Eating junk food may make you crave even more junk food

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By Melissa Dahl

We know that eating highly-processed carb-heavy foods is not great for us. But we tend to gobble up the food and move on with our lives, telling ourselves we’ll eat healthier at the next meal.

But evidence is building that, at least for some of us, our brains don’t quite work that way.

A small study published this week shows that hours after we eat a highly-processed meal with lots of carbohydrates – think white bread, or potato chips – we start to crave more of these junk foods.

“Highly processed carbohydrates might program us to crave food in general,” says David Ludwig, lead author of the paper published this week in the American Journal of Clinical Nutrition. “And fast-acting carbohydrates in particular might be setting up these cycles of high blood sugar followed by low blood sugar, food cravings -- and then overeating.”

Researchers gave 12 obese young men two test meals – milkshakes that were not only identical in calories and carbohydrates, but also contained indistinguishable sweetness, appearance, smell and overall deliciousness. The only difference was the nature of the carbohydrates – one shake contained high-glycemic index carbs, and the other had carbs of low-glycemic index. (Foods with a high glycemic index, or GI, raise blood sugar or blood glucose levels higher and faster than foods with lower GIs.)

But as far as the young men who participated in the study knew – the shakes were the same. The two shakes were given to the young men in a lab environment on two different days, and researchers tracked their blood sugar and hunger levels periodically for four hours after the meal. At the four-hour mark, the men were given brain scans using functional magnetic resonance imaging, or fMRI – basically, a way of measuring brain activity.

They found that the areas of the brain related to reward, craving, and addictive behavior lit up for the men in the high GI group as their brains reacted to the fast-acting carbs.That’s important, Ludwig explains, because four hours after a meal is the key time when decisions are made about how much to eat at the next meal.

We’ve seen the connection between junk food and cravings before, as past studies have shown a link. But previous research used foods like cheesecake versus virtuous fare like boiled vegetables – in other words, foods you know and would have a strong psychological reaction to. This new research is an attempt to divide the biology from learned behavior, Ludwig explains.

Madelyn Fernstrom, NBC News diet and health editor, advises that this study comes with a few limitations. For one, it’s a very small study, looking at the brain scans of just 12 obese men – plus, the fact that all the subjects were obese, something that Ludwig says was done because the overweight among us are, in general, the ones having the most trouble maintaining a healthy weight.

Still, at this point, it’s too early to say that these cravings lead to obesity, says Fernstrom, who adds that an important follow-up study would be to do the same experiment using lean people.

In real life, we don’t exactly have the option of choosing between a healthy and less healthy milkshake – unless you’re planning on switching to an all-Soylent diet. Elisa Zied, a registered dietitian in New York City, advises dieters to think about all the delicious and nutritious foods you can eat – rather than trying to immediately eliminate all processed foods at once.

“Of course limiting processed foods--especially the kinds that are loaded with trans or saturated fat, sugar, and sodium--is a good idea,” says Zied. “But taking a positive, inclusionary approach to food and eating may very well be a more effective way to alter your diet than being more extreme.”

So if junk food lit up the areas of the brain involved with addiction – then is junk food addictive?

It’s not that simple. The idea of a “food addict” is a bit ridiculous, in the first place, says Fernstrom, Ph. D., CNS. We have to eat to survive; we don’t have to consume alcohol or smoke cigarettes, for example, so these terms are not interchangeable.

“The notion of food addiction is controversial, and the study doesn’t answer that question,” Ludwig says. “But it provides, I think, evidence for the possibility that highly processed food -- apart from its calories, apart from its tastiness -- could be tricking our brain into craving more food than we actually need.

“So the bottom line is: Limiting these fast-acting carbs that have invaded our diet in the last 30 years may help overweight people avoid overeating,” Ludwig says.