On Tue, 20 Aug 2002 16:19:09 +1000, Balzer, Ben <[log in to unmask]> wrote:

>Todd,
>The dietary protein ceiling is said to be due to the limit on the body's
>ability to produce urea.

Or ammonia.
Urea is produced in the liver and has the advantage not to disturb the
acid/base balance when excreting nitrogen.
(good information for this at http://www.zonehome.com/met/metprotnit.htm )
Ammonia is produced in the kindeys and both are excreted by the kindeys.
In a high protein diet (that's >35% or even 18% protein of calories)
kidneys seem to enlarge, to enhance it's functionability (I never heared
that the liver can enlarge as well).
The excreting capacity of the kindeys is also limited - they have to filter
all that nitrogen out. (they filter ketone bodies out as well to decrease
the acid load).
One limit is the maximum acidity (~ph 3) of the urine, which would be
enhanced by drinking much water.
The other main limit certainly is the urea and ammonia synthesis capacity of
these organs. As animals and humans will get sick and can die of this
("rabbit starvation") this capacity is definitely limited and not easy to
increase.

>I think the estimates may be flawed because muscle
>meat and organ meats behave differently. eg Liver contains high levels of
>many vitamins which may be needed to metabolise protein. I would hypothesise
>that the protein ceiling is higher on organ meat-rich diets than on muscle
>meat diets, and would like to discuss this on the list.

In a high protein diet the kidney also has to excrete the purins from all
the cell kernels (uric acid arising from nucleic acid catabolism). Humans
cannot metabolize nucleic acids as a energy or nitrogen source - in contrast
to carnivores like cats.
Organ meats have definitely much more nucleic acids than other cells.
That's annother limit for a high protein diet - purins (btw that's not for
dairy protein, because it has protein without cell kernels).
Purins are worsening or causing gout and worsening athritis.

> I would like to know
>any evidence that feeding trials to determine the upper limit of protein
>considered the relevance of organ meats.

Additional minerals and vitamins could be able to increase urea/ammonia
synthesis capacity. As far it's not the *excreting* capacity that's the
limit eating high vitamin items will be helpfull, I think.
At least if some mineral or vitamin deficit is limiting the synthesis
capacity of the liver/kidney. But also the amount of emzymes and mere size
of the organs will be a limit.

A word on the 80% aboriginal time: That was only 2 weeks.
That's not worth to mention. These diabetics probably had protein depletion
(from the bad kind of ketosis) which will have allowed more absorbetion of
the amino acids.
Then their whish to leave the coast and increase carbs.
That's natural. 14 days of ketosis in a rabbit starvation condition will put
a very strong desire for annother source of carbs. Brain demands 500 kcal of
carbs per day (well, at least 250 in ketosis).
So long all carbs from proteins - that puts a gigantic acid load by
gluconeogenesis on the body. Or simply a lack of glucose, because liver
can't manage this additional gluconeogenesis, in addition to producing urea.
Brain starvation hurts.

The references tell that protein over 50% on the long run can be deadly.
Stephansson had only 18% calories from protein.
18%, because eating more produced nausea.
35% calories from protein is the upper limit the Speth study reports.
50% are from reported cases (with big kidneys probably).
Thanks God these poor ill aboriginals survived the 80% for 14 days.

Regards from

Amadeus S