But doctor, I'm biologically designed to like salt

As an ecologist, Lee Mick Demi hadn't spent much time pondering taste. But now, after co-authoring a study published in the journal Ecology and Evolution examining the connection between taste and the elemental needs of animals, he thinks about it a lot.

Humans recognize five basic tastes — bitter, salty, sweet, sour and umami — but there are other dimensions to the gustatory experience of animals.

“Livestock and rodents both appear to have the ability to taste and enjoy the taste of phosphorus, for example,” says Demi, postdoctoral researcher in North Carolina State University's Department of Applied Ecology. “But I don't know if we taste phosphorus. So I think about that sometimes: `Do I like this because there's a phosphorus taste in here that we don't even really recognize or characterize as a different type of taste?'”

Much of Demi's previous research has focused on trying to understand what limits the growth of organisms in the wild. It's a nutritional question, he says, although ecologists don't necessarily think about it that way.

In initial conversations with fellow study co-author Rob Dunn — also the co-author of Delicious: The Evolution of Flavor and How It Made Us Human (Princeton University Press, 2021), and professor of applied ecology at NC State and in the Center for Evolutionary Hologenomics at the University of Copenhagen — they began to consider how the framework of taste could be used to understand the nutritional imbalances that limit growth. “Why do we taste certain things? Why do certain things taste good? Why do certain things taste bad?”

Taste is among the primary tools animals use to evaluate the nutrition of their food, Demi explains. Other sensory cues such as olfaction lead us to food through scent and external factors such as texture also influence our perceptions.

“Taste is not the only tool, but it is the primary screening mechanism for most animals to assess the quality of their food,” says Demi. “And for that reason, it's a really fun one to try and see if we could think about why that tool works the way it does, and why it works differently in different animals.”

There's a large body of research examining the neurobiology of taste, the researchers highlight, but theirs is the first study to focus on taste as an evolutionary tool. Animals need elements such as sodium, nitrogen and phosphorus to survive, but foods with high concentrations of these components can be hard to find outside of a supermarket.

These nutritional requirements, the team found, have played into our cravings for salty snacks such as potato chips or the umami bomb of an aged steak (which is high in amino acids, “the primary nitrogen-containing molecule in cells in many plant and animal tissues”). The researchers posited that since animals require these elements, they should have adapted a taste for them; a reward incentivizing them to seek out and enjoy foods containing a healthy dose.

“Taste is a very obvious mechanism for that, because we can put something in our mouth, and very quickly evaluate the quality of it, or the composition of it based on these chemical signals that we interpret as different tastes,” says Demi. “We should have some mechanism to help us find foods that are high in concentrations of those potentially limiting things. So that was the evolutionary link that I think was novel in our project.”

Demi was especially interested in the similarities they identified among diverse animal groups. One of the challenges of taste research, he highlights, is that the body of literature deals with a limited number of animal models, including humans, livestock and mice. In comparing mammals, fish and insects, the researchers found that though each animal group's taste systems are “completely independently evolved,” there is strong evidence of a shared preference for certain tastes.

This suggests that natural selection has been acting on these different animals to hone their taste preferences, says Demi. The animal groups have independently evolved to prioritize the same elements such as sodium, phosphorus and nitrogen, which can be hard to come by in nature.

“That really strong evolutionary convergence around those certain tastes, and for certain types of molecules, was really, really cool for us and a really interesting result,” adds Demi. “And I think one of the best pieces of evidence that helped support our hypothesis for why we taste these different things.”

Demi attributes this convergent taste evolution to the fact that animals are relatively similar in their composition. Even among diverse species, the amounts and types of elements don't vary as much as they do in plants. Sodium, for example, is more highly concentrated in animal tissue. Knowing that, the researchers expected there to be “strong evolutionary pressure” for the ability to taste sodium and enjoy it at certain concentrations — and that's exactly what they found.

“We see salt preferences in all those diverse animal lineages,” says Demi, adding that in this way, “eat what you are” is more accurate than “you are what you eat” from an elemental composition perspective.

Having taste receptors that reward the eater for seeking out foods containing these infrequent but necessary elements is especially important for herbivores and omnivores, like humans. Because “their diets are way more biased towards foods that are much different than their elemental composition,” it's more critical to be able to seek out and determine even small differences in the chemical makeup of foods.

It can be tempting to read studies like theirs and go directly to humans as an example, says Demi. But we're outliers rather than the norm. Salt, for instance, was scarce in the environment during the majority of human evolution, and thus light in the diets of early humans. Since salt is necessary for survival, it follows that humans would have developed an affinity for it.

The same can be said for sugar, adds Demi. We evolved a taste for foods high in carbohydrates because we needed to be able to recognize them when we found them. On supermarket shelves and restaurant menus, however, salt and sugar are now abundant. In Western societies especially, we've become “divorced” from the evolutionary context in which we developed our taste senses.

“Now these preferences and our access to foods that contain those particular compounds, salts or sugars, cause pretty serious health and economic consequences in dealing with health issues,” says Demi. “So humans are a really interesting case and it's hard to make sense of that sometimes. But I think if you consider our evolutionary history and why we crave those particular tastes, then it makes sense.”

With their paper, “Understanding the evolution of nutritive taste in animals: Insights from biological stoichiometry and nutritional geometry,” the researchers hope to stimulate more taste research in a wider variety of animals. From an ecological perspective, Demi is interested in the implications of the ways humans have “dramatically changed” the global cycling of nutrients such as nitrogen and phosphorus.

“Especially if you think about it in the context of human taste, (nitrogen and phosphorus) are two major components of agricultural fertilizers that lead to all types of environmental issues when they're overused,” says Demi. “And so you can think about how our tastes for phosphorus- and nitrogen-containing foods have driven this push for changes to agriculture that have affected the global cycling of these elements with far-reaching consequences for ecosystems across the planet.”