very interesting reading:

http://www.animal-science.org/cgi/content/full/81/13_suppl_1/E27

It is a long treatise so I'll leave it to you guys to read.

But here are a few interesting excerpts:

*******
"The general equation ME = RE + HE has been recognized since the days
of Von Liebig, but, for many years, the primary effort of energetics
researchers was to describe and quantify the ME of food and heat
produced (HE), with retained energy (RE) seemingly a secondary
consideration. Lawes and Gilbert (1861) first employed the comparative
slaughter method in experiments. Those experiments were of considerable

interest because they demonstrated for the first time that
carbohydrates were the major source of energy leading to the synthesis
of fat."
****

The term efficiency demands a numerator and a denominator along with
terms and units of each. All have taken many forms when used to define
"beef cattle energetic efficiencies" particularly when gross, partial,
or net efficiencies are defined. The numerator is the caloric content
of the product (megacalories of product) or its proxy, whereas the
denominator is defined in units of diet (diet input). The units of diet

can be weight, or megacalories or joules of GE, DE, TDN, ME, or NE.
Additionally, the diet input can be divided into that provided for
animal maintenance and that provided for product above maintenance;
e.g., product/(total ME minus ME required for maintenance). Thus,
enumerable ratios have, and are, being used to describe "energy
efficiency of beef production." These efficiency ratios always embody
three components:

Diet energy cost of maintaining the animal per unit of time.

Diet energy cost per unit of product.

Rate of product per unit of time (product/fixed maintenance cost).

However defined, the determination of partial efficiencies, e.g., body
tissue energy gain/ME above maintenance, would appear to be a
straightforward, simple process. But in practice, it becomes a complex
problem with multiple levels of confounding, making it difficult, if
not impossible, to precisely define the partial efficiency or
maintenance energy requirement of the producing animal. A prime example

of this complication is the frequently observed shifting maintenance
requirements as animals adapt to changing levels of alimentation. For
example, Marston (1948) reported a shifting of fasting heat production
(FHP) of sheep in direct proportion to their prior plane of nutrition.
Additional frequent confounders include changing diet digestibility,
pattern of fermentation, microbial growth, and protein supply
concomitant with changing levels of production or alimentation. Add to
these the changing nutrient flux, metabolism, hormonal control, and
product composition likely with changing level of alimentation and the
simplicity of measuring or calculating "partial efficiency" becomes
even murkier.
*********

Thermogenesis of individual human subjects associated with activities
that are not purposeful exercise has been shown to be highly variable,
heritable, and predictive of weight gain (Snitker et al., 2001) and low

in obese individuals (Schoeller, 2001). Snitker also found that the
measurement in respiration chambers of these "activities of daily
living" correlates (r = 0.53) to individuals' free-living activity.
These types of movements, sometimes called fidgeting, can elevate
sitting or standing thermogenesis by 50 to 80% (Levine et al., 2000)
and can be monitored in free-living individuals with inclinometers and
accelerometers (Levine et al., 2001a). General usefulness of these
monitors is apparently limited by the need to calibrate them to
individual subjects (Levine et al., 2001b).
*********

A lot of the treatise talks about cattle, but a lot of the introductory

stuff talks about the development of our understanding of energy
efficiecies in animals in general which applies to the human animal.

It seems to confirm a few of my points,

1) "Those experiments were of considerable interest because they
demonstrated for the first time that carbohydrates were the major
source of energy leading to the synthesis of fat."

2) "However defined, the determination of partial efficiencies, e.g.,
body tissue energy gain/ME above maintenance, would appear to be a
straightforward, simple process. But in practice, it becomes a complex
problem with multiple levels of confounding, making it difficult, *if
not impossible*, to precisely define the partial efficiency or
maintenance energy requirement of the producing animal."

3) "These types of movements, sometimes called fidgeting, can elevate
sitting or standing thermogenesis by 50 to 80% (Levine et al., 2000)
and can be monitored in free-living individuals with inclinometers and
accelerometers (Levine et al., 2001a). General usefulness of these
monitors is apparently limited by the need to calibrate them to
individual subjects (Levine et al., 2001b). "

So carbs have a major role to play in fat genesis and it is virtually
impossible to apply calories to accurately predict weight gain or loss.

TC