On Sat, 12 Jan 2002 14:02:34 -0000, Peter Smith <[log in to unmask]>
wrote:


>I am unsure whether the upper limit to avoid protein toxicity is
>absolute, percentage, or of some other type. If it is a limit on
>percentage of calories obtained from protein (as opposed to a fixed
>number of grams of protein per day), then someone who takes in
>sufficient calories to support a large muscular body and a
>strenuous life can eat a larger number of grams of protein per day
>than a slender sedentary person who takes in fewer calories.

Codrain said that protein toxicity arises  by the inability of the body to
upregulate the urea synthesising enzymes above a certain limit.

Urea is built for the purpose to excrete the excess nitrogen from the unused
(or better used calorically) amino acids.
This capacity would be determined by the size of organs who do the job.
Liver and kidney.
The process is nicely described in
http://www.zonehome.com/met/metprotnit.htm

If the excess nitrogen cannot be excreted, toxicity arises.

In addition to the urea synthesis capacity also the kidneys have to have
enough excreting capacity for the urea.
If urea cannot be excreted fast enough it would likewise cause toxicity.
In a high protein diet of living material (milk is nor living) the cells
eaten also contain cell kernels, with the DNA (genetic material).
In humans (as opposed to cats) this excess DNA also has to be excreted (it's
called purines).
A job wich requires additional work by the kidney.
Animal protein tends to have more DNA in it than plants because animals have
 more genetic information - they are complexer.
Very rapidly growing tissues also have more DNA.
This is the case for fish skin, and organs. Or generally the stuff not
recommended for gout.

Then in degrading the protein also the problem of ammonia toxicity for the
brain will come up (described in the same paper in point 3. a recommended
reading on the topic). The symptoms  "if the brain becomes deprived of its
source of ATP" (energy) should be similar to those of hypoglycemie (low
blood sugar).

In short, the absolute amount of protein is what matters or limits.
For example Australian aboriginals, which were reported to temporarily eat
some 75 percent of calories as protein, did so with a diet of only 1000-1200
kcal.
If you eat so little energy, the protein percentage can be high.
If you eat a high calorie diet (>2400 kcal) the protein energy percentage is
to be lower.

I think that the organs involved can be trained to increase the absolute
amount by becomeing enlarged, to a certain extent.

In any condition of problems in the liver or the kidney I think
it would be a bad idea to eat protein up to the limits.

regards,
Amadeus S.