When it comes to ways to burn calories, few people think of chewing. But about 3 percent of the daily energy we burn comes from chewing gum, kibble and other treats, a new study finds — and maybe more if you’re into salads and celery sticks. This is far less than walking or even digestion, but it may have been enough to change the faces of our distant ancestors.
The study adds concrete data to the debate about why human jaws are so different from those of our distant ancestors and modern primates, said Calum Ross, an anatomist at the University of Chicago who was not involved in the study. “That gives us a number we can start working with.”
Scientists have long suspected that our jaw size and tooth shape evolved to make chewing more efficient. As our hominid ancestors shifted their diet to easier-to-chew foods and developed technologies such as slicing and cooking to reduce the time and effort spent chewing, the shape of the jaw and teeth also changed, shrinking compared to other primates . But without knowing how much daily energy we expend on chewing, it is difficult to determine whether energy conservation is also a factor driving these evolutionary changes, says Adam van Casteren, a biological anthropologist at the University of Manchester.
So in the new study, van Casteren and his colleagues put 21 men and women into a balloon-like helmet. The device measures the amount of oxygen they consume and carbon dioxide (CO2) they exhaled. The researchers then gave the participants tasteless, odorless, calorie-free gum to chew for 15 minutes.
While chewing, CO2 levels in the volunteers’ breath rise, indicating that their bodies are working harder. (Because the gum had no smell, taste, or calories, it did not trigger the digestive system, which also consumes energy.) When the gum was soft, the volunteers’ metabolisms increased by an average of 10%; harder gum requires 15% more energy than when resting. “It’s not huge, but it’s still significant,” said study co-author Amanda Henry, an archaeologist at Leiden University.
Overall, chewing gum accounted for less than 1% of the participants’ daily energy budget, the team concluded today in Scientific progress. But chewing gum in a lab was essentially a proof of concept: Before the advent of cooking and the use of tools, early humans likely spent much more time chewing. If ancient humans spent as much time chewing gum as gorillas and orangutans, the authors estimate that they would have expended at least 2.5% of their energy budget chewing. “If you eat harder food and chew longer, you end up with a much larger proportion of total energy expenditure,” says Henry.
The findings came as a surprise. Henry says that even some of her collaborators were skeptical that the energy required to chew would be sufficient to measure in the lab. “I think this is a great study. It shows that there is a measurable amount of energy being used,” says Ross.
The finding supports the idea that more efficient chewing, tailored to diet, may have been an evolutionary advantage, Henry says. “By saving energy in the chewing category, you have more energy to spend on other things like rest, recovery and growth.”
Calculating the energy costs of human chewing may provide insight into the evolutionary strategies of other hominids as well. For example, Australopithecus— a hominid that lived in Africa between 2 million and 4 million years ago — had teeth with chewing surfaces four times the size of modern humans and massive jaw muscles. They must have expended more energy chewing, and the new study is a first step toward calculating just how much. “It is assumed that they took advantage of a very energy-dense food,” says Henry. “We have the first evidence that explains this pattern.”
Still, Ross isn’t convinced that energetics alone can explain the way jaws and teeth have evolved over time. Other factors—such as a jaw shape that minimizes tooth breakage or wear, for example—may have been more important. “Natural selection is probably more interested in not wearing out your teeth than it is in energy efficiency,” he says; an animal without any teeth would run out of energy quickly.
Compared to Australopithecus or primates living today, humans are outliers: Some estimates suggest we spend just 7 minutes a day chewing. In contrast, mountain gorillas can spend up to 90% of their waking time chewing, on par with ruminants such as goats and cows. “Modern people are the strangest. We have really soft foods and little chewing time,” says van Casteren. “Decreasing the amount of energy you expend chewing is another element of those stages in human evolution or in agriculture where you choose foods that are less fibrous or chewy.”