previous studies have shown methionine restriction had effects like this and further that glycine increase which help regulate methionine also had a good effect. some folks then felt that you needed a certain level of glycine (or another simple amino acid that acted similarly) in relation to methionine uptake. Im not sure if it functions similarly for isoleucine but it and methionine seem to be in a lot of muscle meat whereas glycine and other simple amino acids are in collegen. So the meat broth people may have something there.