In the long list of ways that obesity is bad for the body—hypertension, diabetes, heart disease, to name a few—a diminishing sense of taste can seem almost trivial. But the loss of taste could be fueling obesity itself—or at least help to explain why losing weight is so hard—by driving humans to consume more and more of the sugary, salty, and fatty foods that trigger the reward center in the brain. Understanding exactly how obesity affects taste is a critical first step to identifying an intervention. And now, a new study in mice, published today in PLoS Biology, shows that obesity-related inflammation leads to a rapid loss of taste buds.
Researchers have known for some years that obesity seems to weaken taste sensations, but it's a hard phenomenon to study in humans. "It's not controlled at all. You can't test people before they've gained weight and then after they've gained weight, because you don't know who is going to gain weight, you don't know when they're going to do it, you don't know how much they're going to gain," says Robin Dando, an assistant professor at Cornell University and a co-author on the new study. "So we moved to the mouse model."
In order to zero in on the effects of diet and weight gain on taste buds, Dando and his colleagues split a group of genetically similar male mice into two factions; over the course of eight weeks, they fed the lean, control group a standard chow, and the other a high-fat diet. The latter group gained a significant amount of weight very quickly. Then the researchers tallied up taste buds for both groups. "What we saw was a really powerful effect, where these [obese] mice were losing maybe 25 percent of their taste buds in just two months," Dando says.
Next, the researchers zeroed in on the growth and death of taste buds, to determine how obesity might be driving their decline.
Taste buds are actually a collection of roughly 50 to 100 cells that come in three varieties, (each responsible for detecting different tastes—salty, sweet, bitter, umami, and sour) and have a lifespan of only about 10 days. "Taste buds turnover very quickly, you will get a whole new set of taste buds in probably four weeks, all the way through your life," Dando says. Fueling that turnover are stem cells, which sit at the base of taste buds and continuously churn out new cells. "You can imagine it's a balance of new cells being born and old cells being broken down and dying," he says. "What we saw is both sides of that balance being tipped." In the obese mice, apoptosis increased in the taste buds, and the number of cells responsible for producing taste bud cells declined.
The researchers believe obesity-related inflammation is to blame. Fatty tissue, for example, secretes inflammatory cytokines—a type of signaling molecule that shuttles information between cells. The team found that, in obese mice, the amount of one such inflammatory molecule, called tumor necrosis factor-alpha (TNFα), increased around taste buds.
The researchers repeated the experiment with two groups of mice that were genetically modified so that they couldn't make any TNFα. Once again, the group that ate the standard chow for eight weeks was lean, and the group that ate the high-fat diet gained weight. But this time, the obese mice had the same number of taste buds as the lean group.
Dando is cautious about what this work might mean for humans, but there are reasons to believe that the same processes are underway in our own taste buds when we gain weight. "At the moment, this remains work performed in mice, but we chose mice because they have they a pretty similar taste bud to our own," he says.
The good news is that the effects on taste bud stem cells are reversible, though exactly how long that recovery takes is unclear. "This could be one potential explanation for why sticking to a diet is so difficult," Dando says, "because you just don't get the taste buds re-establishing fast enough for you to start enjoying your foods with a complete sense of taste."
Taken together, these findings suggest the taste bud could be a target to treat obesity in the future, and Dando and his colleagues are currently looking for ways to encourage taste bud growth or even block taste bud loss. "Taste remains the No. 1 driver of our food choice," Dando says. "We choose our foods based on taste ahead of convenience or price or even how good they are for us, so if we can change how people perceive their foods then maybe they'll be more open to a healthier diet."