Putting Fat On The Taste Map

Putting Fat On The Taste Map

We've asked or answered the question innumerable times: What do you feel like eating?

Beef or fish? Chinese or Mexican? Pasta or potatoes? Chocolate or vanilla?

Perhaps the more important question is, what does your body need?

Researcher Tim Gilbertson
Clues to answering those questions are right on the tip of your tongue. Actually, they're all over your tongue, in your stomach, small intestine, and your brain. Taste receptor cells that send and receive signals from the brain allow us to taste our favorite things, help us avoid eating some potentially dangerous ones, trigger searches for certain foods and, neurobiologist Tim Gilbertson believes, prompt us to eat things our bodies need.

Why we like some foods, dislike others, crave certain flavors or textures and sometimes eat more than we should are all extremely complicated questions. Finding answers requires a careful blending of molecular biology, physiology, biochemistry and behavioral studies.

"The primary goals of the research in my lab are to understand how taste receptor cells function," Gilbertson says. "We also want to know how the body recognizes carbohydrates, proteins, fats, and the essential nutrients we need for survival."

People have long believed that the taste system is passive, that it does nothing until a stimulus comes along and then it reacts. By contrast, Gilbertson says he believes the taste system has a very active role in controlling what we choose to eat.

Fat is among the nutrients we need, though it has gotten a bad name as obesity and its accompanying health problems have become more prevalent in the United States. Of course, in the days when people expended more calories doing physically demanding tasks just to survive—hunting for food, carrying water, walking miles while guiding a plow horse—fat was an especially important part of the diet.

"If you're an animal foraging in the wild or someone out there trying to find food the best thing you could find is fat," Gilbertson says. "It is the most energy dense food at nine kilocalories of energy per gram. Protein and carbohydrates have
only four."

graphic, tongue map
Scientists long believed that fat did not have a taste. Gilbertson explains that flavor—the combination of taste, smell and texture—is a complex thing and people have long known that when you add fat to food it enhances the flavor. But most believed that fat contributes only to the texture portion of the flavor equation, enhancing what food scientists and chefs refer to as mouth feel.

Several years ago Gilbertson and his research team set out to challenge the notion that fat has no taste.

"We talk about salty, sweet, sour, bitter and umami (Japanese for 'delicious,' describing a unique taste blend of salty, meaty and sweet associated with proteins), but we've left out a big one and that's fat," Gilbertson says. "You don't hear people say, 'That tastes like fat.' We talk about mouth feel and historically assumed that fat had no taste."

Researchers in Gilbertson's lab isolate individual taste receptor cells, primarily from rodents, and attach tiny glass electrodes (1/10 of a micron) and record the electrical activity that results when the cells contact a nutrient or taste stimulus. Taste cells, like many neurons, deal in changes of electrical activity as they signal the brain.

"Most taste stimuli activate the cells and the electrical activity changes according to the concentration and how long the stimulus is on the cell," Gilbertson explains. "For example, saliva has about 50 millimoles of sodium chloride, but you don't taste that as salt because you've adapted to it. Whereas, if we rinse your mouth with distilled water and then you drink a solution with 50 millimoles of sodium chloride you find it tastes salty."

Most fats are in the form of triglycerides. When Gilbertson's team applied triglycerides to taste receptors the electrodes registered no change. But in most foods that contain fat there are other compounds, including free fatty acids.

"We put fatty acids on taste cells and saw huge increases in activity," Gilbertson says.

"It was really the first report of anyone being able to show that something in fat activated the taste system. We jumped on the findings and looked to find the parameters of the response, looked to see if all fatty acids activated the system or just specific ones."

What they found was that the response was limited to polyunsaturated fats, an interesting discovery in light of Gilbertson's belief that a primary role of the taste system is to detect nutrients that are important for survival.

"We must have polyunsaturated fats in our diet in order to survive," he explains. "Monounsaturates and saturated fatty acids did not activate the taste receptors. But there are enzymes in the body that can change polyunsaturates in the other forms so we don't have to have them in our diet to survive."

Gilbertson says one reason fat substitutes may not have become wildly popular is that they are designed to imitate the texture of fat while ignoring that there is a taste factor as well. The research team further discovered that fatty acids send messages to the brain through a particular ion channel and modulates the taste system's response to other stimuli. That may be why we like things that combine fat and sweet, like chocolate, or fat and salty, like chips. The fats come in and block the ion channel which extends and heightens our response to the sweet or salty stimuli.

"Ask people about fat-free foods and most of them say, 'They don't taste right,' or 'They taste like cardboard,'" Gilbertson says. "That is partially texture contributing to the flavor, but we think it also involves the taste. We're looking at the implications of what we've found and how we might be able to trick the body into thinking it's had fat when it hasn't. Down the road, knowing which molecules activate our receptor cells may help develop a fat substitute that will satisfy consumers and may help us get our average 40 percent fat diet down to 30 or 20 percent so we might see a decrease in heart disease, diabetes and obesity's other attendant problems."

Contact: Tim Gilbertson (435) 797-7314, tag@biology.usu.edu
Reprinted from Utah Science
Story by Lynnette Harris (435) 797-2189. lynnette@agx.usu.edu
Photo by Gary Neuenswander, garyn@agx.usu.edu
Illustrations by Mary Donahue, mdonahue@cc.usu.edu

Would You Like Fat With That?

graphic, cake and fat
Figuring out why people and animals choose certain foods and pass on others is a complicated endeavor. Clearly, people have emotional and economic reasons for some of their food choices. But does biology tip the scale and, consequently, cause us to tip the scale?

Neurobiologist Tim Gilbertson set out to find whether the way individuals sense fat in combination with other tastes might affect their food choices. His research team bred a group of rats that were closely related, but with slight and specifically selected genetic differences. They allowed the rats to choose meals from among food in three dishes—one containing protein, one with sugars or carbohydrates, and one with fat—and measured how much each rat ate every day.

What they found were clear differences between "fat preferers" and "fat avoiders" that soon became visible differences in body composition.

"Fat preferers took fifty percent or more of their calories from fat," Gilbertson says. "We call this one our 'typical American' rat. This is the rat that given the opportunity would run downtown for a burger or pizza every day."

Their fat avoiding cousins, however, ate mostly carbohydrates and got about 10 percent of their calories from fat. Next, the rats were offered only the high fat food. The fat avoiders ate the right number of calories to maintain normal weight, but the fat preferring rats ate themselves quickly into a state of morbid obesity and continued eating as long as food was available.

When the researchers isolated taste receptor cells from the rats and measured the response to fatty acids they found dramatic differences. Taste cells from the fat preferers had a much lower response to the fatty acids, while the other rats were very sensitive.

"We know the brain activates some negative feedback pathway that tells you to stop eating," Gilberson says. "It took a lot longer for the less sensitive rat to get there and know he'd eaten enough. For example, you may love chocolate cake, but if you were given a whole cake and forced to eat it all you wouldn't like it much by the end. The fat preferer could eat more of the cake before it got the signal to stop."

It's a leap to extrapolate findings from rats to humans, but the signal-transporting potassium channels that tell you when you've had enough may work in similar ways. Those channels may offer another target for appetite-suppressing drugs. Another interesting twist of the research will be investigating whether feeding very young animals a high fat diet will change the way their taste receptor cells and cells in the small intestine respond to fat and predispose them to being fat preferers as adolescents and adults.

Reprinted from Utah Science
Story by Lynette Harris