As teens gain weight, they find high-fat foods less pleasurable

New study links weight gain with lower activity in their brains’ taste and reward centers

Teens’ brains register high-fat, high-sugar foods, such as milkshakes, as very rewarding. Those positive responses to fatty foods dropped in teens who gained excess weight, a new study shows.

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People eat when they’re hungry. But they also eat simply because something tastes good. That can lead to weight gain if someone overindulges too often. A new-brain imaging study finds that healthy-weight teens get a strong positive boost from high-fat foods. However, teens who gain excess weight don’t get the same pleasure from those foods.

Sonja Yokum and Eric Stice are psychologists at Oregon Research Institute in Eugene. They had noted that animal studies had shown that the pleasure centers in the brain light up when rodents eat foods they like. But once those animals plump up from too much of a good thing, their brains respond differently. Now their brains send fewer reward signals. Yokum and Stice suspected the same thing might happen in people.

In fact, Yokum notes, there’s evidence that eating foods high in fat and sugar changes the brain. That may prompt people to overeat as they work harder to get the same rewarding feeling, she says.

To find out, Yokum and Stice recruited 135 high school students for a study on eating and enjoyment. All were 14 to 17 years old. The researchers measured each teen’s height and weight. They used these numbers to calculate each person’s body mass index, or BMI. As a measure of body density, it points to potentially healthy or unhealthy levels of body fat (which is less dense than bone or muscle). All participants had healthy BMIs when the study began.

The teens rated pictures of different types of food on a scale from least appetizing to most appetizing. The images included cupcakes, fruits, vegetables and glasses of water. Yokum and Stice then selected those photos that the teens had rated as the 32 most- and 32 least-appetizing foods.

They showed these images to the teens as their brains were scanned with a magnetic resonance imaging, or MRI, machine. MRI records the amount of blood flowing to different parts of the brain. Blood flow shows which areas are active. While the teens viewed photos of appetizing and unappetizing foods, the MRI machine mapped their brains’ activity.

Yokum and Stice also wanted to know how the brain responds to the tastes of food. So as the teens viewed pictures of a glass of water, they’d get a taste of water. When the milkshake appeared, they got a taste of milkshake. All the while, the scanner kept mapping how their brains responded.

There were four different milkshakes. All were chocolate flavored but with different amounts of fat and sugar. One had high levels of both fat and sugar. Another had low levels of each. The others were only high in fat or sugar. The teens knew they would be tasting four different milkshakes but weren’t told how they might differ.

Yokum and Stice repeated the tests with the same teens one, two and three years after the first scan. They then focused on a subset of these students. One group had put on excess weight. Their BMIs increased by at least 10 percent from year one to year three. Another 31 teens had BMIs that changed by two percent or less over that time.

Changes in the brain

Yokum and Stice now looked for any differences in the brain scans between the two groups. Changes emerged in two areas of the brains in the now-heavier teens. One region processes taste. The other is known as the reward center. It becomes active when we enjoy something, such as eating yummy food.

At first, the taste center responded strongly in teens who went on to gain weight. That was especially true when they tasted the high-fat milkshakes. But three years later, scans showed those parts of the brain responded less in these teens after their BMIs had climbed. Their brains weren’t able to detect fat as well as they had before. Teens with steady BMIs showed the opposite effect. Their brains started off less responsive to fat. But over time they got better at detecting it. By year three, the taste areas showed a burst of activity when the teens sipped the fattier milkshakes.

Changes emerged in the teens’ reward centers, too. Those whose BMIs later rose started off the study with a strong reward response to tasting high-fat milkshakes. They enjoyed it more than the other students. In contrast, teens who maintained a steady BMI throughout initially enjoyed the fattier drinks less. By year three of the study, the response of both groups switched. Higher-BMI teens enjoyed the fattier milkshakes much less, while those with steady BMIs enjoyed it far more.

Interestingly, these changes only happened in response to fat. None of the teens’ brains showed a similar response to sugar.

The brain’s response to looking at pictures of foods also flip-flopped. Teens whose BMIs rose showed more brain activity in response to the appetizing foods in the last year of the study compared to the first year. In teens whose BMIs remained steady over time, their brains’ reward centers became less active when they viewed tasty foods.

The Oregon team shared its findings in the December issue of The American Journal of Clinical Nutrition.

What does this mean?

People encounter ads for sweet and fatty foods every day, Yokum says. Those who eat a lot of such foods may have strong brain responses, finding these images super appetizing and inviting. But when they actually eat these foods, the taste and enjoyment might not live up to expectations.

Indeed, Yokum says, plenty of evidence now links eating a lot of high-calorie food to a diminished reward response to those foods.

The new study suggests that the loss of enjoyment “might drive people to overeat particularly tasty, high-energy foods to get the same rewarding ‘kick’ in their brains,” says Amy Reichelt. She’s a brain and nutrition specialist at Western University in London, Ontario, Canada. “However, BMI isn’t the best measure of obesity,” she cautions. And that’s true, she says, “particularly in young people.” Although higher BMI can indicate overweight or obesity, that’s not always true. It also can be linked to more muscle and general growth, she says.

Yokum agrees that BMI is not a foolproof measure of health. But the brain changes her study detected in response to food are worth noting, she says. “High-fat, high-sugar foods are engineered to be as irresistible as possible,” she says. They “include highly rewarding food ingredients in doses that our brain is not really wired for.” She now suspects that overeating them “can change your brain’s reward system in a manner that contributes to unhealthy eating habits and unhealthy weight gain.”

Alison Pearce Stevens is a former biologist and forever science geek who writes about science and nature for kids. She lives with her husband, their two kids and a small menagerie of cuddly (and not-so cuddly) critters.

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