Of our chemical senses, taste is the one we likely think of as the most obvious. A sweet, salty, sour or bitter taste on our tongue is quick and unambiguous, although umami and others can be more nuanced. The last thing we would expect, though, is a parallel taste system in our bodies operating entirely behind the scenes.
It’s called the diffuse chemosensory system, and is an amazing example of how evolution repurposes biological tools for whatever task is at hand. Here, sensory systems we normally think of as aimed at the outside world turn their attention to the world inside, where they manage metabolic activity and more. Much remains unknown as well.
The fact that they run in the background a good thing. Insulin, for example, is part of a complex system that anticipates incoming sugars, measures their levels and directs the pancreas to squirt out a dose that meets the demand. For diabetics, managing this has been cumbersome and error-prone. No wonder automated delivery systems that mimic the body are the cutting edge.
The gastrointestinal tract doesn’t just passively digest. It is a massive sensory organ as well, with a variety of different receptor types. Their signals aren’t processed like the ones from our tongue, but instead go to parts of our brain that unconsciously control behavior, a system known as the gut-brain axis. Sweet receptors are generally located on hormone-secreting endocrine cells. Their presence in the gut makes it the largest endocrine—in other words, hormonal—organ in the body.
One system modulates appetite. As receptor cells in the mouth and upper gut sense incoming sweetness, endocrine cells in the gut release a cascade of hormones, stimulating insulin production and other processes. Satiation hormones like leptin are released as glucose builds. When this reaches our brain, sweet taste becomes less appealing, suggesting it’s time to put the fork down. Another signaling chemical, glucagon, is modulated by the new GLP1 drugs, helping to address overeating and manage diabetes.
The body’s reaction to artificial sweeteners demonstrates the power of this system by showing how it can go wrong. Taste receptors in the gut are sensitive to all sweet compounds whether or not they have any nutritive value. Artificial sweeteners like sucralose, aspartame and saccharin are false flags. Rather than satiating hunger as carbohydrates do, actually increase the hunger hormone, ghrelin. As a result, calorie-free soft drinks can cause some adverse health effects in the form of “metabolic derangements” that can lead to problems like excess weight gain, glucose intolerance, diabetes and cardiovascular disease.
With capabilities and effects like these, it’s easy to understand the presence of sweet receptors in our digestive system, but bitter receptors are also active in digestion, stimulating the release of gastric acids for digestion. This would suggest a scientific basis for the folk medicine view that bitter herbs such as gentian and wormwood help settle the stomach and aid digestion of a heavy meal—the perennial pitch for products like Underberg® and other bitter digestifs. For unknown reasons there are sweet receptors in the brain, bones and elsewhere.
Unexpectedly, taste receptors also play important roles in our respiratory system. Located in our nasal mucosa, they’re the basis of a rapid defense against airborne pathogens. Two types of taste cells participate: one sensing bitterness and another for sweet, which also produces glucose as a kind of bait for bacteria. As long as the sugar level remains high, the sweet cells inhibit the bitter-sensing cells, basically saying: “hold your fire.” Any drop in glucose is interpreted by the sweet cells as a sign that bacteria are feeding in the vicinity, so they remove their inhibition of the bitter-sensing cells. Unleashed, these search for bitter chemicals pathogens use to communicate. When they find it, they release nitric oxide gas, lethally penetrating the bacteria and also signal the sweet-sensing cells to release antimicrobial peptides. This is called innate defense, and it all happens in a flash.
Bitter receptors are also present on immune cells called leukocytes. There, they can recognize bacterial communication compounds and target the invaders for attack. In our airways, bitter sensors on smooth muscle cells cause the little cilia (tiny finger-like projections) on them to beat faster, presumably to help clear out the noxious bitter compounds along with the bacteria that produce them from our esophagus.
Bitter sensors in our circulatory system may be involved in blood pressure regulation. In our bone marrow, bitter receptors modulate calcium signaling and more. It’s not as obvious what bitter receptors are doing in our genitourinary, thyroid and central nervous systems.
Umami- and fat-sensing receptors are also widespread. Umami receptors can trigger intestinal endocrine cells to release hormones to stimulate digestion and signal satiety. There may be a complex system capable of identifying the nine amino acids necessary for our survival, driving behavior when any are missing in our diet. It’s been demonstrated in rodents like rats, who lose interest in food with just one essential amino acid missing. The pathways of communication are not entirely clear, and this experiment hasn’t been done on humans.
Further Reading:
Jing Liu, Amino acid sensing in the gut and its mediation in gut-brain signal transduction, Animal Nutrition 2, no. 2 (2016): 69-73. https://doi.org/10.1016/j.aninu.2016.03.007.
Hojoon Lee, Rewiring the Taste System, Nature 548 2017 Aug 17 (2017): 330–333.https://doi.org/10.1038/nature23299
Lee and Owyang, Sugars, Sweet Taste Receptors, and Brain Responses, Nutrients 9 no. 7 (2017): 653. https://doi.org/10.3390/nu9070653
Susan E Swithers, Artificial sweeteners produce the counterintuitive effect of inducing metabolic derangements. Trends in Endocrinology & Metabolism 24, no. 9 (2013): 431-441. https://doi.org/10.1016/j.tem.2013.05.005
Schiffman and Troy, Revisited: Assessing the in vivo data on low/no-calorie sweeteners and the gut microbiota, Food and Chemical Toxicology 132, October (2019): 110692.https://doi.org/10.1016/j.fct.2019.110692
Nicholas M Dalesio, Olfactory, Taste, and Photo Sensory Receptors in Non-sensory Organs: It Just Makes Sense, Frontiers in Physiology 27, November (2018). https://doi.org/10.3389/fphys.2018.01673