The anabolic response to protein intake appears to be mainly mediated by an elevation of plasma essential amino acids. In addition, ingestion of carbohydrate leads to an attenuation of muscle protein breakdown by virtue of an elevation in plasma insulin, further promoting positive net protein balance. This is mediated by insulin's known action of increasing muscle blood flow and thereby augmenting muscle substrate delivery, which may lead to further stimulation of muscle protein synthesis. It has been shown that there is an insulin resistance with respect to muscle protein anabolism in elderly compared to young subjects during conditions of high plasma amino acid and insulin availability. This suggests that the reduced ability of insulin to augment muscle microvascular perfusion decreases the amino acid stimulus.