Dr. William Wong is showing me down a hallway at the Children’s Nutrition Research Center at the Baylor College of Medicine in Houston. “We call this the stable isotope laboratory,” he says. There are five isotope-ratio mass spectrometers on this floor. They can analyze blood, saliva, water, and, as it turns out, orangutan pee.

For the last quarter century Dr. Wong has been using mass spectrometers to examine something called doubly-labeled water. Doubly-labelled water provides a way for scientists to measure how many calories primates burn. Orangutans, bonobos, gorillas, chimpanzees, humans: we’re all primates.

In doubly-labelled water, deuterium and oxygen-18, two isotopes not in common water, are added to the liquid. The water is then drunk by the subject—whether it be an orangutan, human, or gorilla. As the animal expends energy, the change in deuterium and oxygen 18 when the water comes out of the subject in the form of urine tells the scientist how much energy has been consumed.

“This methodology has really given us a handle on measuring energy expenditure under free living conditions,” said Dr. Wong.  “That means the subject can be eating anything they want. They can be doing anything they want.  There’s no restriction on their physical activity or dietary habits.”

Until doubly-labeled water was perfected, the best way to measure energy expenditure was whole room calorimetry.

In this method, the subject was enclosed in a small room and their output of carbon dioxide was measured. Carbon dioxide is an index of how many calories an animal burns. Whole room calorimeters are restrictive for humans, and even trickier for apes.

But the goal is worthwhile. Measuring how many calories primates burn can yield valuable information about human metabolism. ”You can look at things like obesity. Are you eating too much? Are you exercising enough?” Dr. Wong says.

Dr. Herman Pontzer, an anthropologist at Hunter College in New York, is in the forefront of the science on the subject. What he’s discovered after more than a decade of work raises questions about many of the assumptions we make about obesity and exercise.

Despite the similarities between apes and humans, Pontzer discovered they’re really very different. In general, sedentary apes despite the fact that they have less body fat than people, burn fewer calories than sedentary humans. And humans that exercise a lot, like hunter gatherers, don’t burn more calories than humans with office jobs.

Orangutans were the starting point of this research because of the unique relationship between Dr. Rob Shumaker and an orangutan named Azy. Both were then at the Great Ape Trust in Des Moines, Iowa. Azy and Dr. Rob, as Shumaker is known, have now been acquainted for thirty years. At the time the studies began, Azy was accustomed to regular physical check ups, including urine samples, so he was an ideal candidate for doubly-labeled water research.

Dr. Rob would hand Azy a large cup of water, the animal would drink all of it down--an essential for the research because doubly-labeled water is very expensive--and the orangutan would later provide a sample. Herman Pontzer was there for the sampling, Shumaker recalls. “He was really worried than none of the water be spilled.”

Four orangutans ultimately participated in the study. The ape and the man are still together, now at the Indianapolis Zoo, where Azy is famous for understanding seventy-two spoken words.

So began a methodical collection of doubly labeled water from apes and humans all over the world.

William Wong’s lab in Houston received hundreds of frozen urine samples via overnight courier. The day I visited, the lab was analyzing samples from Sri Lanka. Wong is now an expert on best practices, sample preparation and instrument calibration for doubly-labeled water.

The Upshot So Far. Humans Are Big Calorie-burners.

Even accounting for the fact that apes and humans have different body sizes, the data reveal that humans burn calories 27% faster than other primates.

Herman Pontzer theorizes that humans use more calories because our brains burn a lot of energy. One out of every four breaths we take, he says, goes to provide oxygen for our brain.

Pontzer has moved on from comparing apes and humans to comparing humans with other humans. The data flies against conventional wisdom. He spent weeks with the Hadzas, a hunter-gatherer society in Tanzania, who track game for food and have little developed agriculture. He collected doubly-labeled urine samples from Hadzas and sent them back to Wong’s lab in Houston. Pontzer found that despite their survivalist lifestyle, the Hadzas use fewer calories than western men and women.

He isn’t sure what all this means yet, except that diet may be more important in fighting obesity than exercise.

Back in Houston, Dr. Wong runs Kamp Kaana in the summertime. KAANA stands for Kids Achieving Activity and Nutrition Awareness. The goal is to fight childhood obesity through diet and exercise. There’s no doubly-labeled water involved.

But the lab is still taking samples.