Fifteen years ago, Peter Lucas ate nearly five pounds of carrots. For science.
Lucas researches feeding in mammals, and he wanted to see if he could eat enough root vegetables to meet his body’s energy needs — but this was not an off-the-wall test of human endurance. Rather, Lucas sat munching in a sealed chamber at Maastricht University that measured how much energy he expended while he chomped. And chomped. And chomped.
“Life as a rabbit,” Lucas recalls. “The problem was that I just physically couldn’t consume enough carrots.” Ultimately, he gave up. The experiment was a failure.
While some might dismiss Lucas’ experiment as bizarre, others found the infamous carrot story inspiring. Two of Lucas’s former post-doctoral researchers decided to give the study another go (although they nixed the carrots in favor of something even less appealing).
What’s new — Armed with better measuring technology and some flavorless chewing gum, the researchers calculated how much energy humans spend chewing food. The findings were published Wednesday, August 17, in Science Advances.
They discovered that chewing ramps up your body’s base-level energy consumption by 10-15 percent, depending on the stiffness of the stuff between your teeth. A separate research group in Thailand found similar results in 2013.
A 10-15 percent increase in basal metabolic rate — the energy you burn just to keep your body functioning daily — is not nothing, but it is not exactly a workout, either.
“It’s considered a ‘light activity’ similar to writing, sitting at a desk, using a computer, or standing,” says Amanda Henry, one of the co-authors of the new study. Henry researches the archeology of food at Leiden University in the Netherlands.
But before you pat yourself on the back for making it through an extra-tough kale salad (still, well done!), remember that you don’t spend all day chewing. You likely chew for less than an hour every day, about 35 minutes or so, these scientists say.
“Humans don’t spend much time chewing at all.”
“It’s remarkable how small the costs of chewing are for humans,” says Herman Pontzer, an evolutionary anthropologist at Duke University. He was not involved in the new research.
This means that modern humans spend well below 1 percent of our daily energy masticating (yeah, that’s the technical term). How little time we chew every day is even something that helps distinguish us from other primates. Orangutans, for example, spend 6.6 laborious hours chomping on their daily sustenance. Other animals spend longer. Cows can chew for eight hours, and pandas may spend 12 hours a day gnawing on bamboo.
“Humans don’t spend much time chewing at all because we cook our food,” says Callum Ross, who researches the biomechanics of feeding systems at the University of Chicago. He was not involved in the recent study.
Why it matters — As hinted in the comparisons to primates, how much energy we spend chewing our food can tell us about our evolution. By devising ways to process and cook food, modern humans set themselves apart from our nearest animal relatives, suggesting that saving time and energy on chewing food has enabled us to develop other skills and abilities.
“This study is important because it tells us how much energy is at stake,” Pontzer says.
The study also tells us more about how we stack up to our ancient human relatives, like Neanderthals. Archeologists have a lot of fossilized hominin teeth from modern humans, Neanderthals, and other ancient peoples because they’re generally the best-preserved body part, Pontzer says. From these fossils, we know that our teeth have changed as our diet shifted.
“These data indicate chewing likely composed a larger portion of fossil hominin’s daily energy budget than previously expected,” says Myra Laird. She studies ancient anatomy at the University of Southern California and was not involved in the new study.
The findings also shed light on how our anatomy evolved, says Ross. Since we know how much energy we spend chewing today, we can compare our modern-day jaws to those of our fossil ancestors, who didn’t domesticate or cook their food.
“Those changes presumably make chewing more efficient, saving the organism precious calories that it can use for survival and reproduction,” Pontzer explains.
How they did it — Our bodies store energy in molecules like glucose and fat. The mitochondria (the famed “powerhouse” of the cell) use oxygen to convert fat and glucose into usable energy. This process releases carbon dioxide, which we exhale.
Scientists can estimate how much energy our bodies use by measuring how much oxygen we consume during a task or at rest. To do this, Henry and her team asked 21 people to lie with their head inside an enclosed dome reminiscent of a goldfish bowl. Tubes kept the oxygen supply flowing in and the carbon dioxide flowing out.
Inside the dome, the participants lay still for 45 minutes to set a baseline for how much energy their body consumes at rest. Then, they chewed two types of flavorless gum, one stiffer than the other. The gum was flavorless because taste stimulates the gut to prepare for food, which could throw off the researcher’s estimates.
“If you smell or taste food, your digestive system kicks in, and it is a very energy hungry system,” says study co-author Adam van Casteren. He researches biomechanics at the University of Manchester.
Then, the researchers placed a sensor on each participant’s jaw to measure the force of their muscle contractions.
Using chewing gum isn’t as ideal as using actual food to estimate how much energy you spend chewing. Gum doesn’t break down in your mouth like food and it requires different jaw movements, but it works well enough for a first estimate, says the study’s inspiration, Lucas. Lucas now conducts research at the Smithsonian Tropical Research Institute in Panama.
If you’re thinking what we are thinking... If chewing is a light activity on par with standing, is there such a thing as a negative calorie food — a food that requires so much energy to chew it negates the energy the food brings into the body?
Despite tales of celery’s powers, this idea is mostly a myth. Inverse spoke with a dietician to lay this claim to rest in a previous story.
“Lawn grass might be calorie negative.”
While there may be negative-calorie substances, they won’t be very nutritious, study co-author Henry says.
“Lawn grass might be calorie negative,” she says. That’s because we can’t digest it.
“Most foods that we commonly consume have been domesticated and bred for years to achieve the highest calorie content per unit food as possible, so I would be skeptical about claims that ‘normal’ foods are calorie negative,” she adds.
What’s next — Future studies could investigate how much energy it takes to chew real food instead of gum, says van Casteren. And to better understand how our jaws and teeth evolved, similar experiments could be done with non-human primates, Ross says.
Henry is currently investigating how much energy it takes to collect and process wild plants using a portable version of the fishbowl helmet apparatus.
We have become very efficient food processors over the millennia, both inside our bodies and before the food even reaches our mouths. We can hardly imagine the life of a great ape or cow — let alone a bamboo-loving panda.
“Having to chew all day like they do, it just sounds like a lot of work to me,” Ross says.