Fiber can loosely be defined as any carbohydrate which is not digestible to humans. It is found mainly in plants, and it is important in the human diet because it assists in gastrointestinal health.

We divide fiber into two types: soluble and insoluble. Soluble fiber is soluble in water, and insoluble fiber is not. The digestive function of soluble fiber is that it is digested (fermented) by gastrointestinal bacteria, creating various physiologically significant byproducts. The digestive function of insoluble fiber is to provide bulk to food as it moves through the digestive system, ultimately easing defecation. Insoluble fiber may also ferment, like soluble fiber. Both types of fiber provide the subjective benefit of making meals feel more filling.

The AI of fiber is set at 14g for every 1,000 calories consumed. So for example a person on a 2,000 calorie diet should consume 28g of fiber daily. This should include both soluble and insoluble fiber.