Dear all,

I have two lines of contribution to the discussion on high protein diets,
in which I have tried to summarize what is in some of the literature.

Firstly, from a metabolic point of view, is there a maximum limit on
protein intake by the body? If one considers the physiological outcome of
eating protein, Ben Balzer is right in that it is the body's rate of
production of urea that is the primary limiting factor. Excess intake of
nitrogen leads in a short space of time to hyperammonaeia, which is a
build up of ammonia in the bloodstream, and this is toxic to the brain. To
avoid ammonia build-up, nitrogen must be excreted as urea in mammals, and
urea is produced in the liver. Urea production requires oxygen, in a ratio
of one mole of oxygen to one mole of amino acid nitrogen. Estimates of
oxygen consumption by the human liver are somewhat lacking, but
extrapolating from in vitro and animal studies, the liver of a 73kg man
would use between 2 and 5 moles of oxygen per day. So if the liver did
nothing else but produce urea, a 73 kg man could process 2 to 5 moles of
amino acid nitrogen a day, which is 28 to 70g of N which is equivalent to
175 to 438 g of protein a day (figures from McGilvery, 1983, p791). This
is if the liver ONLY produced urea. But as it has other important
functions, these values can be considered as extreme upper limits, and a
more realistic limit would be half this. Incidentally 175 to 438 g of
protein a day for the mythical 73kg man equates to 2.3 to 6 g/kg body
weight/day, which compares to typical UK intake of 1.2 g/kg body
weight/day. An intake of 175 to 438 g of animal protein a day would
generate 3.1 to 7.9MJ of energy.

Now turning to whole organism studies, be they lions, dogs or humans. I
would suggest that when most people think of an animal eating a
carnivorous diet, they tend to think of it as being a protein-based
one. This is false. A carnivorous diet is more fat than protein, and with
little or no carbohydrate. I consider that this is why lions fare better
on a diet high in organ meats than one with lots of steak: it is not the
prevalence of vitamins, minerals, essential fatty acids etc in organ meats
that makes them a better source of food but fat in general.
There is much anecdotal evidence in the literature that humans can survive
on a purely animal product diet, but only if it is high in fat. Speth et
al (Speth & Spielmann 1983; Speth 1987) discuss this, as do Noli & Avery
(1988). The general pattern appears to be that a lean meat diet leads to
nausea in 3 days, symptoms of starvation and ketosis in 7-10 days, severe
debilitation in 12 days and possibly death in weeks.
Perhaps one of the best documented study is that of the Arctic explorer,
Vilhjamur Stefannson and some contemporaries (see McClelland et al
1930-31). They ate an animal meat diet for more than a year to see whether
such a diet could be healthy. Everything was fine until they were asked to
eat only lean meat. McClelland wrote:
'At our request he began eating lean meat only, although he had previously
noted, in the North, that very lean meat sometimes produced digestive
disturbances. On the third day nausea and diarrhea developed. When fat
meat was added to the diet, a full recovery was made in two days. This
disturbance was followed by a period of persistent constipation lasting 10
days. The subject had a craving for calf brain, of which he ate
freely. (Calf brain is 9g of lipid and 11g of protein per 100g.) On March
12, poor appetite, nausea, and abdominal discomfort were present and a
second milder attack of diarrhea occurred which responded quickly to a
proper proportionment of lean and fat meat.'
When one looks in the literature at typical modern diet compositions from
'western' populations to hunter-gatherers, most derive 12 to 25% of their
energy from protein, and the maximum (Canadian and Greenland Eskimos) is
45% (see Ulijaszek & Strickland, 1993, table 3.8), again suggesting that
the optimum 'carnivorous' diet has to be higher in energy
(fat/carbohydrate) than in protein.

I would also argue is that when discussing high protein diets, one must be
careful to clarify whether it is a high absolute consumption of meat
(limited by urea production probably to an absolute max of 6g/kg body
weight/day) or high proportion of protein relative to carbohydrate and
lipid (limited to probably about 40% max of calorific intake). Otherwise
one can get into a tangle. It is particularly noticeable when reading
about modern 'fad' diets that are presented as 'high-protein' that it is
rarely indicated whether the diet means consumption of large quantities of
protein or a lot of protein relative to other macro-nutrients.

Hope some of this is useful,

Tamsin O'Connell

Refs

McClelland et al (1930-1931) 'Clinical Calorimetry: XLV, XLVI,
XLVII. Prolonged Meat Diets......' J.Biol.Chem. 87:651, 87:669, 93:419
McGilvery, (1983) A Functional Approach to Biochemistry, 3rd edition,
p791.
Noli & Avery, (1988) 'Protein poisoning and Coastal Subsistence'
J.Archaeol.Sci. 15:395-401
Speth & Spielmann, (1983) 'Energy source, protein metabolism, and
hunter-gatherer subsistence strategies' J.Anthr. Archaeol. 2:1-31
Speth, (1987) 'Early Hominid Subsistence Strategies in Seasonal Habitats'
J.Archaeol.Sci. 14:13-29
Ulijaszek & Strickland, (1993) Nutritional Anthropology

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Dr Tamsin O'Connell
Research Laboratory for Archaeology
University of Oxford
6 Keble Road, Oxford, OX1 3QJ, UK
tel:01865-283641
fax:01865-273932
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