The two most abundant biomolecules (molecules produced by a living organism) in breast milk are proteins and lipids, which babies digest and convert to energy. The third most abundant biomolecule is something the human baby lacks the enzymes necessary to digest. In other words, it goes in their mouths and out into their diapers with no digestion along the way.

Turns out, the indigestible matter is a slew of complex sugars called oligosaccharides that are extremely difficult to detect and analyze.

What do the oligosaccharides do? Researchers theorized that they fed bacteria in the baby's gut since they didn't nourish the baby, but what strain of bugs do they feed and why?

That puzzle was solved by professor David Mills, the UC Davis Peter J. Shields Endowed Chair in Dairy Food Science. Mills pinpointed one particular gut bacterium called Bifidobacterium infantis, which is uniquely able to break down and feed on the specific oligosaccharides in
mother's milk.

Lactating mothers produce oligosaccharides to help B. infantis proliferate and dominate in the baby's gut, keeping their babies healthy by crowding out less savory bugs before they can become established. Perhaps more importantly, the oligosaccharides help B. infantis nurture the integrity of the lining of the babies' intestines, playing a vital role in protecting them from infection and inflammation.

Researchers believe providing oligosaccharides and specific bacteria could also be used to treat gastrointestinal disease in adults, restoring microbial balance in their digestive tracts. Similar treatments could soon be used to boost the immune defenses of people with compromised immune systems, such as people with the human immunodeficiency virus, patients undergoing chemotherapy, the elderly and others.

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