The latest trick to fighting obesity isn’t a focus on eating less fat or sugar (although that would probably help): It’s eating more fiber. And not just any kind of fiber. It should be the fermentable type. Microbes in the gut chow down on this type of fiber. As they break it down, they release a chemical that moves to the brain. There it curbs appetite.
Researchers at Imperial College London, in England, say their study is the first to link eating fiber to the brain hormones that help you feel full. They published their findings April 29 in Nature Communications.
Scientists have known that obese people tend to eat foods low in dietary fiber. At some level, that made sense. Fermentable fiber — fiber that is broken down by gut microbes — makes people feel full after eating somewhat less. Such fiber is found in fruits, vegetables, oats and barley.
Hundreds of thousands of years ago, our paleolithic ancestors ate nearly 100 grams of fermentable fiber each day, says Gary Frost. He’s a dietician who led the new study. Today, people’s diets are very different. We eat only 10 to 20 grams of all types of fiber each day. Most comes from whole-grain wheat and bran. Those types do not break down in the gut, Frost notes. Modern diets also are full of convenience foods, such as snacks and frozen dinners. These tend to be high in fat and sugar. Add in our couch-potato lifestyle and it’s a recipe for obesity.
Eating more fermentable fiber reduces the risk of becoming overweight or obese. But until the new study, exactly how the fiber did that was unclear.
In the new study, researchers fed one group of mice a high-fat diet. It also was high in fermentable fiber. Another group received a high-fat diet with non-fermentable fiber. After eight weeks, the first group weighed less. Those mice also had eaten less food than the second group.
Next, the scientists looked at the effect of one particular chemical: acetate. Also known as acetic acid, it is a common by-product of the breakdown of sugars and starches by gut microbes.
To learn how the chemical affects weight, the scientists injected the mice with acetate. But first they labeled the acetate with carbon-11. This radioactive isotope of carbon has only five neutrons, rather than the usual six. The radiation it gave off allowed researchers to track its location. To do that, they put each mouse in a PET-scanner. This device creates a three-dimensional image showing the location of the radioactive “label” on each acetate molecule.
The acetate traveled through the blood and into the mice’s brains. There it collected in a region known as the hypothalamus, which controls appetite. By releasing hormones, that brain region can shut off hunger and promote satiety — a feeling of fullness. As a sign that’s indeed happened, mice injected with acetate ate less food than untreated mice.
Researchers then labeled fermentable fiber with carbon-13. This non-radioactive form of carbon contains an extra neutron. That bonus neutron causes the nucleus of this atom to spin. And that spin allowed investigators to track its movement through the body.
After feeding mice the labeled food, the researchers examined the rodents’ brains. Microbes in the gut had broken the fiber down into acetate. Again, the labeled acetate traveled to the hypothalamus. There it accumulated. This was clear evidence that chemicals released during fiber fermentation — or breakdown — act on the brain to control appetite, says Frost.
It’s a good study showing the benefits of fiber intake, says Satya Kalra. A neuroscientist at the University of Florida in Gainesville, he was not involved in the new study.
Should people start eating pure acetate to reduce food consumption? Absolutely not, says Frost. The best way to control weight is by eating food from natural sources. That means eating more foods with fermentable fiber. “The bugs in the gut will change the fiber into acetate in a way the body has always done,” he says.
So be sure to load up on those fruits and veggies.
acetate (also called acetic acid) A short-chained fatty acid that is a common byproduct of fiber fermentation in the gut. Acetate appears to play a role in preventing obesity.
atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
carbon-11 Carbon atoms with 6 protons and 5 neutrons in the nucleus. This radioactive isotope of carbon allows researchers to track its location in the body using a PET scanner.
carbon-13 Carbon atoms with 6 protons and 7 neutrons in the nucleus. This isotope of carbon has a magnetic spin that allows researchers to detect its presence.
fatty acid Large molecules made of up chains of carbon and hydrogen atoms linked together.
fermentable fiber The type of dietary fiber found in fruits, vegetables, oats and barley. As microorganisms in the gut feed on it, they break it down through a process known as fermentation. Fermentable fiber contributes to a feeling of fullness after eating.
fermentation 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.
hormone A chemical produced in a gland and then carried in the bloodstream to another part of the body. Hormones control many important body activities, such as growth. Hormones act by triggering or regulating chemical reactions in the body.
hypothalamus A region of the brain that controls bodily functions by releasing hormones. The hypothalamus is involved in regulating appetite through release of appetite-suppressing hormones.
isotope A variant of an element. Isotopes of an element have the same number of protons but different numbers of electrons and therefore different masses.
microbe Short formicroorganism. (see microorganism)
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.
neutron A subatomic particle carrying no electric charge that is one of the basic pieces of matter. Neutrons belong to the family of particles known as hadrons.
nucleus Plural is nuclei. (in physics) The central core of an atom, containing most of its mass.
paleolithic A prefix meaning old age of stone, it refers to events or residents of the early Stone Age. This period began about 750,000 years ago and ended around 200,000 years ago. A hunter-gather period, it predates when people began farming.
PET-scan (short for positron emission tomography) A test that uses radiation to creates three-dimensional images of the inside of the body. The individual receives a radioactive “tracer” chemical in the blood that shows up during the scan. As the tracer moves through the body, it will accumulate in certain organs. This allows researchers and doctors to see create X-ray-like details of those organs.
satiety A feeling of fullness. It’s the opposite of being hungry. The body tends to register satiety through the release of certain brain hormones. Someone who is full, after a meal, is said to be sated.