Repeated injections of human growth hormone caused 12 men in their 60s and 70s to lose fat and gain lean body mass in a six-month study that suggests the well-known drug can restore a more youthful body to an older man.

The findings have led some to speculate that growth-hormone injections for the elderly may one day be prescribed much as estrogen "replacement" therapy is prescribed for women after menopause.

The report, published in today's New England Journal of Medicine, is the first to assess the effects of growth hormone on healthy, older adults. Although researchers called the findings preliminary, they said the results suggest that the hormone may turn back the clock on some aspects of aging, reversing the normal pattern of muscle loss and fat gain -- and potentially making some frail, elderly people more vigorous.

The annual cost of growth-hormone injections for an average-sized man is almost $14,000 a year now, according to Mary Lee Vance, a professor of medicine at the University of Virginia who wrote an editorial accompanying the report. The injections would need to be taken three times a week indefinitely to maintain the youthful effect.

"This gets into the whole area of what aging is," said Gordon Cutler, chief of developmental endocrinology at the National Institute of Child Health and Human Development.

Noting that growth-hormone levels normally decline with age, he added, "Should that be reversed? Is the loss of growth hormone part of the way in which nature programs the gradual enfeeblement, and eventually the death, of older people? Does fixing this improve on nature?"

Cutler warned that long-term studies may reveal a "down side" to growth-hormone treatment. "Growth hormone makes almost everything grow," he said. "Old age is also the time when cancers grow. Is there going to be an increased frequency and growth of malignancies?"

In high doses, human growth hormone's potential side effects are high blood pressure, arthritis, diabetes and heart failure.

Growth hormone is a protein manufactured by the pituitary gland. It stimulates normal bone growth in children and also has profound effects on muscle, fat and general metabolism. Synthetic growth hormone, made by genetically engineered cells living in vats but identical to the natural substance, can be prescribed by any physician, and injections are routinely administered to improve growth in children whose pituitary glands make too little of the protein. The treatment also has been tried for short children who are not hormone-deficient.

Growth hormone is almost never prescribed for adults except in experimental studies, yet about one-third of people over 60 produce little or no growth hormone, said Daniel Rudman, a professor of medicine at the Medical College of Wisconsin and principal author of the study. In the past, experts have not considered low growth-hormone levels abnormal in this age group, but Rudman suggested there may be such a thing as growth-hormone deficiency in the elderly.

"Anything that improves functional capacity in frail, elderly people translates into better quality of life," he said.

But Rudman and other experts cautioned that much larger and longer studies will have to be performed to assess the hormone's effects in adults before its long-term use can be recommended. This study was funded in part by Eli Lilly & Co., which sells human growth hormone.

For their study, Rudman and his coworkers enrolled 21 healthy men between 61 and 81 years of age, all of whom had blood tests indicating they were producing very low levels of natural growth hormone. Twelve of the men were randomly chosen to receive growth-hormone injections three times a week, at doses that raised their hormone levels into the normal range for healthy young adults. The other nine men served as the control group, receiving placebo injections. Men in both groups followed similar diets and maintained their usual activity levels. Neither the men nor their doctors knew who was getting the hormone until after the study.

In the group receiving the hormone, lean body mass -- the non-fat and non-bone portion of the body -- increased by almost 9 percent, and fat decreased by more than 14 percent. Skin thickness increased by 7 percent. There were no significant changes in body composition in the placebo group.

Rudman said X-rays showed that the arm and leg muscles of the treated men got bigger. "Some of the men said they had more energy and didn't get tired as easily," he added.

The group receiving the hormone showed slight rises in blood pressure and blood sugar but suffered no significant side effects. Rudman said the men receiving the hormone received frequent blood tests to make certain they were not getting too high a dose.

Rudman said the increase in lean body mass probably reflects growth of both muscles and internal organs. Organs such as the kidneys normally become a little smaller and work a little less efficiently in elderly people. He said studies must be done to find out which organs enlarge in response to the hormone and whether such growth improves their function.

Cutler noted that, while the hormone's effects on muscle and fat may be visible and dramatic, its ability to retard aging may depend more on how it affects the brain, heart, lungs, immune system and other vital organs.

"Aging involves every organ system," he said. "It's hard to believe all of this is going to be reversed. But you never know."