Probiotics are live microorganisms (e.g., bacteria) that are either
the same as or similar to microorganisms found naturally in the human
body and may be beneficial to health. Also referred to as “good
bacteria” or “helpful bacteria,” probiotics
are available to consumers in oral products such as dietary supplements
and yogurts, as well as other products such as suppositories and
creams. The U.S. Food and Drug Administration (FDA) has not approved any
health claims for probiotics. This fact sheet provides a general
overview of probiotics, with an emphasis on oral products, and suggests
sources for additional information.
The concept behind probiotics was introduced in the early 20th
century, when Nobel laureate Elie Metchnikoff, known as the “father of
probiotics,” proposed in The Prolongation of Life: Optimistic Studies
that ingesting microorganisms could have substantial health benefits
for humans. Microorganisms are invisible to the naked eye and exist
virtually everywhere. Scientists continued to investigate the concept,
and the term “probiotics”—meaning “for life”—eventually came into use.
Picturing the human body as a “host” for bacteria and other
microorganisms is helpful in understanding probiotics. The body,
especially the lower gastrointestinal tract (the gut), contains a
complex and diverse community of bacteria. (In the body of a healthy
adult, cells of microorganisms are estimated to outnumber human cells by
a factor of ten to one.) Although we tend to think of bacteria as
harmful “germs,” many bacteria actually help the body function properly.
Most probiotics are bacteria similar to the beneficial bacteria found
naturally in the human gut.
Various mechanisms may account for the effects of probiotics on human
health. Possible mechanisms include altering the intestinal
“microecology” (e.g., reducing harmful organisms in the intestine),
producing antimicrobial compounds (substances that destroy or suppress
the growth of microorganisms), and stimulating the body’s
immune response.
Probiotics commonly used in the United States include Lactobacillus and Bifidobacterium.
There are many specific types of bacteria within each of these two
broad groups, and health benefits associated with one type may not hold
true for others.
In the United States, probiotics are available as dietary supplements
(including capsules, tablets, and powders) and in dairy foods (such as
yogurts with live active cultures). According to the 2007 National
Health Interview Survey, which included a comprehensive survey on the
use of complementary health approaches by Americans,
“prebiotics/probiotics” ranked fifth among natural products used for
children, but were not among the top-ranking products for adults.
Although probiotic products are more popular in Europe and Japan than in
the United States, the U.S. consumer market for probiotics is
growing rapidly.
Although the FDA has not approved any health claims for probiotics,
they are used for a variety of gastrointestinal conditions such as
infectious diarrhea, diarrhea associated with using antibiotics,
irritable bowel syndrome, and inflammatory bowel disease (e.g.,
ulcerative colitis and Crohn’s disease). Probiotics are also being used
for preventing tooth decay and for preventing or treating other oral
health problems such as gingivitis and periodontitis. Some—but not
all—probiotic formulations have been widely studied and show
considerable promise. However, the rapid growth in marketing and
consumer interest and use has outpaced scientific research on the safety
and efficacy of probiotics for specific health applications.
The potential of probiotics to benefit human health in many different
ways has stimulated great interest and activity among researchers. For
example, the National Center for Complementary and Alternative Medicine
(NCCAM) is part of the National Institutes of Health (NIH) Probiotic and
Prebiotic Working Group, a trans-NIH effort to identify gaps and
challenges in prebiotic/probiotic research.
Probiotic research is moving forward on two fronts: basic science
(laboratory studies) and clinical trials to evaluate the safety and
efficacy of probiotics for various medical conditions. Many early
clinical trials of probiotics have had methodological limitations, and
definitive clinical evidence to support using specific probiotic strains
for specific health purposes is generally lacking. Nevertheless, there
is preliminary evidence for several uses of probiotics, and more studies
are under way. In particular, a recent review of the scientific
evidence on the effectiveness of probiotics in acute infectious diarrhea
concluded that there was evidence that probiotics may shorten the
duration of diarrhea and reduce stool frequency but that more research
was needed to establish exactly which probiotics should be used for
which groups of people.
In 2008, the journal Clinical Infectious Diseases published a
special issue on probiotics, which included an overview of clinical
applications. Based on a review of selected studies, the authors
classified several applications according to the strength of evidence
supporting the efficacy of probiotics in prevention and/or treatment.
For example, the authors concluded that strong evidence exists for acute
diarrhea and antibiotic-associated diarrhea, and substantial evidence
exists for atopic eczema (a skin condition most commonly seen in
infants). Promising applications include childhood respiratory
infections, tooth decay, nasal pathogens (bacteria harbored in the
nose), gastroenteritis relapses caused by Clostridium difficile
bacteria after antibiotic therapy, and inflammatory bowel disease. The
authors also discussed various potential future applications.
Studies also indicate that probiotics may reduce side effects associated with treatment for Helicobacter pylori
infection, the cause of most stomach ulcers. A systematic review
suggests that there is strong evidence that probiotics may reduce the
risk of necrotizing enterocolitis, a severe intestinal condition of
premature newborns. Other potential future applications include use in
reducing cholesterol levels, treating obesity, and managing irritable
bowel syndrome.
It appears that most people do not experience side effects from
probiotics or have only mild gastrointestinal side effects such as gas.
But there have been some case reports of serious adverse effects, and
research on safety is ongoing. A 2008 review of probiotics safety noted
that Lactobacillus rhamnosus GG has been widely studied in
clinical trials for a variety of conditions and generally found to be
safe. Nevertheless, a recent review of Lactobacillus and Bifidobacterium
noted that the long-term, cumulative effects of probiotics use,
especially in children, are unknown, and also pointed to evidence that
probiotics should not be used in critically ill patients. Similarly, a
2011 Agency for Healthcare Research and Quality assessment of the safety
of probiotics, partly funded by NCCAM, concluded that the current
evidence does not suggest a widespread risk of negative side effects
associated with probiotics. However, the data on safety, particularly
long-term safety, are limited, and the risk of serious side effects may
be greater in people who have underlying health conditions.
Concerns have also been raised about the quality of probiotic
products. Some products have been found to contain smaller numbers of
live microorganisms than expected. In addition, some products have been
found to contain bacterial strains other than those listed
as ingredients.
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