For many people, a high sugar diet has become almost accidental.
According to Monica Dus, a researcher at the University of Michigan, three-quarters of food in supermarkets contains added sugar, leading the scientist to wonder if our sense of taste can become dulled. Now, in a study on rats, a collaboration of UM researchers found that, yes, a high-sugar diet reduces the ability of the taste system to detect sweetness.
Dus wanted to further examine whether this phenomenon was a physiological effect occurring in the senses of rats consuming the high-sugar diet. The group, a collaboration that included UM scientists Robert Bradley, Charlotte Mistretta and Carrie Ferrario, found that the responsiveness of the nerve that transmits information about tongue softness to the brain was reduced by nearly 50% in rats fed a high sugar diet. Their results are published in the journal Current Biology.
A few years ago, a study showed that decreasing levels of sugar in the human diet led people to perceive sugar as more intense. But how does it happen? Several of our previous articles have shown that sugar dulls the sweetness of fruit flies by decreasing nerve responsiveness and reprogramming taste cells, but flies are still just flies. I really wanted to understand if and how this happened in mammals.
Monica Dus, Associate Professor of Molecular, Cellular and Developmental Biology
Dus contacted UM School of Dentistry professors Bradley and Mistretta, who have studied how taste works and how it is altered by diet and disease. The team also connected with Ferrario, an associate professor of pharmacology at UM School of Medicine, who studies the neurobiology of ingestive behavior in rats.
Researchers gave a group of rats their typical diet and access to sugar water. A control group of rats received their usual diet and had access to plain water. After four weeks, the researchers used electrodes to record nerve responses as the tongue was stimulated with different solutions. This nerve carries taste information from the front of the rat’s tongue, which contains the taste bud cells that sense chemicals, to the brain.
Next, the researchers fed the rats sweet, bitter, sour, salty, and umami-flavored solutions, and measured the rats’ responses to touch and cold. While they were doing this, they were testing the response of the front of the tongue to these flavors and sensations.
The researchers found no difference in the rats’ reaction to salty, bitter, sour and umami flavors, touch or cold, but they found a 50% reduction in nerve reactivity to the sucrose sugar solution. (table sugar).
“It’s not a subtle effect,” Dus said. “It’s really strong, and it only took four weeks.”
At the end of the four weeks, the researchers also found that the rats had not gained weight from the sugar. The scientists then returned the rats to their normal diet with plain water. After another four weeks, they retested the rats’ sensitivity to sweetness and found that it had been restored.
“This is potentially good news for us,” Dus said. “If you eat a lot of sugar, but decide to cut it out, that shows you can get your taste back (assuming the system works the same way).”
This result was somewhat expected, Dus said. Previously, Mistretta and Bradley found that the taste system is highly plastic and can recover from drug treatments that disrupt it, such as chemotherapy.
The group then looked at why this might be happening. Looking at the rats’ tongues, they found no changes in the rats’ taste buds, the structures that house the taste buds in the tongue. That makes sense, Dus said: If they had, they likely would have found changes in the rats’ sensitivity to other tastes as well.
The researchers also found no change in the number of taste buds on the tongue, or how the nerve is connected to the taste buds. But, by looking inside the taste buds, they found fewer cells that detected sweetness in rats fed a high-sugar diet.
In future studies, Dus and Ferrario will examine how these taste changes affect the feeding and activity of cells that receive information about sweet taste deep in the brain. For example, Dus previously found that in flies, lower sweetness dampens dopamine release, decreasing satiety and triggering overeating. Does this also happen in rats?
There is ample evidence that sugary and fatty diets affect dopamine and food learning systems in human and mammalian brains, but the causes of these alterations remain largely unknown. Do they come from the changes in our senses, in particular the changes in taste observed here?
“Because we are mammals and our taste systems are similar to those of rats, this is the best available evidence that a high-sugar diet alters the sensory system,” Dus said. “So it could affect your food choices. It could affect your metabolism. But also, the other important implication is that if your taste system is really plastic, it’s likely that if we reformulate foods to have less sugar, our taste buds will learn to eat and like food as much that we enjoyed this sweet supplement. things today.
The group wrote an internal collaborative grant proposing to study how sugar alters sweetness in rats, which funded their work. Work began in 2018, first with undergraduate student Hannan Driks, then with MCDB postdoctoral researcher Hayeon Sung and dental school postdoctoral researcher Iva Vesela. The COVID shutdown and subsequent research restrictions halted the project for nearly a year and a half.
Sung, H. et al. (2022) Exposure to a high-sucrose diet is associated with selective and reversible alterations in the rat peripheral taste system. current biology. doi.org/10.1016/j.cub.2022.07.063.