Background/Objectives: HMOs are the third most abundant solid component after lactose and fats in human milk. This study aimed to examine the relationships between maternal diet and HMO composition and concentration in human milk among lactating women in Latvia. Methods: Pooled 24 h human milk samples, 72 h food diaries, and questionnaires on anthropometric and sociodemographic characteristics were collected from 68 exclusively breastfeeding women residing in Latvia. HMOs were analyzed by UHPLC/FLD, and dietary data were analyzed using the Estonian NutriData program. Results: The eight most abundant HMO structures were determined with total concentration ranging between 178.66 and 32,910.09 mg L-1. 2'-FL was the most prevalent HMO in human milk (median concentration-3647 mg L-1), followed by 3'-FL (1436.74 mg L-1). Participants had an insufficient intake of vegetables, fruits, berries, milk and dairy products, and fish, leading to vitamin A, vitamin C, folate, and iodine intakes lower than recommended for lactating women. Limitation or exclusion of milk and dairy products from the diet was associated with a higher 2'-FL concentration in human milk (p = 0.037). Preference for "zero sugar" products was associated with a higher 3'-FL, 6'-GL, LNnT, 6'-SL, LNDFH II concentration in human milk (p < 0.050). Dietary supplement use (e.g., vitamin D, calcium) was also associated with differences in HMO composition and concentration in milk (p < 0.050). Conclusions: The findings highlight the importance of dietary habits and supplement use in shaping HMO profiles, though more human milk samples and dietary data need to be evaluated to draw further conclusions.

Galactooligosaccharides (GOS) are prebiotic carbohydrates that impart changes in the gut bacterial composition of formula-fed infants to more closely resemble that of breast-fed infants. Consuming human milk oligosaccharides (HMOs) provides specific bacterial strains with an advantage for colonizing the infant intestine. These same effects are seen in infants after GOS consumption, however GOS are very complex mixtures and the underlying molecular mechanisms of how GOS mimic HMOs are relatively unknown. Here we studied the effects of GOS utilization on a prominent gut symbiont, Bacteroides thetaiotaomicron, which has been previously shown to consume HMOs via mucin O-glycan degradation pathways. We show that several pathways for targeting O-mucin glycans are activated in B. thetaiotaomicron by GOS, as well as the galactan utilization sytem. Characterization of the endo-galactanase from this system identified activity on various longer GOS substrates while a subset of GOS compounds were identified as potential activators of mucin glycan metabolism in B. thetaiotaomicron. Our results show that GOS functions as an inducer of mucin-glycan pathways while providing a nutrient source in the form of β-(1 → 4)-galactan. These metabolic features of GOS mixtures may serve to explain the beneficial effects that are seen for GOS supplemented infant formula.