Milk mineral composition and not maternal dietary intake  is strongly associated with the human milk microbiome
Associations between maternal mineral intake and milk mineral concentrations and their interactions with the milk microbiota remain understudied, especially in low-and middle-income countries. To understand potential interactions and obtain insight into the dynamics of milk composition, we explored associations of milk mineral concentrations with maternal mineral intake and human milk microbiome in an indigenous community of Guatemala.Methods: In this cross-sectional study, milk samples were collected from 77 Mam-Mayan mothers and classified into early and established lactation. Concentrations of 9 milk minerals were analyzed, maternal dietary intake was obtained from 2 non-consecutive 24 hr recalls at early or established lactation. Milk microbiome diversity was obtained by 16S rRNA gene amplicon sequencing of the V5-V6 region.Results: Maternal mineral intakes of Ca, Fe, Mn, K and Se were deficient throughout lactation, as was Cu in established lactation. Correlations between maternal mineral intake and milk mineral concentrations throughout lactation were negative and weak. Importantly, milk mineral composition was closely linked to the milk microbiota and had a less strong connection to the maternal diet. Bifidobacteria and Lactobacillus were not associated with any milk or maternal mineral intake at either stage of lactation. At the phyla level, Bacillota was similarly present in early and established lactation. Early lactation was dominated by Pseudomonadota, while Actinomycetota, Bacteriodota, and Fusobacteriota dominated at established lactation.Conclusion: Our findings reveal a complex and dynamic interplay between milk minerals and the milk microbiome, which seems to operate independently of maternal diet. Specific minerals may or may not be associated with the microbiome depending on the lactation stage.