According to some researchers, probiotics have enormous potential to improve our GI tract, balance the immune system, fight infections and allergies, and improve our mood and cognitive function. What does the current science say? Read on to learn more.
What are Probiotics?
At birth, the sterile human gut is immediately colonized with several types of microorganisms from both the mother and the environment. By the time they reach one year of age, each individual develops a unique bacterial profile .
It is estimated that only 10% of the cells in the human body actually belong to the body itself. The overwhelming majority of the cells consist of the diverse microbiota of nonpathogenic bacteria, 1-2 kg of them living in the gut alone .
Gut microbiota includes ~30 species of Bifidobacterium, 52 species of Lactobacillus, and others, such as Streptococcus and Enterococcus .
The genome of the entire gut microbiota named as “microbiome” exceeds the human nuclear genome by at least 100 times .
Human gut microorganisms are strongly involved in diverse metabolic, nutritional, physiological, and immunological processes . They play an important role in energy homeostasis and through the microbe-gut-brain axis  by impacting our mood and cognitive abilities. They also stimulate the immune response, prevent pathogenic and opportunistic microbes/bacteria, and produce vitamins such as B and K .
Diet can exert a profound effect on the gut microbiota profile, and people in different parts of the world have different bacterial profiles . For example, there is an association of Bacteroides and high animal fat or protein diets, while Prevotella is associated with a high carbohydrate diet .
Probiotic bacteria are live microorganisms known as “friendly gut bacteria” which when present and/or administered in adequate amounts can have potential health benefits .
The term “probiotic” comes from the Greek term ‘for life’ .
Probiotics have been regarded as beneficial since ancient times (particularly lactic acid bacteria – Lactobacilli and Bifidobacteria), but they came into the spotlight in the late 1800s and early 1900s, when it was proposed that consuming yogurt containing Lactobacillus would decrease toxin-producing bacteria in the gut and increase longevity .
The most common probiotic strains belong to the species Lactobacillus and Bifidobacterium, followed by the genera Streptococcus, Enterococcus, Propionibacterium, Bacillus, and Escherichia. In addition, some yeast species are used as probiotics, for example, S. boulardii and S. cerevisiae [7, 1].
According to some studies, probiotics could potentially enhance the immune system, improve the skin’s function, enhance resistance against allergens, and decrease body pathogens .
Note that in clinical studies, probiotic mixtures are often demonstrated to be better than a single strain for improving indigenous microflora .
Prebiotics are specific carbohydrates, such as polysaccharides, fructans, and inulins, that can exert beneficial effects on the composition and metabolic activities of gut microorganisms. A diet high in inulin and related fibers, for example, has been shown to increase Bifidobacteria .
A prebiotic and probiotic combination is referred to as a synbiotic when the net health benefit is synergistic .
Purported Health Benefits of Probiotics
Despite the promising evidence we discuss below, remember that the FDA has not yet approved probiotics for any condition or health claim. Take any single study – especially an animal or cell study – with a grain of salt, and remember to talk to your doctor before adding probiotics to your daily health plan.
Likely Effective For
1) Gut Health
Given the importance of the microbiota in gut health, it’s unsurprising that this is the clearest and best-studied area in which probiotics could have benefits.
Probiotics can decrease the number of potentially pathogenic gastrointestinal microorganisms and pathogens, reduce gastrointestinal discomfort, flatulence and bloating, and improve bowel regularity [8, 12, 13, 14, 15].
In various clinical studies, probiotic supplements have also:
- Improved the composition of the gut flora [20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32]
- Prevented multiple types of diarrhea [33, 34, 35, 36, 37, 38]
- Relieved constipation [39, 40, 41, 42, 43, 44]
- Relieved the symptoms of IBS [45, 46, 8, 47, 48]
- Relieved the symptoms of IBD [49, 50, 51]
- Strengthened the gut barrier 
- Prevented drug-induced GI damage [8, 52, 53, 54]
- Promoted healing of gastric ulcers 
- Prevented and helped resolve necrotizing enterocolitis [55, 56, 57, 58]
- Relieved the pain associated with various gut disorders [59, 60, 61]
For an in-depth exploration of how probiotics might improve gut health, check out this post.
Possibly Effective For
2) Immune Function
Certain probiotics may help the human immune system fight off different types of infection, from Candida to H. pylori and even the common cold. The gut microbiota cooperate with the host immune system through an extensive array of signaling pathways .
Studies with germ-free animals show that the microbiota are necessary for the development and regulation of immunity in the gut, where it prevents the development of inappropriate inflammation .
Perhaps most impressively, probiotic supplementation has helped people fight off a number of infectious diseases, including upper respiratory tract infections like the cold and flu [73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84].
Saccharomyces boulardii has also specifically produced benefits against:
- E. coli infection 
- B. hominis infection 
- Giardiasis 
- H. pylori [4, 88, 89, 90]
- Candida [91, 92, 93, 94]
Finally, HIV-positive patients appeared to develop stronger immune responses after supplementation with S. boulardii .
The possible benefits of probiotic supplementation on immune function are broad and diverse. For a deep dive, check out this post.
Inflammation may be the widest-ranging area in which probiotics have potential benefits. Broadly speaking, probiotic and synbiotic supplementation reduced biochemical markers of inflammation in multiple clinical trials [11, 96, 97].
More specifically, probiotics have benefited people with:
- Autoimmune disorders like lupus [62, 98]
- Mucositis, a complication of cancer treatment 
- Celiac disease [100, 101, 102]
- Allergies [103, 104, 105, 106]
- Asthma [107, 108]
- Development of oral tolerance [109, 110, 111]
- Eczema [110, 33]
- Atopic dermatitis [33, 112]
- Arthritis [113, 114]
There is a huge amount of information available for each of these individual potential benefits; check out this post to learn more!
4) Heart Health
High cholesterol and blood pressure are among the most important risk factors for cardiovascular disease, and certain probiotics have produced beneficial changes to both in clinical trials.
In humans, probiotic supplementation has:
- Decreased LDL cholesterol [5, 115, 5, 116]
- Decreased total cholesterol [5, 116, 117]
- Increased HDL cholesterol [3, 11, 118]
- Decreased triglycerides [5, 116, 119]
- Decreased blood pressure [120, 121, 122, 123]
- Reduced arterial stiffness 
For more about probiotics and their potential benefits to heart health, check out this post.
5) Nutrient Absorption
Some species of probiotic bacteria directly produce nutrients like folate, while others may create an environment in which more nutrients are available to be absorbed from the gut .
Among the nutrients either produced or made more available by probiotic bacteria are:
- Riboflavin (vitamin B2) 
- Vitamin B12 
- Folate (vitamin B9) [127, 130]
- Vitamin D3 
- Iron [132, 133, 134]
- Calcium [135, 136]
- Phosphorus [135, 127, 137]
- Zinc [135, 127, 137]
- Magnesium [127, 137]
For more about how probiotics might benefit nutrient status, check out this post.
Insufficient Evidence For
6) Weight & Metabolism
According to a few studies, consuming probiotics may reduce body weight and BMI. A greater effect was achieved in overweight subjects, when multiple species of probiotics are consumed in combination or when they are taken for more than 8 weeks .
In other studies, various Lactobacillus probiotics have:
- Induced weight loss [139, 140]
- Decreased visceral and subcutaneous fat [140, 141, 142]
- Reduced waist and hip circumference [143, 144]
Daily ingestion of milk containing B. animalis ssp. lactis significantly reduced the BMI, total cholesterol, low-density lipoprotein, and inflammatory markers in humans .
For a complete exploration of probiotics’ effect on weight and metabolic health, check out this post.
7) Reproductive Health
Pregnancy and birth are among the most vulnerable processes in the life of both mother and child, and probiotics supplements may support immune function and prevent reproductive dysfunction during this time.
The use of a specific set of probiotics during the first 1,500 days of life appeared to help prevent infections and inflammatory events in infants .
Probiotics have improved immune function in both mother and newborn; mothers taking probiotic supplements were less likely to develop breast infections, and preterm babies were better able to develop feeding tolerance [150, 110, 151, 152, 153, 154, 155].
For more about the amazing potential benefits of probiotics in reproductive health, and especially in pregnancy and preterm birth, check out this post.
8) Cognition & Mental Health
There is an intriguing connection between the gut flora and the brain, which scientists have yet to fully unravel. In the meantime, some intriguing evidence suggests that beneficial probiotic species could potentially improve mood, reduce stress, and even make it easier to fall asleep.
Gut probiotics play a major role in the bidirectional communication between the gut and the brain , referred to as the “microbiota-gut-brain” axis. It is now generally accepted that microbiota can affect behavior and modulate cognitive function, especially in older adults .
The vast number of microorganisms in our intestines may have a major impact on our state of mind . Gut microorganisms are able to produce and deliver such neuroactive substances as serotonin and gamma-aminobutyric acid (GABA) .
In human studies, probiotic supplementation has:
- Reduced negative thoughts in non-depressed individuals .
- Improved mood and reduced depression scores 
- Decreased anxiety, anger, and hostility 
- Decreased cortisol and physical symptoms of stress [164, 165, 166]
- Improved sleep quality and duration [167, 168]
To learn more about the potential of probiotics in brain health, check out this post.
9) Skin & Bones
Certain probiotics may improve markers of aging and damage to the skin, teeth, and bones. According to one study, probiotics may be able to restore acidic skin pH, alleviate oxidative stress, attenuate photoaging, improve skin barrier function, and enhance hair quality .
Some researchers believe that the topical application of probiotic bacteria may enhance the skin’s natural defense barriers. Additionally, probiotics, as well as resident bacteria, can produce antimicrobial peptides that benefit skin immune responses and eliminate pathogens. Finally, probiotic creams increased skin hydration and clearness [174, 175, 176, 177, 178].
Long-term consumption of L. rhamnosus containing milk reduced caries development in children .
For more about the potential benefits of probiotics to the skin and bones, check out this post.
10) Physical Stress & Wounds
The human body is very, very good at recovering from physical stress: from exercise to wounds to surgery, it heals and does its best to come out stronger on the other side. However, when the body struggles, could gut flora and probiotics pick up the slack? Some research suggests a role for probiotic supplements in exercise , wound healing, and even postoperative recovery.
Probiotics have had other benefits to athletes as well. In various clinical studies, they:
- Enhanced protein absorption 
- Reduced muscle damage 
- Accelerated physical recovery 
- Increased vertical jump power 
- Increased time to fatigue 
For more about how probiotics can improve the response to various types of physical stress, check out this post.
Certain probiotic bacteria have demonstrated an ability to bind to some toxins (like heavy metals) and even degrade others (like certain organophosphate pesticides). Could probiotic supplements help prevent damage to the human body caused by such toxins? Some research suggests that this could be the case.
Some species of probiotic bacteria may bind and prevent the absorption of:
And others may degrade:
- Organophosphate pesticides [199, 200, 201]
- Heterocyclic aromatic amines [200, 19]
- Aflatoxin [202, 203, 204]
- Other mycotoxins 
For more about how probiotic supplementation could benefit the human detox system and prevent uptake of toxins, check out this post.
12) Healthy Aging
When we age, the composition of our gut flora shifts, and our immune response declines. Are these connected? Some researchers think they might be. For one thing, levels of Bifidobacteria decrease as we age .
In multiple studies of elderly subjects, Bifidobacterium and Lactobacillus probiotics enhanced immunity, increased white blood cell activity, and prevented harmful inflammation [217, 218, 219, 220, 221].
Some researchers attribute these benefits, at least in part, to the antioxidant activity of probiotic bacteria like L. casei, L. helveticus, L. fermentum, B. bifidum and B. subtilis [222, 223, 224, 225, 226, 227].
For a more detailed look at the interactions between probiotics and aging, check out this post.
13) Food Sensitivities
Certain probiotics may help the human body to distinguish between safe and dangerous compounds and avoid allergic reactions to otherwise harmless foods. Probiotic bacteria can degrade:
Some researchers have therefore suggested that probiotic supplementation could reduce symptoms of intolerance to these compounds.
For a deeper discussion of probiotics in food sensitivities, check out this post.
Probiotic bacteria have shown antitumor activities, and some studies suggest they could potentially reduce the incidence of cancer. They may delay cancer onset and progression as well as regulate cell growth mechanisms .
Consumption of soy isoflavones in combination with L. casei decreased the risk of breast cancer among Japanese women .
L. casei administration significantly reduced the recurrence rate of bladder cancer and colorectal cancer in cancer patients .
Beyond this, research into probiotics and cancer has been restricted to animals and cells. For a deep dive into the future of this research, check out this post.
Remember to talk to your doctor before adding a probiotic to your daily regimen.
Most probiotics are considered safe. However, we do not recommend taking probiotics if you are severely ill or immunocompromised. There have been rare incidents of sepsis, endocarditis, and liver abscess during the use of Lactobacilli; additionally, fungemia has been reported with the use of S. boulardii, primarily in patients with severe comorbidities .
The most common reported side effects of probiotics are constipation, flatulence, hiccups, nausea, infection, and rash .
Some probiotic bacteria can produce biogenic amines: L. brevis and L. lactis can produce tyramine and putrescine [237, 238, 239]. S. thermophilus can produce low amounts of histamine and tyramine . L. reuteri is able to produce histamine [241, 242].
There is at least one case of S. cerevisiae inducing an allergic response .
B. bifidum cell-surface biopolymers (BPs) can interact selectively with human serum thyroid peroxidase (TPO) and thyroglobulin (Tg) autoantibodies (anti-TPO and anti-Tg, respectively). There is a possibility that Bifidobacteria play a role in the pathogenesis of autoimmune thyroid diseases (ATD) in those with a genetic predisposition to ATD .
Probiotic products can be formulated as capsules, tablets, powders (which are regulated as a dietary supplement), and food ingredients (e.g., yogurts, kefirs) .
L. acidophilus and B. longum can survive and adhere better to the gastric mucosa than S. thermophilus and B. infantis/adolescentis/bifidum .
L. acidophilus can survive a pH of 3 for 3 hours, and L. rhamnosus can survive 4 hours of incubation at pH 2.5. The viability of several strains of Bifidobacterium was maintained for about 3h in the pH range of 1.5 – 3.0. On the other hand, L. delbrueckii and S. thermophilus do not readily survive stomach acidity .
To overcome the inability of some probiotics to survive transit through the stomach, microencapsulated or coated probiotic strains have been developed .
Furthermore, even though some viable probiotic strains do not survive gastric transit, their dead forms remain beneficial. These nonviable probiotics are now known as ‘para probiotics’ or ‘ghost probiotics’. Non-viable probiotics ameliorated the anti-inflammatory response in rats with colitis, protected against Candida, and prevented the growth of cancer cells .
Each probiotic species comes in many different strains. Some of the properties of each species may be strain-specific or vary between strains. You can find information about each strain in the references linked above.
We’ve compiled much deeper dives into each potential benefit of probiotics. Check them out here:
- Gut Health
- Immune Function
- Inflammation & Autoimmunity
- Cardiovascular Health
- Weight & Metabolic Health
- Reproductive Health
- Cognition & Mental Health
- Skin & Bones
- Physical Stress & Wound Healing
- Healthy Aging
- Nutrient Absorption
- Food Sensitivities
For more technical information, check out these individual probiotic chapters:
- B. animalis (B. lactis)
- B. bifidum
- B. breve
- B. coagulans (L. sporogenes)
- B longum
- B. subtilis
- C. butyricum
- L. acidophilus
- L. brevis
- L. casei
- L. crispatus
- L. delbrueckii (L. bulgaricus, L. lactis)
- L. fermentum
- L. gasseri
- L. helveticus
- L. johnsonii
- L. lactis
- L. paracasei
- L. plantarum
- L. reuteri
- L. rhamnosus
- L. salivarius
- P. freudenreichii
- S. boulardii
- S. cerevisiae
- S. thermophilus