MadSci Network: Biophysics

Re: What is the efficency of digesting food.

Date: Wed Oct 13 20:50:41 1999
Posted By: Dian Dooley, , Associate Professor, Food Science and Human Nutrition, University of Hawaii at Manoa
Area of science: Biophysics
ID: 939071578.Bp

Aloha, Jonathan,
     A piece of bread contains 70 kilocalories OF USEABLE ENERGY.  That's 
the answer.  The kilocalorie (Calorie) amount you find on food labels or in 
food composition tables is based on energy value corrected to reflect 
useable energy only.  The 'true' energy in the bread is probably a bit 
more.  Then that number is reduced to represent useable, metabolizable 
energy for the human body.
     The body should be able to absorb/use almost all of that 70 
kcals...regardless of whether it comes from carbohydrate, fat, or 
protein...and bread contains mostly carbohydrate, some protein, and a bit 
of fat (depending upon what kind of bread it is).  It might contain fiber, 
too...if it is made from whole-grain flour or has fiber added.
     As to your question about the different efficiencies for bread, 
apples, and pure sugar...again the amount of energy (useable) is what you 
find listed in food composition tables or on food labels.  There should be 
very little variation in the efficiency of the use of this energy, 
regardless, once it is absorbed.  Now, quickness of absorption is another 
matter.  The energy-containing macronutrients (carbohydrate, protein, fat) 
would probably be more quickly absorbed from the pure sugar, since it only 
has to be hydrolyzed (enzymatically broken apart) from sucrose to glucose 
and fructose...and these are quickly absorbed in the small intestine.  The 
energy macronutrients from the bread and apples might be more slowly 
absorbed, especially if the bread was whole-grain.  The difference is that 
both whole-grain bread and apples contain dietary fiber...and dietary fiber 
tends to slow both digestions and absorption down.
     The question about the dung beetle is a bit out of my line, but I'll 
take a stab at it.  I'm a human nutritionist, not an entomological 
nutritionist.  Anyway, I think that dung beetles probably live on not just 
the left-over macronutrients in dung, but on all the other good stuff in 
it:  bacteria, fiber from the food the elephant? has eaten, by-products 
made by the bacteria (such as organic acids), sloughed-off cells from the 
lining of the elephants digestive tract, etc.  By the way, did you know 
that you replace the cells that line your own digestive tract on the 
average every 2-3 days.  Amazing, isn't it?
     Wow, I really got carried away with this answer.  Thanks for asking 
such interesting questions.

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