Prenatal supplementation with fish oil, rich in omega-3 polyunsaturated fatty acids, is widely recommended in Canada. The aim of this observational, prospective cohort study is to determine the effects of maternal fish oil supplements on the development of their infants' gut microbiota and immunity.

During infancy, the gastrointestinal tract is colonized with microbiota, a community of microorganisms, which play a significant role in the development, regulation, and maintenance of immune functions. Fish oil, rich in n-3 polyunsaturated fatty acids (PUFA), are thought to be anti-inflammatory and consequently may increase susceptibility to infection by hindering their body's ability to produce an adequate inflammatory response in defense against infectious disease. Currently, women in Canada are supplementing their diets with n-3 PUFA during gestation and lactation despite their being conflicting clinical evidence regarding the beneficial effects on infant development. It is currently unknown how maternal fish oil supplementation impacts the offspring's gut microbiota and immune functions. Considering that diet influences microbiota and maternal microbiota is passed from mother to infant, the investigators hypothesize that maternal fish oil supplementation will affect the participants' infant gut microbiota and immunity.

This study will evaluate the effects of postnatal n-3 PUFA supplementation in breast milk on the fecal microbiome in infants over a six-month period. The investigators will analyze the fecal microbiome of infants born to mothers in the fish oil and non-fish oil groups via next generation sequencing. As short-chain fatty acids (SCFA) are produced by gut bacteria and affect immunity, the investigators will analyze SCFA in stool through gas chromatography. Markers of inflammation like fecal calprotectin and sIgA in stool samples will also be identified.

Inclusion Criteria:

• All mothers of infants in the Okanagan Valley are invited to participate in the study between the first day of delivery to until solid food is introduced into the infant's diet.
• Healthy, full-term infants, who are predominantly breastfed by mothers who are or are not supplementing with fish oil. Mothers must also be healthy, i.e., asymptomatic and with no clinical indication of disease.
• To remain within the inclusion criteria for this study, participants will be expected to be consistent (i.e. maintain the same type and amount of omega-3 PUFA intake or no intake) with whichever supplementation group they have decided to join for the entirety of the breast milk and infant stool donation period. In other words, the investigators will be asking the participants not to switch from one group to another which would happen by changing their omega-3 PUFA supplementation patterns.

Exclusion Criteria:

• Any infants who are ingesting juice or solid foods as part of their regular diet will be excluded from the study.
• Any infants who are clinically ill (fever, contagious diseases, or active diarrhea) will be excluded from the study.
• Any participants who decide to drastically change their omega-3 PUFA supplementation intake patterns (i.e. causing them to switch between study groups) will result in all subsequent samples to be excluded from the study.

Healthy women in the Okanagan Valley who predominantly breastfeed their healthy, full-term infants while self-administering fish oil supplements or not.

Infant stool microbial data will be made open access upon publishing.