Philippic wrote:
- quote -

That their food is so unpalatable that they swollow whole bites of it without
chewing, and hence, derive less energy from it?

-Jay

"Jay Tanzman" <jtanzman[at]sph.llu.edu> wrote in message news:2na7d8Fu47s4U1[at]
- quote -


And the relevance of the above to the *Atkins diet* might be.....?

;-)

(Sorry; couldn't resist that! I think your posts are great, Jay! Thanks a
lot for the time you put in!)

Philippic

Josh wrote:

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I was hoping it wouldn't come to this, but you also have to reduce what I've
called "metabolic energy" for incomplete absorption of the food. After you've
done that, you get "physiologic" or "metabolizable energy," which is the
energy value listed in food tables. That's what the 4 kcal/g for protein is.

Since it seems like we're inexorably headded down this path to its inevitable
inglorious conclusion, I might as well mention now that the energy figures in
the food tables are estimates, most of which have been arrived at using a
myriad of interpolations and extrapolations. One consequence of this, for
example, is that whole peanuts and pure peanut butter have been assumed to
have the same physiologic energy per gram; however, recent studies have shown
that the more processed peanut butter has higher physiologic energy than the
less processed whole peanuts. For that matter, how thoroughly you chew your
peanuts probably affects how much physiologic energy you derive from them.
Likewise, the physiologic energy of a food eaten alone likely differs from its
physiologic energy when eaten as part of a mixed meal, since, for instance,
its absorbability will be enhanced or reduced by other components in the meal.

-Jay

In <2n9qg4Fue696U1[at]uni-berlin.de> Jay Tanzman <jtanzman[at]sph.llu.edu> writes:



- quote -

Got it.

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Hmmm. I got some more reading to do... All along I thought that
the calorimetric value for protein was around 4 kcal/g (along with
the widely quoted 4 kcal/g for CHO, 9 kcal/g for fat, and 7 kcal/g
for EtOH).

Josh

josh_kerchenko[at]DO_THE_yahoo_OBVIOUS.com.INVALID wrote:

- quote -

Strictly speaking, no, but in this context, yes.

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No. As you've noted, most of the energy content of the food is not converted
to chemical energy in ATP. The energy content of the food is just that: the
amount of energy liberated when the food is completely oxidized; that is, its
heat of combustion. However, there is one major exception. Protein is not
completely oxidized in the body, since one of its physiolgic end products is
urea. Protein's metabolic energy is 4.7 kcal/g, compared with its calorimetric
value of 5.6 kcal/g.

-Jay

In <2n88o9Fu7k3tU1[at]uni-berlin.de> Jay Tanzman <jtanzman[at]sph.llu.edu> writes:

- quote -

You may be right; I used the figures given in Stryer's Biochemistry
(3rd ed., p. 421).

- quote -

Thanks, that's very helpful. Just to be clear, "digestion" means
absorption and storage? Also when you write "energy content of
the food" you mean precisely the energy contained in the "ATPs the
food yields during oxidation" (so this "energy content" serves as
a normalizing factor)?

Again, many thanks!

Josh

Josh wrote:
- quote -

I think it's 38 molecules of ATP (did you forget the 2 molecules produced in
glycolysis), but, actually, I think that that is beside the point.

When expressed as a percentage, the thermic effect of food is defined as "the
increment in energy expenditure during digestion, above baseline rates,
divided by the energy _content_ of the food" [1]. The number of ATPs the food
yields during oxidation does not enter into the definition.

-Jay

Ref:
1. Dietary Reference Intakes for Energy, Carbohydrtes, Fiber, Fat, Protein and
Amino Acids (Macronutrients) (2002). The National Academy of Sciences.

Please, help me sort out a major discrepancy I have noticed
surrounding the concept of the thermic effect of food. In the
paper "Pathways to obesity" by E Jequier (Int J Obes (2002) 26,
Suppl 2, S12-S17) the author writes (p S13):

The efficiency of nutrient utilization is inversely related to
the nutrient-induced thermogenesis, a phenomenon caused by the
energy cost of absorbing, processing and storing nutrients.
Expressed as a percentage of their energy content, the thermic
effect of nutrients is 25-30% for proteins, 6-8% for carbohydrates
and 2-3% for lipids.

This paragraph seemingly suggests that the "fuel efficiency" of
carbohydrates is 92-94%. But this flatly contradicts the fact,
found in any undergraduate biochemistry textbook, that the metabolic
oxidation of glucose is 38% efficient. Specifically, glucose yields
36 molecules of ATP, each worth 7.3 kcal/mol, for a total of 263
kcal/mol of glucose, which translates to 1.46 kcal/g. In contrast,
the full oxidation of 1 gram of glucose (in a calorimeter) yields
3.81 kcal/g. 1.46/3.81 = 38%.

How is this huge discrepancy resolved? I assume it has to do with
the precise definition of "thermic effect", but I have not been
able to find a definition of this concept that is both authoritative
and sufficiently specific to resolve this discrepancy. Any help
would be much appreciated. (A citation to a textbook or scientific
literature would be particularly helpful.)

Thanks!

Josh