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United States Department of Agriculture

Agricultural Research Service

To your health: Probiotics and Prebiotics
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by Wesley Canfield

Most of us think of bacteria as harmful and have taken antibiotics for the infections they cause.

However, not all bacteria are harmful in fact some are helpful. For example, some bacteria make vitamins, including B12, folic acid, biotin and K, which may be used by the host. Other bacteria have been implicated in prevention and treatment of certain disorders, including allergies, inflammatory bowel disease, diarrhea, irritable bowel syndrome and even colon cancer. We depend on these helpful bacteria even if we don't realize it.

The bacteria that live in the human intestinal tract play an important role in maintaining good health. It is estimated that more than 500 types of bacteria may be found at any time in any particular person's colon. Some are potentially harmful, but are only present in small numbers, and are kept at bay by the other numerous bacteria. If certain conditions change, for example, prolonged treatment with antibiotics, then the balance between the harmful and helpful bacteria may change, resulting in overgrowth of the harmful types. That condition holds potential for disease.

The term biotic is from the Greek word, meaning "life." Antibiotics are, therefore, "against life." Probiotic means "for life." This term refers to a viable bacterial supplement that benefits the health of the host.

For example, lactose or milk sugar intolerance is relatively common. People who lack the intestinal enzyme, lactase, cannot digest lactose to its component sugars (glucose and galactose), which can then be absorbed. As a consequence, the lactose travels intact to the colon, where bacteria ferment it to hydrogen and carbon dioxide gas. This may result in some intestinal discomfort, and at times socially embarrassing consequences. By consuming a supplement containing lactase-producing lactobacillus bacteria found in some yogurts, the symptoms of lactose fermentation can be decreased.

Prebiotics ("before life") are nondigestible or fiber components of foods, usually complex carbohydrates that beneficially affect the host by stimulating the growth of intestinal bacteria. Certain bacteria prefer a particular prebiotic to use as a source of energy. The bifidobacteria family prefers complex sugar chains of fructose or galactose. Including such prebiotics in the diet provides a competitive advantage to these bacteria compared with others.

What health advantage is there for humans to have more of these bacteria in the colon?

The answer may be found in a fact of nature. Breast-fed infants carry bifidobacteria as their predominant colon bacteria. Their dominance is maintained in part by a prebiotic (long-chain galactose) present in breast milk. Infants exclusively breast-fed may be protected from rotavirus infection because of the dominance of bifidobacteria (estimated at 10 times greater than in formula-fed infants). Rotavirus is a major cause of severe diarrhea in infants. Feeding pregnant mice bifidobacteria while nursing their pups, provided protection against rotavirus infection. This protection was in the form of immunoglobulins directed against the virus.

In addition to making vitamins, intestinal bacteria also produce other nutrients of benefit. We've all heard advice about limiting our intake of fats, especially saturated ones. However, not all saturated fats are bad for us. Many of the gut bacteria ferment digestion-resistant fiber to short-chain saturated fatty acids. These fats have two to four carbon atoms.

Acetate, propionate and butyrate all are examples of SCFA. Some scientists estimate that up to 10 percent of a person's daily energy needs can be met by using the SCFA as fuel. Butyrate, in particular, has been shown to have anti-cancer effects in animals. In addition, a probiotic containing butyrate-producing bacteria also reduced cancer formation in an animal model.

Another way of increasing butyrate production by intestinal bacteria is to eat prebiotics similar to the high-galactose type found in breast milk. Inulin, for example, is a type of high-fructose, nondigestible fiber that is present in wheat and onions. Many studies in both humans and animals have demonstrated the bifidobacteria-promoting benefits of inulin and other high-fructose prebiotics.

Are probiotics and prebiotics available? Yogurts fortified with probiotics (lactobacillus bacteria) are locally available. Probiotics in capsule and powder form also are available in local health-food and nutritional supplement stores.

Last Modified: 6/12/2006
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