Todd,
The dietary protein ceiling is said to be due to the limit on the body's
ability to produce urea. 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. I would like to know
any evidence that feeding trials to determine the upper limit of protein
considered the relevance of organ meats.

I also hypothesise that O'Dea's estimates may be correct if the Aboriginal
diets were rich in organ meats- which other data suggest they are.

While I can accept that there is a metabolic ceiling on urea production,
there are other important constraints on the utilisation of meat as a food.
Let us consider 2 true carnivores- lions and dogs.

Lions and dogs cannot live on steak (muscle meat) alone.

Weston Price (N+PD) reports that lions could not breed in captivity when fed
on steak alone. In the early 1900's Lions used to cost $4000. When it was
realised that in the wild, the liver is the first thing they eat, liver was
added with excellent results, such that they bred easily and they cost a
mere $40.

Crawford and Marsh (Nutrition and Evolution) report lions in Africa fed on
steak alone suddenly becoming cripples due to spinal collapse from
osteomalacia due to high Po4/Ca ratio. The solution then was meaty bones
such as ribs that the lions could crush (unable to crush the larger bones).
Thus they reinvented Price's wheel to some degree (I know for sure that
Marsh has
read Price and I presume Crawford has too).

Ina Billinghurst (Give Your Dog a Bone (A Paleo Diet for dogs)
www.drianbillinghurst.com) reports dogs cannot thrive on steak
but do extremely well on a diet of steak, organ meats and raw meaty bones.

On an empirical basis, organ meats become essential for high protein diets
for carnivores. Thus, if dogs and lions cannot live on steak alone, how
would one expect an omnivore to do well on it? Thus, I think that all high
protein diets for humans probably need the addition of organ meats.

Why should this be so? There are several obvious reasons, but I prefer to
simplistically stick to the empirical evidence. I think that Weston Price
presented buckets of good empirical evidence and in a way it's a shame that
we have had to wait until we have evidence at the molecular level to prove
that he was right. The empirical animal evidence is also important.

Liver is a storage depot for vitamins and minerals, and this is probably
crucial. These may provide necessary co-factors for the digestion of
protein, production of urea etc.  It is a good source of essential fatty
acids omega 3 and 6. It is an excellent source of many biochemical including
glutathione lipoic acid biotin etc etc.

"Standard" high protein diets eg Atkins, Scarsdale do not have a formal
source of vitamin A or sufficient omega 3 fats, unless one stumbles onto
fish (omega 3 and 6), pink fish (vitamin A). They also have a low
calcium/phosphorus ratio unless one consumes small bones, or eats marrow.
Salmon and some other foods are high in calcium. Fruit and vegetables also
supply calcium. Acid load is high in these diets but again is ameliorated by
fruit and vegetables, and also by most mineral waters (bicarbonate content
varies greatly- eg Evian is quite high and will neutralise quite a lot of
acid).

Thus, I believe most high protein diets are not viable unless one includes
organ meats, including liver, and preferably marrow too.

Simple feeding trials to establish whether the protein ceiling changes with
the inclusion of liver and marrow would resolve this issue easily.

Ben Balzer