A study of healthy older people shows that being fit doesn't mean you have strong bones. Kerry J. Stewart, EdD, director of clinical exercise physiology at Johns Hopkins University, led a study of men and women age 55-75. Their only problem was blood pressure in the high-normal or mild-high range.

Being normal Americans, the study participants didn't exercise regularly. Stewart's team put them through a series of tests. These included a treadmill test for aerobic fitness, muscle-strength tests, measures of abdominal fat and overall muscle and fat, and several measures of bone density. The findings appear in the November issue of the Journal of Internal Medicine.

The most fit of these "average Joes and Janes," as Stewart calls them, looked pretty good on the treadmill test. But this aerobic fitness meant nothing in terms of bone density. The fittest people were at just the same risk for brittle bones as those who gasped for breath after a short time on the treadmill.

"Moderate exercise might be good for heart health, but more vigorous forms of exercise may be needed for bone health," Stewart tells WebMD. "We are looking at a group of people who are able to exercise -- but they don't. They do some walking, they may do usual household routines, they go to work, and some have vigorous jobs. But there is no evidence that the low-intensity exercise of a normal lifestyle contributes to bone mineral density. That, together with other studies, implies that when there are improvements in bone it is more vigorous exercise that does the trick."

The findings come as no surprise to aging and bone-health expert Robert Marcus, MD, professor emeritus at Stanford University. He says that it takes a lot more than the daily routine to improve bone strength.

"People tend to be in a lazy zone where there isn't much change," Marcus tells WebMD. "The body doesn't respond to subtle changes in day-to-day activity. You would have to see fairly substantial changes to see a skeletal response. Everybody is at a comfort level. If you are doing only mild things over a period of time you are not going to get significant bone growth."

Stewart did find one thing linked to better bone mass: big bellies. The people with the fattest abs had the densest bones.

"We are certainly not going to recommend that people should gain weight for bone health," Stewart says. "Abdominal fat increases the risk of heart disease, high blood pressure, and diabetes and worsens chronic conditions such as knee arthritis. But one protective factor for bone is body weight, particularly fat [around organs]. It appears that having more fat raises levels of leptin, an enzyme that may help bone cells grow."

What the fat finding may mean, Stewart speculates, is that weight-loss diets could weaken bone. If this turns out to be true, it would mean that it's important for people who go on diets to begin a vigorous exercise program at the same time.

Marcus notes that while exercise doesn't make you build a lot of bone, being a couch potato makes you lose bone fast.

"It would be very unrealistic to think a person could gain more than 5% to 6% of bone mass through exercise -- but by being inactive you could lose 40% of bone mass in a year," he says. "We encourage people to do as much as they can do: regularly, frequently, and without injury."