A large part of human milk cannot be digested by babies and seems to have a purpose quite different from infant nutrition — that of influencing the composition of the bacteria in the infant’s gut.
The details of this three-way relationship between mother, child and gut microbes are being worked out by three researchers at the University of California, Davis — Bruce German, Carlito Lebrilla and David Mills. They and colleagues have found that a particular strain of bacterium, a subspecies of Bifidobacterium longum, possesses a special suite of genes that enable it to thrive on the indigestible component of milk.
This subspecies is commonly found in the feces of breast-fed infants. It coats the lining of the infant’s intestine, protecting it from noxious bacteria.
Infants presumably acquire the special strain of bifido from their mothers, but strangely, it has not yet been detected in adults. “We’re all wondering where it hides out,” Dr. Mills said.
The indigestible substance that favors the bifido bacterium is a slew of complex sugars derived from lactose, the principal component of milk. The complex sugars consist of a lactose molecule on to which chains of other sugar units have been added. The human genome does not contain the necessary genes to break down the complex sugars, but the bifido subspecies does, the researchers say in a review of their progress in today’s Proceedings of the National Academy of Sciences.
The complex sugars were long thought to have no biological significance, even though they constitute up to 21 percent of milk. Besides promoting growth of the bifido strain, they also serve as decoys for noxious bacteria that might attack the infant’s intestines. The sugars are very similar to those found on the surface of human cells, and are constructed in the breast by the same enzymes. Many toxic bacteria and viruses bind to human cells by docking with the surface sugars. But they will bind to the complex sugars in milk instead. “We think mothers have evolved to let this stuff flush through the infant,” Dr. Mills said.
Dr. German sees milk as “an astonishing product of evolution,” one which has been vigorously shaped by natural selection because it is so critical to the survival of both mother and child. “Everything in milk costs the mother — she is literally dissolving her own tissues to make it,” he said. From the infant’s perspective, it is born into a world full of hostile microbes, with an untrained immune system and lacking the caustic stomach acid which in adults kills most bacteria. Any element in milk that protects the infant will be heavily favored by natural selection.
“We were astonished that milk had so much material that the infant couldn’t digest,” Dr. German said. “Finding that it selectively stimulates the growth of specific bacteria, which are in turn protective of the infant, let us see the genius of the strategy — mothers are recruiting another life-form to baby-sit their baby.”
Dr. German and his colleagues are trying to “deconstruct” milk, on the theory that the fluid has been shaped by 200 million years of mammalian evolution and holds a wealth of information about how best to feed and defend the human body. Though milk itself is designed for infants, its lessons may apply to adults.
The complex sugars, for instance, are evidently a way of influencing the gut microflora, so they might in principle be used to help premature babies, or those born by caesarean, who do not immediately acquire the bifido strain. It has long been thought there was no source of the sugars other than human milk, but they have recently been detected in whey, a waste byproduct of cheesemaking. The three researchers plan to test the complex sugars for benefit in premature infants and in the elderly.
The proteins in milk also have special roles. One, called Alpha-lactalbumin, can attack tumor cells and those infected by viruses by restoring their lost ability to commit cell suicide. The protein, which accumulates when an infant is weaned, is also the signal for the breast to remodel itself back to normal state.
Such findings have made the three researchers keenly aware that every component of milk probably has a special role. “It’s all there for a purpose, though we’re still figuring out what that purpose is,” Dr. Mills said. “So for God’s sake, please breast-feed.”