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Probiotics are “live microorganisms [that] when administered in adequate amounts confer a health benefit on the host.”

A person’s body contains millions of live bacteria. Many of these are located in the gut. Gut bacteria make up a person’s “microbiome.”

The microbiome is unique to each individual, and studies have shown it is determined from before birth. Research suggests that the microbiome changes throughout a person’s life based on their diet, lifestyle, and exposure to different environmental influences.

Probiotics can help populate the gut with good bacteria. This is a key part of a person’s immune system. Gut bacteria have many functions in the body and affect things such as weight, mood, and inflammation.

In recent years, there has been a lot of interest in how probiotics can support health and reduce a person’s risk and symptoms of certain conditions.

Many probiotics contain mixtures of two or more individual strains of bacteria or yeasts. They may also contain prebiotics, which are compounds that the probiotics can feed on. If a formula contains both probiotics and prebiotics, it is called a “synbiotic.”

Products most often contain Lactobacillus or Bifidobacterium species, although many other species exist. Different strains of the same species of probiotics can act in different ways, according to some research.

Some people take probiotics for maintaining everyday health. When using a probiotic for a specific health concern, people should speak to a healthcare professional about the best strategy. This is because clinical trials show that different probiotics and dosages are effective for different conditions and situations.

Read on to learn more about the best probiotics for specific uses.

People can take probiotics in supplement form or in the form of foods and beverages such as yogurt and drinks that have live cultures added to them.

Fermented foods also naturally contain beneficial bacteria. Examples of fermented foods include:

• miso
• natto
• tempeh
• sauerkraut
• kimchi
• kombucha

Probiotic manufacturers measure amounts of bacteria in colony-forming units (CFUs). Supplements can vary, with some commonly having a CFU of billions. Foods with added probiotics often contain lower numbers of bacteria. Generally, people might take higher-CFU products for specific conditions and a lower-CFU product for general health maintenance.

Some formulas may also contain prebiotics. Including prebiotics can be beneficial, as they provide substances for probiotics to digest. This process can produce short-chain fatty acids, which can help maintain the health of a person’s colon and immune system.

Some research indicates that in order for prebiotics to be effective, they must withstand food processing and reach the colon undigested. Some of the most common types of prebiotic in supplements and foods are called oligosaccharides.

Some people’s bodies can be sensitive to prebiotics, so taking too much of them can cause flatulence or diarrhea. Such people can obtain the health benefits of prebiotics by instead including plenty of fiber and resistant starch in their diets and eating foods such as garlic, onions, bananas, and oats.

A 2018 review states that the “strongest evidence in favor of probiotics lies in the prevention or treatment” of five conditions:

• necrotizing enterocolitis, an inflammatory condition that affects the bowel, mostly in premature infants
• acute infectious diarrhea
• acute respiratory tract infections
• antibiotic-associated diarrhea
• infant colic

The effects of probiotics may be different in males and females.

In a 2014 study, researchers gave males and females with obesity Lactobacillus rhamnosus. The results suggested that females lost weight and fat during the trial, and that they maintained the loss after study completion. The males in the study, however, did not.

A different study in people with Salmonella infection suggested that the effects of taking Lactobacillus plantarum varied according to sex. Although the study authors saw little evidence that this probiotic can treat Salmonella, they noted how males and females experienced the symptoms and clearance of the infection differently.

Scientists need to conduct more studies to establish whether or not probiotic therapy should be different depending on a person’s sex.

The bacteria present in a person’s gut can have an impact on their weight. Research from 2009 shows that people with obesity have lower bacterial diversity.

A small 2019 study found that people following a low calorie diet for 15 weeks had changes in their gut bacteria. They also had a reduction in bacteria associated with atherosclerosis, which occurs when the arteries become clogged with plaque.

The links between weight and gut bacteria could indicate that probiotics may be able to support weight loss.

Some experts suggest that supplementation with synbiotics, using strains such as Lactobacillus gasseri, cause weight reduction in large independent studies. Adding certain fibers may increase these effects.

A 2019 review looking at obesity and probiotics concluded that bacterial strains are associated with obesity, and that mixtures of probiotic strains may be more effective in treating obesity than single strains. In particular, Bifidobacterium may be effective for reducing body mass index (BMI).

Most research into this has used Lactobacillus and Bifidobacterium strains. However, there are still unanswered questions about specific strains, duration of treatment, and appropriate dosages.

One 2018 review concluded that probiotics may result in a reduction of weight and fat percentage. However, there have not been enough high quality studies to confirm that probiotics have this effect.

Probiotics can also be useful to help support a person’s gut health and relieve gut issues.

The gut’s lining can sometimes become damaged, which may lead to intestinal permeability, or “leaky gut.” This can allow molecules to get through into the bloodstream and cause inflammation or immune reactions.

Antibiotics can kill both bad and good bacteria. Sometimes, this can cause a person to experience symptoms of diarrhea when taking them. Taking probiotics at the same time, however, can help repopulate a person’s gut with beneficial bacteria and ease any symptoms of diarrhea.

In particular, the probiotic yeast Saccharomyces boulardiihas been effective in the prevention and treatment of diarrhea that occurs when taking antibiotics or following infection. The yeast may also be effective in treating irritable bowel syndrome and inflammatory bowel disease, but more research is needed to confirm this.

Research indicates that probiotics may reduce gut transit time — that is, the time it takes for food to pass as a bowel movement — by 12.4 hours and increase bowel movements by 1.3 per week.

The Bifidobacterium lactis strain seems most efficient in improving gut transit time, stool frequency and consistency, and flatulence. The researchers advise caution when interpreting these results, however, as there may be bias in some of the studies.

The gut and brain are connected and communicate with each other using the nervous system. One 2016 review suggests that probiotic strains can help combat mood and neurotransmitter conditions.

Several studies have used Bifidobacterium and Lactobacillus to improve the symptoms of anxiety, depression, and obsessive-compulsive disorder, as well as memory, according to the review.

In recent years, the extensive research into probiotics and the microbiome leaves no doubt that these novel products can benefit some aspects of human health.

Specifically, science has identified areas of use for diarrhea, respiratory conditions, and some infant illnesses and conditions. Probiotics may also help with weight management, gut issues and constipation, and mental health.

People can do their own research into which specific strains might work for their particular condition. A healthcare professional can help with this.

They can choose probiotics according to their CFU, the strains included, and whether or not the product also contains prebiotics. It can be more cost effective to choose a product with a lower CFU for general health maintenance.

People can choose from a wide range of probiotic brands online.

Those taking probiotics for the first time might want to monitor any effects and build up their dosage slowly, especially if the product contains prebiotics, which can cause excess gas.

People who have an immune condition or severe illness should speak to their doctor before taking probiotics, as they may not be suitable. Also, people with allergies or intolerances should be sure to read product labels carefully to avoid triggering a reaction.