I thought I'd share a little knowledge about
some common causes of diarrhea, since it's relevant for many IBSers and it will help me solidify it in my brain in preparation for my second biochemistry test tomorrow...I'll try to keep it as uncomplicated as possible.
One cause of diarrhea are defects in the enzymes that break down certain sugars. I'll use lactose as an example: lactose is a "dissacharide" sugar consisting of glucose and galactose, two "monosaccharides." Only monosaccharides can be absorbed by the body, which is why you need an enzyme (in this case, lactase) to break the lactose down.
If you're "normal," lactase (the enzyme) breaks down lactose (the sugar) into glucose and galactose in your small intestine and absorbed. Water is absorbed at the same time. This means there's no "net" water accumulation in the lumen of the small intestine.
If you're "abnormal," either you don't have lactase or the lactase you do have is defective. So the lactose can't be broken down and absorbed. So you get water and unabsorbed lactose flowin' through your small intestine into the large intestine, where even more water is drawn into the lumen. All this extra water = diarrhea! At the same time, just for kicks I guess, the lactose is fermented by bacteria, which produces carbon dioxide and hydrogen gas, and that makes you fart.
Not just lactose can cause this problem. There are plenty of other sugars that need to be broken down by different enzymes and those can be messed up too. Some people can't digest starch because they lack the enzyme that breaks it down.
Another cause of diarrhea is malabsorption of monosaccharides. Fructose is one of them. 33% of adults and 67% of infants do not absorb fructose normally! Fruit juice contains lots of fructose (especially apple juice). This is why we shouldn't drink a whole lot of it. Some people can actually forever mess themselves up by drinking too much fruit juice as a child. This malabsorption can cause diarrhea, flatulence, etc.
You can find out if your diarrhea is osmotic by doing a simple stool test; the lab calculates what's called an "osmotic gap" and if it's high, that means you probably have osmotic diarrhea, which is probably caused by something you're eating.
Hope this was informative!