The food coma effect

Whenever you eat a big meal, the chances are that you probably feel sleepy afterwards. This is dubbed the food coma effect, and, incredibly, it's evident even in flies, as Bill Ja has discovered by filming drosophila eating foods containing different proportions of nutrients. And studying their brains is revealing a lot about why it happens to us too, as he explains to Chris Smith...

Bill - It was about 10 years ago now that we started using these tiny glass straws that we filled with liquid food, sugar water or sugar in yeast solution. We found that the flies were able to suck food from the straw – sort of like a hamster feeder. And so, as you watch the liquid volume go down in the capillary, you could calculate the volume that they consumed. And so, by putting these two systems together, the camera for watching them sleep and a second camera for watching these capillaries so that you could constantly monitor how much these flies are eating, we would have a constant readout of food intake and activity and sleep. From this dataset, we would see that flies eat a certain number of meals per day. So I simply ask, what does the sleep of these animals look like before and after eating? Is there a difference? We didn’t really believe it at first but then we went through a long series of experiments to try to confirm that what we were seeing was real – that the flies really do sleep more after meals than before, and all of the components of that meal that make postprandial sleep stronger or weaker.

Chris - I was going to ask you about that because there is this claim that if over your Thanksgiving table, one overconsumes turkey, you're much more likely to feel sleepy than if you have, say, pasta. Is this true in your flies if you vary the composition of what you feed them so they have, say, a protein dominated dinner? Would this make for a sleepier fly?

Bill - Meal size or meal volume strongly induces the food coma effect. But independently, protein and salt intake also do it. And so, even if the fly is in a small meal, if there's a lot of protein or salt in that meal, it will induce just almost as much food coma or sleep as the large meal. And so, if this is analogous to what happens in humans and all the same nutritional triggers are the same between flies and people, yes, anecdotally would be turkey over cake that would cause food coma.

Chris - How does this work? Have you got some insights into what is going on in the fly’s body? What its physiological mechanism is for detecting what it’s had for dinner and that then having this sleep effect?

Bill - In the gut, there are certainly lots of receptor and different mechanisms for sensing the different nutrients, how the signals are integrated to dictate appetite and satiety or this food coma effect, we’re not really sure. The mechanisms that we identified first were all in the brain and we found a few different circuits close to each other so we think they form part of a network that regulate this food coma behaviour but only to protein. And so, we think that the other circuits that must be used to regulate food coma in response to salt or meal size, still have to be found.

Chris - So in essence, there is some kind of mechanism for detecting what is being consumed and how much has been consumed. This is being fed on to a certain population, probably a mixed population of cells that are responsive to different micronutrients in the brain and that is then impacting on sleep and behaviour.

Bill - Right. If you look at where these cells are in the brain, a lot of them pointing to regions of the brain that are known sleep centres and so, it makes some sense that that’s where we found these circuits.

Chris - Now, from an evolutionary point of view, why is it beneficial to have this behaviour?

Bill - That’s a great question. We’ve been wondering that ourselves for a while. It could be something as simple as digestion. Someone pointed out to me that perhaps, protein and salt are limiting nutrients in the wild for flies or nutrients that the flies really prioritise. And so, if that’s true, and the food coma effect is there to enhance or optimise the efficiency of assimilation or absorption of these rare or prioritised nutrients, maybe that’s why the fly has this effect.

Chris - Thinking about it from the other direction, if one manipulates those circuits in the brain that you’ve uncovered that appeared to be sensitive to proteins and to salt, if you fiddle with those neuronal circuits, can you shutoff this effect, thus, proving that they must be doing what you think they're doing?

Bill - Yeah. So the tools we used to do this specifically allow us to turn on and off these different circuits in the brain. And so, these circuits that we find are responsive to the food coma effect to protein, we could turn them on or off to make the effect stronger or weaker. When you shut off these or silence these neurons that are responsive to protein, feeding protein gave a huge waking response. So it actually suggests that protein induces both wakefulness and sleepiness, and usually, the wakefulness part of that is shut down by these leucokinin receptor neurons.