Thanks to the keto, Atkins, and paleo diets, the world we live in is a carbohydrate-hostile one. Celebrities like Beyoncé and Lebron James have claimed that their low-carb diets provide an energy boost and eliminate extra weight. And in a lot of cases, that seems to be true. But these diets seem to be at odds with the human body, which, as scientists report in an eLife study published Tuesday, rapidly evolved mechanisms to be a better carb-consuming machine.
Many mammals, humans included, carry the gene encoding an enzyme called amylase, which breaks down carbohydrates — long strands of sugars — into smaller, simpler sugars that the body can use for energy. In the new study, the University at Buffalo scientists and their colleagues investigated the genomes of 46 mammals with this gene, discovering a crucial connection between the genomes and diets of these species.
Carbohydrates may have a bad rap now, but long before they were ubiquitous in french fries, pizza, and soda, they were, for many animals, a lifeline. Starch, or complex sugar, “is essentially an energy bomb,” study co-lead author and human evolutionary biologist Omer Gokcumen, Ph.D., tells Inverse. “It’s almost like a battery.” In the carbohydrate family, sugars are “simple” and easily absorbed; starches are “complex” and take longer to process.
The team, which also included lead researcher Stefan Ruhl, Ph.D., and first author Petar Pajic, showed that mammals with starch in their diets expressed amylase in the pancreas, where it could help digest carbohydrates in the gut. Furthermore, animals that had extra-starchy diets, like humans, rats, mice, and dogs, tended to have more copies of the amylase gene; carnivores like mountain lions, in contrast, do not.
But what seems to set us and a few other mammals apart is the amylase that’s also expressed in our saliva. This, says Gokcumen, may prime us to do something special: To recognize starch as a good thing as soon as it’s in our mouths.
Salivary Amylase and the Role of Pizza Rat
When food is scarce, it’s important to know when you’ve come across an energy-dense carbohydrate source, like a potato or an ear of corn. Since animals and ancient humans weren’t equipped with a list of carb-rich foods, they had to rely on their instincts to tell them.
This is where salivary amylase comes into play. Animals that have amylase in their spit have the essential ability to detect carbs when they’re in their mouths. Amylase breaks down the starches in, say, a yam, releasing the simple sugars inside it. Gokcumen thinks this may have led to a taste preference for starchy foods over time.
Humans have salivary amylase, as do some pet dogs, mice, some great apes, and brown “sewer” rats, allowing him to illustrate this idea with a viral meme.
“I am fascinated by the New York Pizza Rat,” he says, referring to the 2015 video of a rat hoarding a slice between its teeth. “This animal has choices like any urban dweller has choices.”
He hypothesizes that despite the wealth of city trash, Pizza Rat chooses the pizza because, as a result of the salivary amylase releasing sugars from its crust, its brain can detect that it’s a carb-rich — and therefore energy-rich — source of nutrition.
“For it to realize that, since it’s not conscious as we are, it really needs to instinctively like pizza more,” he says. Like rats, we may have evolved a mechanism to recognize carbs when they are in our mouths because, at one point in our evolution, it was important to eat them when they were available.
(Pizza Rat turned out to be staged using a trained animal, but the idea still stands.)
Which Came First, Carbs or the Enzyme?
The team’s analysis of the 46 genomes showed that, among the different lineages that have many copies of the amylase gene, amylase duplication happened independently. But it’s still not clear whether amylase copies or access to carbs came first. The ability to digest starch might have led to more copies of the amylase gene, or having copies of the amylase gene to begin with could have shaped a starchier diet.
What is clear is that there was some kind of interaction between animal genomes and their diet, and that adaptation to carb-eating (getting more copies of the amylase gene) happened fast. “This happens in thousands of years in some instances, and that’s remarkable,” Gokcumen says. “There are very few examples of evolution happening that fast.”
As for humans, previous research suggested that our number of salivary amylase genes spiked around the time we invented agriculture, sometime in the “last 10,000 years,” says Gokcumen. Others argue that we evolved it even earlier, around the time that historical populations started cooking starches. “It may actually coincide with cooking of tubers and roots, which are rich in starch and only after cooking can be consumed,” says Gokcumen.
How We Deal With Carbs Now
While carbohydrates were an important source of energy in the past, today we risk consuming more calories than we need. Low-carb diets help people cut back on the number of calories they get from starches, putting more emphasis on calories from protein and fat. The effects vary among people; that’s why Gokcumen hesitates to endorse or refute any particular diet.
“I think we shouldn’t put that much emphasis on a particular universal solution to these kinds of health problems because they are variable, among us,” he says.
He does, however, point to the “thrifty gene hypothesis,” which is the idea that evolution preserved genes that were useful in the past but harmful now (the original “thrifty genes” allowed humans to collect and preserve fat). It’s the idea that “we change our environment so much that some of the things that we evolved to do, biologically speaking, are not compatible with our current standards of living,” he says.
Skyrocketing obesity rates suggest that, on the whole, we are consuming more than we can expend. Whether low-carb diets are a healthy and safe way to mitigate this effect remains to be seen. But if there’s anything to be learned from the evolutionary history of amylase, it’s that organisms can adapt — rapidly — to their changing environments. It leaves open the possibility that future humans will evolve to be less enamored with carbs than we are today.
“Our work and many others’ work shows that our genes are evolving with our culture,” says Gokcumen. “It’s kind of a fascinating situation that we are subconsciously, in a way, domesticating ourselves.”
The amylase gene (AMY), which codes for a starch-digesting enzyme in animals, underwent several gene copy number gains in humans (Perry et al., 2007), dogs (Axelsson et al., 2013), and mice (Schibler et al., 1982), possibly along with increased starch consumption during the evolution of these species. Here, we present comprehensive evidence for AMY copy number expansions that independently occurred in several mammalian species which consume diets rich in starch. We also provide correlative evidence that AMY gene duplications may be an essential first step for amylase to be expressed in saliva. Our findings underscore the overall importance of gene copy number amplification as a flexible and fast evolutionary mechanism that can independently occur in different branches of the phylogeny.