Sugary foods and drinks, white bread and other processed carbohydrates that are known to cause abrupt spikes and falls in blood sugar appear to stimulate parts of the brain involved in hunger, craving and reward, a new research shows.
The findings suggest that limiting 'high-glycaemic index' foods could help obese people avoid overeating. The study, published in the American Journal of Clinical Nutrition, investigated how food intake is regulated by dopamine-containing pleasure centres in the brain.
"This research suggests that based on their effects on brain metabolism, all calories are not alike," he said. "Not everybody who eats processed carbohydrates develops uncontrollable food cravings. But for the person who has been struggling with weight in our modern food environment and unable to control their cravings, limiting refined carbohydrate may be a logical first step."
In addition to raising blood sugar, foods that are sugary and highly caloric elicit pronounced responses in distinct areas of the brain involved in reward. Earlier imaging studies have shown, for example, that the main reward and pleasure center, the nucleus accumbens, lights up more intensely for a slice of chocolate cake than for blander foods like vegetables, and the activation tends to be greater in the brains of obese people than it is in those who are lean.
But Ludwig said those studies typically compared “grossly different foods,” such as cheesecake versus boiled vegetables. Instead, Dr. Ludwig and his colleagues recruited a dozen obese men and then fed them milkshakes on two different occasions separated by several weeks. In each case, the milkshakes were nearly identical: flavored with milk and vanilla, and containing the same amount of calories, carbohydrates, protein and fat. But on one occasion, the shakes were made with high-glycemic corn syrup; on the other, a source of low-glycemic carbohydrates was used.
"These test meals were identical in appearance and tastiness, and we verified that our subjects had no preference for one or the other," Dr. Ludwig said. As expected, blood sugar levels rose more quickly in response to the high-glycemic milkshake. But the researchers were especially interested in what happened several hours later, about the time most people are ready for their next meal.
What they found was that four hours after drinking the high-glycemic shake, blood sugar levels had plummeted into the hypoglycemic range, the subjects reported more hunger, and brain scans showed greater activation in parts of the brain that regulate cravings, reward and addictive behaviors.
"We showed for the first time that refined carbohydrates can trigger food cravings many hours later, not through psychological mechanisms — a favourite food is just so tasty, you need to keep eating — but through biological effects" on the brain, said Dr. David Ludwig.
Previous research suggests that when blood sugar levels plummet, people have a tendency to seek out foods that can restore it quickly, and this may set up a cycle of overeating driven by high-glycemic foods, Dr. Ludwig said. "It makes sense that the brain would direct us to foods that would rescue blood sugar," he said. "That’s a normal protective mechanism."
"Humans need food to survive," Ludwig added. "But in the last few decades, our food supply has been transformed by highly-processed, hyper-palatable food products." As a result, the glycemic load of the typical diet has risen substantially, he said. "Our research suggests that some of these foods might hijack the reward systems of the brain and produce symptoms related to addiction."
Christopher Gardner, a nutrition scientist at Stanford University who was not involved in the new study, said that after decades of research but little success in fighting obesity, "it has been disappointing that the message being communicated to the American public has been boiled down to ‘eat less and exercise more.’"
