Past, Present, and Future of Human Milk Research

Human milk is the best source of infant nutrition and is recognized as a biological fluid vital for optimal growth and development of the infant. Exclusive breastfeeding during the first 6 months of infant life not only provides optimal nutrition but also significant short- and long-term benefits to infant and mother [1]. Sapiens’ milk has coevolved with mammalian species for millions of years which has resulted in this remarkable secretory product of nutrient-rich milk [2].

The nutritional composition and nonnutritive bioactive factors in human milk are uniquely appropriate for the infant, which promote survival and healthy development [3]. Human milk is also comprised of many hundreds to thousands of distinctive bioactive molecules that protect against infection and inflammation and contribute to immune maturation, organ development, and healthy microbial colonization [3].

Research over the past 2 to 3 decades focused on increasing the understanding of the factors that influence human milk composition. The first fluid produced by the mother is colostrum, a fluid rich in immunologic components. Five days to 2 weeks postpartum, transitional milk is produced, while mature milk is produced by about 2 to 4 weeks postpartum. The macronutrient composition in human milk varies within mothers and across lactation [3]. The mean macronutrient composition of mature term milk is estimated to be approximately 0.9–1.2 g/dL for protein, 3.2–3.6 g/dL for lipid, and 6.7–7.8 g/dL for lactose [3]. Another important factor that highly influences the volume and composition of human milk is maternal diet. Several studies highlighted the influence of maternal diet on nutrients in human milk [4]. Geographical location is also contributory to human milk fatty composition and frequently attributed also to the dietary intake of the mother [4]. The composition of human milk also varies depending on the gestational age at infant birth. No differences were observed for lipid and energy content, but lower lactose concentrations were found in preterm compared to term human milk [4]. Preterm milk was higher in true protein than term milk with differences up to 35% in colostrum [5]. Regarding changes in circadian8 variability, lipid and lipolytic enzymes in human milk show a circadian rhythm and peak at midday [4].

Presently, different groups are working on the establishment of reference from mother’s milk in the form of databases that involve cross-continent collaborations using reference and growth standards methodology. Collaborative efforts are ongoing in communicating the clinical outcomes of human milk nutrients to public health. Understanding these health benefits will further improve breastfeeding practices.

Toward the future, continuous efforts should be made in exploring the structure-function relationship of human milk. Cellular agriculture is another exciting field of human milk research in the near future.

References

1. Martin CR, Ling PR, Blackburn GL. Review of infant feeding: key features of breast Milk and infant formula. Nutrients. 2016;8:279.
2. Oftedal OT. The evolution of milk secretion and its ancient origins. Animal. 2012; 6:355–68.
3. Ballard O, Morrow AL. Human milk composition: nutrients and bioactive factors. Pediatr Clin North Am. 2013;60:49–74.
4. Samuel TM, Zhou Q, Guiffrida F, et al. Nutritional and non-nutritional composition of human milk is modulated by maternal, infant and methodological factors. Front Nutr. 2020;7:576133.
5. Gidrewicz DA, Fenton TR. A systematic review and meta-analysis of the nutrient content of preterm and term breast milk. BMC Pediatr. 2014;14:216.