Why Dieting Is So Hard

New year, new you. For many people, a new you really means a new diet, shorn of white carbs, fried foods and ice cream. (Losing weight is, by far, the most popular New Year’s resolution.) Alas, the new you has to struggle against the habits of the old you, which knows perfectly well how delicious French fries taste. Most diets fail because the old you wins.

Why is the new you so weak? A recent study in Psychological Science by Deborah Tang, Lesley Fellows and Alain Dagher at McGill University helps reveal the profound challenges faced by the typical dieter, struggling for a slimmer waistline. We were not designed to diet; the mind does not crave celery. We were designed to gorge.

The study began by asking 29 people to evaluate pictures of fifty different foods, some of which were healthy (fruits and vegetables) and some of which were not (chocolate bars, potato chips, et. al.) The subjects were asked two questions about each picture: 1) How much they wanted to eat it, on a twenty point scale and 2) How many calories it contained.

The first thing the scientists found is that people are terrible at guessing the number of calories in a given food. In fact, there was no correlation between subjects’ estimate of calories and the actual amount of calories. This failure of dietary intuition means that even when we try to eat healthy we often end up eating the wrong thing. A Jamba Juice smoothie might seem like a responsible choice, but it’s actually a speedball of energy, with a large serving of the Orange Dream Machine clocking in at 750 calories. That’s 35 percent more calories than a Big Mac.

But here's the fascinating twist: although our conscious assessments of calories are not to be trusted, the brain seems to contain a calorie counter of its own, which is pretty reliable. (This calorie counter learns through personal experience, not nutritional labels.) In short, part of you knows that the low-fat smoothie contains more calories than the double burger, even if the rest of you is in sweet denial.

The scientists revealed this internal calorie counter in two ways. First, they showed that the amount people were willing to bid in an auction for a familiar food was closely related to its true caloric content, and not their liking ratings or the number of calories they thought the food had. In short, people were willing to pay larger amounts for food with more energy, even if they didn’t particularly like the taste of it.

The second source of evidence featured fMRI data. After showing the subjects the food photos in a brain scanner, the scientists found that activity in a part of the brain called the ventromedial prefrontal cortex (vmPFC) was closely correlated with the actual number of calories, and not individual preferences or the estimated number of calories. And given previous scanning research linking the vmPFC to assessments of subjective value - it helps determine the worth of alternatives - this suggests that, for certain parts of the brain, “the reward value of a familiar food is dependent on implicit knowledge of its caloric content.” Kale juice is for suckers.

This research comes with a few unsettling implications. The first is a sobering reminder that the mind is a calorie-seeking machine. Although we live in a world of cheap glucose and abundant fats, part of us is still terrified of going hungry. That, presumably, is why we assiduously track the amount of energy in certain foods.

But wait - it gets worse. Not only does the brain ascribe high value to calorically dense foods, but it also seems to get a lot of pleasure from their consumption, regardless of how the food actually tastes. A 2008 study by researchers at Duke, for instance, showed that mutant mice who can’t taste sweet things still prefer to drink sugar water, simply because their gut enjoyed the fuel. (The ingestion of calories triggers a release of dopamine regardless of how the calories taste.) This suggests that we’d still crave that Jamba Juice smoothie even if it wasn’t loaded with fruit sugars; energy makes us happy. 

There are no easy fixes here, which is why losing weight is so hard. This is true at the individual level - the cravings of the old you are difficult to resist - and at the societal level, as the government seeks to persuade people to make healthier eating choices. In fact, this study helps explain why calorie labeling on menus doesn’t seem to work very well, at least in some early trials. Although the labels attempt to educate consumers about the true caloric content of foods, the brain is already tracking calories, which makes it hard for the fine-print on menus to compete. And even if we did notice the energetic heft of the smoothie it’s not clear how much we’d care. Simply put, we are wired to prefer those foods with the most fuel, even when that fuel makes us fat.

The old you wins again.

Tang, Deborah W., Lesley K. Fellows, and Alain Dagher. "Behavioral and Neural Valuation of Foods Is Driven by Implicit Knowledge of Caloric Content." Psychological Science 25.12 (2014): 2168-2176.