MadSci Network: General Biology

Re: How long does it take carbohydrates to be digested?

Date: Thu Apr 21 12:43:35 2005
Posted By: Steve Denham, Faculty, Animal Agriculture, Monsanto
Area of science: General Biology
ID: 1113853684.Gb

Good question, Sylvan--and the more I think about it, the messier my 
answer gets.  I'm pretty sure we're talking human digestion here, not 
sheep or something else that gets bacteria to help them digest 
carbohydrates.  First, we need to recognize that "carbohydrate" is a 
general term that includes several different chemical entities.  First is 
simple sugars, or monosaccharides.  Glucose is the prime example here.  
These have a zero digestion time, and are rapidly absorbed into the 
bloodstream in the small intestine--and by rapidly, I mean large amounts 
absorbed in less than a minute.  Once absorbed, the various 
monosaccharides are metabolized differently, and trigger different 
hormonal responses.  The next category is the disaccharides--made up of 
two monosaccharides.  Examples are sucrose (table sugar) and maltose.  
These are the first kinds of carb that need digestion down to 
monosaccharides to be absorbed.  They must get to the small intestine for 
digestion to begin.  Digestion is rapid if you have the enzymes to do it 
(people who are lactose intolerant have low levels of lactase, the enzyme 
that digests lactose into its components).  Now comes another messy part--
there is a limited amount of digestive enzyme capacity, so the time it 
takes to digest completely depends on the amount of stuff to digest.  But 
let's say we're talking the amount in a small candy bar, about 100g.  100g 
of maltose will be digested within 5 minutes, 100g of sucrose within 7 
minutes, and 100g of lactose within 10 minutes.  You probably need to add 
in transit time through the stomach, as well, but this isn't much for 
sugars eaten alone--about two minutes.  Add in other nutrients and this 

On to complex carbohydrates--starch and fiber.  Okay, fiber is fiber 
because, essentially, it's indigestible by the human system.  Cows and 
sheep and termites can digest it, otherwise we'd be in over our heads in 
dead plant stuff.  This leaves starch.  Starch digestion begins in the 
mouth and continues in the small intestine (and sometimes, embarrasingly, 
by bacteria in the large intestine).  The problem here is that the starch 
doesn't just sit in one place and get digested.  It's moving on along the 
digestive tract.  In most people, transit through the small intestine 
takes about an hour and fifteen minutes.  During that time, between 80 and 
99% of starch will be digested, depending on the individual.  If the 
transit time goes up, the percent digested goes up.  If it is shortened, 
the digestibility goes down.  So, back to the 100g.  Of that about 10-20g 
will be digested to maltose in the mouth, and we saw that maltose was 
completely digested in about 5 minutes.  Of the remaining 80g to 90g, all 
but about 8 or 9 grams will be digested and absorbed in an hour and a half.

Essentially all of the absorption is as simple sugars.  Glucose is ready 
immediately for metabolism, as is fructose (the other part of sucrose).  
Galactose (the other part of lactose) has to be converted by the liver to 
glucose before use.  But this is extremely rapid--less than a minute to 

Hope this helps.  If anything is confusing, a good Google on that term 
should clear things up rapidly.

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