The minimum average protein needed is 0.6 g/kg lean body mass/day. This is to compensate for the irreducible nitrogen loss that must be replaced. Note, however, that this is the mean average, and the study that determined it showed a fair amount of individual variation in nitrogen loss. Some people can get away with less protein, others require more. The point, however, is that inadequate protein intake causes severe problems and eventually starvation. The minimum daily recommended protein intake is 0.8 g/kg LBM/day in order to provide some cushion over the average minimum.
The maximum recommendation I’ve seen is 2.0 g/kg LBM/day, even for body builders. Since the body has no way of storing more than a minimal amount of amino acids in the labile pool, the excess must be deaminated, and the resulting nitrogen waste is turned into nitric acid, which is excreted in the form of urea. If the nitric acid cycle gets overwhelmed, there is a serious risk of toxicity and even death from ammonia. If you smell ammonia on your breath, or your friends tell you that your body odour reeks of ammonia, you are eating too much protein and must cut back immediately. The figure of 6.0 g is twice what I’ve seen as the maximum intake the body can tolerate without overwhelming the nitric acid cycle. I suppose it is possible that there is an outlier who can safely tolerate that much protein, but it’s far too much for the rest of us.
Muscle building requires three of the essential amino acids, leucine, iso-leucine, and valine, all of which belong to the class of so-called branched-chain amino acids (BCAA’s). Although these three are considered essential amino acids because the body cannot manufacture them from other amino acids, people who are not body builders need only a very small amount and no more, since in quantity they cause inflammation. Body builders, of course, not only tolerate but need these BCAA’s in greater quantity.
All proteins in the diet are broken down into their constituent amino acids in the digestive tract (just as all carbohydrates are broken down into their constituent glucose molecules). These amino acids are then either re-formed into the proteins that the body needs at the time, to build or repair tissue; or else deaminated in order to produce the minimal amount of glucose needed (in the case of gluconeogenesis) or nitric oxide for regulating blood pressure (too much carbohydrate produces too much insulin, which interferes with the production of nitric oxide, which is why high blood pressure is part of the metabolic syndrome).
Whether any amino acids get used for gluconeogenesis depends on the amount of carbohydrate in the diet. Above a certain threshold, glucose from carbohydrate intake stimulates the production of insulin, which inhibits gluconeogenesis by raising the insulin/glucagon ratio, and excess protein intake also increases serum insulin significantly. When carbohydrate intake is below the threshold, excess protein intake has no effect on the insulin/glucagon ratio (because glucagon secretion rises proportionately along with insulin) and therefore gluconeogenesis is not inhibited. It used to be believed that the amount of gluconeogenesis depended solely on the amount of available amino acids, but the more nuanced view is that that the glucose produced more closely matches the need for it (for instance, the red blood cells cannot metabolise fatty acids or ketone bodies and therefore require glucose in order to live; on the other hand, just how much glucose the brain actually requires is currently in dispute).
