There are two types of food fiber: soluble and insoluble. Soluble can dissolve in water and insoluble can't. All fiber-containing foods contain a combination of both types of fiber. While you need both, it's the soluble fibers that serve you as true prebiotics.
3 Foods, 3 Carbs
In the foods you eat, there are three main nutrient groups: carbohydrates, fats and proteins. And among carbohydrates there are three groups: starch, sugar and fiber. Fiber was once viewed as less important, since your body doesn't digest it, and because it has no calories, vitamins or minerals. The thinking was that it was just for bulk, helping move everything through your digestive system. But the more we've learned, the more we see how critically important fiber is to us.
Soluble fibers are naturally found in grains, fruits and vegetables. While these fibers are not digested in your stomach or small intestine, they travel to your large intestine where they are fermented. Fermentation breaks down the fibers to provide nutrients that feed the bacteria in your gut. All 100,000,000,000,000 (100 trillion) of them.
Fiber That Absorbs
Another function of soluble fiber, is to absorb water and give you a sense of being full, which helps you desire less food. It slows the rate at which sugar is absorbed into your bloodstream, which can be important in avoiding or managing diabetes. Fiber also helps to keep blood cholesterol levels lower by absorbing bile acids, which leave our body as waste along with the fiber. The liver then must produce more bile acids and therefore works harder, which uses up more of its supply of cholesterol. That's good.