Originally posted July 2017 / Updated July 2023

The term probiotic is derived from Greek and literally means “for life.” The term is most often used to describe the beneficial bacteria that inhabit the human intestinal tract. Manufacturers of probiotics typically provide safely freeze-dried live forms of many of these beneficial bacteria so that they can be put in capsules, liquids, or food and brought back to life in the human intestinal tract. Probiotics are also found in fermented foods such as yogurt, aged and raw cheeses, kefir, kimchi, sauerkraut, and miso. The specific microorganisms found in these products will vary but usually include lactobacilli and bifidobacteria, which are the major probiotics found in dietary supplements.

Probiotics are helpful in promoting the health of the “microbiome,” the collection of genetic material within the microbes that we harbor in our bodies. The number of microbiota - bacteria, viruses, and funguses - that live on or in the human body is enormous. Estimates are that approximately 100 trillion microbial cells from 1,000 different species of microorganisms live within or on us in a truly symbiotic relationship.

To a very large extent, the human intestinal microbiome plays an integral role in our overall health. In an effort to improve the intestinal microbiome and thus support overall health, many people look to probiotic supplements. This use of probiotics is supported by significant clinical research with over 500 double-blind, placebo-controlled studies. Much of the research has focused on the use of probiotics to promote gastrointestinal health and support immune function, but clinical research also supports the use of probiotics for supporting many other health goals.1

Health Benefits of Probiotics

Probiotics have been shown to possess many mechanisms to improve the gastrointestinal environment and make it less hospitable for less desirable microbes. For example, many undesirable organisms must bind to the lining of the gastrointestinal tract in order to colonize effectively. Some strains of probiotics can adhere to the epithelium and act as “colonization barriers” by preventing undesirable bacteria (and yeast) from adhering to the lining of the gastrointestinal tract.

Another mechanism of action is that probiotics produce antimicrobial compounds known as bacteriocins. The release of these compounds by probiotic organisms results in a beneficial modification of the microbiome. Some of the antimicrobial activity of probiotics has also been shown to be due to their production of hydrogen peroxide and organic acids like lactic and butyric. The production of these small organic acids also helps to maintain an optimal intestinal pH as well as nourish other health-promoting organisms and, in the case of butyric acid, provides the key energy source for the cells that line the large intestine.2,3

Probiotics also compete for nutrients that would otherwise be utilized by undesirable microorganisms. Probiotic organisms in sufficient numbers can utilize most of the available prebiotic nutrients, resulting in the inhibition of the growth of undesirable microbes.2,3

Lastly, probiotics also help to stimulate the immune response. For example, probiotics can increase the secretion of immunoglobulin (Ig) A – an antibody that lines our intestinal. IgA is a nonspecific antibody that can bind to and neutralize unwanted microorganisms. Certain probiotics have also shown an ability to activate key cells of our immune system (natural killer cells, macrophages, and T-lymphocytes).4

There are many more mechanisms that have been discovered as well that deal with effects outside the gastrointestinal effects. However, the focus here will be on the gastrointestinal effects of probiotics.

Probiotic Supplements: Quality Matters

The quality of probiotic supplements depends on two main factors: (1) the characteristics of the strains contained in the supplement and (2) adequate viability so that sufficient numbers of bacteria are viable at the point of consumption. Viability at consumption depends on factors such as proper manufacturing and the “hardiness” of the strain, as well as packaging and storage of the product in the right amount of moisture and at the correct temperature.

Strains of bacteria can be likened to different breeds of dogs. All dogs belong to the genus Canis and the species familiaris. Within this one species is great diversity in size, shape, strength, and other physical characteristics — ranging from Saint Bernard to the Chihuahua. A similar division occurs within species of bacteria; each species of bacteria comprises a multitude of strains. Some probiotic strains are resilient and strong, able to survive passage through the upper gastrointestinal and inhibit pathogenic bacteria and others are weak and can’t survive or kill pathogenic bacteria.

What this analogy means is that consumers must utilize products developed and manufactured by companies that have done the necessary research to ensure the viability of their product and, ideally, tested their product in human clinical trials. With probiotics, as with other supplements, be a smart consumer and choose products that will provide the benefits that you are looking for. The information above provides some guidance as to the best probiotic strains to help in the prevention of antibiotic-associated diarrhea and traveler’s diarrhea and the treatment of Helicobacter pylori infections and irritable bowel syndrome.


The dosage of probiotic supplements is most often based on the number of live organisms present in the product. It is, therefore, important to use products that list the number of live organisms at expiration versus at the time of manufacture (which means nothing). Successful results are most often attained by taking between 5 billion and 20 billion viable organisms per day. Surprisingly, based on the clinical studies on these gastrointestinal issues, dosages in this range usually provide better results than dramatically higher dosages.


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