What a baby eats

A long time ago, I learned in school about the human digestive system. I learned how the food I ate, composed of proteins, carbohydrates, and fat, would be broken into less complex structures: amino acids, peptides, simple sugars, and fatty acids, by the enzymes present in my digestive tract. Those simpler structures would then be absorbed and used by my cells for energy and building material. It was exciting. As I came to know as I got older, that was a very simplified version of the dynamics for what was going on inside my gut.

Many complex interactions happen inside a persons’ gut. It is estimated that the human body contains ten times more cells from microorganisms than human cells, which make up to 1 to 3% of the total body’s mass of a person (1). A good portion of those microbes resides inside the gut. Our cells constantly interact with the microbes in the gut, and vice-versa (2) and more and more researchers are finding links between the gut microbiome, health conditions and diseases such as autism (3,4,5), diabetes (6), irritable bowel syndrome (7), and obesity (8,9,10).

Here, I will go back in time in each of our life timelines, and discuss the complexity of the gut and microbiome for the specific case of babies. The first food babies have access to is human breast milk. So, what does the baby eat when is fed in such a way?

Researchers have known for decades that, besides fat, proteins, lactose and minerals, which provide nutrients and energy for the baby’s growth and development, human breast milk contains a variety of oligosaccharides, some of which were described as early as the 1950s (11). The human breast milk has a rich mixture of complex oligosaccharides, with higher content and more complicated structures compared to other mammals. Over 200 different oligosaccharides have been identified in human breast milk (12). Those oligosaccharides are not used directly by the babies: most pass through the stomach and upper intestine intact, and less than 1% is absorbed by the baby’s intestines (12). So why, after millions of years of evolution, the mother’s milk contains such a variety of these compounds? Part of the answer is that the microbes present in the baby’s gut (the gut microbiome, previously referred to as gut flora) feed on such compounds. Oligosaccharides act as natural prebiotics for specific genera of beneficial bacteria, which, as a result, are enriched in the baby’s gut, also protecting against the pathogenic ones (12).

Researchers believe that breast milk contains microbes as well, which are actively being fed to the baby, making their way to the gut. The microbes in the milk can originate from “cross-contamination” from the mother’s skin or the baby’s mouth (13). There are even lines of evidence which indicate that the mother’s gut microbes could make their way internally through the mother to the mammalian glands and the milk through translocation (13), but the subject is still controversial.

Besides, the human breast milk contains several bioactive proteins such as antibodies, cytokines, growth factors and lactoferrin, which protect the baby against harmful infections, and hormones such as leptin and adinopectine (14, 15).

In summary, breastfed babies are not only actively fed nutrients and protective elements for themselves, but their diet also includes the microbes that will colonize their guts, and the food specially designed for these bacteria. Breast milk is a complex food source, optimized through years of evolution, to deliver much more than nutrients to the baby. Even with recent and constant development, the industry is yet to develop a substitute as rich and complex as the one provided by nature.


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  10. Turnbaugh, et al. (2017) Diet-Induced Obesity Is Linked to Marked but Reversible Alterations in the Mouse Distal Gut Microbiome, 21, 3, 278-281. https://www.sciencedirect.com/science/article/pii/S1931312808000899
  11. Malpress and Hytten. (1957) The oligosaccharides of human milk. Biochem J. 68(4): 708–717. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1200422/#
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  15. Amoros, et al. (2019) Reviewing the evidence on breast milk composition and immunological outcomes. Nutrition Reviews, nuz019, https://doi.org/10.1093/nutrit/nuz019