Between the upcoming flu season, seasonal allergies, and the ongoing COVID-19 pandemic, there are lots of reasons to give your immune system a boost. For that big job, more people are turning to a tiny resource.

Probiotics are living microorganisms, and you already have some of them in your body. Others come from food, specifically fermented foods such as yogurt, according to the National Center for Complementary and Integrative Health (NCCIH).

Probiotics are sometimes called good bacteria: They can help your body digest food and even fight cells that cause disease. Probiotics have shown promise in helping ease diarrhea, ulcerative colitis, and periodontal disease, according to the NCCIH. Research about other benefits of these beneficial bacteria is ongoing, and one area of particular interest is how they impact immunity.

Do Probiotics Keep You From Getting Sick?

It makes sense that probiotics have proved effective in treatment of digestive diseases. More than half the cells that create antibodies for fighting sickness are in the intestines and other organs in the gut, according to InformedHealth.org. If you have healthy gut flora ?— the balance of helpful bacteria and other organisms in your intestines ?— you may be less likely to get sick.

Scientists are interested in probiotics’ potential to support immune function, but so far research has not produced clear evidence that probiotics are truly effective in preventing things like a cold or the flu. A review published in October 2020 in the journal Nutrients looked at the most recent research on probiotics and the immune system, specifically with regard to respiratory illness. The authors concluded that probiotic use is associated with a lower rate and duration of mild respiratory tract infections in children and adults, but again, more research is needed. There is also early evidence to suggest that probiotics might help prevent COVID-19 infection when used alongside other treatments, according to a study published in August 2021 in the Archives of Medical Research.

More research is needed to determine what specific kinds of bacteria may offer benefits, and in what amounts, as well as whether they will work the same for everyone the same way, says Libby Mills, a national spokesperson for the Academy of Nutrition and Dietetics and registered dietitian nutritionist near Philadelphia. “Everyone has different gut bacteria and a different diet,” Mills says. “What works for one person might not work for the next.” She points out that there are hundreds of species of bacteria and even more strains, and the benefits of each alone or in different combinations have not been thoroughly researched.

Which Probiotics Are Best for Your Immune System?

You might already be getting probiotics without realizing it. Most of the research on the benefits of probiotics have focused on these common strains of bacteria:

Acidophilus, which is found in yogurt, milk, miso, and tempeh, might support the immune system, according to a meta-analysis published in the journal Food Bioscience in August 2020.
Bifidobacterium is in yogurt, kefir, buttermilk, miso, tempeh, pickles, kimchi, cured meats, some wines and vinegars, sauerkraut, and sourdough bread. It may reduce inflammation, according to one meta-analysis in the journal Nutrients, from January 2017. And a study in the February 2017 Nutrients suggested it may support immune health in older adults, too.
All yogurt with live and active probiotics contains Lactobacillus delbrueckii subsp. bulgaricus (or L. bulgaricus) and S. thermophilus, according to the FDA, and these have both been associated with supporting the immune system.

Conversely, there are foods that can weaken your immune system and make you more susceptible to illness. Research published in the January 2018 issue of Oncotargetshows that regularly consuming alcohol, sugar, and ultra-processed foods can lead to cellular inflammation, your immune system’s response to potential harm. Inflammation is a natural process, but when it occurs inappropriately or for sustained amounts of time, it can trigger chronic diseases and conditions including depression, cancer, heart disease, and Alzheimer’s disease, according to Cleveland Clinic.

Are Probiotic Supplements Effective?

In addition to naturally being found in many foods, probiotics are sold as supplements and added to other foods where they don’t naturally occur, like orange juice and peanut butter. Mills says a balanced diet with foods that naturally feature probiotics may be the best bet.

“Getting probiotics from food is something most people can easily do and may have an edge over taking supplements,” Mills explains. “Food seems to have the fastest effect on changing the gut biome. Plus, the fermentation of the food may make other nutrients more bioavailable.”

Right now, probiotics’ efficacy is theoretical. Probiotics might have some effect on the immune system, whether it’s good, bad, or neutral to people avoiding diseases is unknown, says Pieter Cohen, MD, an associate professor of medicine at the Cambridge Health Alliance and Harvard Medical School in Cambridge, Massachusetts. There’s a growing body of evidence that probiotics may boost immunity, but that varies with the type of probiotics, a person’s health and other factors. They’re usually not necessary.

“If you take a healthy person, someone who doesn’t have any health issues to begin with, [they have] no need to take a probiotic supplement. There’s no evidence that that’ll improve your health,” says Dr. Cohen. If you’ve been a little unhealthy lately, probiotics can’t fill those gaps, he adds.

“We have no evidence that putting live bacteria in your body can make up for skipping a healthy meal, or a few extra hours of sleep,” says Cohen. The best way to keep your gut flora healthy is to eat a variety of healthy foods. “Food is good for the immune system,” he says. “If you don’t eat, you’re malnourished, and your immune system is going to be low.” He recommends home-cooked meals of fruits, vegetables, grains, and nuts. A study from the March 2020 issue of Nutrients indicates that fiber intake may be associated with an improved gut microbiome, too.

Why Should You Not Take Probiotics?

There’s really no reason to take an over-the-counter probiotic unless it's prescribed by your doctor for a specific ailment. There are risks: Starting probiotics or taking too many can cause an allergic reaction, gas, bloating, constipation, or diarrhea, according to Cleveland Clinic. If you have a compromised immune system, introducing new bacteria to your body could make it harder for your body to heal, and even make you sicker. Don’t take a probiotic unless it's recommended by your doctor.

Another potential concern about OTC probiotics? Regulation of their sale in the United States is convoluted. They can be classified as a food, a drug, or a supplement. Any probiotic supplement you buy at the store or online is not FDA-approved. Cohen noted the murky regulation, or lack thereof, in an article in the September 2018 JAMA Internal Medicine titled “Probiotic Safety ?— No Guarantees.”

How Long Does It Take Probiotics to Work for Your Immune System?

It’s tough to say exactly how quickly probiotics might start working, Mills says. Different strains of probiotics, the amount you take and the quality of the food or supplement will affect when ?— or if ?— you see results. More research may bring answers.

There’s reason to be hopeful. “There’s enough evidence to say probiotics may have some validity and it’s worth pursuing, both in research and in the foods that we choose in a balanced diet,” Mills says.