Alpha-ketoisocaprotic acid is the metabolic product of transaminated leucine and some have implied that α-KIC is responsible for the suppression of proteolysis observed with leucine supplementation. In an early study, Tischler et al. reported that α-KIC inhibits proteolysis in skeletal muscle (Tischler et al., 1982). However, their conclusions were not definitive enough to directly implicate α-KIC as the only factor responsible for their anti-catabolic observations. Nakashima et al., tested whether leucine itself or α-KIC was responsible for its anti-catabolic effects (Nakashima et al., 2006). In this animal based and comprehensive molecular level investigation, they compared the effects of L- and D- leucine and α-KIC on myofibullar proteolysis in skeletal muscle of chicks (baby chickens, not girls). Their results indicated that L-leucine had an inhibitory effect on proteolysis, but D-leucine and, even more so, α-ketoisocaprotic acid had greater anti-catabolic effects. Their results clearly indicate that α-KIC is responsible for the anti-catabolic effects of leucine supplementation and that D-leucine is more effective in converting to α-KIC. This does not discount leucine as a supplement to increase anabolism, as leucine is widely recognized for this property. However, if you want to throw catabolism out the window, then skip the middleman and add α-KIC to your supplement regime.
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