L-arginine is a conditionally essential amino acid and it plays an important role in cell division, immune function, the release of hormones and wound healing [1,2]. Arginine is the immediate precursor of nitric oxide (NO), urea, ornithine, and agmatine, it is also necessary for the synthesis of creatine. It is found in red meat, poultry, fish, and dairy products.
L-Arginine as Performance Enhancer
L-arginine is broadly marketed (usually as arginine alphaketoglutarate [AAKG]) in order to promote widening of blood vessels by increasing nitric oxide (NO) production in muscles during exercise, improving strength, power and muscular recovery. However, studies contrast each other . Many studies show no significant results with L-arginine as a performance enhancer [4-8] while some show beneficial effects [9-11].
Two more recent studies [20,21] both reported no effect of L-arginine on exercise performance or recovery in healthy individuals. Furthermore, L-arginine supplementation was unable to increase the effects of exercise on nitric oxide production in runners .
L-arginine supplementation only seems to aid in nitric oxide synthesis when a body is in L-arginine deficiency .
Effects on Human Growth Hormone
Orally supplemented L-arginine has been shown to cause a significant increment in resting growth hormone levels (by at least 100%) at doses from 5 to 9 grams . A higher dose of 13 grams did not augment growth hormone response, probably due to intestinal distress which prevented absorption of the L-arginine . In contrast to this study, Forbes and Bell  reported that neither high nor low dose or orally supplemented L-arginine was able to promote a significant increase in IGF-1, growth hormone, nitric oxide or insulin at rest.
A study measuring 24-hour growth hormone secretion reported no significant differences with two daily doses of 2 g . This may be explained by modulatory effect on growth hormone known as the auto-negative feedback loop.
Both exercise and L-arginine have both been shown to independently augment growth hormone concentrations; however, their combined effect is controversial. The combination of oral arginine and exercise was reported to attenuated the growth hormone spike caused by exercise alone [13,15]. Exercise alone can increase growth hormone levels by 300-500%, while oral L-arginine plus exercise only increased growth hormone levels by around 200% . This diminished response is seen in both younger and older individuals ; however, it seems to affect youth more than older persons .
In support of these findings, a more recent study by Forbes et al.  reported that L-arginine ingested prior resistance training attenuated plasma growth hormone in bodybuilders. Researchers also noted that growth hormone suppression was not due to a growth hormone or IGF-1 induced auto-negative feedback loop.
Possible L-Arginine Side Effects
Low oral doses are well tolerated and clinical side effects are rare in healthy subjects . At high doses (13 grams) arginine was reported to cause considerable gastrointestinal distress . Other side effects may include low blood pressure, diarrhea, abdominal pain,…
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