The negative health claims of the high protein diet on kidney function is based on information gathered from people who have preexisting kidney problems. You see one of the jobs of the kidneys is the excretion of urea (generally a non toxic compound) that is formed from ammonia (a very toxic compound) which comes from the protein in our diets. People with serious kidney problems have trouble excreting the urea placing more stress on the kidneys and so the logic goes that a high protein diet must be hard on the kidneys for healthy athletes also.
Now for the medical and scientific facts. There is not a single scientific study published in a reputable peer - reviewed journal using healthy adults with normal kidney function that has shown any kidney dysfunction what so ever from a high protein diet. Not one of the studies done with healthy athletes that I mentioned above, or other research I have read, has shown any kidney abnormalities at all. Furthermore, animals studies done using high protein diets also fail to show any kidney dysfunction in healthy animals.
Now don't forget, in the real world, where millions of athletes have been following high protein diets for decades, there has never been a case of kidney failure in a healthy athlete that was determined to have been caused solely by a high protein diet. If the high protein diet was indeed putting undo stress on our kidneys, we would have seen many cases of kidney abnormalities, but we don't nor will we. From a personal perspective as a trainer for many top athletes from various sports, I have known bodybuilders eating considerably more than the above research recommends (above 600 grams a day) who showed no kidney dysfunction or kidney problems and I personally read the damn blood tests! Bottom line? 1-1.5 grams or protein per pound of bodyweight will have absolutely no ill effects on the kidney function of a healthy athlete, period. Now of course too much of anything can be harmful and I suppose it's possible a healthy person could eat enough protein over a long enough period of time to effect kidney function, but it is very unlikely and has yet to be shown in the scientific literature in healthy athletes.
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