Mice that chowed down on too many calories gained weight. In another sense, these plump animals became big losers: One in every four of their taste buds disappeared. The findings come from a new study of animals on a high-fat diet.
A taste bud is a collection of 50 to 100 cells. Found on the surface of the tongue, these cells sense whether a food is sweet, sour, bitter, salty or umami (savory). Taste bud cells help identify safe and nourishing food. Some also turn on reward centers in the brain.
Each bud lasts only about 10 days. So the tongue must constantly make new ones. Special progenitor (Pro-GEN-ih-tur) cells will give rise to the replacement taste buds.
Studies had suggested that a sense of taste can dull in people with obesity. Scientists don’t know why this might happen. But if taste does weaken, “then maybe you don’t get the positive feeling that you should [from eating],” says biologist Robin Dando. He works at Cornell University in Ithaca, N.Y. There, he’s been using animals to probe the biology of taste.
His team’s new study is looking into whether a diminished sense of taste might explain some overeating. People might find less satisfaction in a healthy portion of food.
The issue is an important one. Nearly one in five U.S. children and teens are overweight to the point of obesity. Heart disease, diabetes and cancer are among health problems that have been linked with being so seriously overweight.
For one new study, Dando’s group compared two groups of mice from the same litter (group of siblings). Over eight weeks, one group ate normal mouse food. At the end, their weight was in a healthy range. In addition, their tongues hosted a normal number of taste buds. The other animals ate only high-fat meals. During the trial they quickly plumped up — and lost about one-fourth of their taste buds.
The researchers described their findings March 20 in PLOS Biology.
The inflammatory findings
Cells in the taste buds of obese mice died off faster than they did in healthy-weight mice, the study showed. At the same time, the obese mice made fewer new cells to take their place.
Inflammation is a process in which the body makes immune cells, proteins and other materials in response to injury or infection — or obesity. It can cause redness, pain and swelling. Inflammation usually goes away once the body has healed or fought off an infection. But obesity tends to be a chronic condition. Not surprisingly, many studies in humans and other animals have shown that obesity seems to trigger persistent, low-level inflammation (In-fluh-MAY-shun). And it can occur throughout the body.
Brief bouts of inflammation can be helpful. They can kill off germs and sick cells. If inflammation persists, however, it risks harming healthy cells. Such nonstop inflammation now appears to underlie the loss of taste buds.
Taste buds in the obese mice had excess levels of a type of protein known as a cytokine (SY-toh-kyne), the researchers found. This cytokine regulates inflammation. Called tumor necrosis (Neh-KRO-sis) factor alpha, this protein also seems to damage taste buds, the researchers report.
They figured this out by running two different tests. In one, they investigated what happens in obese mice that can’t make the protein. And numbers of taste buds in them never dropped.
In the second test, they fed high-fat food to mice given genes that would not let their bodies gain extra weight. Because these mice never became obese, they never developed the inflammation linked to obesity. These mice, too, had a normal number of taste buds.
Taken together, these data suggest the fat mice lost their taste buds due to obesity-linked inflammation.
The take-home message
Indeed, that conclusion “provides a possible link between obesity and taste,” says Kathryn Medler. She works at the University at Buffalo in New York and was not involved with the new study. But she knows much about the subject. She’s a physiologist who studies how taste works.
Along with learning more about how inflammation damages taste buds, Dando is interested in looking for new treatments for obesity. One approach might be to restore the diminished sense of taste. “These mice lose taste buds,” he says. “Can we bring them back?”
biology The study of living things. The scientists who study them are known as biologists.
cancer Any of more than 100 different diseases, each characterized by the rapid, uncontrolled growth of abnormal cells. The development and growth of cancers, also known as malignancies, can lead to tumors, pain and death.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
chronic A condition, such as an illness (or its symptoms, including pain), that lasts for a long time.
constant Continuous or uninterrupted.
cytokine A substance secreted by certain cells of the immune system which the body uses to have some particular controlling effect on other cells. Examples include interferons, interleukins and growth factors.
diabetes A disease where the body either makes too little of the hormone insulin (known as type 1 disease) or ignores the presence of too much insulin when it is present (known as type 2 diabetes).
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.
fat A natural oily or greasy substance occurring in plants and 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 also is a vital nutrient, though it can be harmful if consumed in excessive amounts.
inflammation (adj. inflammatory) The body’s response to cellular injury and obesity; it often involves swelling, redness, heat and pain. It also is an underlying feature responsible for the development and aggravation of many diseases, especially heart disease and diabetes.
link A connection between two people or things.
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
obese Extremely overweight. Obesity is associated with a wide range of health problems, including type 2 diabetes and high blood pressure.
obesity (adj. obese) Extreme overweight. Obesity is associated with a wide range of health problems, including type 2 diabetes and high blood pressure.
overweight A medical condition where the body has accumulated too much body fat. People are not considered overweight if they weigh more than is normal for their age and height, but that extra weight comes from bone or muscle.
progenitor Ancestor, parent or originator. (in biology) Some individual — or thing (perhaps even a cell) — from which all others descend.
protein A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.
ratio The relationship between two numbers or amounts. When written out, the numbers usually are separated by a colon, such as a 50:50. That would mean that for every 50 units of one thing (on the left) there would also be 50 units of another thing (represented by the number on the right).
reward (In animal behavior) A stimulus, such as a tasty food pellet, that is offered to an animal or person to get them to change their behavior or to learn a task.
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.
sibling An offspring that shares the same parents (with its brother or sister).
taste One of the basic properties the body uses to sense its environment, especially foods, using receptors (taste buds) on the tongue (and some other organs).
taste buds A collection of 50 to 100 or so taste receptors. They’re found on the tongues of land animals. When certain chemicals in food or other materials trigger a response in these receptors, the brain detects one or more flavors — sweet, sour, salty, bitter or umami.
tissue Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.
umami One of the five major tastes (along with sweet, sour, salty and bitter). It has been described as savory but most people find the mild flavor hard to characterize. It is particularly prized as a flavor in Japanese cuisines.