Sweet taste cells that respond to sugars and sweeteners on the tongue also contain digestive enzymes capable of converting more complex sugars like sucrose into simple sugars like glucose and fructose, say researchers writing in the Proceedings of the National Academy of Sciences (PNAS).
By breaking down more complex sugars in our food in to simpler ones, the digestive enzymes in sweet taste cells enable the activation of a different set of receptors – which then triggers a second pathway of sweet taste perception.
Working together, these two sweet pathways identify sweet substances with a caloric value, and provide a potential explanation for why humans and other mammals respond positively to the taste of sucrose when compared to non-caloric sweeteners, said the Monell led team.
"Sucrose is the perfect sweet compound. As a complex sugar, it activates the 'classic' main sweet receptor, but after being broken down by sucrase in the taste cells, the released glucose also activates the second sweet pathway," said study senior author Dr Robert Margolskee.
"Through these insights we are better able to understand how sweet taste works, why sucrose is so appealing, and even perhaps what would be needed to make a sucrose substitute that tastes good but has no calories," he added.
- Sweet taste receptors called T1R2+T1R3 are the primary mechanism for sensing sweet tastes from complex sugars and sweeteners.
- Simple sugars like glucose and fructose also activate a ‘secondary pathway’ involving glucose transporters (GLUTs), sodium glucose cotransporter 1 (SGLT1), and the ATP-gated K+ (KATP) metabolic sensor.
- The discovery of enzymes that breakdown more complex sugars to simple sugars within sweet taste cells means that they activate both pathways, while non-caloric sweeteners only activate the primary pathway.
Sensing sugar
Previous research by the same Monell-led team in 2011 used knowledge of how sugars are sensed in the gut to identify a second class of sweet taste sensors on the tongue which are sensitive to simple sugars like glucose but not to sucrose and other complex food-related sugars.
In the new study, the authors again turned to the gut for the answer. Using a mouse model, they found that the intestinal digestive enzymes sucrase and maltase are also expressed in sweet taste cells on the tongue.
These ‘gut enzymes’ found on the tongue are in the ideal location to break down complex sugars from the foods into simple glucose and fructose sugars, which can then activate the secondary sweet taste pathway, said Dr Sunil Sukumaran, who co-led the research.
Zero-calorie ‘sugars’?
The team noted that current non-caloric sweeteners, which only activate the T1R2+T1R3 receptor, are limited by their inability to replicate the full sweet taste of sugars.
Sukumaran and his team suggest this is because current non-caloric sweeteners do not also target the secondary sugar sensors, which are activated by the breakdown of complex sugars and therefore contribute to the full taste and unique perception of sugar.
Sukumaran told FoodNavigator that in order to develop sweeteners that better mimic the taste of a complex sugar, industry should focus on the development of classes of ingredients that activate both the T1R2+T1R3 pathway and the secondary pathways.
"Our study potentially enlarges the arsenal to tackle them, especially because many pharmacological agents that target the secondary sugar sensors are already available," he said.
“There are several drugs that target these pathways already, and have been studied in the context of diabetes,” the first author told us. “This information could be used could be used to develop new artificial sweeteners.”
“It is better to target both pathways at the same time rather just one or the other,” he added.
Source: Proceedings of the National Academy of Sciences
Published online ahead of print, doi: 10.1073/pnas.1520843113
“Taste cell-expressed α-glucosidase enzymes contribute to gustatory responses to disaccharides”
Authors: Sunil K. Sukumaran, et al