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.
Prehistory: Storing sushi in the early days of fermentation
About the time our hunter-gatherer ancestors took up farming
, 11,000 years ago, they started to consume probiotics without even realizing it, said Dr. Cate Shanahan, a Newtown, Connecticut-based family physician who also consults as a nutritionist with the Los Angeles Lakers.
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.
, when microorganisms grow on and break down food, can make that food item rich with probiotics. The process also may increase the shelf life of a food and make some foods more digestible, Shanahan said.
For instance, in Asia, sushi was originally a fermented food, Shanahan said.
"They had so much fish that they'd catch all at once. ... So they would store it, and they discovered that packing it in rice would help store the fish so that it would basically rot, but in a way that wasn't disgusting and more controlled," she said. "We now know it was because there was special bacteria, called Bacillus bacteria
, in the rice that was helping out."
In other words, the bacteria in the rice helped store the fish.
Around the same time, many other examples of fermentation were emerging in other parts of the world. Some research suggests that ancient Egyptians fermented their beverages through complex brewing methods
"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.
13th century: Did Marco Polo drink kefir?
Marco Polo, the Venetian merchant traveler who ventured across Asia, was known to speak of probiotic-rich fermented beverages
such as kefir in his travels.
It is believed that when nomadic shepherds and travelers journeyed with raw milk carried in leather
, the milk would accidentally ferment over time.
Some studies suggest that this process led to the creation of kefir
, a fermented milk drink that originated in the Caucasus Mountains
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.
19th-20th centuries: The 'father of probiotics'
In the late 1800s, French biologist Louis Pasteur
identified the bacteria and yeasts responsible for the process of fermentation, but it was Russian biologist Elie Metchnikoff
who linked those microorganisms to health outcomes.
Metchnikoff had long theorized that the microorganisms in our guts
could have beneficial or adverse effects on our health.
In 1905, Metchnikoff studied how many residents of poor communities in Eastern Europe
were centenarians, living to be 100 or older. He associated their longevity with the type of bacteria used to ferment the yogurt they would eat, and he became known as the "father of probiotics
"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.' "
In 1908, Metchnikoff shared a Nobel Prize in physiology or medicine
with German physician Paul Ehrlich for their research on the immune system.
Many other scientists followed Metchnikoff's research efforts into the bizarre micro-world of bacteria, including Henry Tissier, a French pediatrician who discovered "good" bacteria
in the guts of infants. He proposed that the bacteria could be used to treat patients with diarrhea.
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.
1950s-1980s: Making a name for probiotics in Europe
In 1953, German bacteriologist Werner Kollath first used "probiotic" to describe various supplements believed to restore the health of malnourished patients. The term
was derived from Latin and Greek, meaning "for life."
In the next year, German scientist Ferdinand Vergin used the term to describe "active substances
" beneficial for health.
Then, in 1965, a paper in the journal Science
used the term probiotics to describe substances produced by one microorganism that stimulate the growth of another.
In 1974, a paper published in the journal Animal Nutrition Health
used the term probiotics in the context in which we use it today, to "contribute to intestinal microbial balance."
Then, in 1989, Roy Fuller, a researcher of gut microbial ecology, redefined probiotics
as "a live microbial feed supplement
, which beneficially affects the host animal."
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.
"About the late '80s, early '90s, we started to understand antibiotics had a downside," she said, such as leading to antibiotic resistance
and killing off beneficial bacteria. Meanwhile, Americans started to understand probiotics.
1990s: Probiotic popularity spreads across US
"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."
In 1994, the US Food and Drug Administration
implemented the Dietary Supplement Health and Education Act, which allowed dietary supplements to be regulated under a different set of rules than prescription drugs, foods and beverages.
"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."
2000s: Health officials crack down on probiotics
In 2002, the Food and Agriculture Organization of the United Nations and the World Health Organization drafted guidelines for the evaluation of probiotics in food in a joint consultation
The guidelines recommended that the definition of probiotics
be "live microorganisms which when administered in adequate amounts confer a health benefit on the host."
Around this time, various ways to administer probiotics and prebiotics, which promote the growth of beneficial bacteria
in the gut,
emerged, McFarland said.
"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.
2010: A spotlight on the gut
Studies suggest that some probiotics might help with symptoms of certain chronic gastrointestinal tract conditions, such as irritable bowel syndrome. However, such benefits have not been conclusively demonstrated, and not all probiotics have the same effects, according to the National Center for Complementary and Integrative Health
In 2010, a systematic review paper in the journal Gut
found that probiotics were helpful in treating the symptoms of irritable bowel syndrome, compared with a placebo or no treatment. Yet the magnitude of benefit and most effective species of probiotic remained uncertain.
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."
2010s: His and hers probiotics emerge
By 2012, the use of probiotics
had skyrocketed in popularity in the US, with nearly 3 million more adults
using probiotics or prebiotics than in 2007, when only about 865,000 adults used such supplements
Among American adults, probiotics ranked as the third most commonly used dietary supplement behind fish oil, and glucosamine and chondroitin, according to a 2015 National Health Statistics Report
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.
"The thing to remember is that your vagina is a very acidic environment, and it's colonized with lactobacillus," she said. "There's only a couple strains that have been shown to be really good -- Lactobacillus reuteri and rhamnosus
-- and others have been tried but haven't really been that effective."
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."