What is the best form of exercise for older adults who are dieting?Doctors do not yet know the answer.But one study suggests that elderly dieters may be able to protectagainst bone thinning by doing resistance exercise.Medical experts often use body mass index, or BMI,to measure obesity – the condition of being very overweight.BMI compares a person's height and weight.A BMI of around 30 or higher is considered obese.Worldwide the number of obese people has nearly tripled since 1975,notes the World Health Organization.Obesity problems among older adults are difficult to deal with.This is because weight loss efforts can create health problems,such as an increased risk of bone thinning.Thin bones are more likely to fracture, the study team noted.Their findings appeared in The Journal of Bone and Mineral Research.For the study, researchers randomly put 141 obese adults into one of four groups.The first group had dieters who did only aerobic exercise.The second group had dieters who only did resistance exercise.The third had dieters who did both forms of exercise.The final group, the control group, did not exercise or change their diet.The subjects met with dieticians and did supervised workouts over 26 weeks.The control group went to some classes to educate them about healthy eating.All of the subjects in the study had a BMI of 30 or greater.The average age was 70 years old.In the exercise groups, subjects were given custom dietsthat created a deficit of about 500 to 750 calories a day.They also did supervised one-hour workouts three times a week.The aerobic exercises included walking on an exercise machine,riding a stationary bicycle, and stair climbing.Resistance exercises included upper- and lower- body exercises using weight machines.All of the exercisers also did some activities designed to improve balance and flexibility.People in all three diet groups lost about 9% of their body weight during the study.Members of the control group lost about 1% of their body weight.At the end of the study, all of the participants had a decrease in bone density.But bone density decreased less in the older adults who did resistance exercise.Bone density also decreased less in those who did resistance exercise and aerobic workouts.Doctor Dennis Villareal was the lead writer of a report on the study."When undergoing weight loss therapy," he said, elderly patients should use"some resistance exercise to protect against bone loss.""Resistance exercise should be at least moderate in intensity," Villareal added in an email."We do not know if less-intense exercise would be effectivein protecting against bone loss during weight loss."Villareal is with the Baylor College of Medicineand the Michael E. DeBakey VA Medical Center in Houston, Texas.One limitation of the study is that researchers only followed the subjects for about six months.That length of time is too brief to confirm whether one type of exercisemight be better than another for fall or bone fracture prevention, the report said.Cynthia Brown is a doctor with the Integrative Center for Aging Researchat the University of Alabama at Birmingham."Based on this study, one would conclude that (for) an older adultwho is obese and trying to lose weight, the combination of aerobic and resistance exercisewill help the most to maintain their bone mineral density and their general physical fitness," she said by email."Older adults who are able should do muscle-strengthening exercises (resistance exercise) at least twice a week."Brown was not involved in the study.She also suggested that older adults with health issues"make sure they understand how these conditions affect their ability to exercise."She added that "all older adults should be as physically active as their abilities and conditions allow."