If you made a New Year’s resolution a few weeks ago, you probably decided to get fit or lose weight – two goals that pretty unavoidably involve a pledge to eat less. Perhaps you’ve stuck with it so far, through some combination of brute will, guilt, and the deployment of winning slogans at spots of greatest temptation. But unless you’re one of the rare successful long-term dieters, your assault on adiposity will be short lived. Sooner or later, you’ll find your way back to foods that are sweet, fat, and synthetically tinted.
Why do we eat bad stuff, and too much of it?
Naturally, “because we’re hungry and it tastes good” is an answer, but this just begs the deeper question of what call, exactly, we’re answering when we relent to hunger. Is hunger just the stomach’s empty light, pinging the brain with motivation to refill the tank? Maybe hunger works more like a balance sheet, telling us how many calories we should seek out, relative to how many we have in the bank. Or perhaps hunger is opportunism, indulged in any case where the expected ‘taste-payoff’ is high enough.
In fact, all of these are known and well-studied facets of the drive experienced as hunger, and they all speak to the idea of appetite as a complex regulatory process.
According to a recent study in PLOS One, however, by Jeffrey Brunstrom and his colleagues, hunger is also a trick of memory. In effect, these scientists argue that the remembrance of meals past can help fill an empty belly.
What makes this work especially interesting is that it cleanly demarcates a psychologically complex component of hunger. Naturally, we suspect that our thoughts about food must surely play a role in our eating (and overeating) decisions. But nailing these processes down has been trickier than, say, identifying the peptides and hormones that enhance or suppress appetite. By understanding the hunger’s cognitive components better, we might come up with more effective mind hacks to limit overeating.
Early evidence that memory plays a role in eating came from subjects whose memory was impaired. More specifically, both amnesic rats and people with damage to the hippocampus – a key structure in memory formation – tend to eat in short sporadic bouts, rather than in a few concentrated meals. If you put food in front of an amnesic person, they’re almost always game to eat, whether their last large meal was 5 minutes ago, or 5 hours ago. Other evidence for the “memory for eating” idea comes from memory cueing experiments. If people who are about to eat are given reminders of their previous meal (as opposed to reminders of a meal from long ago), they tend to eat smaller portions. In other words, we seem to keep tabs on what we’ve eaten not just by computing signals of physical fullness – which are presumably unperturbed in all of these cases -- but also by keeping a mental tally of what was eaten when. It’s as if you have a vigilant inner parent, reminding you that “you just ate 15 minutes ago – how could you possibly be hungry!”
While interesting, these experiments have their problems. In the amnesic studies, for example, it’s dangerous to infer too much about normal brain function from behaviors that are the product of a damaged brain. Likewise, its unclear what the memory cueing experiments tell us about normal, everyday eating, since we’re not typically given overt reminders of our previous meals. In order to test if the memory of a previous meal comes unbidden, and has effects on subsequent eating, you have to perturb that memory in a normal individual. But how do you do this?
Basically, you trick people, and you do it in a carefully controlled way. In Brunstrom’s experiment, this was done using a device with an infomercial-worthy name: the self-refilling soup bowl apparatus. Experimental subjects would approach the otherwise normal looking soup bowl, and were told to eat what was in it (creamed tomato, for the curious). Half of the subjects were shown a small portion of soup (300 mL, or about one and a quarter cups), and the other half was shown a considerably larger portion (500 mL, a bit more than two cups).
What the subjects didn’t see, however was that the soup bowl was rigged up to system of unseen tubes and valves that let the experimenters covertly add or take away soup as people ate. Some subjects ate an amount of soup equal to what they were presented with (either the small or the large portion), but others ate a different amount from what they saw. In these ‘incongruous’ cases, people thought they consumed the large portion, but really ate the small portion, or vice versa. With this setup, the experimenters could then test how hungry the subjects were some time later, and tease apart the purely physiological aspects of hunger (having to do with the volume consumed) from the cognitive aspects of hunger (having to do with impressions and judgments about what was consumed).
The first result is that there’s no fooling your stomach immediately after a meal. When tested shortly after eating the soup, subjects who had eaten the larger portion were more sated than those who had eaten the smaller portion, and it mattered comparatively little how much people thought they ate. Two cups is more than one cup, and your stomach gets it right, despite any visual trickery.
Two and three hours after eating, however, a different sort of pattern emerged. The subjects were all hungrier, of course, but their hunger had little to do with the volume of soup they had actually eaten. Instead, it was what they remembered seeing in the bowl that mattered. In fact, those who ate the small portion and thought it was large were more sated than those who ate the large portion and thought it was small. When it comes to the feeling of fullness, the eyes are more important than the stomach.
Overall, this work helps clear up our thinking on an important component of hunger that’s historically been tough to study. Of course, this doesn’t say that hunger is the same thing as a memory of having eaten (or rather, having not eaten). Just that our drive to eat can be biased by memory, as well as possibly the contexts in which we encounter food. On the practical front, this work also opens the door for some possible cognitive-based approaches to dieting. Memory is notoriously fickle, and we may be able to use this to our advantage to enhance our feelings of fullness. In fact, previous work has already suggested that distracted, or “mindless” eating leads people to feel hungry, while more deliberate and mindful eating leads people to feel full. One possibility is that deliberative eating leads to stronger food-associated memories, in turn providing a stronger antidote against future hunger.
So when you sit down for your next meal, pay close attention and remember what you eat – you may pull off that New Year’s resolution yet.
Are you a scientist who specializes in neuroscience, cognitive science, or psychology? And have you read a recent peer-reviewed paper that you would like to write about? Please send suggestions to Mind Matters editor Gareth Cook, a Pulitzer prize-winning journalist at the Boston Globe. He can be reached at garethideas AT gmail.com or Twitter @garethideas.