TASTE receptors might not strike you as very mysterious – they are on cells in the taste buds primarily found on your tongue and in your mouth and throat. When they bind to food molecules, they alert us to different kinds of tastes, like sweet, salty or sour.
So far, so ordinary. But in 2013, newspapers went wild over the discovery of taste receptors in our testicles. What on Earth are they doing there? Of course, TikTok has subsequently been awash with people trying to taste things with their nether regions. The truth of the matter is both less and more exciting. It turns out there are taste receptors throughout our bodies, and while they don’t allow us to taste with anything other than our mouths, a better understanding of their role could lead to new ways to combat disease.
The first sign that something strange was going on came in 1996, when researchers uncovered . Subsequent studies revealed that sweet, umami and bitter taste receptors are all present in the gastrointestinal tracts of rodents and humans.
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That was a bit of a surprise, but shouldn’t seem too odd, says , “because the mouth and the tongue are part of the gastrointestinal tract”.
Where else are there taste receptors and why?
Yet in the years since, taste receptors have popped up in organs and tissues further afield. Alongside those in the aforementioned testicles, they were also found in the heart, brain, bladder, lungs and body fat.
From an evolutionary standpoint, it would be useful for animals to develop sensors where they ingest food. This is partly to , but also to help their bodies respond better to what nutrients are available to them. Balanced diets would have been unlikely, says . “It’s like, well, today we ended up with a ton of raspberries because that’s what we could get.” Animals might not encounter more berries for months, so detecting a compelling sweet taste would have helped them stay in those bushes and make the most of this short-lived bounty.
The rest of the body must also tune in to this dietary mayhem, says Kirk. Much as the taste receptors in the mouth bind to food molecules in saliva, receptors elsewhere sense what molecules are in gastric juices or the blood and respond appropriately.
Calling them taste receptors may be obscuring the bigger picture, though. The truth is that they are sensing, but we aren’t tasting, so they aren’t actually taste receptors. “That is the purpose that they serve on taste buds,” says Kirk. Elsewhere, they are “nutrient sensors”.
Kyriazis says their primary role is to assess how much energy is available outside the cell and to help maintain the right metabolic environment. They do this, in many cases, by stimulating the release of calcium when they have detected a nutrient. This helps trigger a cellular response that sends a different message depending on where the receptor is.
In the intestines, this message alters the behaviour of cells that absorb nutrients from food, making them more efficient, says Kirk, while his team has shown that sweet and umami nutrient receptors in the heart . This is probably to boost the flow of blood to help cells in the gastrointestinal tract digest the food.
Taste receptors help produce healthy sperm
Meanwhile, nutrient receptors in the testicles appear similar to those involved in detecting bitter tastes in the mouth and are crucial in enabling sperm to fully develop. Mice that have had these receptors knocked out . While there isn’t any direct evidence for their role in humans, there is a significant correlation between human male infertility and some alterations in taste receptor genes, suggesting this might contribute to fertility problems.
It is possible that malfunctions in our nutrient receptors have a role in other medical conditions, too. In the gut, for example, they are involved in regulating nutrient uptake and the release of molecules such as hormones and neurotransmitters that help keep organs in peak physical condition.
Disturbances to these receptors, and thus this signalling, have been hypothesised as one factor , type-2 diabetes and irritable bowel syndrome. For instance, studies have shown an . “Given the central role of nutrient sensing in metabolism, it makes sense that these receptors have a role in disease if they start responding abnormally,” says Kirk.
All of which suggests that understanding more about their role could result in big benefits to our health. For example, when Kyriazis and his colleagues genetically eliminated sweet-sensing nutrient receptors from the skeletal muscle of mice, they found something odd. Normally, these receptors help animals move or maintain their posture. When these were knocked out in the mice, there was , the powerhouses of cells. “These mice can run faster, they have healthier muscles and they can maintain these healthier muscles during ageing,” says Kyriazis.
These muscle-strengthening effects are similar to what you see happen after prolonged fasting and caloric restriction, says Kyriazis, probably because in both cases the cell is detecting much less glucose than normal. This shortage of a vital energy source prompts muscle cells to fine-tune themselves, breaking down and renewing internal structures so they function more efficiently. Perhaps one day we may be able to dial down our sweet receptors using drugs to keep muscles strong with no fasting required. At the very least, it is a tantalising taste of what might be to come.
