Probiotics and Prebiotics
The World Health Organization defines probiotics as 'live microorganisms which when administered in adequate amounts provide a health benefit on the host'. Although most people probably think of probiotics as a capsule full of viable bacterial cells that is swallowed to influence the intestinal microbiota, the definition is general enough to include microbes delivered via food (e.g. in yogurt), to include other microbes as well as bacteria (e.g. the yeast Saccaromyces boulardii), and to include body sites other than the gut (nasal cavity, skin, vagina). Prebiotics are defined as food substances that are undigested by humans but that stimulate beneficial microbial activities in the gut. These substances (all those commonly used are plant-derived polysaccharides) are usually thought to provide growth substrates for beneficial microbes so they become more abundant, but prebiotics might also include things that change the activities of microbes without changing their abundance. Synbiotics, by the way, are preparations containing both prebiotic compounds and probiotic microbes.
The long history of probiotics is closely tied to food microbiology, dating back to early microbiologists such as Ilya Metchnikoff who proposed in 1907 that lactic acid bacteria in fermented milk products (yogurt, kefir, etc.) promoted health and longevity. Other fermented foods such as sauerkraut, kimchi, and miso are made via the activity of bacteria, and sometimes yeasts, that also have been claimed to be probiotics. It's still the case that most of the probiotic products found on store shelves or online contain food-derived microbes, not microbes adapted primarily to the gut (or other body sites). This may account for a common finding that probiotic strains are detectable in the stool only while they are being ingested and for a short time afterwards, suggesting that these strains don't permanently colonize the intestine (or that they persist at only an undetectably low abundance). However, it's quite possible that food-adapted microbes could provide health benefits as they pass through the system, even if they don't become permanent residents. Furthermore, a microbe could be a permanent (and even prominent) resident of the small intestine where the total microbial density is relatively low, and still be rare or undetected in stool samples because the population of microbes in the large intestine is perhaps 10,000-fold higher.
Most probiotic products, and until quite recently almost all probiotic research, involve members of two unrelated bacterial genera, Lactobacillus and Bifidobacteria. Consequently, both are widely described as beneficial bacteria, or even 'the beneficial bacteria' as if they are the only good guys, but that's an oversimplification. While some strains in both genera have certainly been shown to be beneficial in some contexts, it's unlikely that all strains in these genera are always or even sometimes beneficial. And despite the marketing claims of yogurt makers and supplement manufacturers, it's certainly not true that products with one or a few of these bacterial strains will help resolve any digestive or intestinal problems, nor that their abundance is the only hallmark of a healthy gut microbiota. More recently a wider range of microbial types, including many that are truly native to the human body, are being investigated as potential probiotics; it's a pretty short intellectual jump from discovering a beneficial activity of a particular microbe to wondering if that microbe could be used as a treatment for people whose health seems to be suffering from a lack of that activity. While quite promising, it may turn out that some of these newer potential probiotics can't easily be delivered where they need to go via food or an easily-manufactured supplement (e.g. a microbe that is easily killed by exposure to oxygen). Providing prebiotics (through food choices or as a supplement) that selectively enhance the growth of these beneficial strains may turn out to be a more effective strategy.
Current scientific evidence about probiotics provides a curious mixed bag of results. On the one hand, numerous mechanisms that could relate to probiotic health benefits have been convincingly demonstrated, such as the physical blockage of sites in the small intestine where pathogens might otherwise attach, and the secretion of molecules that specifically attack particular pathogens. Some probiotic strains have also been shown to send anti-inflammatory signals to our immune system, which may be of benefit when an overactive immune system rather than a pathogen is the source of disease (e.g. allergies, asthma, IBD, and other auto-immune diseases). On the other hand, clinical trials of probiotics have often been small or had other methodological limitations, so the evidence for actual health benefits remains weak in many cases. Typically, studies that show a probiotic benefit in a particular context find that some people get no measurable benefit at all - which is also true of most other drugs and treatments - so probiotics shouldn't be seen as a certain cure or preventative, even where the evidence shows that they can help.
At present, the strongest evidence for the benefits of probiotics is with reducing the duration and/or severity of diarrhea, including 'traveller's diarrhea', antibiotic-associated diarrhea, and persistent childhood diarrhea. Good evidence also exists for reducing the risk of atopic eczema and other allergies in children. There are some promising results for prevention of Clostridium difficile infection, upper respiratory infections and middle ear infections in childhood. Studies of probiotics in inflammatory bowel disease show mixed results. Probiotic research is currently being conducted into health topics as diverse as bacterial vaginosis, dental cavities, colon cancer, diabetes, wound healing and depression...and these aren't crackpot studies on the fringes of scientific research. There's no guarantee that such research will result in useful treatments or preventative strategies, but there are good reasons to investigate each of these areas.
Given the exciting potential but sometimes weak evidence, in conjunction with overblown and poorly substantiated marketing claims that contributed to perceptions of 'snake oil' and quackery among some scientists and regulators, the International Scientific Association for Probiotics and Prebiotics was formed in 2000 to promote rigorous scientific research and good public policy in this area. While ISAPP is sponsored by industry, it's run by researchers and has, indeed, improved the quality of research in this area. It's still the case, though, that probiotics and even prebiotics are sometimes caught in a regulatory gap between food and medicine, needing a higher, drug-like standard of evidence to substantiate health claims (implying expensive clinical trials) than is demanded of food or food ingredients. However, such products may still be marketed if they don't make explicit health claims, which has the unfortunate effect of leaving many consumers bewildered with lots of choices but little information.
My own feeling is that with (at least) two complicated and individually-variable biological systems involved (the human microbiota and human physiology), and how recently we've realized the scope of microbial effects on health, it's not surprising that we have inconsistent results thus far. We usually don't have the detailed knowledge of biological mechanisms to make informed decisions that this particular probiotic strain will likely help this person (given their existing microbiota and physiology) in this situation. (Again, a similar statement could be made for many traditional drugs.) I expect that we'll be getting closer to that goal in the coming years and decades as research continues. For now, probiotics are not terribly expensive (at least relative to many medicines) and have low (but not zero) risks, so it's not unreasonable for someone in a situation where there's a plausible benefit to try something to see if it might help. However, I'd hate to see people use probiotics in a situation where their benefit is uncertain, if that means they aren't considering other treatments where the benefit is clear, due to a belief that trying to change their gut microbiota through probiotics must always be a better approach.
For long term maintenance of health, as opposed to treatment of an acute condition, I doubt that regular probiotic supplements are important...eating fermented food with live microbes seems like a better strategy to me, although that's not known to be essential either. Prebiotics, on the other hand, to ensure that a healthy microbiota has the resources to maintain itself, are quite likely to be beneficial for most people. (People with IBD who need to get a flare under control are probably an exception.) When I say 'prebiotics', though, I'm not advocating nutritional supplements as the best or only option. The prebiotics you can buy in the supplement aisle are plant-derived, and I think it usually makes more sense to go to the source. A diet that includes a wide variety of complex plant carbohydrates (whole grains, many different types of fruits and vegetables) is likely to provide a greater amount and diversity of prebiotics to the gut than supplements. There may be some specific prebiotics that are hard to get from regular food, for which supplements might make sense...but I'm pretty sure that a bunch of supplements added to a lousy diet with lots of refined grains and sugar isn't going to be anywhere near as healthy as a diverse, plant-rich diet without any supplements at all.
The long history of probiotics is closely tied to food microbiology, dating back to early microbiologists such as Ilya Metchnikoff who proposed in 1907 that lactic acid bacteria in fermented milk products (yogurt, kefir, etc.) promoted health and longevity. Other fermented foods such as sauerkraut, kimchi, and miso are made via the activity of bacteria, and sometimes yeasts, that also have been claimed to be probiotics. It's still the case that most of the probiotic products found on store shelves or online contain food-derived microbes, not microbes adapted primarily to the gut (or other body sites). This may account for a common finding that probiotic strains are detectable in the stool only while they are being ingested and for a short time afterwards, suggesting that these strains don't permanently colonize the intestine (or that they persist at only an undetectably low abundance). However, it's quite possible that food-adapted microbes could provide health benefits as they pass through the system, even if they don't become permanent residents. Furthermore, a microbe could be a permanent (and even prominent) resident of the small intestine where the total microbial density is relatively low, and still be rare or undetected in stool samples because the population of microbes in the large intestine is perhaps 10,000-fold higher.
Most probiotic products, and until quite recently almost all probiotic research, involve members of two unrelated bacterial genera, Lactobacillus and Bifidobacteria. Consequently, both are widely described as beneficial bacteria, or even 'the beneficial bacteria' as if they are the only good guys, but that's an oversimplification. While some strains in both genera have certainly been shown to be beneficial in some contexts, it's unlikely that all strains in these genera are always or even sometimes beneficial. And despite the marketing claims of yogurt makers and supplement manufacturers, it's certainly not true that products with one or a few of these bacterial strains will help resolve any digestive or intestinal problems, nor that their abundance is the only hallmark of a healthy gut microbiota. More recently a wider range of microbial types, including many that are truly native to the human body, are being investigated as potential probiotics; it's a pretty short intellectual jump from discovering a beneficial activity of a particular microbe to wondering if that microbe could be used as a treatment for people whose health seems to be suffering from a lack of that activity. While quite promising, it may turn out that some of these newer potential probiotics can't easily be delivered where they need to go via food or an easily-manufactured supplement (e.g. a microbe that is easily killed by exposure to oxygen). Providing prebiotics (through food choices or as a supplement) that selectively enhance the growth of these beneficial strains may turn out to be a more effective strategy.
Current scientific evidence about probiotics provides a curious mixed bag of results. On the one hand, numerous mechanisms that could relate to probiotic health benefits have been convincingly demonstrated, such as the physical blockage of sites in the small intestine where pathogens might otherwise attach, and the secretion of molecules that specifically attack particular pathogens. Some probiotic strains have also been shown to send anti-inflammatory signals to our immune system, which may be of benefit when an overactive immune system rather than a pathogen is the source of disease (e.g. allergies, asthma, IBD, and other auto-immune diseases). On the other hand, clinical trials of probiotics have often been small or had other methodological limitations, so the evidence for actual health benefits remains weak in many cases. Typically, studies that show a probiotic benefit in a particular context find that some people get no measurable benefit at all - which is also true of most other drugs and treatments - so probiotics shouldn't be seen as a certain cure or preventative, even where the evidence shows that they can help.
At present, the strongest evidence for the benefits of probiotics is with reducing the duration and/or severity of diarrhea, including 'traveller's diarrhea', antibiotic-associated diarrhea, and persistent childhood diarrhea. Good evidence also exists for reducing the risk of atopic eczema and other allergies in children. There are some promising results for prevention of Clostridium difficile infection, upper respiratory infections and middle ear infections in childhood. Studies of probiotics in inflammatory bowel disease show mixed results. Probiotic research is currently being conducted into health topics as diverse as bacterial vaginosis, dental cavities, colon cancer, diabetes, wound healing and depression...and these aren't crackpot studies on the fringes of scientific research. There's no guarantee that such research will result in useful treatments or preventative strategies, but there are good reasons to investigate each of these areas.
Given the exciting potential but sometimes weak evidence, in conjunction with overblown and poorly substantiated marketing claims that contributed to perceptions of 'snake oil' and quackery among some scientists and regulators, the International Scientific Association for Probiotics and Prebiotics was formed in 2000 to promote rigorous scientific research and good public policy in this area. While ISAPP is sponsored by industry, it's run by researchers and has, indeed, improved the quality of research in this area. It's still the case, though, that probiotics and even prebiotics are sometimes caught in a regulatory gap between food and medicine, needing a higher, drug-like standard of evidence to substantiate health claims (implying expensive clinical trials) than is demanded of food or food ingredients. However, such products may still be marketed if they don't make explicit health claims, which has the unfortunate effect of leaving many consumers bewildered with lots of choices but little information.
My own feeling is that with (at least) two complicated and individually-variable biological systems involved (the human microbiota and human physiology), and how recently we've realized the scope of microbial effects on health, it's not surprising that we have inconsistent results thus far. We usually don't have the detailed knowledge of biological mechanisms to make informed decisions that this particular probiotic strain will likely help this person (given their existing microbiota and physiology) in this situation. (Again, a similar statement could be made for many traditional drugs.) I expect that we'll be getting closer to that goal in the coming years and decades as research continues. For now, probiotics are not terribly expensive (at least relative to many medicines) and have low (but not zero) risks, so it's not unreasonable for someone in a situation where there's a plausible benefit to try something to see if it might help. However, I'd hate to see people use probiotics in a situation where their benefit is uncertain, if that means they aren't considering other treatments where the benefit is clear, due to a belief that trying to change their gut microbiota through probiotics must always be a better approach.
For long term maintenance of health, as opposed to treatment of an acute condition, I doubt that regular probiotic supplements are important...eating fermented food with live microbes seems like a better strategy to me, although that's not known to be essential either. Prebiotics, on the other hand, to ensure that a healthy microbiota has the resources to maintain itself, are quite likely to be beneficial for most people. (People with IBD who need to get a flare under control are probably an exception.) When I say 'prebiotics', though, I'm not advocating nutritional supplements as the best or only option. The prebiotics you can buy in the supplement aisle are plant-derived, and I think it usually makes more sense to go to the source. A diet that includes a wide variety of complex plant carbohydrates (whole grains, many different types of fruits and vegetables) is likely to provide a greater amount and diversity of prebiotics to the gut than supplements. There may be some specific prebiotics that are hard to get from regular food, for which supplements might make sense...but I'm pretty sure that a bunch of supplements added to a lousy diet with lots of refined grains and sugar isn't going to be anywhere near as healthy as a diverse, plant-rich diet without any supplements at all.
Selected Scientific Literature
Sanders et al., 2013:
Good, brief overview of the most active areas of probiotic research;
the references in this paper would be good starting points for
investigating particular diseases. (full text)
Slavin, 2013: Review of the benefits of dietary fiber and prebiotics. (full text)
Kuitunen, 2013: Review of probiotics and prebiotics for preventing eczema and food allergy. (abstract only)
Bernaola Aponte et al., 2013: Systematic review of probiotics for persistent diarrhea in children. (abstract only)
Mugambi et al., 2012: Review of prebiotics and probiotics in infant formula. (full text)
Kellow et al., 2014: Review of the use of prebiotics to treat metabolic syndrome, type 2 diabetes, and cardiovascular disease. (abstract only)
Slavin, 2013: Review of the benefits of dietary fiber and prebiotics. (full text)
Kuitunen, 2013: Review of probiotics and prebiotics for preventing eczema and food allergy. (abstract only)
Bernaola Aponte et al., 2013: Systematic review of probiotics for persistent diarrhea in children. (abstract only)
Mugambi et al., 2012: Review of prebiotics and probiotics in infant formula. (full text)
Kellow et al., 2014: Review of the use of prebiotics to treat metabolic syndrome, type 2 diabetes, and cardiovascular disease. (abstract only)