|MadSci Network: General Biology|
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 increases. 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 accomplish. Hope this helps. If anything is confusing, a good Google on that term should clear things up rapidly.
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