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Keep anabolic during cardio by taking leucine-enriched essential amino acids!
Essential amino acids are those that cannot be synthesized in the body and, as a result, must be taken in the diet or with supplementation.
more info about leucine
We have known for years that the essential (and branched chain) amino acid leucine plays a significant role in turning on anabolism, especially after heavy weight training. It has been suggested that there is an abundance of essential amino acids drawn out of skeletal muscle during sustained exercise to provide energy, which promotes a catabolic environment.

In a recent randomized controlled trial it was shown that ingesting an essential amino acid drink, with high levels of leucine, during cardio enhances protein synthesis after your workout. In this study subjects underwent 2 separate 60 minute bouts of cycling exercise at 60% (VO2 peak) intensity. During each exercise bout, subjects were instructed to sip a different cocktail made up of essential amino acids. Each of the cocktails contained 10 grams of essential amino acids, with one containing high levels of leucine (3.9 grams) and the other containing lower levels of leucine (1.87 grams). Muscle biopsies were taken at baseline and twice during recovery and were used to assess expression of mTORC1 (an anabolic regulator). At baseline and throughout recovery, whole body protein turnover and muscle protein synthesis were assessed by amino acid tracer infusions into the blood. At the end of the study the scientists observed 33% greater muscle protein synthesis and decreased whole body protein turnover, when leucine-enriched drinks were consumed. This is the first study to publish the anabolic benefits of taking leucine-enriched essential amino acids during cardio.

Pasiakos SM, McClung HL, McClung JP, Margolis LM, Andersen NE, Cloutier GJ, Pikosky MA, Rood JC, Fielding RA, Young AJ. Leucine-enriched essential amino acid supplementation during moderate steady state exercise enhances postexercise muscle protein synthesis. Am J Clin Nutr. 2011 Sep;94(3):809-18. Epub 2011 Jul 20.