There is no doubt that the branched chain amino acid (BCAA) leucine is a valuable asset to bodybuilders.
In fact, among the BCAAs leucine promotes anabolism (Anthony et al., 2000;Yoshizawa et al., 2002;Yoshizawa, 2004) and suppresses catabolism (Li & Jefferson, 1978;Fulks et al., 1975;Buse & Reid, 1975) better than any of them. Until recently, the mechanisms by which leucine is anti-catabolic remained a mystery. So now you may be saying to yourself "great, but what does that have to do with KIC?"
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.
Anthony JC, Anthony TG, Kimball SR, Vary TC, & Jefferson LS (2000). Orally administered leucine stimulates protein synthesis in skeletal muscle of postabsorptive rats in association with increased eIF4F formation. J Nutr 130, 139-145.
Buse MG & Reid SS (1975). Leucine. A possible regulator of protein turnover in muscle. J Clin Invest 56, 1250-1261.
Fulks RM, Li JB, & Goldberg AL (1975). Effects of insulin, glucose, and amino acids on protein turnover in rat diaphragm. J Biol Chem 250, 290-298.
Li JB & Jefferson LS (1978). Influence of amino acid availability on protein turnover in perfused skeletal muscle. Biochim Biophys Acta 544, 351-359.
Nakashima K, Yakabe Y, Ishida A, Yamazaki M, & Abe H (2006). Suppression of myofibrillar proteolysis in chick skeletal muscles by alpha-ketoisocaproate. Amino Acids.
Tischler ME, Desautels M, & Goldberg AL (1982). Does leucine, leucyl-tRNA, or some metabolite of leucine regulate protein synthesis and degradation in skeletal and cardiac muscle? J Biol Chem 257, 1613-1621.
Yoshizawa F (2004). Regulation of protein synthesis by branched-chain amino acids in vivo. Biochem Biophys Res Commun 313, 417-422.
Yoshizawa F, Hirayama S, Sekizawa H, Nagasawa T, & Sugahara K (2002). Oral administration of leucine stimulates phosphorylation of 4E-bP1 and S6K 1 in skeletal muscle but not in liver of diabetic rats. J Nutr Sci Vitaminol (Tokyo) 48, 5