Why do we get tired after a big meal?

Chris Smith asks physiologist, Sam Virtue, to digest this question from Don.

Chris - We've all been there. As the Americans like to put it the effect you get with too much turkey at Thanksgiving. Why does this happen? Can you help Don out.

Sam - What happens when you eat a large meal is that you take on board a lot of nutrients and the sum of the nutrients directly but also indirectly by causing the production of hormones from the gut can signal to the brain. And for reasons which are slightly hard to understand how they are evolved, sleeping and eating are very closely related. When you've eaten a large meal it activates active neurons in the brain which are also associated with sleep. And you can think about this, if you've just eaten a large meal and you're out in the wild, you're already pretty fat and slow from all this food and now you can fall asleep? That might possibly make you really easy prey, but perhaps a more positive ways to think maybe you crawl off to a nice hole because you don't need to go looking for food.

Chris - Assuming you can still fit into it , like Winnie the Pooh got stuck into too much happening.

Sam - Oh yes yes I have small children so I've been reading that quite recently. So the neurons that everything seems to converge on in the brain are called orexin neurons and it's quite interesting because these were found by two different groups of researchers at the same time. One group find them in mice and they called them orexin neurones because the mice were fat. So they assume that this was something involved in eating. However, the other group that found them, found them in dogs and the much more striking feature of these dogs was the dogs were bound along and then have a narcoleptic fit and just pass out and roll over and then suddenly just get back up as though nothing had happened and so they thought these were to do with sleep, and it turns out it’s both.

Chris - Where they Labradors, these dogs? Because in my experience Labradors eat just about anything until they pass out and then they sort of sleep it off and then come back to life and do the same again - rinse and repeat until they've gained about 30 stone.

Sam - I think they're doberman pinschers the dogs with narcolepsy. But yeah it's really amazing if you see a video of a narcoleptic dog because they will be in like the middle of a game of catch and then just fall over and sleep. So you think from your gut like glucose and insulin after a meal signal these and they make you feel sleepy.

Chris - And the purpose of doing that is you need to divert a lot of resources to processing that enormous male and then absorbing the calories and distributing it around the body I presume.

Sam - Yes so that's certainly one theory as to why it's evolved, so for example the snake increases its metabolic rate five fold after ingesting something like a piglet or something.

Chris - Big snake!

Sam - A python? Something like that or even or whatever it’s eaten, a mouse or whatever.

Chris - But the interesting thing about a snake is that that might dine once a month once every several months whereas we have to eat regularly. Is that just a reflection on the fact that a snake is cold blooded, has a lower metabolic rate and can literally sit there not burning off much energy so it doesn't need to eat. Whereas you and me are ferociously burning off calories so we need to replace them.

Sam - There's certainly an element of that but it is kind of interesting when you think how long humans can survive without eating because we can go many many days if not months without eating. So why we have evolved these patterns of eating more regularly versus other animals which do not eat very regularly - I mean there are mammals that don't eat for months at all like a bear when it hibernate doesn't eat. So why we have these specific patterns is quite an interesting question. I'm not really sure I have a good answer for it someone might.