“Two people can sit down at the same table and eat the same amount of food,” says Michael Power, a scientist at the National Zoo’s nutrition lab, which is based at the Smithsonian Conservation Biology Institute in Front Royal, Virginia. “But metabolically, one will end up depositing more of that as fat than the other one.” This fact is the bane of millions of dieters everywhere. For scientists, it prompts an important question: What makes certain people more likely to put on weight?
Power teamed with Suzette Tardif, Corinna Ross and Jay Schulkin of the Southwest National Primate Research Center in San Antonio, Texas, to take an unusual tack in exploring this question. They looked at one of our relatives in the primate family: a small South American monkey known as the white-tufted common marmoset.
“The marmoset does seem to fit with the potential models of obesity for human beings,” says Power, the lead author of the research team’s paper, published in the March issue of the American Journal of Primatology. “If they become obese, they get the same sort of metabolic symptoms that a human would, so they could potentially be a good model for testing drugs, or other treatments.”
The team of researchers first began thinking about using the marmosets as a model for obesity because, well, the animals started getting obese. The research center’s colony of marmosets was started in 1993, and for several years, their average weight remained roughly the same, with each animal somewhere in the range of 300 grams. But then, says Power, “We started getting 400 to 450 gram marmosets. And, in these later years, we’ve been getting 500, 550, even 600 gram animals.”
“It looked like some sort of ceiling had been lifted off, and we suddenly started getting these very large animals,” he says. “But we hadn’t changed anything in our management.”
The mystery prompted the researchers to begin closely examining the marmosets as a model for human obesity. Because they are able to frequently weigh each animal, calculate its body fat percentage and precisely track its food intake and feces output, the species presents a promising opportunity to probe the mechanisms by which primates put on fat. Additionally, both humans and marmosets begin life as relatively fatty infants, as compared to most other animal species.
Metabolic analysis of the obese marmosets’ blood further indicated their similarity to humans. Obese marmosets had higher levels of glucose and triglycerides, in particular. “These are basically the same things one would get with an obese human being,” Power says. “If it was a person, and you looked at those numbers, you’d say the person is at risk of developing diabetes or cardiovascular disease.”
The researchers tracked these measurements, among others, over the course of years. Their most striking finding, for many, may be disheartening. Overweight marmosets—those with more than 14 percent body fat—had more body fat almost from the start, at just one month old, as compared to normal animals. “It seems like these animals are dividing into two groups at a very early age,” Power says. “It appears that developing obesity is something that can happen to an animal or a human before they have a real choice.”
Examining the marmosets’ feeding habits further complicated the picture. The team began offering higher fat food, in addition to the conventional fare, attempting to see if a preference for fat or an overall tendency to eat more was responsible for the obesity. No such luck.”We noticed that the animals that got fat did not seem to be eating more food, not in any dramatic fashion,” says Power. “One animal could be eating twice as much as another animal, and they could weigh the exact same. So clearly, there are other things going on.”
What, exactly, are those things? Power believes that energy usage, not just food intake, plays a huge role in determining obesity outcomes. “Energy balance is what you take in minus how much energy you expend,” he says. “Clearly, there are two sides of the equation, and it didn’t look to us that the intake side was what was causing the differences.”
The team is currently studying this second half of the equation among the marmosets, and is noticing significant variation in energy use within the population. “When you look at the animals, you notice some are always moving, always bouncing around the cage, and other seem much more relaxed and calm,” says Power. In coming years, they plan to publish studies examining how these differences affect the marmosets’ fat storage, as well as other relevant factors, such as feeding habits throughout the day and endocrine markers in the blood.
Despite these advances, the exact conditions that lead marmosets—or, for that matter, humans—to put on weight are still not well understood. “The energy balance equation looks incredibly simple, but the biology behind it is so complex,” Power says. “It’s really tricky to figure out how all these things fit together.”