Stunning new research suggests that human growth hormone – mentioned specifically in December’s Mitchell Report to the Commissioner of Major League Baseball – may not improve athletic performance after all.

For more than a decade, illegal steroids and other performance-enhancing substances have been commonly used by players in the MLB, including HGH. HGH is undetectable through urine testing, perhaps explaining its increased usage, according to the Mitchell Report.

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Soon after the report was released, New York Yankee pitcher Andy Pettitte admitted to using HGH in 2004 when he was playing for the Houston Astros to combat problems with his elbow.

The Mitchell Report also named retired baseball player Roger Clemens as a user of HGH (as well as anabolic steroids), although he firmly denies the usage.

However, studies of controlled trials of individuals between the ages of 13 and 45 who took HGH versus those who did not, show some surprising evidence, said Dr. Hau Liu, associate chief of endocrinology at Santa Clara Valley Medical Center, who authored the report in the Annals of Internal Medicine.

“There’s no evidence that growth hormones improve athletic performance,” Liu said. “We found just a hint that it could worsen your performance.”

The study by Dr. Liu and his team found that people who took HGH experienced soft tissue swelling and fatigue more frequently than those who did not.

HGH is naturally produced by the pituitary gland and is secreted in decreasing amounts throughout one’s life. Without it, children wouldn’t grow and our bodies wouldn’t be able to repair themselves.

However, there are some caveats to this new revelation.

First, more research needs to be done, Liu said.

“This is certainly not the final word in growth hormone,” he said. “We don’t know how the hormone was administered in the studies compared to the athletes in the real world.”

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For example, athletes who take HGH probably take it over a longer period of time and they might be mixing it with other substances or steroids.

“There is a much harder question to answer, and that is; ‘What are the effects of multiple agents on athletes?’” Liu said.

Perhaps, "Rocky" star Sylvester Stallone, who has also admitted to taking HGH, knew what he was talking about when he was recently quoted as saying: “HGH is nothing ... Mark my words. In 10 years it will be over the counter.”