Evidence suggests that compounds such as caffeine, ephedrine and clenbuterol may be able to alter mammalian growth rates because they can alter intracellular levels of cyclic AMP (cAMP). This research has perhaps been exemplified by studies on cattle fed high intakes of the β-2 agonist clenbuterol. Animals administered clenbuterol exhibited greater growth rates and a larger muscle mass compared to controls, and this phenomenon has been repeated in chickens, pigs and sheep. At the same time as increasing muscle mass and growth rates, clenbuterol also decreased the body fat of the animals. These effects were further confirmed by other studies using other β-adrenergic agonists such as albuterol, cimaterol and ractopamine. Interestingly, studies found that effects in chickens were the least, the sheep and cattle experienced the greatest effects, and pigs fell somewhere intermediate to these two extremes. One explanation for this is that some species of animal are naturally much higher to their total genetic potential for growth and so β-adrenergic agonists do not affect them to the same extent as other species. This likely reflects the breeding of chickens being aimed towards rapid growth, whereas sheep and cattle are not bred primarily for these effects. Another reason proposed for these differences is that β-adrenergic receptor numbers vary significantly between species.
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Mersmann, H. J. 1998. Overview of the effects of β-adrenergic receptor agonists on animal growth including mechanisms of action. Journal of Animal Science. 76(1): 160-172
