At 10:54 +1000 12/7/03, Andrew wrote:

>< Note though that you need to increase protein intake slowly to
> reach this amount. The liver maintains just enough enzymes to enable
> the metabolization of dietary protein, and as protein intake
> increases it needs to build more enzymes in response.>
>
>I'm skeptical of this, but I'm all ears if you can prove it.
>What are the limiting factors you imply exist?

We've been over this before.

I refer you to this page for a classic textbook description
of nitrogen metabolism:
http://www.indstate.edu/thcme/mwking/nitrogen-metabolism.html


The relevant bit is The Urea Cycle, just over half-way down:

"Regulation of the Urea Cycle
The urea cycle operates only to eliminate excess nitrogen. On high-
protein diets the carbon skeletons of the amino acids are oxidized
for energy or stored as fat and glycogen, but the amino nitrogen must
be excreted. To facilitate this process, enzymes of the urea cycle are
controlled at the gene level. With long-term changes in the quantity of
dietary protein, changes of 20-fold or greater in the concentration of
cycle enzymes are observed. When dietary proteins increase
significantly, enzyme concentrations rise. On return to a balanced
diet, enzyme levels decline. Under conditions of starvation, enzyme
levels rise as proteins are degraded and amino acid carbon skeletons
are used to provide energy, thus increasing the quantity of nitrogen
that must be excreted."



And also a quote of a quote from Cordain from a web site which
has since disappeared:

"I know of no health problems associated with high (250-300 grams/day)
protein intakes that you describe. In fact, to the contrary. High
protein lowers total blood cholesterol, increases HDL, reduces
triglycerides, improves insulin metabolism, facilitates weight loss in
the obese by blunting the appetite and increasing resting metabolism.
Further high protein intakes have been shown in epidemiological studies
to reduce the risk of high blood pressure, coronary heart disease, bone
mineral loss as well as lengthening survival for women with breast
cancer. As long as sufficient fruit and vegetable intake (35% energy)
accompanies high protein intake, there is no increased risk for
osteoporosis or bone mineral loss. Finally, a recent clinical trial has
shown that high protein diets (25-30% energy) have no adverse effects
upon kidney function. There is no absolute ceiling such as 300 grams
for all persons. The physiological protein ceiling is based upon body
weight and also varies from person to person with the same body weight.
The physiological protein ceiling is primarily determined by the
ability of the liver to get rid of (metabolize) the dietary nitrogen (a
toxic compound) that is a by-product of protein metabolism. Nitrogen is
eliminated from the body by first converting it to urea in the liver
and then excreting it in the urine and feces. In order to convert
nitrogen to urea in the liver, a number of enzymes must be up-regulated
(built) for this conversion. The rate limiting (controlling) enzyme for
the conversion of nitrogen to urea is argininosuccinate synthetase.
This enzyme cannot be infinitely up-regulated in the face of increasing
dietary protein intake. Isotopic tracer studies in humans show that the
mean rate of maximal urea synthesis is 65 mg nitrogen per hour per kg
body weight to the 0.75 power and the range is (55-75). If the dietary
protein intake exceeds the ability to convert nitrogen to urea, then
excess nitrogen spills into the blood stream as ammonia and amino acids
which sets up a series of metabolic events that leads to nausea,
diarrhea, electrolyte imbalance and ultimately death if high protein
intakes continue for weeks and months. We have calculated the mean
maximal protein intake for an 80 kg subject consuming 3000 kcal to be
250 g/day (range: 212-292 g/day)."

 ...R.