|MadSci Network: Biophysics|
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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