Richard Archer <[log in to unmask]> wrote:
 >about the mechanism of protein toxicity

Hi Richard, I read and like the link you provided and I think your
statements (below) are correct. I'd like to add some things and links.

Tamsin O'Connell also wrote a good text at
http://maelstrom.stjohns.edu/CGI/wa.exe?A2=ind0208&L=paleodiet&P=984

Protein metabolism is also strongly connected to acid/base balance
excretion capacity of the kidney and K+ metabolism.

As you write the limit is the capacity of the body to get rid of all the
superfluous  nitrogen (protein is carb with nitrogen, the carb is fuelled,
the nitrogen needs to be excreted).

First the nitrogen is made into Ammonia. This is sour (low ph) and toxic
for the brain. Good news is that excreting ammonia lets the body get rid of
some excess acid. That is why on paleodiet Roland Rohde calls protein even a
very basic food. (See
http://maelstrom.stjohns.edu/CGI/wa.exe?A2=ind0209&L=paleodiet&P=456
good to read)

But the amount that can be made to ammonia is limited because of it's
toxicity to the brain and the rate the kidneys can get rid of it
(to drink a lot might help).

So soon Urea is formed, it's not acidic and less toxic.
The amount of ammonia and urea that can be produced *and* excreted
determines the amount of protein (in grams per day) that can be eaten.

The K+ topic is also interesting. (See Rolands link above)
If the blood becomes acidic (I note, also from ammonia) a shift of K+
from the cells to the blood occurs. This K+ is lost.
K+ is *very* determining for a lot of functions particularly of nerve and
brain cells. And insulin sensitivity(!).
That's a reason why ammonia production is limited and must be switched to
urea. Which in turn increases blood acidity because less acid can be
excreted with the ammonia. Acidity which arises from the PRAL (net acid
load) of a food item.

Better to keep gluconeogenesis low.

In this way protein toxicity rotates about the facts of blood acidity, K+
levels, ammonium toxicity, kidney kapacity.
The maximum is in grams per day - not percent.
Tamsin (link above) infers to a ceiling of 100-400 g (probably ~200g) per
day because of liver data.

Adding fat or carbs help to avoid protein toxicity only because it can
displace additional protein for energy. It doesn't increase the maximum
protein grams per day. It increases just the calories edible per day.
Better fuel clean burning fat of carbohydrate than protein.

regards

Amadeus