[King Grub]

In conclusion, using AA-tRNA as the precursor pool, it is demonstrated that, in healthy humans in the postabsorptive state, insulin does not stimulate muscle protein synthesis and confirmed that insulin achieves muscle protein anabolism by inhibition of muscle protein breakdown.

Am J Physiol Endocrinol Metab. 2006 Oct;291(4):E729-36.

There appears to be a requirement for a minimal concentration of plasma insulin to stimulate muscle protein synthesis in response to resistance exercise and AA ingestion.

Eur J Appl Physiol. 2005 May;94(1-2):1-10. Regulation of mTOR by amino acids and resistance exercise in skeletal muscle.

The nature of the involvement of insulin, classically thought of as the most important anabolic hormone, is bizarre because it appears that in adult humans insulin does not do what it does in the growing animals traditionally used for metabolic research, such as rats, mice and even pigs. In people, it appears that it is possible to stimulate muscle protein synthesis by supplying exogenous amino acids alone while maintaining (using the insulin clamp techniques) basal blood insulin concentration at the overnight fasted level. Furthermore, if amino acids in amounts capable of causing a maximal response in protein synthesis are given, adding further insulin has no further stimulatory effect on protein synthesis.

Exp Physiol. 2005 Jul;90(4):427-36.

At insulin of <5 muU/mL, AAs appear to stimulate MPS without increasing mTOR signaling. Further increasing availability of insulin to postprandial values increases signaling activity, but has no further effect on MPS.

J Nutr. 2006 Jan;136(1):264S-8S.