"It's no longer a question whether the bone marrow cells work or don't work; they do work when you have healthy stem cells," Dr. James Willerson of the Texas Heart Institute said in an interview. "The search now is to find the very best stem cell type or types.

"And at the moment, I'd say the best stem cells are fat-derived cells and stem cells that reside in the heart called c-KIT positive stem cells," he said before presenting findings of his latest research at the annual scientific sessions of the American College of Cardiology in Chicago.

Willerson's newest 92-patient, government-funded trial, is the largest study of bone marrow-derived stem cells ever done in patients with severe heart failure - a condition in which the heart is too weak to adequately supply the body with blood.

The study confirmed favorable results seen in the past decade from two of his previous studies of bone marrow-derived cells in similar patients, Willerson said.

He said patients enrolled in his latest trial called FOCUS had ejection fractions, a measure of the heart's pumping strength, of 45 percent or less, far below normal readings of 55 percent or greater.

By removing stem cells from bone in the hip, growing them in culture and then injecting them directly into the heart, he said ejection fractions were nudged 2.7 percentage points higher.

"It was a small but statistically significant improvement and it was even greater if patients were younger," he said, referring to those in their early 60s or younger, when stem cells are more functionally normal.

The patients will be followed for five years to assess the success of the treatment.

Although impressive, Willerson said the heart improvements in the FOCUS trial paled in comparison to favorable results seen in a recent study presented last year. They involved the use of fat-derived stem cells called mesenchymal cells to treat patients with severe heart failure.

In that trial, he said patients had improved heart function that actually translated into a reduction in death, heart attack, and the need for rehospitalization.

"They were more dynamic and impressive results," he said, noting that patients with severe heart failure typically cannot walk any distance for lack of energy, are very short of breath and often sleep sitting up because they cannot breathe well laying down.

Willerson said he and his colleagues hope that continuing favorable trial results will convince U.S. health regulators that stem cells provide considerable benefit to heart patients and are safe.

"We're very enthusiastic about stem cells in the repair of injured hearts," he said. "There will be a day in the not-too-distant future that the U.S. Food and Drug Administration comes to believe the same thing."

(Reporting By Debra Sherman; Editing by Paul Simao)

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