I'm taking the liberty of pasting Loren Cordain's recent message
(from Paleodiet Symposium) on macronutrient composition of
kangaroos (and other mammals) in its entirety, since it's highly
relevant to recent (and not recent) discussions about available
paleo energy sources.

Todd Moody
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----------------------------------------------
Date:    Sun, 6 Aug 2000 13:24:45 -0600
From:    Loren Cordain <[log in to unmask]>
Subject: Re: Kangaroo fat and protein poisoning

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.