You’ve surely heard some people say that, a few hours after eating a meal high in simple carbohydrates (white bread, regular pasta, potatoes), they “crash”—feeling hungry, cranky and craving more food. Exactly what do they typically yearn for? More simple carbs (cookies, cake or some other processed snack food)—because these foods provide a “quick fix” that satisfies their craving…if only for the short term. And then that cycle repeats itself again and again, ad infinitum.
Such people often jest that they’re “addicted” to carbs…and though you laugh at their joke, you may secretly suspect that they simply lack the willpower to control what they put in their mouths. But the truth is more complicated. In fact, an intriguing new study suggests that certain types of carbs may indeed trigger addictive behaviors. It all has to do with what happens in the brain when a person eats carbs. Here are the fascinating details…
Simple carbohydrates are digested quickly because they’re so simple that our bodies have little left to do with them before converting the food to energy. Such foods are said to have a high glycemic index (GI) because they trigger a fast and pronounced increase in blood glucose levels. But soon after high-GI foods are eaten, blood glucose begins to plummet again…and by about four hours later, it winds up even lower than after a 12-hour fast. That’s the crash that people complain of—and it triggers big hunger.
In contrast, low-GI foods (most vegetables, fruits, legumes, nuts and minimally processed grains) cause more gradual and less extreme increases and decreases in blood glucose levels. That why they keep you feeling fuller longer and don’t trigger intense cravings. (To look up the GI for various foods, visit GlycemicIndex.com.)
For the study: Researchers wanted to gain a better understanding of what goes on inside the brain after a person eats carbs. Participants included men who were overweight or obese but otherwise healthy. Each came to the lab on two separate mornings, several weeks apart, after fasting for 12 hours, and was given a milkshake-type breakfast that was either high-GI or low-GI, depending on the ingredients used. Cornstarch and regular 1%-fat milk were included in the low-GI (slow-digesting) drink…corn syrup and lactose-free 1%-fat milk were used in the high-GI (fast-digesting) drink. Both breakfasts contained about 500 calories, had the same basic nutrients, and looked, smelled and tasted virtually identical. If a man was served the high-GI meal on his first visit, he received the low-GI meal the second time…and vice versa.
Before the meal and then every 30 minutes thereafter, blood was drawn to measure participants’ glucose and insulin levels. The men also rated their hunger levels on a scale of one to 10. The blood tests and hunger assessments continued for five hours.
Four hours after the meal, which is when blood glucose levels typically are at their lowest point—and which is a time likely to influence eating behavior at the next meal—each participant had a functional MRI (fMRI). This special scanning technique traced and measured actual amounts of blood flow through various brain regions.
What the scans showed: Blood flow in a brain area called the right nucleus accumbens was 8.2% higher four hours after the high-GI meal than after the low-GI meal. Now, you might think that extra blood flow would be a good thing—but this particular brain area is a “reward center” that’s involved in cravings and substance abuse/dependence. This suggests that eating high-GI foods may trigger a neurophysiological response similar to that involved in addiction. After all, cravings are one of the hallmarks of addiction.
In addition, this increased blood flow in the brain’s reward center directly corresponded with changes in glucose and hunger levels. As expected, compared with the low-GI meal, the high-GI meal caused glucose to spike faster and higher and then to drop farther…and led to significantly higher hunger ratings afterward.
For those reasons, the researchers suggested that the sharp decrease in blood sugar that occurs a few hours after high-GI foods are consumed not only stimulates greater hunger, but also makes the brain find the very idea of high-GI carbs more pleasurable and rewarding. Thus the cycle of overeating is propagated.
This study is not the first to show that specific foods cause special changes in the brain, but it is unique for several important reasons. For one thing, the two test meals looked and tasted virtually identical and contained the same number of calories and nutrients. Neither the participants nor the researchers collecting the data knew which meal was being served. Earlier studies compared dissimilar foods—a piece of cheesecake versus vegetables, for example—so those test results could have been skewed by participants’ liking for or aversion to a particular food.
Also, by using new technology, the researchers could measure direct brain responses. Many previous fMRI studies were unable to quantify the amount of the blood flow changes in the brain.
This study actually brings some good news: The findings suggest that, if people succeed in avoiding high-GI foods, they may be able to interrupt the vicious self-perpetuating cycle of carb cravings. In other words, if you can muster the strength to stay away from simple carbs for a while, it will get easier for you keep off of them. And that is motivating in itself! For additional advice on overcoming cravings, see “More on Food Cravings,” above.
