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.”
(for more about Power Words, click here)
behavior The way a person or other organism acts towards others, or conducts itself.
brain scan The use of an imaging technology, typically using X rays or a magnetic resonance imaging (or MRI) machine, to view structures inside the brain. With MRI technology — especially the type known as functional MRI (or fMRI) — the activity of different brain regions can be viewed during an event, such as viewing pictures, computing sums or listening to music.
calorie The amount of energy needed to raise the temperature of 1 gram of water by 1 degree Celsius. It is typically used as a measurement of the energy contained in some defined amount of food.
carbohydrates Any of a large group of compounds occurring in foods and living tissues, including sugars, starch and cellulose. They contain hydrogen and oxygen in the same ratio as water (2:1) and typically can be broken down to release energy in the animal body.
colon (in biology) The majority of the large intestine, it runs between the cecum (a pouch below the small intestine) and the rectum. Foods are not digested in the colon, although this tissue lubricates wastes that will be excreted. Some liquids and salts, however, will be removed from materials stored in the colon before excretion.
diet The foods and liquids ingested by an animal to provide the nutrition it needs to grow and maintain health. (verb) To adopt a specific food-intake plan for the purpose of controlling body weight.
digest (noun: digestion) To break down food into simple compounds that the body can absorb and use for growth. Some sewage-treatment plants harness microbes to digest — or degrade — wastes so that the breakdown products can be recycled for use elsewhere in the environment.
fat A natural oily or greasy substance occurring in animal bodies, especially when deposited as a layer under the skin or around certain organs. Fat’s primary role is as an energy reserve. Fat is also a vital nutrient, though it can be harmful to one’s health if consumed in excess amounts.
fatty acid A large molecule made of up chains of carbon and hydrogen atoms linked together. Fatty acids are chemical building blocks of fats in foods and the body.
fermentation (v. ferment) The metabolic process of converting carbohydrates (sugars and starches) into short-chain fatty acids, gases or alcohol. Yeast and bacteria are central to the process of fermentation. Fermentation is a process used to liberate nutrients from food in the human gut. It also is an underlying process used to make alcoholic beverages, from wine and beer to stronger spirits.
fMRI (functional magnetic resonance imaging) A special type of body scan used to track brain activity. It uses a strong magnetic field to monitor blood flow in the brain. Tracking the movement of blood can tell researchers which brain regions are active. (See also, MRI or magnetic resonance imaging).
magnetic resonance imaging (MRI) An imaging technique to visualize soft, internal organs, like the brain, muscles, heart and cancerous tumors. MRI uses strong magnetic fields to record the activity of individual atoms.
microbe Short for microorganism . A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.
microbiome The scientific term for the entirety of the microorganisms — bacteria, viruses, fungi and more — that take up permanent residence within the body of a human or other animal.
nutrition The healthful components (nutrients) in the diet — such as proteins, fats, vitamins and minerals — that the body uses to grow and to fuel its processes.
oxygen A gas that makes up about 21 percent of the atmosphere. All animals and many microorganisms need oxygen to fuel their metabolism.
processed foods Foods purchased from a grocery story that are substantially different from the raw materials that went into them. Examples include most foods that come in cans, bottles, boxes or bags. Examples include breakfast cereals, frozen pizzas, canned tuna, jars of spaghetti sauce and dill pickles.
propionate A short-chain fatty acid that is released from dietary fiber in the gut. Propionate in the colon can cause people to choose foods with fewer calories.
recruit (in research) New member of a group or human trial, or to enroll a new member into a research trial. Some may receive money or other compensation for their participation, particularly if they enter the trial healthy.
resistant starch A form of starch that is not broken down in the gut. Instead, it is fermented by microbes, releasing fatty acids. Some of those fatty acids may help to protect against cancer.
reward center (Also reward system) A region of the brain that processes the pleasant reactions we get when we get smiles, gifts, pleasurable stimuli (including food) or compliments.
supplement (verb) To add to something. (in nutrition) Something taken in pill or liquid form — often a vitamin or mineral — to improve the diet. For instance, it may provide more of some nutrient that is believed to benefit health.
Journal: C.S. Byrne et al. Increased colonic propionate reduces anticipatory reward responses in the human striatum to high-energy foods. American Journal of Clinical Nutrition. Published online May 11, 2016. doi: 10.3945/ajcn.115.126706.
Journal: E.S. Chambers et al. Effects of targeted delivery of propionate to the human colon on appetite regulation, body weight maintenance and adiposity in overweight adults. Gut. Published online December 10, 2014. doi: 10.1136/gutjnl-2014-307913.