Boston Children’s Hospital researchers who scanned the brains of men after they drank milkshakes containing rapidly digesting, highly processed carbohydrates found the men experienced a surge in blood sugar followed by a sharp and sudden crash four hours later.
That plummet in blood sugar activated a powerful hunger signal and stimulated the brain region considered ground zero for addictive behaviour.
“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 lead author Dr. David Ludwig.
The study was small and focused exclusively on men. As well, the notion of food addiction is highly controversial and “vigorously debated,” the team writes.
Still, the findings suggest that limiting foods high in highly processed, “high glycemic index” carbs such as white breads, white bagels, white rice, potatoes and concentrated sugars could help overweight and obese people control the urge to overeat, they said.
The research was inspired by the work of renowned University of Toronto researcher Dr. David Jenkins, who, in 1981, together with colleagues, first proposed the concept of the glycemic index — a measure of how fast, and by how much, foods raise blood sugar and insulin levels.
In the new study, the Boston team wondered whether the sudden rapid surge then crash in blood sugar from eating high GI foods could directly affect the brain.
“Overweight people, by definition, overeat. They’re consuming too many calories to keep themselves at a healthy body weight,” said Ludwig, director of the New Balance Foundation Obesity Prevention Center at Boston Children’s Hospital.
“Despite many people’s best efforts they’re unable to stick to a reduced calorie diet over the long-term,” he said. People tend to regain whatever weight they lost, “and then some,” after six months or a year.
“We wondered whether that could, in part, be driven by changes in brain function caused by refined carbohydrates.”
Earlier studies have shown that tasty, high-calorie foods can trigger the pleasure centre in the brain, raising the notion of “food addiction.”
But Ludwig said those studies typically compared “grossly different foods,” such as cheesecake versus boiled vegetables.
His team performed functional MRI brain scans — machines that capture the brain at work in real-time — on 12 overweight or obese men aged 18 to 35 after they consumed two liquid test meals that looked and tasted identical, and contained the same amounts of calories and carbohydrates.
The only difference was that one shake contained fast-digesting, high-GI carbs, the other slow-digesting carbs.
After the high GI liquid meal, blood sugar surged initially, but then crashed four hours later. The men not only reported greater hunger, their MRI scans also showed intense activation in the nucleus accumbens, the part of the brain involved in reward and craving.
“Humans need food to survive,” Ludwig said. “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.”
The study appears in the American Journal of Clinical Nutrition.
Currently, about 62 per cent of the Canadian adult population is overweight, and the heaviest weight classes are growing the fastest.