Does your brain know you're drinking Diet?

Sep 22 2010 Published by under Behavioral Neuro, Neuroscience

Let it be known to the masses (all two of you, hi!) that Sci LOVES Diet Coke.


I mean, not as much as coffee or Pumpkin Spice Lattes, but it's a close second.

And I always heard what I'm sure everyone has heard, about how Diet Coke is HORRIBLE for you and the ASPARTAME!! The ASPARTAME will come and EAT YOUR CHILDREN. And then it will give you AUTISM because it's full of MERCURY. Or some such crap. I did worry slightly about aspartame, but as it's supposed to increase feelings of fullness, and not of hunger (though apparently women's health magazines would tell you otherwise), I'm not fretting overly much.

However, I do wonder. We can TASTE the difference between Diet Coke and regular, and maybe our gut doesn't know the difference. But what about our brains? Does your brain KNOW you are drinking Diet? Smeets et al. "Consumption of caloric and non-caloric versions of a soft drink differentially affects brain activation during tasting" NeuroImage, 2010.

The question comes down to this: are the same areas of the brain activated when you drink sugary drinks as opposed to drinks sweetened with non-caloric sweetener? And does this VARY by the SIZE of the tasting you are doing?

Why size, you ask? Well it turns out that smaller sips of sweet things are more satisfying (to your brain) than huge glomphs. Small sips give you more taste exposure, and may end up increasing sensory satiety faster than large gulps. What they were interested in here was a DECREASE in sensory activation in the brain. Basically, after you drink a ton of something sweet, you should get decreases in sensory activation for that sweet thing. The question is whether or not your brain does this differently in response to non-caloric sweetener, and how much this varies by how big you're sipping.

To test both of these hypotheses, they took 10 dudes, and had them either eat normal breakfast or lunch, or fast for two hours. They put them in an MRI, and gave them sips of regular Orangeade or diet (this is pre-exposure). They then FED them either regular Orangeade, or diet, in a chunk. They fed them quite a bit of soda, almost half a liter (the actual volume was 450mL), served in either small sips of 5mL or big gulps of 20mL. Then, they plunked the guys BACK in the MRI (this is post-exposure), gave them tastes of regular soda or diet, and watched what happened. They also got rinses with water, really quite a bit of liquid.

They then scanned their brains, and also asked them about how full they felt, how hungry, how much Orangeade they wanted, etc. No notes on whether people completely hated Orangeade by the end of the study (or whether hatred of orangeade was in their screening).

We'll start with the behavioral data: obviously, after drinking ALL that soda, their fullness decreased, and so did their thirst. No ratings on how badly they had to pee. The largest change was in the desire to eat, which went down the MOST when the men had the full calorie soda. The participants also found the regular soda more tasty immediately, but didn't end up preferring either one.

Now, ONWARD to the BRAIN!


First off, they got no real effects of sip size, so that hypothesis goes out the window. But they DID get differences in brain activity in response to caloric vs non-caloric sweetener. In particular, they got three main brain areas affected; the orbitofrontal cortex, the amygdala, and the striatum.

The orbitofrontal cortex is an area of the brain located right behind your eyes, and it plays a lot of roles in decision making, reward, and emotion.


In this study, both types of drinks produced activation in the orbitofrontal cortex (reflective of either rewarding stimuli, or possible "hey...I wonder which one this one is..."), but the diet drink actually activated this area MORE than the regular drink.

In the amygdala was where they got some rather interesting stuff. The amygdala is an area (they say it's almond shaped but I don't see it) located right about two inches in or so, behind your ears. This area is mostly known for things like emotional memory, but it also plays a role in processing sensory cues (like taste) as they relate to reward (like sugar). When the participants were given regular or diet soda at pre-exposure in the MRI, the diet soda actually activated the amygdala MORE than the regular. Not only that, the REGULAR soda actually decreased activation! But after drinking that half liter of fluid I mentioned before, the activation went away (the post condition). This shows that the amygdala got used to the presence of the sensory cue of the soda when the soda was given over time, but doesn't really explain that deactivation of the amygdala they got with the regular, sugary drink.


Finally, we come to the striatum, where things turned out the total opposite of the amygdala. When the people were exposed to a single sip of soda (the pre condition), the striatum responded VERY strongly to the regular sugary drink, but didn't respond at all the to the diet drink.


After the exposure to the copious amounts of soda, the striatum apparently had nothing more to say on the matter.

So what does this all SAY!? Well, it does say pretty clearly that you brain can tell the difference between caloric and non-caloric drinks. Your brain, in a way, knows you are drinking Diet Coke, and not regular.

But what does that matter? Sci's not so sure, and she wishes they had addressed this a little in the discussion. In my opinion, a study like this, looking at brain activation, doesn't say as much as it can until it has an effect on behavior. The study participants may have had brains that knew they were drinking diet, but they sure didn't act like they knew or cared. So basically, this study seems like it's a little preliminary, and raises lots more interesting questions:

1) What are the effects on subsequent food intake? Do the participants eat more or less, and how do their brains respond? The authors looked at tomato juice and milk, and didn't get anything in terms of taste activation, but I wonder how they would respond to volume as well. Or solid foods for that matter.

2) What about the LADIES?! These were all men, and I want to see if women react differently, and how this could relate to changes in the menstrual cycle.

3) What do the activations in these areas really tell us about how these people are responding to regular or diet drinks? Does this mean your brain recognizes and imposter? Or does it just respond to "different"? Do the participants have preferences for a drink in a blinded taste test that would let you know?

And of course there are more questions. But it's a cool finding, that somehow, your brain knows your guilty secret. Your brain...KNOWS you're drinking Diet.

Smeets PA, Weijzen P, de Graaf C, & Viergever MA (2010). Consumption of caloric and non-caloric versions of a soft drink differentially affects brain activation during tasting. NeuroImage PMID: 20804848

20 responses so far

  • Janne says:

    Well, striatum is generally implicated in reward learning, and amygdala in aversive learning (though at least amygdala is somewhat involved in positive stuff too). So striatum is signaling a rewarding substance for caloric stuff, and amygdala gives less negative reaction for it.

    OFC is at least partly involved in inhibiting inappropriate responses from those subcortical structures. So if it reacts stronger to the non-caloric stuff that _may_ be that it's preventing the striatum (or other areas) from being too upbeat about something that turns out not to give you any nutrition. But that depends on the part of the OFC too; it's not homogenous or anything.

    So it's at least possible that striatum is reacting to both ("hey, this stuff is sweet!") but OFC inhibits it when it finds out there's no nutrition. Perhaps amygdala reacts negatively to the fake stuff (it has a lot of connections with the thalamus and similar areas), which in turn triggers OFC to inhibit any enthusistic response to it. But all of these (and a number of other areas of course) are interconnected in pretty complex ways so there's no way to really know what is happening here without a lot more data.

    Which is great, beacause, you know, without the need for more data and models we'd be out of a job.

    • scicurious says:

      Oooh, these are great thoughts, Janne! I especially like the point about the amygdala, and the idea of the suppression of aversive stimuli, and being more suppressed by sugar than the non-caloric sweetener...

  • John says:


    After moving my rss from scienceblogs to scientopia because of the pepsi advertising fiasco, I see this piece in my inbox, mocking the (well documented) effects aspartame has on children.

    You should be ashamed of yourself.

  • becca says:

    Agh. The study I wanna see is whether the brains of people who DISLIKE diet soda are different in any generalizable way from those who LIKE it.

    • scicurious says:

      OH! Yes. Well...I actually don't know if it'd be different from people who like vs hate other normal foods, but with the differential stuff in the striatum...possible.

  • I'm with John. Aspartame and HFCS are teh evil! I know that because I read it in the Huffington Post. πŸ˜‰

    Seriously though: Coke Zero pwns Diet Coke! πŸ™‚

  • chezjake says:

    I too wondered why there were no women in the study. As of the last beverage industry consumption stats I saw (about 10 years ago), women consume better than two-thirds of all diet drinks.

    The other thing that puzzled me was why their non-diet comparison drink was anything other than real Coke. Why introduce an unnecessary potential confounder in the form of a totally different flavor?

    • scicurious says:

      Oh the question was just caloric vs non-caloric drinks, not Coke specifically or any other product. In this case they used Orangeade, but I imagine they could have used anything else. like sugary vs sugar free Tang or something.

  • [...] Does your brain know you’re drinking Diet? [...]

  • Joe says:

    And what of the 10 dudes' previous experiences with diet or sugar sodas? It seems like reward reactions such as these would be keenly dependent on previous interactions with the foods/drinks. This should be clearly laid out in the study design.

    • scicurious says:

      They did actually keep track of that, they required that the guys have on average more than one soda per month, and tracked regular or diet, but didn't provide any more detailed information than that. You're right, it would be REALLY interesting to see if people who drank a ton of sweet soda or a ton of diet soda, differed from people who didn't really drink much soda at all in how they reacted.

  • Stacy Hurt says:

    I'm just curious about something. In our home we have been 'died in the wool' Diet Coke fans forever. We have since stopped drinking the beverage for one reason. We read (in the Sugar Busters book) that drinking soft drinks jacks up your glycemic levels right so then you require more of the thing (sugar) and thereby put on serious poundage. I want to know if that's true? Does regular sugar and (do) artificial sweetners create more hunger by raising your glycemic levels? Does one do that more than the other?

    Love all info you've given!

    • scicurious says:

      Hi Stacy,

      To answer your question, if you're drinking Diet drinks that contain no sugar, you shouldn't be putting on the poundage because of the drinks at all. Regular sodas contain sugar (or high fructose corn syrup) to sweeten them (a regular Coke will have about 120 calories in a can). Those sugars that you drink in a regular Coke will indeed jack up your glycemic index, because the sugar will go into the blood stream. Over time, if you're taking in a lot of extra calories from soda (or ANY other food, it's the extra calories that matter), and not burning them off with activity, you will indeed put on weight.

      But Diet drinks are Diet because they contain no sugar or high fructose corn syrup. Instead they are sweetened with artificial sweeteners, like aspartame or others. Those compounds are SO much sweeter than sugar that it takes a lot LESS of them to make your Diet drink sweet, so the net result is that you Diet drink has fewer (or no) calories at all. If there is no sugar in there, and no caloric intake, your glycemic index cannot go up, and so the soda itself will not cause you to gain weight.

      As far as whether Diet drinks lead to more hunger, if they did (and the jury is still out on that one, some studies show they increase food intake, and some show they don't), they would NOT do it by doing anything to your glycemic levels. They contain no sugar, and so cannot have an effect on your blood glucose.

      I hope that helps!

  • yannisguerra says:

    It is funny that whenever I see a "aspartame kills" poster, they always sound like "antivax-creationism-youngearth-whateveridontlikethatandyoushouldnteither". And they have the same responses whenever they are shown evidence:
    1-they ignore it and start talking about "somebody I know,etc"
    2-they dissipate.
    There are good sociological studies about why this happens, but it doesn't fail to make me laugh.

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