Food-Addicted, or Food-Obsessed?

How the brain's reward circuits misbehave in obesity and anorexia.

Posted May 31, 2012

Here’s an easy question for you. Obesity and anorexia – both disorders involving an unhealthy preoccupation with food. But what makes them different?

Clearly there’s the small matter of body weight – whereas those with anorexia have a BMI less than 18, obese people have a BMI of 30 or more. Aside from that, though, it’s still pretty obvious isn’t it? Obese people love food, while those with anorexia hate it?

Well some new brain imaging results suggest it might not be quite that simple.

In one of the first fMRI studies to directly compare obese individuals with those with anorexia, researchers in Colorado trained recently-fed participants to associate certain shapes with either a sweet taste or a neutral taste engineered to be as similar to one’s normal mouth taste as possible. (You see a square? Whoop, sucrose is on its way! You see a triangle? Yuck, brace for artificial saliva…)

But then they played a little trick specially designed to stimulate the brain’s reward system – which responds most vigorously to unexpectedly pleasant things.

On some of the trials subjects got the sweet taste they were anticipating (Ah, a square…and hooray, sucrose!). But on others they got an unexpected surprise (Yeek, a triangle, here comes the gross saliva…Hey wait - that was actually quite tasty!).

Now if obese people love food and anorexics hate it, you’d think obese people’s reward systems would respond the most to the surprise sugar, right?

Not so. The unexpected sweet taste actually triggered the biggest ventral striatum, insula and prefrontal cortex responses in those with anorexia, medium-level responses in those who were normal-weight, and the smallest responses of all in those who were obese.

How can we interpret these results?

Well they're consistent with rat studies showing that food restriction and weight loss (similar to that seen in anorexia…) are associated with heightened reward-related dopamine activity. And they’re also consistent with other studies showing that high-calorie diets and weight gain (similar to that seen in obesity…) are associated with decreased reward-related dopamine activity – which is unfortunately compensated for by eating even more cheesecake.

In short: starvation sensitizes our reward system, but gluttony blunts it.

What do the findings mean for people with anorexia or obesity?

So far it’s looking like whereas obese brains resemble those of people battling addiction, the brains of individuals with anorexia resemble those of people who – regardless of any simultaneous obsession with thinness and food avoidance – are highly sensitive to food reward.

Scientists have yet to show whether the diminished responses seen in obesity can be restored (although I’m prepared to bet that they can be…).

But the new results are encouraging for those suffering from anorexia: If the natural reward response is intact, and can be automatically triggered in response to an unexpected sweet treat, the potential to rebuild a healthy relationship with food – however difficult it may be in practice – still exists.