That finding comes from a study by Dennis T. Villareal, MD, and colleagues at Washington University in St. Louis.
In other words, they were typical Americans.
For the study, 36 of the participants agreed to lose weight. Half of them ate less -- about 20% fewer calories. The other half worked out more -- burning off about 20% more calories.
For comparison, 10 more participants got advice on healthy lifestyles but didn't diet or exercise.
What happened? Compared with the 10 who only got advice, the dieters and the exercisers both lost weight.
But, unlike the exercisers, the dieters lost more than weight. They lost bone, too.
And they lost it in the areas where elderly people are most likely to suffer fractures: their spines, their hips, and their upper legs.
"Calorie restriction is beneficial, but if you don't combine it with exercise you lose bone," Villareal tells WebMD. "You don't have to do it the way these people did, with exercise only.
"If you change your diet and exercise at the same time, it is a win/win situation," Villareal says.
Leslie Bonci, MPH, RD, director of sports nutrition at the University of Pittsburgh, notes that exercise stimulates bone growth.
If you just cut calories -- and don't exercise -- you harm your bones two ways. You may cut nutrients needed to maintain strong bones, and you don't stimulate bone growth.
"Moving is such a critical component of dieting," Bonci tells WebMD.
"You might get lighter without exercise -- but that's because you have less skeleton than you did before. That is a really big concern for people as they age," she says.
The Villareal study suggests exercise can counteract the bone-loss effect of dieting, says Edward Puzas, PhD, professor of orthopaedics at the University of Rochester in New York.
"If you are trying to lose weight and protect your skeleton, especially if you are an older adult, mixing diet with exercise would be best for protecting your bones," Puzas tells WebMD.
The Villareal study appears in the Dec. 11/25 issue of Archives of Internal Medicine.