Move more, eat less. That’s been the standard prescription for weight loss for many decades. But new research involving nearly 5,000 Americans has found exercise trumps dietary changes when it comes to losing weight as we age.

In fact, the University of South Carolina analysis — published in the Medicine & Science In Sports & Exercise journal — shows the quality of most Americans’ diets tends to improve with age, but it isn’t enough to keep the pounds from adding up. That’s because older Americans tend to be less physically active.

The upshot: It doesn’t matter how nutritious your diet may be; if you don’t engage in a minimum of 30 minutes of moderate to vigorous exercise most days of the week, you will gain naturally weight as you grow older.

“It is well known that U.S. men and women gain weight and fat as they age,” the researchers concluded. “However, to our knowledge, this is the first study to comprehensively document age-related trends in weight status, physical activity, and diet quality using a representative sample of U.S. men and women.

“The current investigation found that … age-related increases in BMI [body mass index] and waist circumference are likely to be explained by the age-related decline in physical activity.”

The findings are based on an analysis of information from the long-running National Health and Nutrition Examination Survey, a federally funded research project that tracks American’s overall health and habits.