**Background:** Human milk plays a critical role in seeding the infant gut microbiome, with up to 88% of genera shared between milk and infant stool. The intestinal microbiome is essential for immune development and is linked to lower risks of asthma, allergies, and autoimmune disease. While multiple factors influence milk microbiome composition—including lactation stage, mode of feeding, gestational age, and geography—maternal diet is of particular interest because it is modifiable and perceived as a way lactating parents can contribute to infant health. This scoping review aimed to identify known associations between maternal diet (during pregnancy or lactation) and the breast milk microbiome and/or the breastfed infant gut microbiome.
**Methods:** The review followed the PRISMA-ScR checklist. Three databases (Web of Science, PubMed, Scopus) were searched on 29 September 2021 using a comprehensive search string linking maternal diet, breastfeeding/human milk, infant/neonate, and microbiome/microbiota terms. Inclusion criteria were journal articles published in English with no date restriction; review articles, books, and letters were excluded. Studies examining only maternal supplement use (without food-based dietary assessment) were excluded. Two authors independently screened titles/abstracts and full texts. From 949 initial records, 808 remained after deduplication, 696 were excluded at screening, and 112 underwent full-text assessment, yielding 19 eligible publications.
**Key Results:** The 19 included studies comprised 14 cohort studies, 4 RCTs, and 1 case-control study, published between 2015 and 2021 and conducted across Canada, the US, Israel, the UK, China, Spain, Taiwan, Tanzania, and Brazil. Twelve studies collected milk samples, ten collected infant stool, and only three collected both. The most common microbiome assessment method was 16S rRNA gene sequencing. Dietary intake was assessed primarily via food frequency questionnaires (9 studies), with others using 24-hour recalls, 3-day dietary records, or controlled interventions.
Regarding the milk microbiome (10 studies): One study (Bisanz et al.) found probiotic yogurt did not alter the milk microbiome. Babakobi et al. reported a negative correlation between maternal unsaturated fat and folic acid intake and Streptococcus in milk. Williams et al. found Lactobacillus abundance negatively associated with thiamin, niacin, folate, vitamin B-6, and chromium, while Proteobacteria was positively associated with nutrient-rich diets. LeMay-Nedjelski et al. found fiber from grains positively correlated with Acinetobacter, while trans fats were positively associated with Staphylococcus and Gemella. Padilha et al. found polyunsaturated/linoleic fatty acid intake showed a slight positive correlation with Bifidobacterium in milk. Two studies (Seferovic et al., Moossavi et al.) suggested maternal diet may indirectly influence the milk microbiome by altering HMOs and other milk components.
Regarding the infant gut microbiome (11 studies): Eight studies identified maternal dietary factors associated with infant gut taxa. Urwin et al. found lower Atopobium in infants whose mothers consumed salmon weekly during pregnancy. Lundgren et al. reported maternal seafood consumption positively associated with Streptococcus and negatively with Clostridium neonatale in infants. Bisanz et al. found maternal probiotic yogurt consumption associated with increased Bifidobacterium and decreased Enterobacteriaceae in infant stool. Laforest-Lapointe et al. found maternal artificially sweetened beverage consumption during pregnancy depleted Bacteroides and increased Prevotella copri in the infant gut, and was positively associated with infant BMI at one year. Chu et al. found maternal high-fat diet during gestation significantly associated with increased Enterococcus and depleted Bacteroides in infant meconium, with the Bacteroides depletion persisting at 6 weeks. Three studies (Babakobi et al., Savage et al., Cortés-Martín et al.) reported no significant effect of maternal diet on the infant gut microbiome.
**Clinical Implications:** The reviewed literature supports that maternal diet can impact both the milk and infant gut microbiomes, though the extent to which maternal diet influences the infant microbiome through microbial changes in milk remains undetermined. Several studies found factors other than maternal diet—particularly mode of delivery and infant feeding method—to have a greater impact on the infant microbiome. The depletion of Bacteroides associated with both high-fat diet and artificially sweetened beverage consumption is notable given this genus's importance for metabolizing complex polysaccharides. The review highlights critical research gaps: only 2 of 19 studies assessed both milk and infant microbiomes, sample sizes were small (11 studies had <100 participants), dietary assessments often excluded supplements, and no studies accounted for host cells in milk that may dilute bacterial DNA. Future research should collect both milk and infant stool samples, include larger sample sizes, and incorporate supplement intake in dietary assessments.