If your friends are taller than you, that's because your hypophysis - a tiny gland at the brain's base - has released less growth hormone in you than in them, in your adolescence.

Around 30, the production of this hormone starts decreasing drastically.

Injections with the growth hormone are a treatment for short children and for adults whose pituitary glands don't produce enough growth hormone to maintain normal metabolism, but when the organism produces overdoses, gigantism can emerge if the subject is in childhood/teenage or acromegaly (photo) in adults.

A study published in 1990 revealed statistically significant increases in lean body mass and bone mineral after growth hormone treatment. The authors just stated that the increase in muscle and decrease in fat were "equivalent in magnitude to the changes incurred during 10 to 20 years of aging."

This triggered a wave of misinterpretation that still persists. Even if the original study warned against the general use of the growth hormone as a therapy in adults, after that the sells of this product took off.

More over, most traders promote the growth hormone as an anti-aging therapy for the healthy elderly. "There is certainly no data out there to suggest that giving growth hormone to an otherwise healthy person will make him or her live longer," said Hau Liu, MD, a research fellow in the division of endocrinology and in the Center for Primary Care and Outcomes Research.

"We did find, however, that there was substantial potential for adverse side effects", like joint swelling and pain, carpal tunnel syndrome and a vulnerability towards diabetes or pre-diabetes. "You're paying a lot of money for a therapy that may have minimal or no benefit and yet has a potential for some serious side effects," Liu said.

Between 20,000 and 30,000 people in the US took the growth hormone as an anti-aging therapy in 2004, ten times more than in the mid-1990s. This therapy is not only costly (more than $1,000 monthly) but illegal also in the US.

The team made a meta-analysis of the previous studies in the field. "The 31 studies had a combined total of slightly more than 500 participants, and the average duration of therapy was about a half-year," said Liu, amazed by the little amount of research on the issue. "These studies were designed to look at what happens when you give growth hormone to a healthy elderly person," said Liu.

"For example, what happens to their bone density, to their exercise levels and to their exercise capacity."

The growth hormone proved to have quite slight effect on body composition, increasing muscle body mass by roughly 2 kilograms and eliminating body fat by the same amount. But, "it did not change other clinically important outcomes, such as bone density measurements, cholesterol and lipid measurements, and maximal oxygen consumption."

This is not increased fitness. "From our review, there's no data to suggest that growth hormone prolongs life, and none of the studies makes that claim," said Liu.

"That finding, according to Liu, highlights one of the fundamental problems in the whole debate over the use of growth hormone to combat the effects of aging-misinterpretation of the data."