Sean poses many good questions in his recent post, and I believe that the
answers to quite a few of these can be found in our most recent paper (1) as
well as in my colleague, Jennie Brand-Miller's classic Carnivore Connection
paper (2).  In the absence of carbohydrate, liver and kidney are the main
organs that can synthesize glucose (gluconeogenesis) from either glucogenic
amino acids or glycerol, a product of adipose tissue metabolism.  Inuit and
other native populations that eat little or no carbohydrate must rely upon
gluconeogenesis for glucose as substrate for brain and nervous tissue
metabolism.  Besides the brain and nervous tissue other tissues
(erythrocytes, and fetus) also require glucose.  Glucose is required in
adipose tissue as a source of glyceride-glycerol, and it probably plays a
role in maintaining the level of intermediates of the citric acid cycle in
many tissues.  It is clear that even under conditions where fat may be
supplying most of the caloric requirement, there is always a certain basal
requirement for glucose.  Finally, glucose is the only fuel that will supply
energy to skeletal muscle under anaerobic conditions.
        The evolutionary selection for peripheral insulin resistance in the
muscles then is a mechanism by which glucose can be efficiently conserved
and routed to brain and fetus - hence providing increased survival value
when there is little carbohydrate in the diet (2).  As early hominids
migrated from the tropics to higher latitudes during the Paleolithic (3) and
became increasingly dependent upon animal foods, it is quite likely that
they would have been forced to reduce their carbohydrate intake because of
the seasonal availability of plant foods that can be eaten and
physiologically digested by non-fire using hominids.  Hence, peripheral
insulin resistance which once had positive survival in a carbohydrate poor
environment for our ancient ancestors, now represents a liability in modern
humans because it increases the likelihood of susceptibility to a number of
chronic diseases (hypertension, type II diabetes, obesity, dyslipidemia and
coronary heart disease) (2).
        Because there is a physiologic protein ceiling that has been
quantified (1), then unlimited consumption of protein from very lean or
small animals is also not possible.  If carbohydrate is unavailable or
scarce, as it is at higher latitudes particularly during winter and early
spring, then the only solution is to eat more fat.  As we have shown (1)
larger species of mammals contain relatively more fat than smaller species,
hence early humans living in carbohydrate poor environments would have
preferentially preyed upon larger animal species.  This principal would have
held true for modern hunter-gatherers (Cro Magnon) living in Europe and
North America (PaleoIndians).  Its does not seem coincidental that megafauna
extinctions occurred roughly the same time modern human numbers dramatically
increased on both of these continents.  In more southernly latitudes where
carbohydrate was available, megafauna persisted into modern times because
there was another dietary solution to the protein ceiling.
        Once megafauna became extinct, human populations must have also
declined because the protein ceiling prevented the consumption of the
available calories from small animals (1).  The adoption of agriculture
(i.e. cereal grain domestication) abrogated the physiological protein
ceiling because cereal carbohydrate effectively dilutes the high protein
content of small or lean animals depleted of fat.  Cereals can be  stored
overwinter without decomposing thereby allowing neolithic humans the ability
to eat all sized animal regardless of their fat content, even when body fat
stores are depleted.  By abrogating the protein ceiling, cereal grains
permitted more people to live on a square acre of land, becaue more of the
available biomass (in addition to the cereal grain itself) could now be
consumed. Hence, humans adopted agriculture, because in Sean's own words,
"they had to".

                                        Cordially,

                                        Loren


1.      Cordain L, Brand Miller J, Eaton SB, Mann N, Holt, Speth JD.
Plant-animal subsistence ratios and macronutrient energy estimations in
worldwide hunter-gatherer diets. Am J Clin Nutr 2000;71:682-92.
2.      Miller JC, Colagiuri S. The carnivore connection: dietary
carbohydrate in the evolution of NIDDM. Diabetologia 1994 37:1280-6.
3.      Gabunia L, Vekua A, Lordkipanidze D, Swisher CC 3rd, Ferring R,
Justus A, Nioradze M, Tvalchrelidze M, Anton SC, Bosinski G, Joris O, Lumley
MA, Majsuradze G, Mouskhelishvili A.  Earliest Pleistocene hominid cranial
remains from Dmanisi, Republic of Georgia: taxonomy, geological setting, and
age. Science 2000;288(5468):1019-25.

Loren Cordain, Ph.D., Professor
Department of Health and Exercise Science
Colorado State University
Fort Collins, CO 80523
tel: (970) 491-7436
fax: (970) 491-0445
email:[log in to unmask]
http://www.colostate.edu/Colleges/CAHS/ess/cordain.htm