Biological feedback and breast milk: an example of simple models being wrong

2010 February 26
by Daniel Lakeland

Biology is complex, and anyone who thinks they have a simple model of how it works is almost always wrong. Usually this model comes in the form of the “all else equal fallacy” The idea is that “if everything else is the same” then “doing x causes y”. The thing is that in biology there are too many coupled feedback systems to ever have “all else equal”.

This is why simple ideas of diet and exercise for weight loss don’t work out (depending on the diet composition and the type of exercise, the body might fight you with hormones for more calories, or store fat instead of burning sugar or whatever…), and it’s why so many observational studies in epidemiology turn out not to work in randomized controlled conditions.

Here’s an interesting example of some of these feedback mechanisms at work: the composition of breast milk changes dramatically throughout the day, if you store breast milk for later use you should put a date and time on the container and then feed the milk at the same time of day that it was produced. Morning breastmilk is like a shot of coffee for the child, and evening breastmilk is soporific. If your baby sleeps badly, is it because you’re amping them up with milk left over from earlier in the day? Assuming that all breastmilk is basically the same seems reasonable at first, especially if you’re not a biologist, but once you find out that it isn’t it’s one of those things that makes you say: “duh, I suppose we should have realized that”.

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