Depending on your meal, your small intestine may contain quite a bit of undigested food. (This isn’t the case with the simple breakfast example because it’s short on plant matter and other sources of fiber.) In modest quantities, this undigested food benefits your digestive system and eases the passage of your meal as it scrapes through your narrow intestinal passageways. We call it fiber. One example of indigestible food is cellulose, a compound that helps form the structure of green plants.
In the dieting world, there’s a completely different class of indigestible food substances that masquerade as the sugar and fats our mouths expect, while dodging the absorption step in the small intestine. One example, sucralose (which is known commercially as Splenda), is a subtly modified version of sugar that triggers the sweet taste buds on your tongue, but can’t be broken down by the carbohydrate-processing enzymes in your body. Another example is olestra, an altered fat molecule that has the same mouth feel as fat but passes unhindered through your small intestine. (Eat olestra in great quantities, and you’ll have a significant amount of unneeded matter moving through your system, potentially leading to the abdominal cramping and loose stools mentioned in the package warning label.) These two indigestible foods are examples of food science at its creative best—and potential health concerns.
The key concern for most sugar and fat replacements is not toxic side effects, but the way they allow non-foods with no nutritional value to take up valuable stomach space. Dieters caught up in the excitement of eating without weight gain may forget that their calorie-free potato chips are displacing real foods, and in the process robbing their bodies of the vitamins, minerals, and other nutrients they need.
Source of Information : Oreilly - Your Body Missing Manual