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Probiotics and human health

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What are probiotics?

Probiotics are live microorganisms that are intended to have health benefits. Products sold as probiotics include foods (such as yogurt), dietary supplements, and products that aren’t used orally, such as skin creams.

Although people often think of bacteria and other microorganisms as harmful “germs,” many microorganisms help our bodies function properly. For example, bacteria that are normally present in our intestines help digest food, destroy disease-causing microorganisms, and produce vitamins. Large numbers of microorganisms live on and in our bodies. Many of the microorganisms in probiotic products are the same as or similar to microorganisms that naturally live in our bodies.

What Kinds of Microorganisms Are In Probiotics?

The most common are bacteria that belong to groups called Lactobacillus and Bifidobacterium. Each of these two broad groups includes many types of bacteria. Other bacteria may also be used as probiotics, and so may yeasts such as Saccharomyces boulardii.

Probiotics, Prebiotics, and Synbiotics

“prebiotics” refers to dietary substances that favor the growth of beneficial bacteria over harmful ones.

“synbiotics” refers to products that combine probiotics and prebiotics.

How Popular Are Probiotics?

Data from the 2012 National Health Interview Survey (NHIS) show that about 4 million (1.6 percent) U.S. adults had used probiotics or prebiotics in the past 30 days. Among adults, probiotics or prebiotics were the third most commonly used dietary supplement other than vitamins and minerals, and the use of probiotics quadrupled between 2007 and 2012.

What the Science Says About the Effectiveness of Probiotics

Researchers have studied probiotics to find out whether they might help prevent or treat a variety of health problems, including:

  • Digestive disorders such as diarrhea caused by infections, antibiotic-associated diarrhea, irritable bowel syndrome, and inflammatory bowel disease
  • Allergic disorders such as atopic dermatitis (eczema) and allergic rhinitis (hay fever)
  • Tooth decay, periodontal disease, and other oral health problems
  • Colic in infants
  • Liver disease
  • The common cold
  • Prevention of necrotizing enterocolitis in very low birth weight infants.

There’s preliminary evidence that some probiotics are helpful in preventing diarrhea caused by infections and antibiotics and in improving symptoms of irritable bowel syndrome, but more needs to be learned. We still don’t know which probiotics are helpful and which are not. We also don’t know how much of the probiotic people would have to take or who would most likely benefit from taking probiotics. Even for the conditions that have been studied the most, researchers are still working toward finding the answers to these questions.

Probiotics are not all alike. For example, if a specific kind of Lactobacillus helps prevent an illness, that doesn’t necessarily mean that another kind of Lactobacillus would have the same effect or that any of the Bifidobacterium probiotics would do the same thing.

Although some probiotics have shown promise in research studies, strong scientific evidence to support specific uses of probiotics for most health conditions is lacking. The U.S. Food and Drug Administration (FDA) has not approved any probiotics for preventing or treating any health problem. Some experts have cautioned that the rapid growth in marketing and use of probiotics may have outpaced scientific research for many of their proposed uses and benefits.

How might they work? (What is their causal mechanism?0

Probiotics may have a variety of effects in the body, and different probiotics may act in different ways.

Probiotics might:

  • Help to maintain a desirable community of microorganisms
  • Stabilize the digestive tract’s barriers against undesirable microorganisms or produce substances that inhibit their growth
  • Help the community of microorganisms in the digestive tract return to normal after being disturbed (for example, by an antibiotic or a disease)
  • Outcompete undesirable microorganisms
  • Stimulate the immune response.

What science says about the safety of probiotics

Whether probiotics are likely to be safe for you depends on the state of your health.

  • In people who are generally healthy, probiotics have a good safety record. Side effects, if they occur at all, usually consist only of mild digestive symptoms such as gas.
  • On the other hand, there have been reports linking probiotics to severe side effects, such as dangerous infections, in people with serious underlying medical problems. The people who are most at risk of severe side effects include critically ill patients, those who have had surgery, very sick infants, and people with weakened immune systems

Even for healthy people, there are uncertainties about the safety of probiotics. Because many research studies on probiotics haven’t looked closely at safety, there isn’t enough information right now to answer some safety questions. Most of our knowledge about safety comes from studies of Lactobacillus and Bifidobacterium; less is known about other probiotics. Information on the long-term safety of probiotics is limited, and safety may differ from one type of probiotic to another.

Quality Concerns About Probiotic Products

Some probiotic 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 on the label.

Source of info: US Dept of Health and Human Services, NIH, NCCIH Pub No. D345


Where did the idea of using probiotics first develop?

The idea came from Nobel laureate Élie Metchnikoff. He postulated that yogurt-consuming Bulgarian peasants lived longer lives because of that custom. He suggested in 1907 that “the dependence of the intestinal microbes on the food makes it possible to adopt measures to modify the microbiota in our bodies and to replace the harmful microbes by useful microbes”.

There is a growing body of peer-reviewed science which indeed shows that there is a link between our gut flora (varieties of bacteria that live in our gut) and our health. But this link is complex, and it may vary widely from person to person, depending on their genes, and their gut biome.

Studies on gut bacteria and physical health


Studies on gut bacteria and mental health


Studies which show that treatment should be personalized

Senior author Eran Elinav, an immunologist at the Weizmann Institute of Science in Israel, and colleagues found that many people’s gastrointestinal tracts reject generic probiotics before they can get to work. Even worse, Elinav’s team found that microbial competition from off-the-shelf probiotics can prevent natural gut bacteria from reestablishing themselves after being wiped out by antibiotic drugs.

“I think our findings call for a fundamental change from the currently utilized one-size-fits-all paradigm, in which we go to the supermarket and buy a formulation of probiotics which is designed by some company, to a new method which is personalized,” Elinav says. “By measuring people in a data-driven way, one would be much better able to harness different probiotic combinations in different clinical contexts.”

… Elinav’s group isn’t claiming that probiotic supplements don’t carry heavy doses of beneficial gut bacteria. In fact, the studies confirm that they do. Because many probiotics are sold as dietary supplements, and thus aren’t subject to approval and regulation by many national drug agencies, including the U.S. Food and Drug Administration, the team first set out to ensure that the probiotic supplements in the study actually contained the 11 main strains they were supposed to deliver.

“All those strains were present and viable to consumption and beyond, following the passage through the GI tract, and even in stool, and they were still viable,” Elinav says.

But uncovering what impact these strains of bacteria have on the people who consume them required more digging, poking through patient’s stool and even inside their guts.

The authors set out to directly measure gut colonization by first finding 25 volunteers to undergo upper endoscopies and colonoscopies to map their baseline microbiomes in different parts of the gut. “Nobody has done anything quite like this before,” says Matthew Ciorba, a gastroenterologist at Washington University in Saint Louis School of Medicine unaffiliated with the study. “This takes some devoted volunteers and some very convincing researchers to get this done.”

Some of the volunteers took generic probiotics, and others a placebo, before undergoing the same procedures two months later. This truly insider’s look at the gut microbiome showed some people were “persisters,” whose guts were successfully colonized by off-the-shelf probiotics, while others, called “resisters,” expelled them before they could become established. The research suggests two reasons for the variability in the natural response of different gastrointestinal tracts to probiotics.

First and foremost is each person’s indigenous microbiome, or the unique assembly of gut bacteria that helps dictate which new strains will or won’t be able to join the party. The authors took gut microbiomes from resistant and persistent humans alike and transferred them into germ-free mice, which had no microbiome of their own. All the mice were then given the same probiotic preparation.

“We were quite surprised to see that the mice that harbored the resistant microbiome resisted the probiotics that were given to them, while mice that were given the permissive microbiome allowed much more of the probiotics to colonize their gastrointestinal tract,” Elinav explains. “This provides evidence that the microbiome contributes to a given person’s resistance or permissiveness to given probiotics.”

The second factor affecting an individual’s response to probiotics was each host’s gene expression profile. Before the probiotics were administered, volunteers who ended up being resistant were shown to have a unique gene signature in their guts—specifically, a more activated state of autoimmune response than those who were permissive to the supplements.

“So it’s probably a combination of the indigenous microbiome and the human immune system profile that team up to determine a person’s specific state of resistance or colonization to probiotics,” Elinav says. These factors were so clear that the team even found that they could predict whether an individual would be resistant or permissive by looking at their baseline microbiome and gut gene expression profile.

This unusual in situ gastrointestinal tract sampling also turned out to be key, because in a number of cases the microbiota composition found in a patient’s stool was only partially correlated with what was found inside the gut. In other words, simply using stool samples as a proxy can be misleading.


The benefits of probiotics may not be so clear cut

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