Sugar desensitises food pleasure
Have you got a sweet tooth? If so, the chances are, you've probably desensitized your taste buds, meaning that you're going to have to eat more tasty things more often to get the pleasantly full feeling that accompanies a decent meal. Talking with Chris Smith is Monica Dus from the University of Michigan, with why...
Monica - My lab is interested in trying to understand how some foods change our behaviour, specifically our feeding behaviour. In other words, why it's hard to stick to serving size when it comes to cookies, but it's quite easy when it comes to kale or broccoli! Why do we eat more of some foods over others?
Chris - And why do we? I'm dying to know!
Monica - We look specifically at how the food environment, the amount of sugar in the diet, rewires our brain, our taste system and our reward system to dull the sense of satiety and make us eat more.
Chris - You're saying to me then that if I were to add extra sugar to my tea everyday, I don't take sugar by the way, then what I would be doing is making my cup of tea and other foods I eat potentially less rewarding to me. So I would want to eat more of them to get the same feel good rush.
Monica - That's correct. So we know that the amount of salt and sugar, in this case, in the food, changes the pleasure we get from the food and also dulls the taste or the sweetness we can perceive. The more salt, sugar and fat we have in our diet, the less likely we are to taste those with the same intensity, as of somebody who eats less sugar, salt, or fat.
Chris - There are lots of steps in the chain of taste though aren't there? There's what goes on on your tongue. And then there's the nerve that carries that information centrally into the brain. And then there are the whole series of circuits that control how full you feel and whether you want to eat more. So where in that chain then does this dulling effect occur?
Monica - That's a wonderful question because feeding it's so complex, like you said. It starts with the mouth and it progresses through different brain systems. In a previous study, we found that when we gave animals extra sugar, their ability to taste sweetness was decreased in the mouth. And so in this study, we wanted to know if that decrease is what caused the decrease in pleasure we know animals from flies to humans get when they have a lot of sugar in the diet.
Chris - And how did you do that then?
Monica - In this study, we opened a tiny window in the fly head and then looked at the activity of the neurocircuits that encode for the sweetness, or the pleasure that we get from sweetness. And then we touched a fly mouth with a little bit of sugar and then looked for the activity of these neurons in real time. And we did that in flies that had a healthy diet and in flies that had a high sugar diet. And what we found in the high sugar diet, these neurons did not fire as much to sweetness. And so the pleasure of the flies get is much less.
Chris - Now, is that because the taste sensors having inputs to that bit of the brain are now less sensitive, or is it because the circuits downstream are now detuned, they're less sensitive to sweet inputs.
Monica - It's the first thing you said because the taste cells in the mouth are dulled they don't function as well, the signal that goes through the mouth to the brain, as you said it before, is not as strong. And so that's why these pleasure neurons are not firing as much and telling the brain that the organism has had enough pleasure or reward from the sugar it's experienced.
Chris - And do you have any idea as to how the inputs get dulled by super supply of sweet things?
Monica - Actually, we do. That was part of the study we did last year and it was quite interesting. It's a metabolite of sugar that caused the taste cells to be dulled or decreased. The sugar molecule itself changes the physiology of the taste buds.
Chris - Therefore, if what we're seeing is a reduction in the input from the sweet detectors in the mouth parts, this then produces a smaller signal in the parts of the brain that process satiety. So therefore you feel less happy. So that means you then demand more sugar input to get the same pleasure centre activity that you would have had originally in the brain. Is that why the intake is effectively escalated?
Monica - That's certainly one interpretation of it, but some people think that on top of being pleasure, this sweetness is also used as a cue to predict how much we eat in order to get full. And so if now we cannot taste the same cue as we did before, right? Because it's more dull. It's not as intense now. We essentially make a bad prediction about how filling that food will be.
Chris - And if the satiety does lie downstream of a reduction in the activity of these particular groups of nerve cells in the brain that you've been studying, if you increase artificially the activity of those nerve cells, does that reverse the phenomenon? So in other words, in a fly, that's very habituated, very used to eating very sweet things that would normally overeat. If you boost the brain activity, does that put it back to normal?
Monica - That's also correct. And that's what we showed in the study. In the study, we use a light to turn on these particular cells, even in flies that had a high sugar diet, and the flies now ate normally and were sated. What we think is happening is the cells are very likely controlling the eating rate. And during the course of the meal episode, we start eating really fast because we're hungry. But then towards the middle, we have to slow down. We can't wait for our stomach to be full, that would take a lot longer. And so the sort of cues we get during the meal are very important to start fast and slow down. And we think that the dulling sweetness prevents the feeding rate for decreasing towards the end of the meal and so you end up overeating,
Chris - Is this reversible? I should think there's probably lots of people now panicking thinking I had ice cream with my dinner, and I've got quite used to that, I quite like it, am I therefore destined to overeat for the rest of my life? Can you wind the clock back?
Monica - In flies we find that this process is partially reversible, but not entirely reversible. In humans certainly there have been studies where people decrease the amount of content of sugar in their diet, and what they found is that their sensitivity to sweetness increases. So I think the process is very likely reversible, how much we don't know and that's what we're going to study next.