These live microorganisms are akin to the valuable microorganisms already residing in our bodies, a vast ecosystem of microbial species, including bacteria and yeast.
Now that products containing probiotics are sold as yogurt, drinks and dietary supplements, there seems to be some confusion around how to define probiotics and how beneficial they really are.
"It's taken a while for the scientific community to actually form a consensus of what we mean when we say probiotics, because people might mean different things," said Lynne McFarland, an associate professor of medicinal chemistry at the University of Washington in Seattle.
"The most recent recommendation and consensus is that they have to be alive. They can be a bacteria or a yeast. They have to be used in an adequate dose, and they have to have some proven beneficial health effect," McFarland said of probiotics.
"Probiotics have been around for a long time," she said. "It took a while for science to catch up with what's going on."
How has our understanding of probiotics changed over time? Here's a look at probiotics' steady rise in popularity, from Europe to America, and where health experts now stand on their benefits.
As farmers settled into communities, they developed the habit of storing more of their food. "With anything that you store, microbes are just going to start growing in it," Shanahan said. This sometimes resulted in the fermentation of foods.
For instance, in Asia, sushi was originally a fermented food, Shanahan said.
In other words, the bacteria in the rice helped store the fish.
"They have the hieroglyphics of the pharaoh being served something in a bowl, and people who have translated those have gone, 'OK, this is sort of a fermented milk product,' " McFarland said.
As agriculture expanded, so did our relationship with probiotics.
It's believed that the word kefir derives from the Turkish word keyif, meaning "pleasure" or "feeling good" after its ingestion. The beneficial health properties of kefir and other dairy products were a part of folklore until the idea of probiotics arose.
"He's the first one who published a book looking at Bulgarians and saying, 'Gosh, they live longer,' and it wasn't due to their diet. It wasn't due to the yogurt that they consumed but actually the bacteria that was used to ferment the yogurt," McFarland said. "That clever Russian. ... He's the one who kind of went, 'You know, bacteria aren't all bad.' "
However, the concept of probiotics quietly drifted to the background of medical focus until it re-emerged in the mid-1950s in Europe.
"They were always more popular in Scandinavia and Europe," McFarland said.
In the United States, however, there was less attention on probiotics and more attention on antibiotics.
"Antibiotics were seen as only beneficial. In the '70s, actually, doctors would just treat people with anything with a shot, like they didn't have any clue about resistance or any clue about side effects," Shanahan said.
"I started doing this research back in the 1990s, and it was very infrequent that somebody in the US would know what we were talking about when we would talk about probiotics," McFarland said.
"It really wasn't until 1994, when the dietary health and supplement law was enacted, that allowed these kinds of products to be sold over the counter," she said. "Suddenly ... people became very aware of what it is. It's truly amazing how quickly the popularity of this spread."
"What changed is that before that law became enacted, probiotics were considered an investigational drug. So it was going through the FDA process, and we had to go through ... very long and expensive drug pathway development through the FDA," McFarland said.
"Then, when the dietary supplement law got enacted ... it opened a floodgate of quote-unquote probiotic products that weren't really probiotic, and the quality of the products were not as regulated as they should have been, having not gone through the ordinary FDA process," she said. "I think that's still the situation today."
"There's now probiotics that come in chocolate; probiotics come in cheese; there's bread. Little sprinkles you can put on ice cream," she said.
Next, scientists started to research how probiotics may benefit your health, specifically your gut.
For the paper, 18 randomized controlled trials on the effectiveness of probiotics as an irritable bowel syndrome treatment were analyzed. The trials, published between 1950 and 2008, involved 1,650 patients total.
"This systematic review indicates that probiotics have a therapeutic benefit in improving IBS (irritable bowel syndrome) symptoms," the researchers wrote. "Future studies need to establish which species, strain and dose of probiotics are most efficacious in IBS."
Additionally, "there's been a big thing on 'this is a women's probiotic' or 'this is a men's probiotic.' There has been a big thing on gender probiotics," McFarland said of one emerging trend.
However, she added that there is no difference between a male or female microbiome, and therefore, there should be no difference in how a probiotic would benefit a man or a woman -- that is, outside of vaginal health.
"The only difference is that there are some probiotic strains that are good for vaginitis, so if they're trying to say 'restores vaginal health,' then that might be OK as a woman's product," McFarland said.
All in all, McFarland said, probiotics may be beneficial if taken to prevent travel-associated diarrhea or to prevent side effects of antibiotics. For any other purposes, however, she recommended consulting with your doctor or checking scientific literature for guidance.
"What we're finding is that a person has their own profile of their microbiome. If that's disrupted, and even if you take probiotics, after you stop taking probiotics, it goes back to what your profile was before," McFarland said.
"So, it's like it remembers who's invited to the party, and it only invites those people," she said. "I think it's still an exciting field for research because, the more we appreciate how much bacteria do for us, the more we appreciate what happens when it gets disrupted."
Shanahan recommends going old-school.
"From my perspective, the more logical thing to do is to eat foods that are good for us and that bacteria can utilize as well," Shanahan said.
"I get foods rich in prebiotics and ready-to-eat fermented foods. I'll eat yogurt or kimchi, and for prebiotics to feed the probiotics, I make sure I always get some kind of fiber-y thing, whether it's nuts or vegetables or beans," she said. "But the probiotic-rich foods, which are the fermented and cultured foods, are more likely to be beneficial than supplements."
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