Whole grains, beans, and other legumes, fruits (especially plums, prunes, and figs), and vegetables are good sources of dietary fiber. Fiber has three primary mechanisms, which in general determine their health impact: bulking, viscosity and fermentation. Fiber provides bulk to the intestinal contents, and insoluble fiber facilitates peristalsis – the rhythmic muscular contractions of the intestines which move contents along the digestive tract. Some soluble and insoluble fibers produce a solution of high viscosity; this is essentially a gel, which slows the movement of food through the intestines. Fermentable fibers are used as food by the microbiome, mildly increasing bulk, and producing short-chain fatty acids and other metabolites, including vitamins, hormones, and glucose. One of these metabolites, butyrate, is important as an energy source for colon cells, and may improve metabolic syndrome.