Exercise can do a lot of good for most people, but it apparently isn't much help to those with heart failure, the fastest-growing heart problem in the United States.

The study — the largest ever of exercise in patients whose hearts don't pump enough blood — left many doctors disappointed. Results were reported Tuesday at an American Heart Association conference.

Although there were some encouraging trends and clear benefits for certain people, exercise failed to deliver on the main goal — keeping people out of the hospital and improving their survival rates.

"It's a shame," said Dr. Harlan Krumholz, a quality-of-care researcher at Yale University who had no role in the study. "Exercise is not that magic elixir that we had hoped," at least for these patients.

About 5 million Americans have heart failure. It kills more than 300,000 of them and accounts for a million hospitalizations each year. Those numbers are expected to grow as the nation's population gets older.

The condition occurs when the heart muscle weakens over time and can no longer pump effectively. Fluid can back up into the lungs, leaving people gurgling and gasping for breath as they struggle to climb stairs or walk around the block.

Exercise has long been known to help prevent the clogged arteries that develop in heart disease and to help heart attack patients recover. But smaller, previous studies have made conflicting findings about whether it helps heart failure patients or even is safe for them.

Doctors had hoped that exercise would prove as effective as drugs for these patients, sparing them the cost and potential side effects.

The study involved 2,231 people with moderate heart failure in the United States, Canada and Europe. It was led by Dr. Christopher O'Connor at Duke University.

All of the patients were getting optimal medical care, with more than 90 percent on an ideal mix of medicines. Those who needed them also had implanted heart devices to maintain good rhythm.

They were randomly placed in two groups — one given usual care and the other usual care plus an exercise training program. Exercisers were given 36 supervised training sessions lasting half an hour three times a week. After 18 such sessions, they were given a treadmill or an exercise bike to use at home, for five 40-minute sessions each week.

Three months into the study, only half were exercising at least three times a week for 40 minutes. After one year, only one-fourth were exercising five times a week.

The fact so few stuck with the exercise program made it difficult to show a positive result, O'Connor said.

There were 796 deaths or hospitalizations among those getting usual care and 759 in the exercise group — a statistical draw.

However, after doctors adjusted for factors like how long people were able to tolerate exercise, how badly damaged their hearts were and rates of depression, they did find a modest but significant benefit for exercise.

"It's disappointing," said Dr. Robert Eckel, a former heart association president and an exercise specialist at the University of Colorado at Denver. "You cannot make strong conclusions about subgroups."

Insurers now do not pay for exercise training for people with heart failure, and "this study is not going to help" convince them to start, Eckel said.

"We all would have liked to see a huge benefit to exercise," said Dr. Lawton Cooper, medical officer at the National Heart, Lung and Blood Institute, which paid for the study.

Still, for most people, "it is worth your while," Cooper said. "We know there are all kinds of benefits of exercise."

Among them: quality-of-life improvements, said Dr. Ann Bolger, a heart failure specialist at the University of California in San Francisco.

"Just the fact that it's safe is a huge deal," she said. "Patients want to be in control and to be active," and this shows them they can.

One study participant — Lise Coleman, 44, of Fayetteville, N.C. — said exercise dramatically improved her life.

"When I first started in the program, I was a pitiful thing. By the time I finished — you know how they time you when you walk around the track — I was the fastest walker," she said.

Doctors in the study gave her an exercise bike, which she still rides sometimes more than once a day. Her husband also bought her a treadmill, and she bought an elliptical trainer.

"I love it. When you have heart failure, your mind wants to do more than your body is able," she said. "I can do more than I used to."