Dietary fiber is the portion of plant foods that cannot be broken down by digestive enzymes. This includes non-starch polysaccharides (cellulose, hemicelluloses, pectin, gums), oligosaccharides, or resistant starches. Dietary fiber is classified according to its solubility in water. Insoluble fiber does not dissolve in water and passes undigested through the digestive system. Soluble fiber dissolves in water to form a gelatin-like substance that can reduce blood cholesterol levels, regulate intestinal functions and prevent constipation. Although there are many forms of dietary fiber, humans require three main types: cellulose, hemicellulose, and lignin. Dietary fibers are often referred to as roughage or bulk, which is a non-scientific term that describes the indigestible portion of food derived from plants. In general, fibers can be soluble (e.g., pectin, inulins, mucilages) or insoluble (e.g., cellulose).

In humans, dietary carbohydrates are found in a wide variety of foods from plant sources. Carbohydrate intake is restricted to those that provide enough essential nutrients and energy for human life without causing significant risk to one’s health. However, the exact definition of “significant” is open to interpretation and debate, as some argue that moderate amounts of certain carbohydrates may be harmful. Others question whether or not restricting dietary carbohydrate intake to a minimum is even possible. According to the Dietary Guidelines for Americans 2010, a healthy diet includes fiber from fruits, vegetables, whole grains, legumes, and nuts with low-fat dairy products. The recommendation was made after conducting an evidence review of health benefits associated with dietary fiber intake.

Dietary fiber may be found in the form of soluble or insoluble fibers. In western societies, it is common to find that most of the dietary fiber used comes from cereal and other plant-based foods, but some cultures have a much higher intake of dietary fiber derived from natural sources. Soluble fiber dissolves in water, forming a gel that helps slow the movement of food through the digestive system. This may help control blood sugar levels and lower cholesterol levels. Soluble dietary fiber includes pectins, gums, mucilages, some hemicellulose and, beta-glucans. Good soluble fiber sources include fruits, oats, barley, legumes, and root vegetables. Insoluble fiber does not dissolve in water and passes undigested through the digestive system. It may decrease how much sugar is absorbed from food. The two major types of insoluble dietary fibers are cellulose and hemicellulose. Examples of good sources of insoluble fiber include cereals, whole wheat, and most other whole-grain foods, vegetables, and fruit.