As a readily available substance and primary source of nutrition for infants, breast milk serves as a reasonable window into adaptive human physiology and human growth differences in physiologically stressed environments. Prior studies in Tibet show a considerable buffering of breast milk composition at high altitude but imply that economic status and cultural differences in breastfeeding practices influence milkfat content significantly. With an elevation of over 4000 meters above sea level, relatively low levels of socioeconomic status, and variability in women’s schedules resulting in high variability of breastfeeding patterns, Nunoa, Peru provides an excellent location for investigating altitudinal and sociocultural impact on the nutritional value of breast milk. A mid-feed, self-expressed collection of breast milk was completed for 23 mothers. The samples were aliquoted into sterile vials within 30 minutes of collection and then frozen prior to analysis, ensuring maximum preservation of the macromolecules. Participants completed a detailed survey assessing sociodemographic characteristics, diet, and infant feeding practices. For some mothers, observations of breastfeeding behavior occurred, providing additional evidence for feeding variability. Significant variability in breastfeeding patterns and composition was observed. Primarily, in comparison to prior observations in highland Tibet, Nunoan mothers fed more frequently and for shorter bursts, impacting total volume and overall feeding time. Furthermore, Nunoan mothers produced milk with lower lactose levels (6.01 ± .89 g/100mL compared with 7.25 ± 0.35 g/100mL in Tibetan High Nubris). Nevertheless, breast milk fat composition in high-altitude populations of Nunoa (4.56.78 g/100mL) was similar to that observed previously in the Tibetan highlands, and represented higher breast milk fat content than that seen in other comparative milk composition studies at lower altitudes. Our results suggest that individual anatomical indices of lower trunk fat are not the strongest predictors of milk fat levels. Instead, the Nunoan data suggests that relative distribution of anatomical fat is a more reliable predictor of milk fat concentrations. Specifically, maternal waist-to-hip ratio may in fact be a more reliable measurement to predict milk fat rather than the general measurements of adiposity currently used. This indicator provides a new method for predicting breast milk composition and opens the door for future studies of breast milk content in physiologically stressed populations and elsewhere.