You’re on a diet, you’ve ramped up your aerobic exercise, and the numbers on the scale are going down. There’s just one problem—this popular weight-loss formula could be causing you to lose so much muscle, as opposed to just losing fat, that you are endangering your strength, stability and, if you’re older, continued independence. In fact, a new study shows that combining dieting with aerobic exercise can make people lose more muscle mass than if they didn’t exercise at all!

The study: Researchers at Wake Forest University in North Carolina recruited 249 people in their 60s and 70s who were overweight and followed them for 18 months. Each participant was assigned to one of three programs—dieting with no exercise…dieting plus aerobic exercise (which, for this study, was walking)…or dieting plus weight training using weight machines. The two exercise groups worked out four days a week for 45 minutes.

The results: Diet plus aerobic exercise actually led to greater muscle loss than dieting alone. Among those who dieted only (no exercise), 16% of the weight they lost was muscle. Among those who dieted and did aerobic exercise, 20% of the weight they lost was muscle. On the other hand, among those who dieted and lifted weights, just 10% of the weight they lost was muscle.

Weight training won out in another way, too. Participants who dieted and weight trained lost 17 pounds of fat, on average, while those who dieted without any exercise lost just 10 pounds of fat. (The group that dieted and got aerobic exercise lost 16 pounds of fat.)

Now, no one, including the researchers, is saying that dieters should refrain from aerobic exercise—it’s essential for heart health and for overall fitness. But this study shows just how important weight training is and why you should add it to your diet plan. For a specific routine to preserve muscle while dieting, see “Finetune Strength Training for Specific Goals.”

The benefits go beyond the scale: We all naturally lose muscle mass as we age, starting in our 30s. The additional muscle loss from dieting is greater in older people. So while losing excess weight is good for overall health, preserving as much muscle mass as you can with strength training will keep you strong and independent, prevent functional declines and help you avoid falls. Learn more ways to fight muscle loss in your senior years.

Important: If you’re new to strength training, ask your doctor if it’s OK for you to start, says Jacqueline Crockford, a certified trainer with the American Council on Exercise. Once you get the green light, begin working out two days a week, and as you get stronger, you can work out every other day (muscles need about 48 hours between sessions to recover).