mean you have providers and recipients, which distinguishes the practices from formal milk donation; see here ○ ○ for an example: DOI: 10.1177/0890334416651067 1 Also, see here for a discussion around terminology 2 (could be referenced in the 2 nd paragraph of the introduction when the difficulty defining breastfeeding is discussed).

Introduction:
Change complimentary foods to complementary foods.

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The sentence beginning "At this juncture" at the end of the second paragraph of the introduction is a little bit hard to follow, it feels unfinished.Also, I initially read "this juncture" to mean the present day rather than 1990.

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In the third paragraph of the introduction, mother's own milk is spelled out in full and abbreviated twice.Same for donor human milk.

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The abbreviation HM is used mid-way through the third paragraph and is not used anywhere else in the paper.Consider writing human milk here.

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The final sentence of the 3 rd paragraph begins with "In recent years…" and I think that this could be modified to indicate that it is the sharing of expressed milk that is new.The next section that goes on to describe wet nursing then builds on that.

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It would be worth noting that there was an element of choice involved in wet nursing also… the British nobility would hire a wet nurse if the chose not to breastfeed themselves.So, it wasn't solely about a necessity for child survival as might be the case of maternal death.

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In the second paragraph on page 4, it would be worth referencing Sheela Geraghty and Sarah Keim, who have published work about milk sharing via the internet.

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Third paragraph on page 4, IHMS is spelled out in full and abbreviated again even though it is on a previous page.

Methods
In the description of the Population it is stated "Studies which include donors and/or receivers of human milk via milk banks will not be included."I suggest a slight modification to "… via milk banks alone will not be included."That way, you can ○ ○ include studies that may have explored both informal sharing and donating to milk banks in the same study.Same for those discussing purchasing of milk and wet nursing.
Could you expand on the rationale for not including wet nursing?It may be hard to distinguish in some cases.As in, there will be studies that comment on both types of behaviours.

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In the "Phenomena" section, should it state "…recipients of human milk…"?○ Also, in the "Phenomena" section, consider removal of the word "an" before "informal means." ○ First paragraph on page 5, remove the word "of" after the word "comprise." ○ Page 5, paragraph 2, consider removing the s of the word confirms.

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In "Search strategy" the tense changes to past tense when describing the search that was undertaken, which is a bit distracting as the first two sentences are in the future tense.

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Under "Assessment of methodological quality" is there a word missing in the following sentence: "Quantitative and qualitative and the relevant components of mixed-methods studies which are selected for retrieval will be assessed by two independent reviewers…" ○ Please review the following sentence for clarity: "Data extraction and synthesis will be performed on all studies, regardless of the results of their methodological quality results, where feasible." ○ Are the datasets clearly presented in a useable and accessible format?

Not applicable
Competing Interests: No competing interests were disclosed.
Reviewer Expertise: Nutrition, breastfeeding, infant feeding, breast milk expression, breast milk sharing.
I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above.
human milk sharing" be abbreviated as IHMS throughout as was introduced in the introduction?○ Here, and throughout, consider being more specific when you say "perspectives" of those involved in human milk sharing.Could you specify what exactly you are interested in exploring?○ ○
Population: donors and recipients of informal human milk sharingInterest: motivations, barriers, facilitators, experiences of of informal milk sharing Context: globally

Is the rationale for, and objectives of, the study clearly described? Yes Is the study design appropriate for the research question? Yes Are sufficient details of the methods provided to allow replication by others? Yes Are the datasets clearly presented in a useable and accessible format? Not applicable Competing Interests:
No competing interests were disclosed.

the rationale for, and objectives of, the study clearly described? Yes Is the study design appropriate for the research question? Yes Are sufficient details of the methods provided to allow replication by others? Yes Are the datasets clearly presented in a useable and accessible format?
○Discussion:In the first sentence, it is unclear what "momentum" means.Authors may consider replacing "gaining momentum in recent times" with "rapidly growing and evolving" if that is what is meant here.○Is

have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard.
Reviewer Report 26 May 2023 https://doi.org/10.21956/hrbopenres.15005.r33659Elizabeth J. O'Sullivan School of Biological and Health Sciences, Technological University Dublin, Dublin, Leinster, Ireland