Bitterness is a sensation often considered unpleasant characterized by having a sharp, pungent taste. Unsweetened dark chocolate, caffeine, lemon rind, and some types of fruit are known to be bitter.
Non-essential nutrients are substances within foods can still have a significant impact on health, whether beneficial or toxic. For example, dietary fiber is not absorbed by the human digestive tract, but is important in maintaining the bulk of a bowel movement to avoid constipation.
A subset, known as soluble fiber, can be metabolized by bacteria residing in the large intestine. Soluble fiber is marketed as serving a Probiotic (nutrition) function - promoting 'healthy' intestinal bacteria - even though there is not yet a scientific consensus on benefits of probiotics. Bacterial metabolism of soluble fiber is also thought to product short-chain fatty acids, which may be absorbed into intestinal cells as sources of calories.