Probiotics are a big and rapidly growing business and are now among the most popular dietary supplements. The term probiotic refers to dietary supplements (tablets, capsules, powders, lozenges and gums) and foods (such as yogurt and other fermented products) that contain “beneficial” or “friendly” bacteria. The organisms themselves are also called probiotics.
They are promoted to improve digestion, strengthen immunity, help in weight loss and even protect against periodontal disease, among other proposed benefits, as well as for general health. Will the friendly bacteria in these supplements and foods keep you healthy?
Proponents claim that probiotics (meaning “for life,” as opposed to antibiotics) confer health benefits primarily by rebalancing the normal microflora in the large intestine (colon). There are many general types of bacteria used as probiotics (two common ones are Lactobacillus and Bifidobacterium), and many different species as well as strains within species. They have different physiological effects—and thus possibly different health benefits (as well as possible risks). Some yeasts, such as Saccharmyces, can also act as probiotics.
Probiotic products may contain a single strain or many strains, and the number of organisms in a daily dose can range from 1 billion to more than 250 billion. Many manufacturers boast that their products contain unique probiotics and/or combinations of strains that make them even better for you—and often charge a premium price.
Probiotics: what the studies show
The large intestine is home to hundreds of trillions of bacteria. Fortunately, most are neutral or even beneficial, performing many vital body functions. For example, they help keep “bad” bacteria at bay, play a role in immunity, help us digest food and absorb nutrients and may even have anticancer effects. But will consuming them as probiotics in foods or capsules make a notable difference to your health—especially if you are already healthy? Here’s a look at the evidence.
Digestive problems. The best evidence for probiotics is for reducing diarrhea, especially following antibiotic use. A 2010 review from the Cochrane Collaboration concluded that probiotics shorten episodes of acute infectious diarrhea. And in 2011, a Health Canada monograph stated that products containing certain probiotics (such as Lactobacillus rhamnosus GG) help manage acute infectious diarrhea and antibiotic-associated diarrhea. A 2012 research review in the Journal of the American Medical Association (JAMA) found that probiotics reduced the risk of antibiotic-associated diarrhea by 42 percent—but many of the studies had flaws, so these findings should be interpreted with caution. A 2013 Cochrane review of 23 trials also concluded that probiotics may be effective for preventing antibiotic-related diarrhea. However, two large, well-designed studies, in the Lancet in 2013 and the Annals of Internal Medicine in 2014, found that probiotics were no better than a placebo in preventing diarrhea in older people taking antibiotics. A review of 19 studies, published in Gastroenterology in 2017, found that probiotics reduce the risk of Clostridium difficile-related diarrhea in hospital patients, especially when the supplements were started during the first two days of antibiotic treatment.
A small Israeli study in the journal Cell in 2018 found that a probiotic supplement (11 strains, including Lactobacillus andBifidobacterium) taken after a course of antibiotics actually delayed the restoration of normal bacteria balance in the gut for as long as five months.
A systematic review of 14 clinical trials, published in the American Journal of Clinical Nutrition in 2014, found that certain strains of probiotics—Bifidobacterium lactis, in particular—can help with constipation. Overall, they improve gut transit time, stool frequency, and stool consistency.
Some studies suggest that certain probiotic strains may also help in mild to moderate ulcerative colitis and possibly for bloating and gas in irritable bowel syndrome (IBS). Probiotic supplements, on the whole, “improve global symptoms, bloating, and flatulence in IBS,” by modifying the gut microbiome, according to a 2014 monograph from the American College of Gastroenterology. As IBS researchers said in a paper in Gastroenterology & Hepatology in 2015, “The concept of manipulating the microbiome is one of the most promising new ways in which to treat patients with IBS, but there is still much to learn.”
One product that has some research behind it for inflammatory bowel conditions is VSL#3 (a combination of 8 different probiotic strains), to be used under a doctor’s supervision.But the studies are hard to evaluate overall because they’ve used different strains, formulas, and doses for varying durations, and no single type may work for everyone.
Immunity and colds/flu.There’s a close connection between the bacteria in your colon and the immune system. Several studies, including one in 2012 in the British Journal of Nutrition, have found that certain probiotic strains boost measures of immune response—but whether this translates into any clinical benefits is uncertain. Studies have been inconsistent, for example, as to whether probiotics will actually curb colds and other upper respiratory infections. In 2014 a review in the British Journal of Nutrition, which looked at 20 clinical trials, linked probiotics to shorter duration of colds, but not necessarily reduced incidence or severity. Similarly, a 2015 Cochrane review of 12 clinical trials concluded that certain probiotics may help prevent or shorten such infections, though the quality of the studies was poor.
In 2018, a review and meta-analysis of 15 clinical trials, published in Age and Ageing, concluded that probiotic supplements do not reduce the risk of infections in older people compared to a placebo. No safety problems were detected. The studies were judged to be of low-quality.
Oral health. An increasing number of probiotic lozenges and gums are promoted for oral health—to reduce periodontal disease, throat infections, and bad breath, for example. There’s preliminary evidence that certain strains may have some benefits, but commercial products may not have the same strains and formulations as those tested in published studies.
Blood pressure. A review of nine clinical trials, in the journal Hypertension in 2014, concluded that probiotics can lower blood pressure modestly, with greater effects seen with higher blood pressure, use of multiple species, longer daily use, and higher doses.
Other uses. Many other claims are made for probiotics—that they lower cholesterol, alleviate allergic skin conditions (like eczema), treat ulcers and urinary tract infections, improve vaginal health, reduce the risk of colon cancer, ease anxiety and depression, and ward off traveler’s diarrhea. Good evidence to support these claims is lacking. Research on probiotics for weight loss has yielded inconsistent results, and even studies with positive results have mostly found very small benefits, as was seen in an analysis of 15 clinical trials in Obesity Reviews in 2018.
Until recently, testing of probiotics by ConsumerLab.com found that some products had far lower amounts of live organisms than claimed on the labels. But in 2018, it found that 17 out of 18 products tested contained the amounts listed; none exceeded limits for heavy metal contaminants. In contrast, in 2016, a study in Pediatric Research found that only one out of 16 Bifidobacterium longum supplements tested contained the species named on the label.
Side effects: Probiotics are considered safe overall for healthy people; short-term side effects may include mild gas and bloating. But risks may be greater in immunocompromised people. And a systematic review in the Annals of Internal Medicine in 2018 found that the reporting of adverse effects is often missing or inadequate. Also in 2018, an editorial in JAMA Internal Medicine by Dr. Pieter Cohen, a well-known critic of the supplements industry, called for the FDA to improve its regulatory standards for probiotics to match those of Canada and the EU.
Keep in mind that if probiotics do have physiological effects, these may not always be good effects. In some people, probiotics may, at least in theory, overstimulate the immune system or adversely affect metabolic pathways.
If you are immunocompromised, have certain bowel problems or are seriously ill in other ways, avoid probiotics unless your doctor has okayed their use. Probiotics should be used cautiously by pregnant women, infants and young children and never given to premature infants.
Bottom line: Probiotics are a promising field of research and may one day be used to treat or help prevent many disorders. But there’s not enough solid evidence to recommend their widespread use. Vague claims that probiotics "support good digestive health" are meaningless. Larger, longer and better studies are needed to test specific strains for specific conditions and to determine the proper doses and regimens.
Originally published March 2014. Updated September 28, 2018.
Published September 28, 2018