Question: 10% energy absorption from food

Drozd Michael DROZD at rzmain.rz.uni-ulm.de
Mon Aug 8 09:47:06 EST 1994


In <31tpgd$lor at remus.rutgers.edu> daveng at remus.rutgers.edu writes:

> my original question was about the energy pyramid.
> 
> actually, I've got a question about this 10% figure now.  Does that mean,
> if I ate a piece of apple pie at 100 calories, I would only be utilizing
> about 10 of these calories, the rest being sent down the toilet?  The
> recommendation of 2600 calories a day, does that mean we really only need
> 260 calories, the other 90% or, 2340 calories being wasted by our bodies?
> 
> I had also thought that I read something (or been told) that biological
> systems (such as ours) are one of the most efficient found on the planet,
> much more efficient than anything we've created, such as the internal-
> combustion engine.  Is this true?  How do they compare (%age wise).
> thanks,
> 
> dave
> 

Hi Dave,

you already got the response to your question. However, there is another
interesting aspect when dealing with calories in human nutrition. It is
the way the calories are measured. If you get them from a calorimeter, 
in which the whole apple is burned, you will also get the calories, you 
cant use at all, the cellulose. This is important if you wont to compare
with a mashine. I hope you will never put bitumen into your car. Bitumen
has to be seperated from the mineral oil that comes from the ground, 
which takes a lot of energy. To get a calculation, that is simple enough
for comparison, you should compare the efficiency of utilizable sources.

So, for comparison with man made engines, one uses the efficiency of
phosphorylation of Adenosinediphosphate (ADP)  to Adenosinetriphosphate 
(ATP) during glycolysis and Krebs cycle. The next step is the efficiency
of the utilization of ATP. In this system, living cells seem to be
superior to most engines as far as I know (hope so).

Of course you would like a balance sheet of the whole animal. But to
value such a balance gets more complicated with increasing complexety
of the subject. As you mentioned nearby "....sent down the toilet...",
this sounds as if you are wasting enegy. However, the cellulose that
you cannot utilize for biomass production, not even for thermoregulation,
is essential for the funktion of your intestinal system. Just remember
how obstipation makes you feel. This effect cannot be calculated in
terms of calories.

good appetite

Michael



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