(Reuters)
Updated: 2008-03-18 09:08

NEW YORK - Growth hormone may increase muscle mass, but there is no evidence it enhances athletic performance, researchers reported Monday.

In a review of 44 previously published studies, researchers at Stanford University in California found that human growth hormone generally seemed to increase muscle mass in young, fit adults.

It did not, however, improve muscle strength or exercise capacity.

"Claims that growth hormone enhances physical performance are not supported by the scientific literature," Dr. Hau Liu and colleagues write in a report published online by the Annals of Internal Medicine.

The body naturally produces human growth hormone to maintain body tissues and organs throughout life. Synthetic growth hormone is prescribed for certain medical conditions, such as growth hormone deficiency in children.

It may be more well-known, however, as a "doping" agent among athletes hoping to boost their performance. The current findings, according to Liu's team, suggest that those athletes may not get what they are after.

Instead, they write, the evidence "suggests that while growth hormone may alter body composition, it has minimal effect on key athletic performance outcomes and may, in fact, be associated with worsened exercise capacity."

The review found that in studies that looked at exercise capacity, participants who used growth hormone showed no gains in endurance, bicycling speed, "power" output or other measures of athletic performance.

In some studies, there was evidence that growth hormone might, in fact, lower stamina. More research, Liu's team writes, is needed to further investigate this possibility.

Along with this lack of benefits came the risk of side effects.

Across the studies, growth hormone users were more likely to have problems like swelling, joint pain and fatigue.

According to Liu's team, a limit to the findings is that the studies were done in controlled research settings.

More studies are still needed, they say, to see what effects human growth hormone has on athletes as it's used in the "real world" -- often in combination with steroids or other doping agents.