Question: when we eat, how does our body know what part of the food is good for us and what part is waste?

David Christensen answered on 18 Nov 2013:

That’s a tough question and not something that I know very much about, but I think the answer is actually quite simple.

During digestion, as the food we eat passes through the stomach and small intestine it is broken down into smaller and smaller bits. This is first done by us chewing and then by the work of enzymes that can break down the food into very very small bits. I think the intestine can then work a bit like a sieve or filter. If the food has broken down enough, then it can be taken into the body to be used, but the parts of our food, including fibre, that can not be broken down into small enough bits are not taken into the cells along the side of the intestine and those parts of our food carry on passing through us as waste. So really the body knows what parts of food it can use because those parts are the bits that can be broken down by enzymes. But this doesn’t mean that all of those parts are good for us. We can still eat a lot of unhealthy food that our bodies will digest and take in, so that we can become overweight if we aren’t careful about what we eat.