The findings, published in the American Journal of Clinical Nutrition, imply that the quick spike and subsequent crash in blood sugar that comes after eating highly processed carbs activates reward and addiction centers in the brain.
The study was small and looked only at overweight and obese men, but if the findings can be replicated in a larger population, they would suggest that avoiding refined carbohydrates might be a good weight-loss strategy because people would avoid not only the calories but also the strong cravings that the carbs they induce.
“Refined carbohydrates seem to be able to provoke food cravings many hours after consumption, at least in susceptible people,” said study co-author David Ludwig, director of the New Balance Foundation Obesity Prevention Center in Boston. “Limiting these foods could help overweight people avoid overeating.”
The body typically breaks down carbohydrates into blood sugar, or glucose. Whole grains and carbohydrates in vegetables take a long time to break down, raising blood sugar slowly, whereas refined carbs such as sugar, corn syrup and white flour cause sharp spikes in blood sugar.
Research suggests that refined carbohydrates are tied to pleasurable feelings, but those studies typically compared enormously different foods: cheesecake vs. steamed broccoli, for instance. That made it difficult to discern whether the carbohydrates, or some other part of the food, caused the cravings.
To tease out this difference, Ludwig and his colleagues gave 12 overweight and obese young men two drinks. One contained cornstarch, which has a low glycemic index, meaning it causes only a gradual rise in blood sugar. The other drink contained corn syrup, which has a high glycemic index. The researchers used artificial, zero-calorie sweeteners to make the two drinks taste identical.
The men who received the high-glycemic-index drink showed a dramatic spike in blood sugar after consuming the drink. Four hours later, their blood sugar levels crashed, and they reported being very hungry.
At the same time, brain scans of those who drank the corn syrup drink showed increased activation in the nucleus accumbens, a region of the brain that has been tied to reward and cravings in past research, compared with the men who drank the low-glycemic-index drink.
Though the study looked only at obese people, further studies might clarify whether avoiding processed carbs could help lean people avoid packing on weight, Ludwig said.
The authors hypothesize that the sharp drop in blood sugar spurred by cookies, chips or cake may not only stimulate hunger but also make the idea of eating seem more rewarding and pleasurable to the brain.