Injections of human growth hormone, which have become increasingly popular as a virtual "fountain of youth," do reverse some of the common physical attributes of aging, a new federally sponsored study has found. But the shots also have potentially serious side effects, including increasing the risk of developing diabetes and carpal tunnel syndrome.

In the most extensive clinical trial so far of the hormone -- which is available at the many "anti-aging" clinics opening around retirement centers -- researchers concluded that the growth hormone treatment was not ready for widespread use, although it showed a "promising" ability to increase muscle and decrease fat in older people.

"There may be benefits to some older people in the use of growth hormone, but the safety is not established, and it should only be used in controlled trials," said Marc R. Blackman of the National Institutes of Health, who led the study. "This is not ready for prime time."

Human growth hormone has been approved by the Food and Drug Administration only to treat severe hormone deficiencies in children and adults, and for wasting in AIDS sufferers. But officials believe it is also being widely prescribed for older people who believe aggressive advertisements that promise the hormone will help keep them feeling and looking young. Black-market, and often counterfeit, growth hormone is used as well by athletes and body builders, FDA officials said.

Federal officials said there are no firm statistics on the overall use of the growth hormone, but some have estimated the number of older adults injecting the hormone to be 25,000 to 35,000. Some doctors involved in the business say the overall use exceeds 100,000 people when black-market sales are included. The black market consists of people getting the hormone without a prescription, or getting versions of the hormone made by counterfeiters. Yearly growth hormone sales have been estimated to be as high as $2 billion.

The new study, published in today's Journal of the American Medical Association, followed 131 men and women 65 to 88 years old. The subjects who were injected with the hormone developed significantly more muscle and lost significantly more fat than the placebo group, the researchers found. But 18 of the men on growth hormone developed diabetes or glucose intolerance, while only seven men who were not receiving the hormone developed those conditions. Carpal tunnel syndrome and swelling of limbs were also notable side effects.

Human growth hormone is made in the pituitary gland, a pea-sized structure located at the base of the brain, and is believed to be essential to the normal development of tissues and organs. The two sex hormones, testosterone and estrogen, work in combination with growth hormone to bring on puberty and maturation in teenagers.

Researchers such as Blackman believe a similar interaction occurs as men and women age, and that the level of both the growth hormone and sex hormones naturally decline. With the decline in growth and sex hormone levels, muscle mass tends to decrease and body fat increases. Some have also linked the hormone declines to decreased sexual drive and mood changes.

When growth hormone was initially developed as a medication, it was harvested from the pituitary glands of human cadavers. It has subsequently been produced by genetically engineered bacteria endowed with the gene for growth hormone.

In the new study, which was conducted by a team directed by the NIH's National Institute on Aging and Johns Hopkins University Medical Center, researchers found that both the positive and negative effects of growth hormone treatment were present in men and women. They also found that men given both growth hormone and testosterone -- but not growth hormone alone -- had significant improvements in their ability to exercise.

Despite the improvements in body composition regarding fat and muscle with the growth hormone, Blackman said that there was no measurable improvement in actual body strength. He said that could be explained by the relatively short duration of the trial.

S. Mitchell Harman, a study leader formerly with the NIH and now with the Kronos Institute for Longevity Research, warned that use of growth hormone should be seen as experimental research, rather than clinical therapy. "If you are taking it as an anti-aging miracle cure, you are fooling yourself," he said.

He called the business in growth hormone pills and sprays is "snake oil." He said the growth hormone molecule is too large to pass into the bloodstream unless it is injected.

Alan P. Mintz, head of Cenegenics Medical Institute in Las Vegas, one of the country's largest anti-aging centers, disputed the study findings. He said his patients have had great success and few side effects with growth hormone treatment, which he always gives in conjunction with broader hormone therapy.

He said that Cenegenics has about 4,000 patients, and about one-third spend $400 to $500 a month for growth hormone injections. He said the injections only bring patients up to a normal level of growth hormone, although he acknowledged there is dispute about what that is. He said his patients have seen improvements in body mass (the ratio of muscle to fat), in ability to exercise, to sleep and to enjoy sex.

The FDA has sent out at least two warning letters to companies advertising growth hormone in recent months, and a spokeswoman said misuse of the product was a clear concern for the agency.