April 14, 2006 -- Active monkeys stay lean, while couch-potato monkeys get fat -- no matter how much they eat.
The findings come from a research team led by Judy Cameron, PhD, senior scientist at Oregon National Primate Research Center and professor of behavioral neuroscience and obstetrics/gynecology at Oregon Health & Science University.
"Far and away the biggest predictor of weight gain was how active the monkeys were -- that overrode how much food they were eating," Cameron tells WebMD.
The study may explain why people who try to lose weight by dieting alone rarely succeed.
"These findings were surprising to us as scientists," Cameron says. "We always assumed food intake was the main control of body weight and that dieting was the best way to control this. From these results, you are forced to think how active you are is more important than how much you eat."
Cameron and colleagues report their findings in the early online edition of the American Journal of Physiology: Regulatory, Integrative and Comparative Physiology.
Exercise Trumps Calories
Cameron studied 18 middle-aged, female rhesus monkeys. All of the animals had their ovaries removed to simulate human menopause. Earlier studies show that such monkeys tend to gain weight, as do middle-aged women.
The researchers put the monkeys on a high-fat diet -- that is, the 35%-fat diet of most women in the Western world. They let the animals eat as much or as little as they liked. And they let them exercise in their cages as much or as little as they wanted.
Over a nine-month period, the researchers measured the monkeys' activity level using a measuring device they wore around their necks. They also kept careful track of the animals' weight, food intake, and metabolic rate.
Some animals ate a lot. Some did not. It didn't make any difference in the monkeys' weight.
Some animals were very active. Some weren't. The difference was remarkable: The most active monkeys were eight times more active than the most sedentary monkeys. It made a huge difference in how much the monkeys weighed.
"We suspected if you ate a lot of food you would be more likely to gain weight. But we did not find that," Cameron says. "Some individual monkeys that ate a large amount did gain a lot of weight, but others who ate a lot gained virtually no weight. Very active monkeys did not gain weight over time. Very sedentary monkeys did gain weight, even if they were not eating a lot of food."
In a new study that hasn't been published, Cameron's team put overweight, inactive monkeys on a strict diet. They didn't lose much weight.
"I don't think most people who try to diet will be surprised," Cameron says. "The body immediately tries to compensate for calories you are not taking in by dropping your metabolic rate and lowering your activity level. There was no way to get around the fact that activity matters a great deal. Altogether, our feeling is that activity is very, very important in controlling body weight."
Can Obese Humans Become Active?
A lot of studies show that obese people aren't as active as lean people are. Cameron's study now suggests that it isn't obesity that turns people into couch potatoes -- it's inactivity that makes people obese.
However, it may not be a simple thing for overweight or obese people to become more active. Cameron says it's possible to predict how active monkeys will be when they're only 1 week old.
"Why are some monkeys more active? You would think it was what situation they were living in," Cameron says. "But we looked at monkeys with acres to run around in and many playmates. You would expect them to be very active. And if you move them indoors with fewer playmates, they should not be as active. But that was not true. A sedentary monkey was just as sedentary, and an active monkey was just as active no matter the setting. And we find this remains true over time."
It remains to be seen whether human couch potatoes can switch to an active lifestyle. But Cameron's work may have put one myth to rest: It's not your metabolism.
People who gain or don't gain weight often point to their inherent metabolic rate as the culprit. For monkeys, at least, this isn't so. Monkeys' metabolic rates didn't vary much. And those with the highest metabolic rate were no more likely to gain or lose weight than those with the lowest metabolic rate.