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  • A systematic review of the literature was performed in August 22, 2016 through Ovid (https://ovidsp.tx.ovid.com/), which allows simultaneously searching of the following databases: MEDLINE, Embase, Allied and Complementary Medicine Database, CAB ABSTRACTS, PsycINFO®, and The Philosopher's Index. By default, Ovid searches the following fields (some of which are database-specific) when all of its databases are searched: Title, Original Title, Title Comment, Abstract, Subject Heading Word, MeSH Subject Headings, Keyword Heading, Keyword Heading Word, Key Concepts, Full Text, Cited Reference Author Word and others. The following search terms were used for breastfeeding: “breastfe$” OR “breast fe$” OR “bottle fe$” OR “formula fe$” OR “infant feeding” OR “human milk” OR “breast milk” OR “formula milk” OR “weaning”. For epigenetics, the search terms were: “epigenetic$” OR “epigenom$” OR “methylat$” OR “methQTL” OR “mQTL”. Using the wildcard character “$” retrieves any number (including zero) of characters after the stem word (e.g., “breastfe$” retrieves “breastfeeding”, “breastfed”, etc). The two group of search terms were combined using the AND operator: “Breastfeeding” AND “Epigenetics”. Study selection and data collection: The aim of our review was to identify studies on DNA methylation differences associated with breastfeeding. Studies were excluded if they met at least one of the following criteria: i) not reporting effects of breastfeeding on DNA methylation (e.g., studies of epigenetic determinants of breastfeeding, such as the association between methylation in promoters of genes involved in breast milk production); ii) being limited to specific breast milk components rather than breastfeeding or breast milk as a whole; iii) not reporting original data. Eligibility was assessed independently by two reviewers (F.P.H. and C.L.M. ), and disagreements were resolved by consensus. Initially, duplicate records were excluded, titles screened and abstracts reviewed. For the remaining studies, full-texts were examined. The following data were extracted from the included studies: First author’s name and publication year.Country where the study was conducted.Study aim and design.Species, number of individuals, % of females and age.Methylation region, DNA source, measurement method and outcome (e.g., proportion of methylated cells).Breastfeeding categorisation (e.g., never vs. ever, duration in months, etc) and age at ascertainment.Covariates.Breastfeeding-methylation association results. Given the lack of consistency between the designs and methods among the studies (as described below), we opted for a narrative review rather than attempting to perform a meta-analysis.
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