Ask A Scientist Zoology Archive

name         Daphna
status       student
age          20s

Question -   My friend says your stomach absorbs 80% of the nutrients
of a meal within 20 minutes of the food being there. I say that is
incorrect. Do you know how long it takes for the food to be absorbed in
your stomach and what percent of it is absorbed before moving to the
small intestine for further absorption?
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Generally very little of any of the food we eat will be absorbed through the
gastric mucosa, except things like alcohol which can in part make its way
through. Fats, oils proteins and carbohydrates will be almost totally be absorbed
in the small intestine after further emulsification and digestion upon
entering the duodenum.

Peter Faletra Ph.D.
Assistant Director
Science Education
Office of Science
Department of Energy
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      I believe you are correct. How fast a substance is absorbed and where
it is absorbed in the digestive tract depends on: what the food substance
is, what other foods are consumed with it, individual differences, what
medications one might be using, and  a host of other considerations. Water
and ethanol are rapidly consumed, even in the mouth and esophagus to some
extent, and are absorbed almost completely in the stomach. Fats, which
require bile need to pass the gall bladder ducts (if you have a gall
bladder) or the liver in order to be digested. This occurs in the small
intestine. Complex carbohydrates require a long time to break down into
simple sugars, such as glucose. That is why long distance runners load up
with carbohydrates several hours before a race, not just 20 minutes. That 
would
make them very nauseated. In addition, their digestion requires insulin that
empties from the pancreas in the small intestine.
     Finally, if your friend's timing were true, anorexia nervosa / bulimia
nervosa would not be the life-threatening conditions that they are.

Vince Calder
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First of all, most nutrients are absorbed into the small intestine, not the
stomach.  The job of the stomach is to add hydrochloric acid and some
peptidases, which are enzymes that help break down protein into smaller
chunks.  Even then, most protein digestion takes place in the small
intestine.  The stomach also churns your food into a mush called chyme, which
passes slowly, bit by bit, into the small intestine through the pyloric
sphincter muscle.  You have amylases in your saliva which begin the breakdown
of carbohydrates, but those too are mostly digested in the small intestine.
So I would say your friend is mistaken.

vanhoeck
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