Fats and sugars make foods especially tasty. That can prompt people to overindulge, leading to weight gain. The good news? Munching on foods that are high in fiber can help us resist fattening alternatives. That’s because fiber-rich diets can change the brain’s response to high-calorie foods, a new study finds.
Fiber comes in two forms — soluble and insoluble. Our bodies can break down insoluble fiber a little bit. They can’t break down soluble fiber at all, but bacteria in the gut can. They do it using a process known as fermentation. During the fermentation of this fiber, gut microbes release short-chain fatty acids. Fatty acids are the building blocks of fats.
One of those fatty acids, called propionate (PRO-pee-un-ayt), can affect how much people eat, notes Gary Frost. He’s a diet and nutrition expert at Imperial College London in England. When bacteria living in a part of the gut known as the colon digest foods high in soluble fiber, they can release propionate.
In a 2014 study, Frost’s team created a dietary supplement that brings high levels of propionate to the colon. When the team fed the supplement to people, the participants altered their eating habits. They downed fewer calories. What’s more, Frost notes, that study “showed that people who ate this ingredient gained less weight.” However, he adds, at the time “we did not know why.”
Frost teamed up with Tony Goldstone to find out. Goldstone, also at Imperial College London, specializes in brain imaging and eating behavior. The two wanted to know if propionate changed food cravings by altering activity in what is known as the brain’s reward center.
To do this, they recruited 18 healthy adult men. Each came to the university lab before breakfast. The research team fed the men their early meal, which contained soluble fiber. Half of these breakfasts also contained Frost’s propionate supplement. Three hours later, the men received a cheese sandwich and a snack bar for lunch. Six hours after starting breakfast, they received a third meal. This time, they were invited to eat as much as they wanted of a tomato-mozzarella pasta entree. Those that had gotten the propionate supplement at breakfast now ate less — almost 10 percent less pasta.
The results were published in the July American Journal of Clinical Nutrition.
Five hours after a recruit had sat down to his breakfast, the researchers scanned the man’s brain. They used a device to perform magnetic resonance imaging, or MRI. It uses a powerful magnet to measure changes in oxygen in the small blood vessels of the brain. Those changes reflect the activity of the brain in these areas. But the researchers didn’t make the measurements just once. They made a whole succession of them as each participant looked at a series of photos. Such scans are known as functional magnetic resonance imaging, or fMRI.
Some of the pictures showed high-calorie foods, such as cake, chocolate or pizza. Others depicted low-calorie foods, like salad, vegetables or fish. Still others showed non-food items, such as clothing or furniture. With each new image, the men rated on a five-point scale how appealing they found it. After one hour in the MRI machine, the participants sat down to their pasta lunch.
Men who had eaten the propionate supplement as part of their breakfast rated the high-calorie foods less appealing than the other recruits had. Areas of the brain associated with rewards also became less active in these men. But that was true only as they viewed some high-calorie food. Their brain activity was no different from that of the other recruits when they viewed low-calorie fare.
The new data fill in a missing piece of the puzzle, Frost says. High levels of propionate changed the brain’s food-reward system and cut how much food the men eat. But, he observes, it would be difficult to eat enough fiber from foods to get as much propionate as the supplement had delivered.
Bacteria and other gut microbes make up much of what’s called the body’s microbiome. “This study connects the microbiome to the brain’s reward systems,” says Nicole Avena. She is a nutrition expert at Mount Sinai St. Luke’s Hospital in New York City and was not involved with the study.
Many foods are “highly-processed and activate brain reward systems,” she notes. They — and everything else we eat — all “contribute to the makeup of our gut microbiome. So that is why it is important to make sure we eat a healthy, balanced diet.”