Its protein breakdown, or the point at which your body doesnt absorb as much. For example, if you take in .75g/lb (say 150g/day) your body will be very efficient with processing it. But if you take in 2g/lb, then your body will increase the enzymes that breakdown protein so a lower % of protein is being processed the way you want it to. Basically its being wasted. So instead of absorbing 9 out of 10 grams of protein (as an example), your body will drop to 8 or 7 or 5, or something definitely lower than the normal function.