It has long been contended that breastfeeding boosts an newborn's immune system. But I've never quite understood how this is supposed to work. Antibodies are made of protein, so wouldn't any antibodies in the milk just get digested before they were any use to the baby? Actually, newborn stomachs are not actually very acidic, and digestive enzymes are targeted at snipping only particular proteins in particular places. We have also known for a while that breastfed babies have different gut flora from bottlefed babies. Now we know that there is a surprising connection.
Trisha Gura at science mag explains that breastmilk contains lots of oligosaccharides, a type of carbohydrate that is too complex for humans to digest. What's that doing in there? Providing a food supply for baby's 'good' bacteria, it turns out.
A baby's intestine is perfectly sterile at birth, but is
bacteria within hours. The nature of the initial microbial flora is
affected by the mother's diet, stress level, her own bacterial flora,
mode of delivery, and whether she breastfeeds or not. The
guts of babies
delivered by Caesarian section are colonised by bacteria from the
hospital environment (which can't be good). The guts of
vaginally birthed babies quickly pick up lots of Bifidobacteria. Breastfed babies then remain colonised mostly, about 90%, by Bifidobacteria along with lactobacilli.
And these Bifidobacteria not only help baby to digest her food more efficiently, they are also essential to maintaining baby's immune system. This is a beautiful story about how humans have evolved a symbiotic relationship with a strain of bacteria - B. longum bv. infantis, to give them their full title - that helps her look after her newborn. Mummy feeds the the bacterium in baby's tummy its favourite food and the bacterium returns the favour by carrying out a huge range of protective, 'baby-sitting' behaviours. It crowds out and produces bacteriocins against 'bad' bacteria, protecting the baby from serious infections such as necrotizing enterocolitis (NEC) and cholera. It serves lunch (in the form of short-chain fatty acids) for other helper strains. It allays inflammation of the intestine, assists in the production of white blood cells, and helps to set the body's base-line immune response setting, via its effects on intestinal mucosa dendritic cells.
Formula fed babies' guts have a totally different microbiome, predominantly Enterobacteriaceae. Formula manufacturers are now racing to synthesize an oligosaccharide to add to their product, along with the latent enzymes, probiotics and others goodies that occur naturally in mother's milk. Presumably they won't be trying to add the stem cells or macrophages any time soon.
After weaning the levels of bifidobacteria fall to just 3% and adult
microbial composition has more Bacteroides and gram-positive anaerobic
cocci. So there is no probably no point in buying a bottle of bifidobacteria off the internet.