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Fiber refers to the parts of plant foods that humans cannot digest. It is generally classified into insoluble and soluble fiber. The American diet is generally very low in fiber because we have focused on refined grain products and eat very few vegetables and fruit. Insufficient fiber in the diet can contribute to constipation. Soluble fiber has been linked to lowering cholesterol levels. Good sources of fiber include bran cereals, beans, vegetables, fruit and whole grains. Flax seed and psyllium seed are excellent sources of fiber. Fiber supplements are available but food sources are generally recommended more often.