In 1981, a young entomologist named Stephen Simpson spent his summer inside a sweltering little room watching 10 locusts. He recorded all the food the insects ate and every fecal pellet they produced. At night, he turned on a dim, red 100-watt lamp. The experiment lasted for six days and six nights. It was practically biblical, he conceded recently. To pass the hours, he listened to Talking Heads cassettes through a single earbud, which he switched from ear to ear. He had set out to learn why locusts eat what they eat. At the time, he had no idea those 10 insects would eventually spawn a theory linking obesity with climate change.
From his experiment, Simpson learned that locusts swarm when they can’t find enough protein. Eventually, he would discover evidence that protein-seeking behaviors appear across the animal kingdom, including in humans, who generally get around 10 to 15 percent of their calories from protein. Simpson now hypothesizes that climate change is not only reducing the percentage of protein in plant-based foods, but, in doing so, is a contributing factor in the obesity epidemic. As he sees it, humans are consuming excess fats and carbohydrates in an attempt to reach their target protein threshold, an idea Simpson calls the protein leverage hypothesis.
"I wanted to understand that transition from harmless grasshopper to plague-forming pest."
“It sounds a bit out there, I know,” he says. “But if you put an organism in an environment that contains lower-percent protein, then you drive overconsumption of total energy.” And this, he says, may be associated with weight gain.
Not all nutrition researchers think Simpson’s hypothesis is a plausible explanation for the rising worldwide rates of obesity. What empirical support there is for the idea comes primarily from studies performed on insects and other lab animals; nobody has conducted long-term studies in people. But Eric Ravussin, a professor at the Pennington Biomedical Research Center in Baton Rouge and editor-in-chief of the journal Obesity, says there’s no reason not to test the question of protein leverage.
“Everybody is intrigued by this theory. Most people are impressed by data in animals,” he says. Experiments show that the balance of macronutrients that flies and rodents eat affects how many calories they consume overall.
One morning last spring, I met with Simpson to discuss the protein leverage hypothesis, which he and David Raubenheimer, his longtime collaborator, first put forth 10 years ago. Simpson is now the director of a research center studying obesity and related diseases at the University of Sydney in Australia. Simpson is an energetic man with floppy, sun-bleached hair. He greeted me on a sixth-floor atrium, and we gazed over the undergraduates and clinicians, some in full gowns, scurrying around on the ground floor. He explained the origins of his theory. “I wanted to understand that transition from harmless grasshopper to plague-forming pest,” he says. “I was working on locusts as the white rat, if you like, to look afresh at nutrition.”
Before long, Simpson cracked open his laptop to show me one of his favorite videos. Shot in Utah, it depicted a clattering phalanx of black Mormon crickets storming, for several miles, across the desert. The video provided a dramatic demonstration of the importance of hitting your target protein level: The crickets were on a forced march in search of protein. Instinctually, they migrated together, and if an individual insect stopped, it was cannibalized—it became protein. That tightly regulated appetite for protein has also been observed in single-celled slime molds, birds, rodents, spider monkeys, and humans.
Humans retain that evolutionary preference for protein—and that has become a problem, according to Simpson’s calculations, because many factors are diluting the percentage of protein in our food. Processed snacks, for example, are often made mostly with carbohydrates, yet seem protein-filled because they’re flavored with MSG and yeast extracts. I began to think of it as the duped-by-Doritos theory. Barbecue chips and cheesy sticks subvert our appetite-control systems and throw our macronutrient consumption out of balance. When Simpson and his colleagues studied volunteers who ate otherwise identical meals, they observed that people generally consumed more savory, salty snacks when their meals contained less protein.
Stephen Simpson hypothesizes that climate change is not only reducing the percentage of protein in plant-based foods, but, in doing so, is a contributing factor in the obesity epidemic.
Since the 1970s, the rising prevalence of obesity has coincided with a shift in the American diet. In one survey Simpson cites, Americans now get approximately eight percent more of their calories from carbohydrates and one percent less from protein. Simpson began to wonder what other factors deplete the proportion of protein in our diets.
One day, Raubenheimer came to him with an idea: Was it possible that more carbon dioxide in the atmosphere meant less protein in plants? Plant metabolism is fueled by carbon dioxide in the air, so plants are especially sensitive to changes in atmospheric carbon, and respond to these changes at a molecular level. The two found robust experimental data demonstrating that plants grown under increased carbon dioxide levels produced less protein and more sugars and starches. In turn, insects and herbivores need to eat more wheat, rice, and other plants to get the same amount of protein they used to. Based on these models, Simpson says, it would be surprising if the effect doesn’t also show up in the human diet.
The protein leverage hypothesis, and its climate-change corollary, remain two of the lesser-studied ideas in obesity research. Ravussin concedes no one has proved or refuted the claims. Nutrition research remains largely focused on weight and the debate over whether fats or carbohydrates are more fattening. But Ravussin believes Simpson’s assertions are credible and deserve further testing. “It sounds crazy at the first place,” he says. “But if you have a higher CO2 content, which we know we have, and it is changing the composition of leaves and plants, this is an experiment of nature—and you have to try to reproduce it.”
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