The term "probiotic" is misused so often that a group of experts has taken a fresh look at what probiotics really are, and examined what scientists have learned about them in recent years.
Probiotics are generally thought of as the "good" bacteria in the body. Experts define probiotics as live microorganisms which confer health benefits when present in adequate amounts. Much of the current research on probiotics is focused on examining which bacterial species may have health benefits, and what those benefits may be.
But there's still enough confusion surrounding the concept of probiotics and what they can offer that some governments do not even allow the term to appear on product labels, to protect consumers from unfounded health claims.
In a new consensus statement, an international group of researchers reviewed the scientific evidence and concluded that for most strains of well-studied probiotics, the evidence has grown enough to say probiotics support digestive health.
"When you look at findings from a variety of studies on different end points that look at some aspect of digestive health, the combined evidence suggests that digestive health is a core benefit of many probiotic species," said Mary Ellen Sanders, a food science researcher and consultant who was one of the experts on the panel, as well as executive science officer for the organization that convened the panel. [5 Ways Gut Bacteria Affect Your Health]
However, the evidence for whether probiotics have health effects on other systems in the body wasn't as strong — the scientific findings were either not as convincing or too strain-specific to be considered core benefits of probiotics in general, Sanders said. For example, many studies have shown that different probiotics can impact immune function in different ways. But because the effects of probiotic strains can vary broadly, and immune-system effects are incredibly diverse, the panel decided that an immune-system boost should not be considered a core benefit of probiotics, Sanders said.
The panel of researchers, convened by an industry group called the International Scientific Association for Probiotics and Prebiotics (ISAPP), published their findings (opens in new tab) this week in the journal Nature Reviews Gastroenterology & Hepatology.
What is not a probiotic?
The researchers recommended that even if a bacterial species has the potential to offer health benefits, it shouldn't be called a probiotic until the effects are demonstrated in studies. This includes undefined bacteria present in fermented foods.
For example, there may be numerous species of bacteria in blue cheese, or kefir (a fermented milk drink), but it is more appropriate to call these products "sources of live cultures" rather than sources of probiotics, the researchers said.
"There's much folklore around the health benefits of fermented food," Sanders told Live Science.
For example, aged cheeses such as blue cheese contain a mix of bacteria. And although cheese makers likely use "defined" cultures of bacteria that include a known mix of species, the microbial content of any given cheese isn't listed on the food's label and, in fact, varies among samples of cheese, she said. "Blue cheese may have lots of really neat microbes in it, and it might be source of live microbes, but you can't really call it a probiotic until some research shows that there is a benefit associated with it," she said.
"We thought that the term 'probiotics' had to be reserved for the microbes associated with fermented products that have been defined, and have been shown to have some health effect," Sanders said.
The researchers also said that fecal microbiota transplants — procedures that involve transferring fecal matter from a healthy person into an infected person, to help patients suffering from hard-to-treat gut infections with the bacteria Clostridium difficile — shouldn't be considered probiotics, because they involve an undefined mixture of microbes.
Does yogurt contain probiotics?
The standard bacteria used for making yogurt are well studied, and they are considered to be probiotics, especially because they help lactose-intolerant people digest yogurt, the panelists said.
However, these bacteria — called Lactobacillus delbrueckii subspecies bulgaricus and Streptococcus thermophilus — don't make it through the intestines alive, Sanders said. That's why many yogurt producers add other probiotics that can survive in the gut.
"When you pick up your yogurt, and it says it contains Lactobacilli and Bifidobacteria in addition to the yogurt cultures, you're getting more bang for your buck," Sanders said. "Those organisms are added because they actually have the benefit of surviving the intestinal transit, so they can make it into your colon."
How much probiotics should be in products?
For probiotics to actually have a health benefit, it is important for them to be ingested in sufficient amounts. In most countries, including the United States, regulations do not require manufacturers to include the amounts on the label. But in Canada and Italy, foods labeled as "probiotic" are required to contain at least 1 billion colony-forming units (CFUs) of probiotics per serving. (Measuring CFUs is a way to assess the living bacteria within a sample, rather than including the dead cells.)
"We believe that a minimum level is reasonable to expect," Sanders said. "You can't do a fairy-dust approach here; it has to be in suitable quantity to have health benefits."
Some products cannot have probiotics
The researchers on the panel noted that the term "probiotics" has been misused on products such as mattresses, shampoos, disinfectants and aftershave, probably as a marketing effort that's not based on any scientific finding.
"The concept of probiotics requires that the organism be alive, and be substantiated to have a health benefit," Sanders said. "But organisms are probably not surviving in a shampoo. And I'm not aware of any evidence that delivery in a shampoo confers a health benefit."