Bones don't pay a price when fat is lost through exercise

05/25/05

Debunking the myth that exercising to lose excess body fat, unlike dieting alone, comes at a cost to bone health, researchers at Johns Hopkins have determined that for those age 55 to 75, a moderate program of physical exercise generally maintains bone mass and, in some cases, offers modest improvements.

The Hopkins team showed, in a study to be published in the June issue of the American Journal of Preventive Medicine, that after six months of aerobic exercise on a treadmill, bicycle or stepper, plus weightlifting, subjects experienced better overall fitness and fat loss without much change in bone mineral density. A more detailed analysis revealed slight gains in bone mass, of 1 percent to 2 percent, for those who exercised hardest and showed the greatest increases in aerobic fitness, muscle strength and muscle tissue.

The Hopkins study is believed to be the first to evaluate the effects of exercise independently from other factors, primarily diet, on bone mineral density, a strong gauge of bone health, against the risk of osteoporosis and bone fracture. Indeed, the researchers believe that more intense exercise may demonstrate significantly increased bone mass.

"Older people are very concerned about how best to reduce their body fat as a means of preventing other health problems, such as heart disease and diabetes," says lead study investigator and exercise physiologist Kerry Stewart, Ed.D., a professor of medicine and director of clinical exercise physiology and heart health programs at The Johns Hopkins University School of Medicine and its Heart Institute. "However, excess fat does have the benefit of maintaining bone mass. But fat loss through diet alone can lead to loss of bone, worsening the body's natural bone loss due to aging, a major risk factor for bone fractures."

According to Stewart, most existing studies about the effects of exercise on bone had several limitations to their findings. Many enrolled only women, for example, who are more prone to bone loss after menopause. Others combined dieting and exercise, obscuring the source of the impact. None examined the effects of exercise on bone while also factoring into account the weight and fat changes resulting from it.

For a six-month period, the Hopkins team assessed the benefits of a supervised program of exercise training in a group of 104 older men and women, measuring both fitness and fatness levels at the start and end of the study. All of the participants were in general good health except for untreated, mild hypertension. Half were randomly placed in a widely recommended moderate exercise program, believed to improve fitness, heart health and body composition, while the rest maintained their usual physical routine and diet.

The active group participated in a supervised series of exercises for 60 minutes, three times per week. The combination of exercises was designed to work all major muscle groups, the heart and circulation.

Substantial improvements were observed in active participants' body fat, and muscle and fitness levels.

Aerobic fitness, as measured by peak oxygen uptake on a treadmill, increased by 16 percent, and strength fitness increased by 17 percent. The average weight loss in this group was only four pounds, because much of the loss of fat was offset by increased muscle mass. The fat in the abdominal region, measured by magnetic resonance imaging, was reduced by 20 percent among exercisers. The group that was not exercising had either no or significantly less improvement than the exercising group.

Bone scans, using an X-ray machine called DXA, were used to assess bone mineral density. Measurements were taken of the total skeleton and in areas most vulnerable to bone loss and subsequent fracture, including three locations on the hip, and one in the spine. In both men and women who exercised and lost weight, overall bone density did not change, although results were mixed for women in specific sites of the hip. However, those who showed the greatest gains in fitness had modest gains in bone density at several sites, and loss of body fat was not associated with bone loss.

According to Stewart, an increase in bone mineral density among older exercisers has been observed in previous studies, but this increase was lacking in the current study because the fitness program was probably of too low intensity or too short in duration.

"Older people will likely have to exercise either harder or for longer than six months for there for there to be a substantial increase in bone density," says study co-author and endocrinologist Suzanne Jan de Beur, an assistant professor at Hopkins who specializes in bone health. "Our results show that moderate-intensity exercise can increase fitness and reduce body fat, which are important for overall health, but gains in bone density were found only among those who achieved the greatest gains in fitness in six months. Fat loss with exercise did not result in a loss of bone mass, a problem commonly seen when patients lose weight with diet alone."

Jan de Beur adds that while bone mass did not increase in this program, there are other benefits that will likely reduce the risk of fractures. Bones commonly break after a fall, a major risk factor for fractures in older people. But exercise makes bodies stronger and improves balance, thus preventing falls.

The Hopkins study's exercise program followed current guidelines from the American College of Sports Medicine. The study was part of a larger, ongoing trial, called the Senior Hypertension and Physical Exercise study (or SHAPE, for short). It is believed to be the first detailed examination of the guidelines' effectiveness and gender differences in the effects of exercise, with nearly an equal number of men and women enrolled.

Source: Eurekalert & others

Last reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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