The belief that the Human Growth Hormone (HGH) enhances athletic performances comes from the observation that it allows an increase in body lean mass and a decrease in body fat %, even in healthy subjects. 

A recent study (Ann Intern Med 2010; 152:568-577) administered 2mg of HGH per day, subcutaneously for 8 weeks, to a group of amateur athletes all aged below 40, eventually comparing the results in double blind to a control group. 

The pharmacological treatment effectively reduced fat mass by an average of 2.1kg (equal to 3% of body weight), increased the lean mass by an average of 3.3kg (equal to 4.7% of body weight), therefore adding 1.2kg in body weight. 

There was no apparent improvement in the measurement of VO2max, in maximal isometric strength and in standing high jump efforts, while the capacity of sprinting for a maximal effort of 30” on a cycloergometer resulted improved by approximately 4%. 

The Authors therefore hypothesize that the treatment with HGH enhances the anaerobic lactacid metabolism. 

Observations: 

- the group of athletes under examination is of mediocre level: VO2max = 3.2 L/min, equal to 45.7 ml/kg/min, with little training and an average body fat % = 23.2% 

- the dose of HGH that was subjected was not so modest (2mg correspond to 6 IU), equal to 6–8 times the average daily production in a healthy young adult. 
In fact, several collateral effects were reported among the subjects: muscular and articular pain, strong perspiration, paresthesia 

- the cycloergometer test was nonspecific for the majority of the participants, who were not cyclists. The Authors observed that an improvement of 4% could correspond to 0.4 seconds in a 100m sprint or to 1.2 seconds in a 50m swimming competition… 
In order to verify such claim, it would be opportune to evaluate top-level athletes performing their own specific sport: frankly such improvements are not presumable 

- the reduction in fat mass pointed out by the study can be easily obtained through adequate nutrition, especially on a group of athletes with a fat percentage above 20%; similarly, an increase in lean mass can occur through proper training and a diet with sufficient protein intake. 

A careful reader of 53x12.com (CM, United States) wondered why the use of HGH is so widespread in sports, if its effects on performance are so dubious. 
I shall try to answer: 

- the (erroneous) belief that HGH is not traceable in antidoping tests surely plays a major role in the decision to take such substance 

- HGH is a very powerful hormone, capable of altering metabolism and likely inducing certain feelings (of strength, muscle “fullness”) in the subjects, although these are not guarantee of any increase in performances. 
Many times the user actually reports increased internal frictions, with the effect of a “blocked” sensation; specific to cycling, this may induce to using low cadences, pushing long gears, consequently overloading muscles and tendons. 

- if athletes using it notice any improvement in their performances, this is due to a placebo effect supported by subjective “feelings” and a stronger determination of the athlete with regard to training, nutrition, lifestyle, in the belief that he/she’s adopting the best strategy in order to get the best results.