When we bite into a nice sugary dessert, the flavor we enjoy is the result of one of the five basic elements of taste: sweet, salty, sour, bitter and umami.
Part of the satisfaction derived from eating sugar starts on the tongue. Another pathway for sensing sugar, however, also is activated within the intestine, and it reinforces the desire for more sugar. Sugar is the basic energy source for animals, including humans, and we’re all motivated to seek it.
Scientists already knew a second sugar-sensing pathway must exist because mice without the ability to taste sugar still preferred it over artificial sweeteners. It’s not the sweetness but specifically sugar that activates both pathways.
Eating sugar activates reward pathways in the brain, making people feel good. Sugar consumption has risen from about 10 pounds per year in the late 1800s to over 100 pounds annually today. Consumption of so much sugar is linked to a variety of health issues, including cavities, diabetes and obesity, yet we can’t seem to resist it.
Sugar and other elements in food bind to specific taste receptor proteins in the thousands of tongue taste buds and in cells of the palate, the roof of the mouth. When a taste bud senses a taste, it starts a cascade of messages that ends with your brain telling you what you are tasting.
Sugar and sweeteners are both sensed as sweet, but sugar is a little different. When scientists offered water with either sugar or sweetener to mice, they drank both at first, but they quickly showed a preference for the sugar water.
The scientists took images of the brains of the mice while they consumed sugar and identified a region of the brain that responds to sugar and not sweetener: the caudal nucleus of the solitary tract. The caudal nucleus of the solitary tract is an area at the base of the brain, and it serves as a conduit for messages from the brain to other parts of the body to regulate basic functions.
The scientists found a gut-to-brain pathway that was activated when sugar was injected directly into the gut, bypassing taste. The scientists proved this was the circuit responsible for sugar preference by turning it off in mice to make them stop preferring sugar water.
The sugar signal in the gut pathway starts when glucose reaches the intestine. The glucose binds to receptors, which initiate signals that travel to the caudal nucleus of the solitary tract through the vagal nerve, the key connection between the gut and the brain. This glucose-specific pathway may explain why sweeteners and other sugars like the fructose found in fruit don’t have the same effect on the body.
The signals from the gut are sent only after the glucose is ingested, to ensure good absorption and consumption of it. This circuit also explains why sweetener use has not reduced sugar consumption, preference and cravings.
Now that we know about this circuit, perhaps new sweeteners could be made that taste sweet, activate the glucose gut-to-brain circuit and avoid some of the health issues that come with sugar. Maybe then we could reduce our sugar consumption.