I’ve just read Giulia Enders book Gut, though I prefer the German title (Darm mit Charm, Intestines with Charm, it rhymes).
She popularises some not well enough known facts (sitting on the toilet is not the way forward, crouching is better, or at least put your feet on small stool and lean forward to get the same result, see: 7 basic things … you’re all doing wrong).
My favourite idea was the observation that the genes of all the huge variety of bacteria in our stomach, add up to more genes than our entire genome. So that when toddlers go through their phase of stuffing everything they can in their mouth, they are actively collecting their bacteria digestion toolbox.
I know we tend to dismiss observations, until we understand WHY they happen, so this adds in to my epidemiology classes.
We learnt that you can tell how a bacteria spreads by looking at the exposure of a population. This graph (dredged out of my memory, google search was not kind),
shows lots of exposure from 1-2 years of age, exposure then levels off (no more people are getting exposed) but then takes off again in the teenage years. We were taught this is characteristic of germs spread through the mouth. Babies/toddlers go through a phase of stuffing everything they can in their mouths, but then stop, until you are a teenager and ‘oral exploration becomes fun again’.
So without thinking about it I just assumed babies have a poor sense of hygiene.
But Giulia suggests they are deliberately collecting as many bacteria as possible to build up their bacterial food digesting toolbox (especially useful when you transition from uniform milk to all the different kinds of food we eat). Japanese gut flora even have a seaweed digesting gene from a marine bacterium, helping them digest seaweed (and some of the benefits of eating soya are much stronger in the Japanese, suggesting you need the right stomach bacteria to get the full benefits). Also as the stomach ties in to the immune system the baby may also be training up it’s immune system.
So babies do have a poor sense of hygiene, but by design, not by accident.
It was also interesting to learn that your stomach doesn’t rumble because you are hungry, but because it has been long enough since your previous meal, that your stomach, and lower intestines, are completely empty and it is spring cleaning (might coincide with you getting hungry again). My stomach was rumbling the very morning after reading this and now I know why.