The bacteria in your body are said to outnumber your body’s cells at a 10-to-1 ratio. However, a recent study says that the ratio is closer to 1-to-1 (1, 2).

According to these estimates, you have 39–300 trillion bacteria living inside you. Whichever estimate is most accurate, it’s certainly a large number.

Much of these bacteria reside in your gut, and the majority are quite harmless. Some are helpful, and a small number can cause disease (3).

Having the right gut bacteria has been linked to numerous health benefits, including the following (4, 5):

  • weight loss
  • improved digestion
  • enhanced immune function
  • healthier skin
  • reduced risk of some diseases

Probiotics, which are a certain type of friendly bacteria, provide health benefits when eaten.

They’re often taken as supplements that are supposed to help colonize your gut with good microorganisms.

This article examines the health benefits of probiotics.

Probiotics are living microorganisms that, when ingested, provide a health benefit (6).

However, the scientific community often disagrees on what the benefits are, as well as which strains of bacteria are responsible (7).

Probiotics are usually bacteria, but certain types of yeasts can also function as probiotics. There are also other microorganisms in the gut that are being studied, including viruses, fungi, archaea, and helminths (8).

You can get probiotics from supplements, as well as from foods prepared by bacterial fermentation.

Probiotic foods include yogurt, kefir, sauerkraut, tempeh, and kimchi. Probiotics should not be confused with prebiotics, which are carbs — often dietary fibers — that help feed the friendly bacteria already in your gut (9).

Products that contain both prebiotics and probiotics are referred to as synbiotics. Synbiotic products usually combine friendly bacteria along with some food for the bacteria to eat (the prebiotics), all in one supplement (10).

The most common probiotic bacteria are Lactobacillus and Bifidobacteria. Other common kinds are Saccharomyces, Streptococcus, Enterococcus, Escherichia, and Bacillus.

Each genus comprises different species, and each species has many strains. On labels, you’ll see probiotics identified by their specific strain (which includes the genus), the species, subspecies if there is one, and a letter-number strain code (11).

Different probiotics have been found to address different health conditions. Therefore, choosing the right type — or types — of probiotics is essential.

Some supplements, known as broad-spectrum probiotics or multi-probiotics, combine different species in the same product.

Although the evidence is promising, more research is needed on the health benefits of probiotics. Some researchers warn about possible negative effects from the “dark side” of probiotics and call for caution and strict regulation (12, 13).

Summary

Probiotics are living microorganisms that boost health when consumed in adequate amounts. There are many different types, and you can obtain them from foods or supplements.

The complex community of microorganisms in your gut is called the gut flora, gut microbiota, or gut microbiome (14, 15).

The gut microbiota includes bacteria, viruses, fungi, archaea, and helminths — with bacteria comprising the vast majority. Your gut is home to a complex eco-system of 300–500 bacterial species (16).

Most of the gut flora is found in your colon, or large intestine, which is the last part of your digestive tract.

Surprisingly, the metabolic activities of your gut flora resemble those of an organ. For this reason, some scientists refer to the gut flora as the “forgotten organ” (17).

Your gut flora performs many important health functions. It manufactures vitamins, including vitamin K and some of the B vitamins (18).

It also turns fibers into short-chain fats like butyrate, propionate, and acetate, which feed your gut wall and perform many metabolic functions (19, 20).

These fats also stimulate your immune system and strengthen your gut wall. This can help prevent unwanted substances from entering your body and provoking an immune response (21, 22).

Your gut flora is highly sensitive to your diet, and studies show that an unbalanced gut flora is linked to numerous diseases (23, 24).

These diseases are thought to include obesity, type 2 diabetes, metabolic syndrome, heart disease, colorectal cancer, Alzheimer’s, and depression (25, 26, 27, 28).

Probiotics and prebiotic fibers can help correct this balance, ensuring that your “forgotten organ” is functioning optimally (29).

Summary

Your gut flora consists of hundreds of types of microorganisms. These microorganisms perform numerous important bodily functions.

Probiotics are widely researched for their effects on digestive health (30).

Evidence suggests that probiotic supplements can help cure antibiotic-associated diarrhea (31).

When people take antibiotics, especially for long periods of time, they often experience diarrhea — even long after the infection has been eradicated.

This is because the antibiotics kill many of the natural bacteria in your gut, which shifts the gut balance and allows harmful bacteria to thrive.

Probiotics may also help combat irritable bowel syndrome (IBS), a common digestive disorder, reducing gas, bloating, constipation, diarrhea, and other symptoms.

Research regarding the effectiveness of probiotics supplementation for the treatment of IBS is mixed. A recent review reported that seven of the studies indicated IBS improvement with probiotic supplementation, but four did not (32).

Research indicates that multi-strain probiotic supplements seem to bring most IBS improvement, especially when taken for longer than 8 weeks.

However, much remains unknown about probiotic treatment for IBS. Questions like the following have yet to be answered (33):

  • Which IBS symptoms improve with probiotics?
  • Which probiotics, or probiotic mixtures, are most effective?
  • What dosages and durations of probiotic treatments are best?
  • Do different types of IBS require different probiotic treatments?

Researchers find early results of IBS probiotic treatment promising but say additional large trials are necessary before healthcare providers can confidently prescribe probiotic treatments consistently for IBS (34).

Some studies also note benefits of probiotic supplementation against inflammatory bowel diseases, such as Crohn’s disease and ulcerative colitis. Again, researchers say further research is needed before the treatment is confirmed to be effective (35).

Probiotics may also help fight Helicobacter pylori infections, which are one of the main drivers of ulcers and stomach cancer (36, 37, 38).

If you currently have digestive problems that you can’t seem to vanquish, a probiotic supplement may be something to consider. However, be sure to consult your healthcare provider first.

Summary

Probiotics may be effective against various digestive problems, including antibiotic-associated diarrhea and IBS.

Some research indicates that people with obesity have different gut bacteria than those who are lean (39).

Research shows a connection between gut microbes and obesity in both infants and adults. It also shows that microbial changes in the gut are a factor in developing obesity as an adult (40).

Therefore, many scientists believe that your gut bacteria are important in determining body weight (41, 42).

While more research is needed, some probiotic strains appear to aid weight loss (43).

Nevertheless, researchers advise caution in rushing to this conclusion, noting that there are still many unknowns.

These unknowns include (44):

  • the specific strains of probiotics to be used
  • the dosage and duration of treatment
  • the long-term effects of treatment
  • the interaction of age, gender, health conditions, and lifestyle

In one study, 210 people with central obesity, which is characterized by excess belly fat, took the probiotic Lactobacillus gasseri daily. Participants lost an average of approximately 8.5 % of their belly fat over 12 weeks (45).

When participants stopped taking the probiotic, they gained the belly fat back within 4 weeks.

Evidence also suggests that Lactobacillus rhamnosus and Bifidobacterium lactis can aid weight loss and help prevent obesity — though more research is needed (46).

Summary

Though more research is necessary, some evidence suggests that certain probiotic strains can aid weight loss.

In the past decade, research has shown that the gut and brain are connected in a system called the gut-brain axis. This axis links the body’s central and enteric nervous systems, the latter of which governs digestion (47).

Some research shows that certain microbes in the gut can affect your brain via this axis in both health and disease. These bacteria are part of an emerging field called “psychobiotics” (48, 49, 50).

Research indicates that psychobiotics can help treat cognitive and neurological disorders, such as autism, Alzheimer’s disease, and Parkinson’s disease (51).

Which microbes these are and how they interact with the brain is the subject of much current research (52).

Some researchers suggest that, for some people, supplementing with certain strains of probiotics may be preferable to taking psychotropic drugs to cope with the mental stress, loneliness, and grief accompanying the current COVID-19 pandemic (53).

Summary

Early research holds promise that certain gut bacteria called psychobiotics may be able to help treat cognitive and neurological disorders, such as autism, Alzheimer’s disease, and Parkinson’s disease.

There are many other benefits of probiotics. They may help with the following conditions:

  • Inflammation: Probiotics reduce systemic inflammation, a leading driver of many diseases (54).
  • Depression and anxiety: The probiotic strains Lactobacillus helveticus and Bifidobacterium longum have been shown to reduce symptoms of anxiety and depression in people with clinical depression (55).
  • Blood cholesterol: Several probiotics have been shown to lower total and LDL (bad) cholesterol levels, although the research remains controversial (56).
  • Blood pressure: Probiotics may also cause modest reductions in blood pressure (57).
  • Immune function: Several probiotic strains may enhance immune function, possibly leading to a reduced risk of infections, including those that cause the common cold (58, 59).
  • Skin health: There’s some evidence that probiotics can be useful for acne, rosacea, and eczema, as well as other skin disorders (60).
  • Anti-aging. Though research is extremely limited, there’s evidence that probiotics have the potential to extend lifespan by increasing the ability of cells to replicate themselves (61).

This is only a small slice of probiotics’ benefits, as ongoing studies indicate a wide breadth of potential health effects.

Summary

In addition to their potential effects on weight loss, digestion, and neurological disorders, probiotics may improve heart health, immune function, and symptoms of depression and anxiety.

Some researchers propose that improving the gut microbiome via probiotic supplementation and diet may be a strategy to fight and treat an infection with the novel coronavirus SARS-CoV-2. This infection can cause COVID-19, which stands for coronavirus disease 2019 (62).

COVID-19 is known to damage the body’s immune defense through a “cytokine storm” of excessive inflammatory cytokines. This is believed to be the main cause of deteriorating health and even death (63).

Because intestinal flora have been shown to strengthen the immune system and fight inflammation, researchers think probiotic supplements may help speed recovery from coronavirus by inhibiting or limiting this “cytokine storm” (64).

Also, people with COVID-19 have reported gastrointestinal symptoms, such as diarrhea, nausea, vomiting, abdominal pain, and loss of appetite (65).

Some researchers theorize that probiotics could help prevent the coronavirus by blocking the angiotensin-converting enzyme (ACE) receptor where the SARS-CoV-2 pathogen enters the body to invade gastrointestinal cells (66).

Another proposed link between COVID-19 and probiotics involves what is called the “gut-lung axis.” This is a system of communication and interaction between the gut and lung tissues, which occurs via microorganisms of the human microbiome (67).

Imbalances of the intestinal flora are known to be related to lung diseases and respiratory tract infections. Researchers suggest that correcting those imbalances may promote optimum lung health, which might help guard against pathogens like SARS-CoV-2 (68, 69).

Other research suggests that probiotic supplementation may promote antiviral activity in general to improve immune, pulmonary, and anti-inflammatory response that might help clear the SARS-CoV-2 infection (70, 71).

All these hypotheses are at the theoretical stage. Researchers say further studies are needed to confirm them.

One study advises caution, suggesting that not all probiotic strains will exert the same effects. It questions whether probiotic supplementation can alter the content of the gut microbiome enough to combat COVID-19 (72).

Summary

Some current research proposes that improving the gut microbiome through probiotic supplementation and diet may help treat the SARS-CoV-2 infection that causes COVID-19. Research is preliminary, and much more data and clinical trials are needed.

Probiotics are generally well tolerated and considered safe for most people. However, regulations differ among probiotics, so you must be cautious when choosing a product.

Choosing probiotics

As you face the large selection of probiotics now available, you may feel overwhelmed. You’re not alone. The choice can be difficult.

In the United States, probiotics are generally sold as food ingredients, drugs, or dietary supplements. While the Food and Drug Administration (FDA) regulates each of these in different ways, most foods and supplements do not require approval before marketing (73).

As a result, some companies take advantage of the buzz around probiotics to sell supplements they label as probiotic and make claims that aren’t backed by evidence (74).

Regulations on probiotics vary greatly around the world, so ordering online from other countries is risky. Unregulated food items, cosmetics, and supplements are easy to find abroad, but their safety is unconfirmed.

Looking for companies that follow best practices, such as third-party testing, can help you find high quality supplements.

The best choice is to run your choice by your healthcare provider or ask for suggestions. They may be able to recommend products they know to be safe and effective.

Probiotics side effects

In the first few days of taking a probiotic supplement, you may experience side effects related to digestion, such as gas and mild abdominal discomfort (75).

However, after you adjust, your digestion should begin to improve.

In people with compromised immune systems, including those with HIV, AIDS, and several other conditions, probiotics can lead to dangerous infections (76).

If you have a medical condition, consult your healthcare provider before taking a probiotic supplement.

Summary

Probiotic supplements may cause digestive symptoms, which should subside within a few days. They may also be dangerous for people with certain medical conditions.

Science has made tremendous strides in the past two decades toward understanding the roles that probiotics play in human health and disease. Still, probiotic research is in its infancy, and there’s much to learn.

Despite widespread studies, researchers are still working on identifying all the individual microbial species that live in your gut. Identifying them is crucial to understanding how they function in human health.

For example, researchers in 2019 reported identifying nearly 2,000 previously unknown gut bacterial species. This was a major step toward categorizing the microbes living in the human gut (77).

After identifying the microorganisms, the next challenge facing researchers is to associate the various species, subspecies, and strains of microbes with their effects on human health — and this is where things get tricky (78).

Though thousands of studies have assessed the health benefits of probiotics for many clinical conditions, the results often contradict each other (79, 80).

One reason for this is that methods for processing and analyzing probiotics data are not consistent around the world. This leads to conflicting research analyses of published data (81).

The standardization of probiotics research is challenging because the human body contains a large and diverse set of microorganisms, which varies among countries — and even among individuals in the same country.

Plus, the bacterial strains themselves are constantly evolving, as are the health and environments of their human hosts.

Probiotics researchers face the task of classifying what might be trillions of ever-changing organisms in varied and evolving environments.

It’s only the development of computational analysis of the genomes of collective groups of microbes (called metagenomics) in the past two decades that makes this Herculean task possible at all (82).

Scientists must standardize a mountain of sometimes conflicting evidence from thousands of studies, and then translate that evidence into clear recommendations for therapeutic probiotic use (83).

Summary

Researchers are challenged with identifying all the microbiota in the vast and changing microbiome of the human gut. They also need to establish a system for standardizing research results to develop clear therapeutic recommendations for probiotic uses.

Maintaining a healthy gut is about more than taking a probiotic supplement.

Day-to-day diet and exercise are just as important, as many lifestyle factors affect your gut bacteria.

However, probiotic supplements may offer a wide range of benefits with few side effects. As such, if you’re interested in improving your gut health, they could be worth a try.

Be sure to talk to your healthcare provider first to make sure you’re trying the right strains in the right amounts and avoid any side effects.

Maybe you’re wondering whether a specific condition you have could benefit from probiotics. If so, you might want to consult the World Gastroenterology Organization Global Guidelines. It lists probiotics, conditions, and recommended dosages.

Caution is always advised when starting with probiotics. Be sure to use a reputable product, start slowly, and get good advice from a trusted healthcare practitioner.