Catechins are a class on molecules that are common in teas, including black and green tea. They display many different health properties, including antioxidant, fat oxidation and carb digestion. However, today I wanted to talk about a new claim being made about epicatechin. In a 2014 study, researchers saw that subjects given 150mg of epicatechin saw a favorable change in the plasma levels of follistatin/myostatin ratio. It's been previously reported that an increase in follistatin may decrease expression of myostatin, but to what degree remains to be seen.
The study didn't measure muscle mass, but did note an increase in hand grip strength. Based on this study, As far as building muscle, more research is needed to see to what degree epicatechin decreases myostatin and how long one would have to decrease myostatin levels to see any gains in muscle mass. Very interesting though. Molecules like these(flavonoids) have a myriad of biological properties that we're still discovering. I'll keep you guys posted if new research comes out!