For many people, a high-sugar diet has become almost accidental.
Three-quarters of the food items in the supermarket have added sugar, according to University of Michigan researcher Monica Doss, leading the scientist to wonder whether our sense of taste can become dull to sweetness. Now, in a study in mice, a collaboration of researchers from UM has discovered that a diet high in sugar reduces the ability of the taste system to sense sweetness.
Doss wanted to further examine whether this phenomenon is a physiological effect that occurs in the senses of mice consuming a high-sugar diet. The group, which included MM University scientists Robert Bradley, Charlotte Mistrita and Carrie Ferrario, found that the response of the nerve that relays sweetness information from the tongue to the brain was reduced by nearly 50% in mice fed a high-sugar diet. Their results were published in the journal Current Biology.
Several years ago, a study showed that lowering sugar levels in the human diet led people to perceive sugar as more intense. But how does that happen? Several of our previous papers found that sugar dampens sweet taste in fruit flies by decreasing the nerve response and reprogramming taste cells, but flies are still just flies. I really wanted to understand if and how that happens in mammals.”
Monica Doss, Assistant Professor of Molecular, Cellular and Developmental Biology
Dus reached out to UM College of Dentistry professors Bradley and Mistretta, who have studied how taste functions and how it is altered by diet and disease. The team also reached out to Ferrario, an assistant professor of pharmacology at UM Medical School, who studies the neurobiology of swallowing behavior in mice.
The researchers gave one group of mice their typical diet and access to sugar water. The control group of mice received their usual diet and regular water. Four weeks later, 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 taste bud cells that sense chemicals, to the brain.
Next, the researchers gave the mice sweet, bitter, sour, salty and umami-flavored solutions, and measured the mice’s responses to touch and cold. While they did this, they tested the response of the front of the tongue to those flavors and sensations.
The researchers found no difference in the mice’s reaction to salty, bitter, sour, and umami flavors, nor to touch or cold, but they did see a 50 percent decrease in the nerve’s response to the sweet sucrose (table sugar) solution.
“This is not a hidden effect,” Doss said. “It’s really strong, and it only took four weeks.”
At the end of the four weeks, the researchers also found that the mice had not gained weight from the sugar. Then the scientists returned the mice to their regular diet with plain water. After another four weeks, they retested the mice’s sensitivity to sweetness, and found that they recovered.
“That’s probably good news for us,” Doss said. “If you eat a lot of sugar, but decide to stop eating sugar, it shows that you can restore your taste (assuming the system works the same way).”
Doss said that result was somewhat predictable. Previously, Mistrita and Bradley have found the taste system to be highly plastic and can recover from drug treatments that disrupt it, such as chemotherapy.
The group then investigated why this happened. Looking at rats’ tongues, they found no change in the rats’ taste buds, the structures that house the tongue’s taste buds. This makes sense, Doss said: If they had done so, they likely would have also found changes in the mice’s sensitivity to other tastes.
The researchers also found no changes in the number of taste buds on the tongue, nor in how the nerve connects to the taste buds. But by looking inside the taste buds, they found fewer cells that detected sweetness in mice on a high-sugar diet.
In future studies, Doss and Ferrari will study how these taste changes affect eating and the activity of cells deep in the brain that receive information about sweetness. For example, Dus previously found that in flies, a decreased sensation of sweetness results in reduced dopamine release, which reduces feelings of satiety and leads to overeating. Does this also happen in mice?
There is plenty of evidence that diets high in sugar and fat affect the dopamine and nutritional learning systems in the brains of humans and mammals, but the reasons for these changes remain largely unknown. Do they arise from changes in our senses, specifically the taste changes observed here?
“Because we are mammals and our taste systems are similar to rats, this is the best available evidence that a high-sugar diet alters the sensory system,” Doss said. “So this may affect your food choices. This may affect your metabolism. But also, the other important implication is that if your taste system is really plastic, it’s possible that if we reformulate the foods so they have less sugar.” Our taste buds will learn how to eat and like food as much as we enjoyed the extra sugar stuff today.”
The group wrote an internal grant collaborative proposing to study how sugar changes sweetness in mice, which funded their work. Work began in 2018, first with undergraduate student Hanan Drakes and then with MCDB Postdoctoral Researcher Hayeon Sung and Dental School Postdoctoral Researcher Iva Vesela. The COVID shutdown and subsequent research restrictions put the project on hold for about 1.5 years.
Song, H.; et al. (2022) Exposure to a high-sucrose diet is associated with selective and reversible changes in the peripheral taste system of rats. Current Biology. doi.org/10.1016/j.cub.2022.07.063.
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