Diet & Nutrition

Probiotics 101: A Complete Scientific Guide

Angela Lemond, RDN
Leslie Barrie
By  and Medically Reviewed by 
Reviewed:
Medically Reviewed
Foods high in probiotics pickles fermented kimchi cabbage miso yogurt
Fermented foods are a common source of probiotics.iStock
Trillions of good-for-you microorganisms reside in your digestive tract, helping you function at your best every day.

Called probiotics, these microorganisms play a role in helping you digest food, fend off diseases, and even create vitamins.

They’re closely related to a term you might be familiar with — the microbiome, which refers to the mysterious microscopic world in your gut that scientists are rigorously studying. Each day, they tie new health effects to this environment of good and bad bacteria, helping fuel what is now a more than $35 billion industry.

Scientists’ fascination with the microbiome is jump-starting an even newer scientific field called pharmabiotics, which refers to the science of using microbiome-based medical products to influence human health.

Although probiotics may seem like a relatively new area of nutrition, scientists discovered them in the early 1900s. Elie Metchnikoff, a researcher many consider to be the “father of probiotics,” observed health benefits from the repeated ingestion of lactic-acid-producing bacteria. Since then, there have been more than 20,000 studies in the area of probiotics and their health implications.

Common Questions & Answers

What are probiotics good for?
Probiotics may be good for issues like diarrhea, irritable bowel syndrome, gingivitis, and pregnancy nausea. Probiotics may lower your risk of certain cancers and COVID-19, and could boost your immune system, prevent food allergies, and aid weight loss.
What happens when you start taking probiotics?
When you start to eat probiotic-rich foods like yogurt or take supplements, you may experience a mild upset stomach or gas within the first few days. Still, probiotics are considered generally safe for healthy people because they are found naturally in your body.
Is it okay to take a probiotic every day?
Probiotic-rich foods can be eaten daily — such as yogurt for breakfast or sauerkraut with dinner. Probiotic supplements are typically considered safe (when used as directed, which may be daily). But the FDA doesn’t regulate probiotic supplements, so talk to your healthcare team before taking them.
Who should not take probiotics?
People with compromised immune systems or serious illness, including premature infants, are at the highest risk for adverse effects with probiotics because they already have a high risk of infections. If you fall into one of these groups, consult your healthcare provider before trying one.
What are the conditions that may benefit from probiotics?
Probiotics may help with problems like diarrhea, irritable bowel syndrome, gingivitis, and pregnancy nausea. So if you are currently dealing with one of those issues, consider talking to your doctor about whether a probiotic is right for you.

What Do Probiotics Do Exactly?

There’s still a lot that scientists don’t understand about probiotics, but studies so far have identified a few functions of probiotics, including:

  • Promoting the health and maintenance of our digestive tract’s cell lining
  • Supporting immunity
  • Managing inflammation
With these findings, researchers are beginning to conduct more expansive studies on the microbiome. They suspect that if probiotics play definitive roles in these body functions, they may also play a role in diseases and conditions that affect digestive health, immunity, and systemic inflammation.

Potential Health Benefits of Probiotics

These living nutrients are the foundation of our health. A healthy microbiome may prevent and treat diseases in the areas of digestive health, yeast infections, oral disease, food allergies, and eczema.

To reap these touted benefits, you may be interested in a quick fix, by way of taking a probiotic supplement (more on when to take one — and the potential risks — later). But first, know that probiotic supplements probably affect everyone differently, and for older people in particular, their touted benefits need more research.

Indeed, the quality of the research on how beneficial probiotics are varies depending on the disease in question.

Improved Digestive Health

Probiotics, whether by way of food or supplements, may help reduce diarrhea caused by things like antibiotic use, cancer therapy, and hospital infections. Bacteria strains of Streptococcus and Lactobacillus can help, but doctors also use yeast strains, such as various Saccharomyces boulardii, to prevent diarrhea.

If dairy gives you bad gas, you may have lactose intolerance, which is caused by a deficiency in the enzyme lactase.

The good news is that probiotics seem to help with lactose digestion. Studies show that the probiotics used to make yogurt (Lactobacillus delbrueckii and Streptococcus thermophilus) release lactase, which takes over the body’s usual responsibility of digesting lactose.

If your gassiness isn’t due to lactose intolerance or associated with recurrent diarrhea from any of the aforementioned reasons, you may have a condition called irritable bowel syndrome (IBS). IBS is a diagnosis that is used for unexplained digestive symptoms that last for at least three months, such as gas, bloating, abdominal pain, diarrhea, and constipation. Some studies suggest probiotics can help relieve symptoms of IBS.

Don’t confuse IBS with a more serious digestive-tract disease called inflammatory bowel disease (IBD). IBD is an autoimmune disease categorized by chronic inflammation.

Crohn’s disease (CD), ulcerative colitis (UC), and indeterminate colitis (IC) are three types of IBD. Clinical trials suggest probiotics may be a promising therapy for UC.

Unfortunately, the same therapeutic effect was not found with CD and IC.

Also, if you’re pregnant, here’s some good news: One study found that taking probiotic supplements (namely Lactobacillus) while pregnant may help reduce nausea, vomiting, and constipation.

In addition, probiotic supplements may ease depression, a review of research found. While the authors aren’t exactly sure why, one theory is that because probiotics can improve conditions like IBS, they may in turn help improve one’s mental health.

Reduced Cancer Risk

Much of the research on the potential effects of probiotics on cancer and immunity has involved lab and animal studies on lactic acid bacteria (LAB), which can be found in dairy products and certain supplements. Put differently, the perks here require lots more study, especially in humans. In the existing studies, this probiotic strain seems to decrease the enzyme activity of other bacteria that produce cancer cells, potentially reducing the risk of liver, colon, and bladder cancer. Time will tell what implications this will have for people’s health.

Prevention of Allergies and Digestive Disorders

Probiotics and prebiotics in general seem to influence an entire immunological network in the body, and tend to have the biggest potential early in life.

If a mother consumes probiotic-rich food while pregnant, for example, she may reduce the child’s risk of allergy symptoms, such as skin rashes, nasal congestion, and watery eyes. Certain strains may also decrease the incidence of chronic digestive disorders like celiac disease, Crohn’s disease, and ulcerative colitis.

Lower Risk of COVID-19

Taking probiotic supplements may lower your odds of getting COVID-19, various studies have shown, though the studies were observational, so more rigorous ones are needed. For example, one study found an association between multivitamins, probiotics, vitamin D or omega-3 fatty acid supplements and a lower likelihood of testing positive for the virus.

Better Oral Health

Probiotics may benefit the digestive tract from beginning to end, though more studies are needed. The perks may start in your mouth, where harmful bacteria, also known as plaque, may be decreased by — yes, you guessed it — probiotics. In a randomized controlled trial, researchers divided 90 children ages 13 to 15 into three groups: One received a mouth disinfectant, one received a probiotic mouth rinse, not swallowed, and one received a placebo. The probiotic mouth rinse contained Lactobacillus acidophilus, Lactobacillus rhamnosus, Bifidobacterium longum, and Saccharomyces boulardii. After two months, the group that received the probiotic had the greatest reductions in plaque volume and gingivitis risk.

Probiotics may also help prevent cavities, because plaque (a buildup of certain bacteria, particularly streptococci, on the surface of the tooth) is what causes a cavity. Probiotics help protect the teeth by lowering the pH in the mouth and making the environment less favorable for these harmful bacteria.

A lower pH (more acidic) environment full of certain helpful bacteria is protective in addition to good oral hygiene and avoiding sugar.

Reduced Risk of Food Allergies

Probiotics may also help prevent food allergies, though more research is needed. One review suggested probiotics may help prevent eczema (a risk factor for food allergies) in children when used by pregnant or breastfeeding mothers or when given to infants.

It's important to note, though, that infant eczema doesn’t always signal a food allergy, and could be triggered by dry skin or a seasonal allergy.

Fermented dairy products are high in probiotics but are also one of the major food allergens. That means some of the very foods that help prevent this food allergy are off-limits for those who already have the issue. The good news is if you have a food allergy, you can choose probiotic sources that are dairy-free or nondairy fermented foods. Examples of nondairy probiotics include kimchi, kombucha, sauerkraut, kefir (when made with nondairy milk), and tempeh.

Improved Weight Loss Success

Another growing area of research on probiotics is weight loss.

For example, one review suggested that lowering inflammation by way of improving gut bacteria may combat insulin resistance, the hallmark of type 2 diabetes, and fat accumulation.

Also, a systematic review and meta-analysis found that using a probiotic supplement was associated with decreases in body mass index (BMI), weight, and fat mass with a probiotic dose of at least 30 billion for greater than 12 weeks.

The authors stated that the amounts, type, and duration of the probiotics require further study, because these measures weren’t consistent across every study. Probiotics and prebiotics showed a significant decrease, while synbiotics did not, more due to the lack of studies, and the large differences between those studies negatively impacted the analysis. Yet the authors concluded that these dietary agents are essential tools in treating obesity (more on how these differ later).

What Are the Best Food Sources of Probiotics?

Certain foods naturally contain healthy bacteria that support a healthy microbiome partly by a process called lactic acid fermentation. This occurs when bacteria converts sugar or other carbohydrates to lactic acid, which then increases the bacterial count. Beer and sourdough bread are fermented, but these foods do not contain the live beneficial bacteria we call probiotics. Foods that contain probiotics include:

  • Yogurt
  • Kefir
  • Fermented cheeses, such as buttermilk cheese and cottage cheese
  • Kimchi
  • Raw sauerkraut
  • Tempeh
  • Miso
  • Kombucha
  • Natto
  • Lacto-fermented vegetables, such as pickles

Should I Take a Probiotic Supplement?

In general, the body benefits most from probiotics it receives via food, which often also contains beneficial nutrients. That said, probiotic supplements may be helpful for certain people, including the aforementioned groups.

Probiotics may also benefit people on antibiotics because they may reduce unwanted symptoms, such as diarrhea, by replenishing the good bacteria killed by the antibiotic.

Nearly all antibiotics have the potential to cause antibiotic-associated diarrhea, but especially:

Ask your healthcare team before you take a probiotic, especially if you’re currently managing a health condition or taking medication.

Probiotic Supplement Side Effects

Though probiotic supplements are generally safe, they can pose health risks for certain people.

If you have a disease or condition that has weakened the immune system and you are sick, the risk of infection may be higher when taking probiotics.

How to Choose a Probiotic Supplement

As for the best single probiotic strain for human health, researchers haven’t identified one. Most of the studies have been done on bacterial strains of Lactobacillus and Bifidobacterium and the yeast strain Saccharomyces boulardii. It’s also likely that certain strains are better for certain conditions.

If you’re using probiotics to treat a disease or illness, a supplement that contains a higher number of strains may be more effective, one review found.

Here are some other things to consider when choosing a probiotic supplement that is right for you.

Be choosy. In particular, work with your healthcare team to identify and buy the best probiotic supplement for you. Certain strains may be best for certain health concerns. For instance, Lactobacillus rhamnosus GG can help with diarrhea, while a combination of Bifidobacteria and Lactobacillus may be helpful for IBS.

Read the label. Make sure any probiotic you choose is free of any ingredients you do not want, such as food allergens. Your healthcare team can help you translate any terms you don’t understand.

Practice caution. Understand that the U.S. Food and Drug Administration (FDA) doesn’t review dietary supplement products for safety and effectiveness before they’re marketed. Therefore, consider following the FDA’s tips for dietary supplement users. Again, consult your healthcare team before adding a probiotic supplement to your diet — especially if you have an underlying condition.

When to Take a Probiotic Supplement

Researchers are still determining the best time to take probiotics, and there are no universal guidelines on this matter.

For now, going by past research, you may reap the most benefits from probiotics if you take them 30 minutes before a meal or while eating. Also important: Take them with a food or drink that contains some amount of fat, because probiotics are fat soluble, meaning fat helps the body properly absorb them.

Probiotic Dosage

Dosing recommendations vary depending on whether you’re taking probiotic supplements for general health or if you are taking them to help with a specific health issue. For general health, one to two million colony-forming units (CFUs) is a reasonable dosage. Most studies for the treatment of IBS or antibiotic-induced and infectious diarrhea include dosages for children starting at 5 billion CFUs per day and more than 10 billion CFUs per day for adults.

If eaten in food, the risks of probiotic overdose are low. With supplements, it’s a different story. Some strains could potentially overgrow and cause infections, so it’s important that you discuss your specific probiotic supplements with your healthcare team before you take any if you are being treated for a current infection. Bacterial overgrowth is more of a risk if you have an immune system disorder (such as cancer and autoimmune diseases), are a premature infant, or have cardiac valve disease, short bowel syndrome, or central venous catheters, according to past research.

Probiotics, Prebiotics, and Synbiotics: There’s a Difference

As mentioned, probiotics are supplements or foods containing live microorganisms in significant enough numbers to produce health benefits to the host beyond basic nutrition.

On the other hand, a prebiotic is a nondigestible food ingredient that promotes the growth or activity of bacteria in the colon. In other words, a prebiotic promotes the growth of a probiotic. It’s easier to remember the difference when you think of the word prebiotic having the prefix “pre,” which means “before.” Dietary fiber is a prebiotic. Other prebiotics are inulin and fructooligosaccharides.

Synbiotics are simply products that contain both prebiotics and probiotics. The prefix “syn” is from the word “synergy” because the prebiotic works in a favorable way to produce the probiotic. One example would be Lactobacilli strains promoting the production of Bifidobacteria in the gut, which both have immune system benefits.

How to Store Probiotic Supplements

Keep in mind that probiotic supplements are sensitive to heat and moisture. Yet many of those in pill form are freeze-dried, so they do not require refrigeration. Supplement companies have to be careful to avoid high heat that might kill bacteria during processing and shipping. For optimal effectiveness, don't take the supplement with extra-hot foods or beverages.

Summary

The probiotics that are part of our natural microbiome play an essential part in our health, and may play a role in fending off certain health conditions as well as food allergies. While eating foods high in probiotics is the best way to reap these benefits, you may be curious about a probiotic supplement. Probiotic supplements are considered safe for most people, but may be risky for those with severe illnesses and individuals with compromised immune systems. Chat with your healthcare team before adding one to your wellness regimen.

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