As we get older our bodies metabolize calories more slowly, and we just aren’t capable of doing the things we used to do. Hangovers get worse, lasting several days, and a night out consuming hamburgers, cheese fries, and beer makes us feel way, way worse than it used to when we were sprightly young 20-year-olds. This also means that, as we age, we might also witness our waistlines getting bigger.

The strange thing is that as people get older, they tend to eat much healthier than they did as college students (the time when cramming down chocolate bars, fluffer nutters, and pizza during study sessions was likely a weekly ordeal). Older people are more likely to turn to responsible food choices like salads and lean meats. But if our diets improve as we get older, then why do we still get fat as we age?

According to a new study, it’s because physical activity has a greater impact on our weight and health than diet does as we grow older. The study examined 4,999 American adults between the ages of 20 and 70 by using data from the National Health and Nutrition Examination Survey, and recorded information about people’s weight, body mass index (BMI), waist circumference, and diet.

“Our study points to the very important impact of physical activity on weight status in U.S. adults, and in particular it points to the critical role of the age-related decline in physical activity on the increasing rates of overweight and obesity that we see with aging,” said Russell Pate, lead author of the study, according to the Huffington Post. “Our findings indicate that increasing fatness with age in U.S. adults cannot be explained by changes in the quality of the diet they consume.”

People who rely only on diet to lose weight end up either gaining the weight back shortly after, or struggle to lose weight because they’re not feeding themselves the proper nutrients. Exercise, on the other hand, helps you keep the weight off longer and will jumpstart your metabolism. Learn more about the diet-versus-exercise debate here.

Indeed, the study is important because obesity continues to be a huge problem in this nation, affecting nearly one-third of the American population, and the importance of exercise can’t be stressed enough. Two-thirds of the American population, meanwhile, is either obese or overweight.

“Americans should meet the federal physical activity guideline: 150 minutes of moderate-intensity physical activity per week,” Tate states. “If most American adults met that guideline, rates of overweight and obesity would be substantially lower than they are today.”

So, ordering a salad at your favorite diner instead of your regular cheeseburger may be a good start, but it’s certainly not the whole deal. You’re going to have to get moving if you want to stay fit and healthy as you get older.

Source: Pate R, Taverno Ross S, Liese A, Dowda M. Associations among Physical Activity, Diet Quality, and Weight Status in US Adults. Medicine & Science in Sports & Exercise. 2015.