Trying to lose weight? Better hit the gym. That’s what a study published in the American Journal of Physiology suggests. Research on monkeys living at the Oregon National Primate Research Center found that, after consuming a high-fat diet for years, these animals didn’t lose weight after they were put back on a normal diet with 30% fewer calories. But they were less active. When calories were reduced even further, these primates were even less active than before.

From this study, you can see why people who diet without exercising aren’t likely to be successful in their weight loss efforts. Consider the fact that a comparison group of monkeys fed an identical diet did manage the shed the extra weight. How? They were trained to run for an hour on a treadmill every day. Researchers didn’t have them pump iron, but other studies suggest that a combination of resistance and cardio exercise can be an effective weight loss and fitness strategy.