Exercise has a profound effect on muscle growth, which can occur only if muscle protein synthesis exceeds muscle protein breakdown; there must be a positive muscle protein balance. Resistance exercise improves muscle protein balance, but, in the absence of food intake, the balance remains negative (i.e., catabolic). The response of muscle protein metabolism to a resistance exercise bout lasts for 24-48 hours; thus, the interaction between protein metabolism and any meals consumed in this period will determine the impact of the diet on muscle hypertrophy. Amino acid availability is an important regulator of muscle protein metabolism. The interaction of postexercise metabolic processes and increased amino acid availability maximizes the stimulation of muscle protein synthesis and results in even greater muscle anabolism than when dietary amino acids are not present. Hormones, especially insulin and testosterone, have important roles as regulators of muscle protein synthesis and muscle hypertrophy. Following exercise, insulin has only a permissive role on muscle protein synthesis, but it appears to inhibit the increase in muscle protein breakdown. Ingestion of only small amounts of amino acids, combined with carbohydrates, can transiently increase muscle protein anabolism, but it has yet to be determined if these transient responses translate into an appreciable increase in muscle mass over a prolonged training period.
I read it as direct support of what I have been saying. To have positive protein synthesis (repair and growth) you need a level of amino acids (protein) greater than what has been broken down by exercise (mechanical process) If the amino acids are not present in sufficient quantities you body will continue to breakdown muscle through protein metabolism (bodies response to exercise).
As for your son and you in your youth, insulin and testosterone are at their peak So protein intake is used for repair most efficiently. A small amount can go a long way. Also both of you may have been practicing proper nutrition timing without even realizing that you were doing it. Getting in protein and carb in the magic hour after a workout to maximize recovery and growth. Also you may have reduced calories in the sense that you were WAY over what your body needed for recovery and growth, you reduced calories down to proper levels so excess storage was no longer happening. You eat and than used it. I would surmise if you continued reducing calroies beyond this balance point you and your son would have experienced the negative affects (catabolism)
One other thing we are both doing in this discussion is limiting it to muscle growth. There is a whole array of functions that are happening in the body that require amino acids to happen correctly. Hence why a higher level of intake is required, one to maintain regular everyday bodily function and two grow above these functions and maintain the new level of growth with regression.