Laura said:
Why cooking counts - Study finds an increase in energy from meat, suggesting key role in evolution
http://news.harvard.edu/gazette/story/2011/11/why-cooking-counts/?utm_source=SilverpopMailing&utm_medium=email&utm_campaign=11_09_111&utm_content
More info from an article by the same authors, Rachel N. Carmody and Richard W. Wrangham. The quote below addresses - although very briefly - the possible effects of cooking fat (http://www.anthro.utah.edu/PDFs/CarmodyWrangham09cookingHumEv.pdf):
Energetic effects of cooking animal foods
Meat is an important item in human evolution, but few studies
have addressed the potential energetic effects of cooking meat; and
in general, the literature on the nutritional consequences of cooking
animal protein is diffuse and inconsistent. Animal foods consist
largely of protein and fat, with a small amount of ash. Protein
represents approximately 70% of muscle tissue by dry matter mass,
with relatively higher importance for lean wild meats (USDA,
2008). Fat is of lesser importance by mass, representing approximately
25% of muscle tissue and approximately 90% of marrow in
meats sold in the USA (USDA, 2008), but its energy value is
disproportionately great owing to the high gross caloric value of
lipids (9 kcal/g) compared to protein (4 kcal/g; Merrill and Watt,
1973).
Cooking by dry heat methods, such as roasting, results in fat loss
due to dripping (Bender, 1992). Table 4 summarises fat losses for
seven common meats, as reported in the USDA Nutrient Database
for Standard Reference (2008), along with their implied reduction
in gross caloric value per gramof dry matter compared to rawmeat.
Reductions in gross caloric value due to cooking were calculated by
comparing the reported protein, lipid, carbohydrate, and ash
contents of raw and cooked samples and multiplying these by the
caloric conversion factors of 4, 9, 4, and 0 kcal/g, respectively
(Merrill and Watt, 1973). As Table 4 demonstrates, the extent of fat
loss can be considerable both in terms of mass and gross caloric
value. Based on these data alone, cooking would appear to have
negative consequences for the energy value of meat. However, it is
not currently known whether the negative effects of cooking on the
gross caloric value of meat due to fat loss are outweighed by
potential positive effects of cooking on the net energy values of the
residual fat and protein (e.g., due to increased intake, increased
digestibility, reduced cost of digestion, and/or lower basal metabolic
expenditure).
As Table 1 shows, there are various mechanisms by which
cooking has been argued to have positive, neutral, or negative
effects on the net energy value of meat. Given this diversity of
possible effects, the question relevant to human evolutionary
biology is whether there is a consistent net consequence. The
simplest way to find out would be to obtain data on people eating
meat-rich diets that differ by whether their meat is raw or cooked.
However, no such studies have been reported for humans. Even
animal data are lacking. It has been claimed that many experiments
show that rats ‘‘thrive better on cooked than on raw meat’’
(Anonymous, 1931), but we have not yet found proof of such
research.
Here, therefore, we review evidence for the impacts of cooking
meat on four contributory factors to net energy: food intake,
digestibility, the metabolic cost of digestion, and basal metabolic
rate. We focus mainly on the effects of cooking on whole meat or
animal protein rather than animal fat. The purpose is not to suggest
possible (Stefansson, 1960; Hayden, 1981; Speth and
Spielmann, 1983; Defleur et al., 1999). Archaeological evidence
suggests that fat derived from bone marrow may have been
preferred over muscle tissue as a source of energy and nutrients
among early Homo (Blumenschine, 1991; Blumenschine and
Madrigal, 1993). Moreover, it is known that diets deriving more
than 50% of calories from lean protein can lead to negative energy
balance, so-called ‘‘rabbit starvation,’’ due to the high metabolic
costs of protein digestion (Speth and Spielmann, 1983; Noli and
Avery, 1988), as well as a physiological maximum capacity of the
liver for urea synthesis (Speth, 1989; Cordain et al., 2000). Rather,
we focus on whole meat or animal protein because virtually no
research to date has addressed the impact of cooking on the energy
value of fat. In the nutritional literature, the energy values of
different lipids are viewed interchangeably, with discussion
focusing instead on fatty acids and their implications for food
texture, preservation, and health. Nevertheless,
we can envisage
two ways in which cooking might positively alter the energy value
of fat. First, to the extent that cooking heats fat to body temperature
or above, less energy will be expended by the body in doing so.
Second, the liquefaction of solid fats into oils may increase the
surface area of lipid globules exposed to amphipathic (i.e., having
both hydrophilic and hydrophobic domains) bile acids in the small
intestine, thus promoting faster emulsification and ultimately
faster absorption. These hypotheses remain to be tested.
It's probably digestive related though.
