Growth hormone injections may increase recreational athletes’ sprinting speed, but does not seem to boost endurance, muscle strength or power, according to a study published Monday.

Human growth hormone, produced naturally in the brain’s pituitary gland, stimulates muscle and bone growth - making synthetic growth hormone a popular “doping” agent among athletes. The World Anti-Doping Agency (WADA) includes the hormone on its list of banned substances for competitive athletes.

However, a number of studies have questioned whether athletes actually gain a competitive advantage from using growth hormone. Researchers have found that while growth hormone injections can change users’ body composition - increasing muscle and decreasing fat - they do not improve athletic performance.

The new findings, reported in the Annals of Internal Medicine, are the first to show benefits for physical performance.

“We found the enhancement in sprint capacity would correlate to a 0.4 second improvement over 10 seconds in a 100-meter dash,” senior researcher Dr. Ken Ho said in a news release from the journal.

“This improvement could turn the last place finisher in the Olympic finals into a gold medal winner,” said Ho, of the Garvan Institute of Medical Research in Sydney, Australia.

No one, however, is advocating that athletes use growth hormone.

Among the short-term side effects are joint and muscle pain, swelling, and breast enlargement in men. The long-term risks are not fully clear, but growth hormone abuse may contribute to heart disease and diabetes.

And while sprint speed improved in this study, it’s not known whether that would actually translate into a better performance during competition.

For the study, Ho’s team recruited 96 healthy recreational athletes, including 63 men and 33 women. The women were randomly assigned to inject themselves with either growth hormone or a placebo (saline solution) every day for eight weeks; the men were assigned to use either growth hormone or the placebo, alone or in combination with testosterone.

After eight weeks, the researchers found, growth hormone appeared to boost the athletes’ sprint speed, as measured on an exercise bike - by an average of 4 percent over their speed at the study’s start.

Men who had received both growth hormone and testosterone showed an 8 percent improvement in sprint speed.

Other performance measures - including endurance, strength and jump height - failed to improve, however. And the gain in sprint speed was no longer apparent six weeks after the study participants stopped using growth hormone.

The most common side effects were swelling - reported by two-thirds of all growth hormone users - and joint pain, reported by 47 percent. Twenty-eight percent reported burning or tingling sensations in their skin.

Despite the apparent sprint capacity improvement, the study, which was funded by WADA and the Australian government, does not prove that the gain in sprint capacity would translate to a competitive advantage, according to Ho and his colleagues.

They note that the study is also limited by the fact that it focused on recreational athletes, rather than the elite athletes who are most likely to use the hormone. (It is considered unethical to give competitive athletes banned substances, even for research purposes.)

In addition, study participants took “modest” doses of growth hormone, below those reportedly used by elite athletes. Higher doses of the substance, taken for a longer period, might have different effects, according to the researchers.

“In our study, we used doses of growth hormone on the low end of what is believed to be abuse in sports,” Ho said. “And for that reason, we think that the real effects of growth hormone could be far greater than what’s reported in our study.”

“Equally,” he added, “the side effects could be much more serious, as well.”

SOURCE: Annals of Internal Medicine, May 3, 2010.