I'd like to respond to Sean McBride's post of July 14th regarding kangaroo
fat and the physiological protein ceiling.  Gould's (1) observation that a
97 lb (44 kg) kangaroo yielded only 4 ounces (114 g) of removable fat would
not give the true picture of the total carcass fat because it doesnt account
for endogenous, non-dissectable structural and storage fat. Additionally,
fat given by weight rather than by energy for wild animal carcasses is
misleading because it doesn't adequately reveal the carcass protein/fat
relationships.
         It is possible to mathematically re-arrange the Pitts and Bullard
regression (2) and solve for % body fat from body weight rather than FFM
such that (log % body fat = 0.357 + 0.177 x (log body wt (g); r =0.75; SEE =
0.27).  So, a 44 kg kangaroo would be predicted to have 8.2% body fat by
weight (360.8 g) if one were to homogenize the entire carcass and chemically
extract the total lipid mass.  If one subtracts the amount of fat that Gould
was able to dissect (114 g) from the predicted total carcass fat (360.8 g),
the remaining figure (246.8 g) represents the predicted non-dissectable
lipid.  The endogenous non-dissectable lipid in the  tissues of wild animals
averages 2.1 % by weight for muscle, 3.8 % for liver, 9.3% for brain, 51%
for marrow and 82.3 % for subcutaneous storage fat (3).  Hence, the residual
fat (~ 250 g) that Gould was not able to extract lies primarily as
structural lipid in muscle and other organs.  Additionally storage
triglyceride would be present in marrow (it is unclear if Gould used marrow
in his estimates).
        Using the 3rd order polynomials that we have developed (4), it is
possible to now examine the true energetic relationships between fat and
protein in our 44 kg kangaroo in light of the physiologic protein ceiling.
The total carcass food by energy would be 46% fat and 54% protein.  Since
the maximal protein ceiling in humans averages about 35% of total energy
(4), then most of the kangaroo edible carcass could be consumed, providing
the fat were divided evenly and that just a small amount of carbohydrate
from plant food were available.  Fat can even be extracted from non-edible
portions (cancellous bone tissues) of the carcass by boiling them.  Also,
remember that the protein ceiling is an absolute number (i.e. grams of
protein, not % total calories) -  consequently, high amounts of protein
could be eaten for a number of consequtive days, as long as either a fat or
carbohydrate source were eventually found to make up the caloric deficiency
that was dictated by the excessive lean meat.  Only when lean meat is the
sole available food source day after day will symptoms of protein toxicity
emerge.
        Human's living at northern latitudes preferentially hunted megafauna
because these beasts contained both absolutely and relatively more fat.
Hence, at northern latitudes wherein carbohydrate (plant food) sources are
seasonally restricted, the fat obtained from larger mammals was sufficient
to dilute the lean protein from muscle tissues.  The fossil record shows
that the worldwide extinction of animals that took place at the end of the
Pleistocene occurred primarily in animals over 100 kg (220 lbs) (5).  Using
the Pitts and Bullard regression (2), a 220 lb mammal would be expected to
have about 15% body fat.  Applying our cubic regressions (4) to this value,
a 220 lb mammal would have 60 % of its total body energy as fat and 40% as
protein.  The protein value then is very close to maximal protein ceiling
(also 40  % of energy) -- hence it is not surprising that the "cutoff"
values for megafauna extinction (100 kg) corresponds almost exactly to the
value for the maximal physiological protein ceiling in humans.  In animals
weighing less than 100 kg, the entire carcass cannot be consumed unless
there is a carbohydrate source, whereas in animals weighing more than 100
kg, the entire carcass can be eaten with no worry about protein toxicity and
with no need to find a carbohydrate source.




                                REFERENCES
1.      Gould RA.  Notes on hunting, butchering and sharing among the
Ngatatjara and their neighbours in the Western Australian desert. Kroeber
Anthropological Society Papers 1966;36:41-63.
2.      Pitts GC, Bullard TR.  Some interspecific aspect of body composition
in mammals.  In: Body composition in animals and man.  Washington D.C.:
National Academy of Sciences, 1968:45-70. (Publication 1598).
3.      Cordain L, Watkins BA, Mann NJ. Fatty acid composition and energy
density of foods available to African hominids: evolutionary implications
for human brain development. World Rev Nutr Diet, in press.
4.      Cordain L, Brand Miller J, Eaton SB, Mann N, Holt SHA, Speth JD.
Plant-animal subsistence ratios and macronutrient energy estimations in
worldwide hunter-gatherer diets. Am J Clin Nutr 2000;71:682-92.

5.      Stuart AJ.  Mammalian extinctions in the late pleistocene of
northern eurasia and north america. Biol Rev 1991;66:453-562.


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