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Probiotics and prebiotics are found in a variety of foods and are considered bioactive food components. They may also be available as dietary supplements. Probiotics and prebiotics combined in the same product are known as synbiotics.                             

Probiotics (“for life”) are live microorganisms (e.g., lactobacilli) that are considered beneficial to humans. These bacteria may help suppress the growth of potentially harmful bacteria in the human body.

Prebiotics (“before life”) are certain nondigestible substances (usually found in carbohydrates, such as fructo-oliogosaccharides) in foods or supplements that are used as fuel by beneficial bacteria already in the body.

Probiotics and prebiotics alter the level of microorganisms in the intestines. The number of bacteria normally present in healthy intestines is estimated to be in the trillions, made up of more than 500 different types of bacteria. Most are beneficial, although a small number may be potentially harmful. These beneficial bacteria are necessary to help fight infection and aid immune system functioning.

The beneficial bacteria in probiotics, fueled by prebiotics, may reduce the number of potentially harmful bacteria in the intestines in a number of ways. They may compete with them for nutrients, thus crowding out disease-causing bacteria. These beneficial bacteria may also produce substances that inhibit the growth of potentially harmful bacteria. They may also restore the balance of beneficial bacteria in the gut due to imbalance caused by disease, stress, poor diet or the use of certain medications (e.g., antibiotics). Probiotics and prebiotics may also aid digestion by slowing the digestive process and encouraging nutrient absorption. 

To be effective, beneficial bacteria in probiotics must be alive when consumed. This allows them to grow and develop in the intestines. They may be freeze-dried if taken in supplement form. These bacteria may die if exposed to heat, moisture or air. Some types of probiotics may need to be stored in the refrigerator. The packaging of probiotic products should indicate if active (live) cultures are present.

Prebiotics must be able to survive the gastric acid of the stomach and the enzymes produced during digestion. Because the human body is unable to completely digest these substances, they pass through the stomach and small intestine into the large intestine, where they stimulate the growth and activity of certain bacteria. Different types of probiotics may prefer particular types of prebiotics. For example, bifidobacteria probiotics may prefer fructose- or galactose-based prebiotics.

Taken together, probiotics and prebiotics are believed to significantly promote the development of beneficial bacteria in the gut. Probiotics and prebiotics are not considered nutrients because they are not necessary for the human body to function. Thus, no deficiency disease occurs due to a lack of probiotics or prebiotics in the human body.

Research regarding the effects of probiotics and prebiotics is ongoing. Due to the vast assortment of bacteria that exist in the intestines, it is difficult for researchers to determine which effects are specific to each type of bacteria. For example, it is unclear which bacterial strains in probiotics have potential beneficial effects on particular diseases or medical conditions.

In addition, when studying probiotics and prebiotics, many different factors must be controlled for reliable results. These factors include bacteria type, potency (amount of bacteria per dose), purity (presence of possible contaminating microorganisms) and disintegration properties of each type of bacteria.