Interest in probiotics is at an all-time high in the United States, driven in part by new products emerging in the market, by US researchers eager to evaluate efficacy claims rigorously, and by consumers interested in potential therapeutic and preventive health benefits. The US marketplace is a mixed bag of products, some well-defined and properly evaluated in controlled clinical studies and others with unsubstantiated claims of efficacy. Validation of probiotic contents in commercial products is needed to ensure consumer confidence. The term “probiotic” should be used only for products that meet the scientific criteria for this term—namely, products that contain an adequate dose of live microbes that have been documented in target-host studies to confer a health benefit. Probiotics must be identified to the level of strain, must be characterized for the specific health target, and must be formulated into products using strains and doses shown to be efficacious. Several characteristics commonly presumed to be essential to probiotics, such as human origin and the ability to improve the balance of the intestinal microbiota, are discussed.
Definition of Probiotics
The internationally endorsed definition of probiotics is live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. Other definitions advanced through the years have been restrictive by specification of mechanisms, site of action, delivery format, method, or host. Probiotics have been shown to exert a wide range of effects. The mechanism of action of probiotics (e.g., having an impact on the intestinal microbiota or enhancing immune function) was dropped from the definition to encompass health effects due to novel mechanisms and to allow application of the term before the mechanism is confirmed. Physiologic benefits have been attributed to dead microorganisms . Furthermore, certain mechanisms of action (such as delivery of certain enzymes to the intestine) may not require live cells. However, regardless of functionality, dead microbes are not probiotics.
The term “probiotic” is sometimes erroneously used as a synonym for putatively beneficial members of commensal microbiota. The context for this misuse is the assertion that certain dietary or environmental factors may “encourage your native probiotics.” Members of human commensal microbiota are often sources from which probiotics are isolated, but, until such strains are isolated and then adequately characterized for content, stability, and health effects, they are not probiotics.
The US Food and Drug Administration (FDA) uses other terms for live microbes for regulatory purposes; live microbes used in animal feeds are called “direct-fed microbials” , and, when intended for use as human drugs, they are classified as “live biotherapeutics” . However, no legal definition of probiotics exists in the United States or in other countries, which allows the marketing of products labeled as “probiotics” that do not meet the fundamental criteria stipulated in the scientific definition. As public awareness of probiotics increases, the use of the term has implications both for the violation of the standard of truthful and not misleading labeling and for consumer confidence in this product category. In the absence of a legal definition of the term “probiotic, ” it is incumbent on industry participants to adopt and adhere to the scientific definition of the term. If the products are not properly characterized and validated, the category will suffer.
Validity of Commercial Products
Reports that the labels on commercial probiotic products are inaccurate with regard to the identity and potency of the contents are numerous [4, 5]. However, care must be exercised in interpreting some of these published reports, since not all arrive at accurate conclusions, because of inappropriate methods used to address the research question being asked [6–8]. Studies are further complicated by the need to obtain statistically representative samples, by the difficulty in differentiating multiple species and strains that may be combined into one product, by the presence of injured cells that may be viable but not culturable under certain circumstances, and by the need to use DNA-based approaches to properly identify species of some probiotic genera, including Lactobacillus and Bifidobacterium. Consumer Reports reported that yogurts are better sources of probiotics than are supplements . But this conclusion may have been in error. Although methods were not disclosed, the results suggested that no differentiation of the different types of bacteria present in yogurt was conducted. Reporting the total number of bacteria in products containing multiple strains can be misleading. In the case of yogurt, both starter cultures (Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus) and extra bacteria added for health effects (e.g., strains of other species of Lactobacillus or Bifidobacterium) can be present. If the health effects are derived from these additional bacteria, then these need to be specifically enumerated before a judgment can be made on the potency of the product. A total count does not indicate the levels of each of the added probiotic strains. For any product containing multiple strains of probiotics, counts of each strain should be provided so that consumers are clear on the relative potency of each added strain.