Tongue sensors seem to taste fat

Receptor may determine desire for dietary fat

As you stand at buffet tables during holiday parties this year, it might cheer you up to know most people don’t gain as much weight over the holidays as once was thought. Instead of five or 10 pounds, most of us actually gain only a pound or two. But it might depress you to know that weight gain happens one pound at a time and in the long run, it may be hard to avoid — especially for some of us, because some of the taste buds in our tongues are programmed to make us crave fatty food — and fat is everywhere in our diets.

The fatty acid receptor CD36 is found in many tissues including the surface of taste cells in the tongue.  CD36 recognizes and facilitates the transfer of fatty acids into cells, by still unknown mechanisms.
The fatty acid receptor CD36 is found in many tissues, including the surface of taste cells in the tongue. CD36 recognizes and facilitates the transfer of fatty acids into cells, by still unknown mechanisms.

French researchers recently reported that mice have a receptor in their tongues that can sense fat, and the presence of that receptor seems to drive the mice to crave fat in their diets. The work was based on research from Nada A. Abumrad, Ph.D., the Dr. Robert C. Atkins Professor of Medicine and Obesity Research at Washington University School of Medicine in St. Louis. She previously had identified a protein receptor for fat and documented its function in recognizing and using fatty food. This led the French researchers from the Taste Institute in Dijon, France, to wonder whether the protein also may have a role in actually tasting fat.

“Fat sensing has been very controversial,” Abumrad says. “It once was thought that we could sense five different tastes: sweet, salty, sour, bitter and what scientists refer to as umami, which is the taste of a protein like monosodium glutamate. There was some indirect evidence that the tongue might be able to identify fat, too, but many scientists thought that involved sensation of texture more than the actual taste of fat.”

Abumrad adds that several scientists had proposed people might not only sense the texture of fat, but also might have fatty acid receptors that lead them to prefer foods containing fat. She studies the molecular mechanisms regulating utilization of fatty acids, and she was the first to identify a protein called CD36 that facilitates the uptake of fatty acids. The CD36 receptor protein is located on the surface of cells and distributed in many tissues, including fat cells, the digestive tract, heart tissue, skeletal muscle tissue and, as it happens, the tongue.

In the mouse experiments conducted by the researchers at the University of Bourgogne in Dijon, the rodents were fed two solutions: one laced with fat and the other containing a gummy, fat-free substance that mimicked the feel of fat in the mouth. Normal mice preferred the fatty solution, but mice that had been genetically engineered without the CD36 receptor protein didn’t have that preference. In additional experiments in laboratory rats, the scientists found that removal of the CD36 gene kept the animals’ intestines from initiating secretion of digestive juices necessary to digest fat.

Abumrad, who authored an accompanying commentary in the Journal of Clinical Investigation, says it’s possible that the amount of the CD36 receptor in our systems might help regulate our cravings for fat.

In her own research, Abumrad studies how CD36 modulates the acute and chronic responses of muscle and fat cells to energy fluctuations and other stresses. The goal is to translate her findings from rodents into humans, where variations in the CD36 gene are common.

Nada Abumrad
Nada Abumrad

“There is evidence that people have different amounts of the CD36 receptor and that mutations in the receptor are quite common,” she says. “Those variations are associated with abnormalities of blood lipids, with high levels of fatty acids in the blood, abnormal blood triglycerides and increased risk of diabetes-associated heart disease. It’s clear that some of us have different amounts of this protein in different tissues, and some individuals are completely deficient for it.”

What no one yet knows is whether variations in the protein might change the way that our tongues sense fat. It’s possible people with very low levels of CD36 can’t taste fat and may not have the preference for fatty food that those with high CD36 levels would have. Abumrad says it will be exciting to find out whether different amounts of the receptor protein correlate with a person’s preference for fat.

“An additional problem with fat is that when you mix it with other things, it makes them taste even better,” she says. “We think of ice cream as being sweet, but I think it wouldn’t be as good if it weren’t creamy. There is something about fat that makes foods more palatable, and as we move forward in this research, we’ll learn more about how big a role this receptor has in how much we want fat.”

It certainly seems to play a role in how much mice want fat, but Abumrad says simply shutting off CD36, as the French researchers did with genetically engineered mice, wouldn’t be a good idea for people. Because the protein has a number of vital functions in the body, getting rid of it completely could have negative effects. But she says as scientists learn more about how CD36 works, they might be able to design artificial fats tailored to the receptor that would taste much more like the real thing.

Future therapies also might be able to activate the receptor in some parts of the body and disable it in others. Although we probably would prefer lower levels in the tongue and the stomach, it might be advantageous to have high levels of CD36 in the muscles. If the muscles could take up and burn more fat, they would be able to work harder for longer periods of time. In fact, mice without CD36 didn’t have as much endurance and seemed to get tired more quickly.

Abumrad says it’s clear that there once were good reasons to prefer fatty foods. When food was scarce, and humans were hunter-gatherers, it wasn’t unusual to go for days between meals, and storing fat helped people through the lean times. In addition, the taste buds not only helped protect us from foods that were toxic, she says they also served the purpose of preparing the digestive system to process foods.

When a fatty acid comes into contact with the CD36 protein on the tongue, a signal travels to the brain and then to the digestive tract, launching the release of bile acids that break up and digest fat so that it can be stored. That was very important when food was scarce, but food isn’t scarce anymore, as we can tell from the fruitcakes, cookies and candies that tempt us at this time of year.

“The good news is that we can still override our taste receptors,” Abumrad says. “Some people have an easy time refusing fatty foods. It’s much harder for others, and this work shows that it may not be purely psychological. Our preference for fat may be related to something much more basic, our ability to sense, and even to taste, fat.”

Abumrad NA. CD36 may determine our desire for dietary fats. Journal of Clinical Investigation, vol. 115:11, pp. 2965-2967, Nov. 1, 2005.

(related article)

Laugerette F, Passilly-Degrace P, Patris B, Niot I, Febbraio M, Montmayeur JP, Besnard P. CD36 involvement in orosensory detection of dietary lipids, spontaneous fat preference, and digestive secretions. Journal of Clinical Investigation, vol. 115:11, pp. 3177-3184, Nov. 1, 2005.