We've had a couple of posts recently about a hypothesis that links the current increase in obesity with an increase in easy access to foods that are designed to trigger reward systems in the human brain. Basically: Maybe we're getting fatter because our brains are seeking out the recurrent reward of food that makes us fat. Scientist Stephan Guyenet explained it all in more detail in a recent guest post.
It's an interesting—and increasingly popular—idea, though not without flaws. To give you some context on how scientists are talking about this, I linked you to a blog post by Scicurious, another scientist who wrote about some of the critiques of food reward and related ideas. In particular, Scicurious questioned some of the implicit connections being made here between body size and health, and eating patterns and body size.
She also talked about another critique, one which came up in a recent article in the journal Nature Reviews Neuroscience. If people are gaining weight because they're addicted to eating unhealthy foods, we ought to see some evidence of that in the way their brains respond to those foods. After all, brains respond to many physically addictive substances in special ways. But we don't see that with junk food. So does that invalidate the hypothesis?
Stephan Guyenet doesn't think it does. In a recent email to me, he explained that he thinks the food reward hypothesis is a bit more nuanced, and can't really be described as "food addiction". At least, not the same way that cigarettes or heroin are addictive.
Addiction is the dependence on a drug, or behavior, despite clear negative consequences. Drug addiction is associated with characteristic changes in the brain, particularly in regions that govern motivation and behavioral reinforcement (reward), which drive out-of-control drug seeking behaviors. Some researchers have proposed that common obesity is a type of "food addiction", whereby drug addiction-like changes in the brain cause a loss of control over eating behavior. Hisham Ziauddeen and colleagues recently published an opinion piece in Nature Reviews Neuroscience reviewing the evidence related to this idea.
The review concluded that there is currently not enough evidence to treat obesity as a "food addiction". I agree, and I doubt there ever will be enough evidence. However, this does not challenge the idea that food reward is involved in obesity, an idea I described in a review article in JCEM, on my blog (1, 2), and my recent Boing Boing piece.
The reward system is what motivates us to seek and consume food, and what motivates us to choose certain foods over others. To begin to appreciate its role in obesity, all we need is a common sense example.
Why do some people drink sweetened sodas between meals, rather than plain water? Is it because sodas quench thirst better than water? Is it because people are hungry and need the extra calories? If so, why not just eat a plain potato or a handful of unsalted nuts? The main reason people drink soda is that they enjoy it, plain and simple. They like the sweetness, they like the flavor, they like the feeling of carbonation on the tongue and the mild stimulation the caffeine provides. It's the same reason people eat a thick slice of double chocolate cake even though they're stuffed after a large meal. The reward system motivates you to seek the soda and cake, and the hedonic (pleasure) system encourages you to keep consuming it once you've begun.
But is this the same as addiction? If I took a person's cola away, would they get the shakes? Would they break into a convenience store at night to get a cola fix? I'm going to say no.
I agree with Ziauddeen and colleagues that the evidence at this point is not sufficient to say that common obesity represents food addiction, and I appreciate their skeptical perspective on the matter. In obesity, as in leanness, the food reward system appears to be doing exactly what it evolved to do: seek out energy-dense, tasty food, and strongly suggest that you eat it. The problem is that we're increasingly surrounded by easily accessible, cheap, commercial food that is designed to hit these circuits as hard as possible, with the goal of driving repeat purchase and consumption behaviors. Our brains are not malfunctioning; they're reacting just as they're supposed to around foods like this.