Athletes who use human growth hormone can sprint faster but the substance does not improve strength, power or endurance, say Australian scientists.
Professor Ken Ho of Sydney's Garvan Institute of Medical Research and colleagues report their findings in the Annals of Internal Medicine.
The researchers say using human growth hormone (hGH) can improve an athlete's sprint speed by 4 to 5% - potentially turning the last-place into a gold medallist in an Olympic track event.
"A 4% improvement over a 10-second period is 0.4 seconds which is a huge time interval," says Ho, who is also chairman of the Department of Endocrinology at St Vincent's Hospital.
He says the study is the first scientific research showing improved physical function from using hGH, a naturally occurring hormone which is important for growth and metabolism and is banned by the World Anti-Doping Agency (WADA).
Human growth hormone is thought to be widely used by elite athletes who believe that injecting themselves with it results in bigger muscles and therefore increases strength, power and endurance.
Ho says while the new findings justify the WADA ban, using growth hormone injections may not improve performance in all areas.
"The advantage is really in the type of sporting event," he says.
"I don't think it would help them if they were a rower or a weightlifter. But it would certainly help them if they were a sprinter."
The study examined 96 recreational athletes aged 18 to 40 over eight weeks.
Men were divided into four groups: the first were given daily injections of hGH, the second were given testosterone, the third received both and a fourth received just a placebo.
Women were given either growth hormone or placebo.
Ho and colleagues found the sprint capacity of those who received the growth hormone rose by 4 to 5%, and that of those who also received testosterone jumped 8%.
Growth hormone recipients did not increase their muscle mass but they did retain body fluid and experienced swelling and joint pain, according to the WADA-funded study.
The research, which used volunteers who had engaged in regular athletic training for at least a year, used lower doses of growth hormone than athletes are reported to use, and for a shorter time.
"We can speculate, therefore, that the drug's effects on performance might be greater than shown in this study, and its side effects might be more serious," says Ho.
Growth hormone test
Ho says the research, believed to be the largest scientific study on hGH as well as the first to study sprint capability, came about as a side project of Garvan's four-year work on a test to detect hGH in athletes.
A test for hGH was first introduced at the 2004 Olympics in Athens but the first athlete believed to have tested positive to the substance is British rugby league player Terry Newton.
Newton, who played for Wakefield Trinity Wildcats, accepted a two-year-ban in February after he was tested in late 2009.
The use of hGH is difficult to detect because it is identical to the hormone which occurs naturally in the body.
Potential side effects from hGH include an overgrowth of bone which can lead to a protruding jaw and eyebrow bones, and abnormal growth in hands and feet as well as the possible development of cancers and a shortened life expectancy.