By Theresa Tamkins
THURSDAY, Oct. 16 (Health.com) — Chocolate milk shakes taste good, and that's why someone might drink more of them, right? Not necessarily: A new study shows that obese women enjoy the taste of food less than women who are not overweight—possibly leading them to compensate by overindulging.
The finding suggests that some people may be born with a blunted pleasure response to food. Alternatively, it could be that overeating causes the brain to turn down the pleasure response.
If either of those is true, then the cause of overeating is more complicated than a simple lack of willpower, and it could result in a vicious cycle of increased eating in response to diminished pleasure.
The study's author says he used to resist thinking about food as a type of addiction, but now believes that approach may make sense.
“This is the first direct evidence that obese individuals, when they eat food, experience less activation of the reward circuitry,” says Eric Stice, PhD, a research scientist at the Oregon Research Institute in Eugene.
For example, if you take cocaine or some other illicit drug, “you’re maximizing how good your brain can make you feel right then and there.” The pleasure of a milk shake is "minor league" compared to the jolt of cocaine, Stice says, but the underlying mechanism may be similar. Over time, "consumption of unhealthy food can be habit-forming.”
Next: How the study worked
In the study, published in the journal Science, Stice and colleagues performed magnetic resonance imaging (MRI) brain scans on 43 female college students while they drank either a chocolate milk shake or a tasteless solution.
The researchers looked at the dorsal striatum, a brain region rich in receptors for dopamine, a pleasure-regulating molecule that floods the area when food is consumed.
When drinking the shake, obese women experienced less blood flow in the dorsal striatum than did leaner women, meaning that the heavier females had reduced dopamine release.
It’s not clear if obese women naturally have fewer dopamine receptors that respond to food, or if overeating causes the brain to reduce the number of dopamine receptors. (The researchers say the same results may apply in men, but they didn't include any males in the study.)
In a second study, the researchers conducted the same brain scans in 33 teen girls and also tested them for a gene linked to obesity and lower dopamine receptor levels.
A year later, they found that girls who had both the blunted response to the milk shake and the obesity-linked gene were more likely to have gained weight than those who did not.
The combination of blunted response and the gene is a double whammy, Stice says. “If you have a blunted response to food and then you have a genotype associated with more compromised dopamine signaling, you’re really in trouble. You’re more likely to gain weight over time.”
Although the research seems to suggest that blunted food response comes before weight gain, more study is needed, says Stice. Many of the girls were overweight to begin with.
Alvaro Pascual-Leone, MD, PhD, a professor of neurology at Harvard Medical School, who was not involved in the study, agrees.
“The one thing that this study doesn’t resolve is the chicken or the egg—what comes first? My gut feeling is that it might be some of each.”
These findings don't explain all cases of obesity. Eating behavior is complex; pleasurable feelings are only one reason people may overeat.
But the evidence does repudiate a simplistic "willpower" explanation, Dr. Pascual-Leone says. Biology influences behavior, and behavior can change biology.
“Behavioral patterns will ultimately change the brain and once changes are in place it becomes very difficult to undo—in a sense you can’t undo them. You need to generate new changes,” he says. “It’s not just simply some psychological process.”
Stice points to the huge differences between individuals. "It's really easy for me to eat a reasonable diet. But people I know who struggle with their weight—it clearly weighs heavily on their mind all the time. I think willpower is a very dangerous notion."
The researchers' next challenge is to see if they can change the pleasure-blunting effect by getting people to change their diet, says Stice.
Other medications or techniques—such as biofeedback—could help increase the dopamine reward that overweight people get from food, and that could lead to new obesity treatments, says Dr. Pascual-Leone.