Probiotics: Benefits, side effects, research and dosage

Do you know that you host trillions of bacteria in your gut? The idea of having bacteria inside your body may sound disturbing, considering the reputation that bacteria have had for decades. The fact that you are home to thousands of species of bacteria may be unsettling.

For many years, bacteria were thought to only cause harm and referred to as ‘germs’. However, “good” bacteria can do wonders for your health, well-being and wellness. 

Bacteria and other microorganisms that naturally reside in your gut are called the gut microbiota. These microorganisms benefit the body and promote health by producing vitamins and nutrients.

Interestingly, these microorganisms also have hormones and neurotransmitters that are important in relaying information and promoting optimal metabolism in the body. 

These beneficial microorganisms are also called probiotics from pro and biota, which mean for life. The probiotics living inside your body can treat and even prevent certain diseases.

In Northern Europe, many people eat fermented food. This dietary habit provides a healthy dose of microorganisms. Eating food fermented by bacteria, such as fermented beverages and yogurt, can help improve longevity and lifespan

Some specialists have recommended using probiotics to treat disorders that are difficult to manage using therapeutic medications, such as irritable bowel syndrome.

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Benefits of probiotics 

Many studies have pointed out the benefits of probiotics. 

Probiotics and gut health 

The gut is not only involved in digestion but also in immunity and the improvement of overall health and well-being.

The gut hosts trillions of microorganisms that are responsible for the following: 

  • Production of vitamin K and B vitamins 
  • Production of neurotransmitters such as serotonin
  • Production and regulation of hormones 

Intake of probiotics has been shown to improve gut health by boosting the diversity of the gut’s microbiota and increasing the population of good bacteria in the colon. It has been shown that the greater the diversity of the gut’s microbiota, the healthier the individual.

An animal model study [1] demonstrated that obese rats tend to have a reduced abundance of Bifidobacterium in the gut compared to rats with normal weight. These findings suggested that lower levels of Bifidobacterium might contribute to obesity. 

In addition, other studies confirmed that obese rats tend to be resistant to Bifidobacterium. Notably, both Bifidobacterium and Lactobacillus are traditional probiotics that are shown to play critical roles in maintaining balance in the gut’s microecology. 

Probiotics and mood 

There is evidence from published studies that probiotics may improve mood. A systematic review and meta-analysis [2] published in the Frontiers of Neurology analyzed and pooled the findings of multiple randomized controlled trials examining probiotics’ effects on stress, anxiety and depression

A total of ten clinical trials were examined and reviewed, with a total of 685 participants. The findings revealed that probiotics significantly reduced the patients’ depression scale with depression and anxiety.

In addition, healthy participants who were under stress and took probiotics also presented with reduced scores in depression and anxiety.

Subgroup analysis showed that probiotics could significantly reduce depressive symptoms in those with depression. 

Probiotics: Benefit to improving mood

Results of this systematic review add to evidence that probiotics might exert significant effects only in those with depression or those diagnosed with anxiety but not in healthy patients under stress.

This is an important finding since many younger patients experiencing depression or anxiety want to undergo natural treatments and veer away from medications. Treating depression with probiotics might help address the concerns of this group of patients. 

The mechanism of how probiotics improve mood may be based on the bacteria’s secretion of neurotransmitters, such as:

  • Serotonin
  • Norepinephrine
  • Dopamine [3]

Bacteria produce about 95% of the body’s serotonin in the gut. Serotonin and dopamine are called happy hormones since these have been shown to boost mood and promote mental well-being. 

Serotonin is both a neurotransmitter and a hormone and is also known as 5-hydroxytryptamine (5-HT). As a neurotransmitter, it carries important messages between the central and peripheral nervous systems.

Besides influencing learning, it has been shown to improve memory and happiness. It is well established in the literature that a lack of serotonin is associated with:

  • Depression
  • Mania
  • Anxiety
  • Schizophrenia
  • Other health conditions

Microorganisms in the gut that produce serotonin include Enterococcus, Streptococcus, and Escherichia. These microorganisms also help make dopamine, with 50% of this neurotransmitter produced in the gut [4].

Dopamine and norepinephrine regulate several functions of the peripheral nervous system, including mood and cognitive abilities. 

Probiotics and cognition 

The effects of probiotics on cognition and memory may be due to the effects of dopamine, norepinephrine and serotonin. These neurotransmitters have been found to improve mood and play critical roles in enhancing cognition and memory. 

Animal model studies [5, 6] suggest that probiotics can influence brain function by producing neurotransmitters, affecting brain chemistry. These studies also demonstrated that probiotics administration could improve memory function in mice.

In particular, administering Lactobacillus strains in aged mice improved memory performance, suggesting that probiotics might improve memory in those with cognitive impairment or aged individuals. 

In another study [7] involving older adults, the administration of tempeh probiotics improved verbal fluency and visuospatial and memory functioning. This study recruited 93 older adults into three groups, with the last group as the control group.

The first two groups were administered probiotic supplementation isolated from tempeh for twelve weeks. The first group received supplementation of probiotics with a concentration of 108 colony-forming units (CFU)/ml.

In comparison, the second group received probiotics at 107 colony-forming units (CFU)/ml concentrations. The third group only received a placebo. 

The study’s findings showed that in the first group, the respondents demonstrated significant improvements in the learning process, cognitive domains of memory, and verbal fluency compared to the placebo group.

Meanwhile, in the second group, the participants demonstrated significant improvements in verbal fluency, visuospatial and memory function compared to the placebo group.

The control group only showed improvements in the memory domain. Notably, only the first group exhibited improvements in the learning process. 

Results of this study showed that supplementation of probiotics containing Lactobacillus fermentum at a concentration of 108 colony forming units (CFU)/ml was better in significantly improving the learning process of older adults in the study. 

Probiotics and the immune system 

The role of probiotics in enhancing the immune system has been extensively studied in animal model studies.

The results of these studies showed that probiotics have the following benefits: 

  • Protection against infections since these beneficial microorganisms inhibit the growth of pathogenic bacteria. These good bacteria compete for food and attachment to the intestinal villi with pathogenic microorganisms. 
  • Relief of irritable bowel syndrome through reducing inflammatory responses of the gut. 
  • Decrease in the gut inflammatory response, which in turn helps relieve symptoms of irritable bowel syndrome. 
  • Prevention of allergies. 
  • Prevention of cancer. 
  • Inhibition of the growth of Helicobacter pylori, the microorganisms that cause peptic ulcer in the upper digestive tract. 

Many probiotics or good bacteria can stick to epithelial cells in the colon or gut. Following these interactions, good bacteria release cytokines such as interleukin-6 and macrophages, essential in initiating the inflammatory response when pathogens or infections are present. 

Probiotics have also been shown to promote the physical barrier between the external environment and the connective tissues of the host in the gut. This physical barrier in the intestine called the intestinal barrier, is strengthened by probiotics.

Lactobacillus sp. has been shown to increase mucus production from the goblet cells. The presence of mucus likewise strengthens the intestinal barrier of the gut. 

In addition, probiotics can cause a shift in the composition of the microbiota of the gut to specific beneficial bacteria.

For instance, the gut’s microbiota can improve the environment for Prevotella sp. and Oscillibacter sp. bacteria growth. These microorganisms are now known to produce several anti-inflammatory metabolites. 

Probiotics have also been shown to promote the growth and production of immunoglobulin A (IgA), which boosts the immune system and helps prevent the growth of pathogenic microorganisms. 

Probiotics and sleeping patterns 

Probiotics also appear to improve sleep quality of patients taking them.

A study [8] published in the Frontiers in Psychiatry journal showed that treatment with probiotics for six weeks in 38 volunteers resulted in a reduction of anxiety and depression scores and an improvement in mood.

Further, the study also revealed that probiotics improved the participants’ sleep patterns

The study participants in the experimental group took a probiotic mixture containing Lactobacillus rhamnosus, L. fermentum, L. plantarum, and Bifidobacterium longum for six weeks. The control group took a placebo. Sleep quality, personality dimensions, and mood were assessed four times during the study.

The first assessment was conducted at the beginning of the study, the second assessment in the third week of the study, and the third assessment during the sixth week. Another assessment was done at three weeks of washout or follow-up period. 

The authors observed:

“Our findings extend previous observations, suggesting that the probiotics mixture we used effectively improved sleep quality in a healthy population. Moreover, the improvement in sleep quality fits well with the reduction in depressive mood state, anger, and fatigue we observed in the experimental group. Previous studies demonstrated a strong relationship between sleep quality and mood. This was also the case of our study.” 

Results further prove that sleep quality was correlated with the following:

  • Depressive symptoms
  • Anxiety
  • Confusion
  • Vigor, anger and fatigue

And some aspects related to sad moods, such as:

  • Hopelessness
  • Risk aversion
  • Aggression

In addition, probiotics improved the participants’ mood by improving sleep quality. It has been shown that sleep quality is associated with improvement in well-being and mental health

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Probiotics and improvement of digestion and absorption of nutrients 

Probiotics feast on food called prebiotics, such as food rich in fiber. Hence, probiotics aid the digestive system by breaking down fiber, which is not digested naturally due to a lack of enzymes that can break down fiber.

Once the bacteria break down the fiber, this increases the intake of nutrients and the production of vitamins and short-chain fatty acids.

These short-chain fatty acids include butyrate, propionate, and acetate, essential metabolites that maintain the balance or homeostasis in the intestine.

Several studies have shown that short-chain fatty acid levels are reduced in patients with inflammatory bowel disease. 

Short-chain fatty acids are also known to strengthen the gut barrier function, which protects the intestines from infections and the development of ulcers.

Further, short-chain fatty acids are recognized to protect against colon cancer. These metabolites protect the body against chronic diseases, such as:

  • Crohn’s
  • Ulcerative colitis
  • Inflammatory bowel disease

Short-chain fatty acids reduce inflammation and increase the body’s ability to absorb nutrients and minerals. It also prevents pathogenic or harmful bacteria from growing in the colon. Since fiber is filling, it lowers appetite and benefits the body’s metabolism.

While animal meat and plants contain soluble fiber, plants contain more fiber than meat. Hence, following a plant-based diet could be more beneficial to your health than an animal-based diet. 

Here are some plant-based foods rich in fiber: 
  • Oates
  • Vegetables 
  • Fruits 
  • Lentils, beans and peas
  • Whole grains 
  • Pasta
  • Cornmeal 
  • Potatoes 
Probiotics: Plant-based foods rich in fiber

Side effects 

Although probiotics have numerous beneficial effects, some may exhibit side effects when first taking probiotics. These may include the following:

  • Digestive symptoms
  • Allergies
  • Skin problems
  • Small intestine bacterial overgrowth
  • Antibiotic resistance
  • Increased risk of infection
Digestive symptoms 

Probiotics are known to improve digestion and aid in the gut’s immune function. However, some people may exhibit digestive symptoms when first taking probiotics.

These symptoms include the following: 

  • The bacteria may produce more gas than usual. 
  • Diarrhea
  • Bloating 

All these symptoms are primarily present in the first few days or weeks of taking probiotics for the first time. These symptoms are expected to clear in the weeks following intervention with probiotics.

However, if these symptoms persist, consult your doctor, who may advise you to change probiotics. 


Probiotics sold as supplements may contain some substances such as:

  • Soy
  • Dairy
  • Gluten
  • Eggs
  • Lactose

If you are allergic to these substances, consult your doctor immediately if you take probiotics containing any of these components. You may also check the ingredients of probiotics before taking these to prevent any allergic reactions. 

Seek medical care if your allergies worsen after taking probiotics. You can choose to change your probiotics to avoid any allergic reactions. 

Skin problems 

The development of rashes after taking probiotics is rare. However, there are isolated cases where individuals taking probiotics report developing an itchy rash. 

Risk of infection 

Probiotics can help maintain a healthy balance of good and bad bacteria in the gut.

Although probiotics may tip the balance to more good bacteria in the gut, it is essential to remember that immunocompromised patients might not respond positively to probiotics.

These patients may develop infections or severe illnesses when taking probiotics use. 

If you have diseases that can compromise your immune system or severe illnesses, always consult your doctor when planning to take probiotics. Your doctor will advise you on the best possible probiotics to prevent the risk of infection. 


Atopic dermatitis 

Atopic dermatitis is a common chronic inflammatory skin disorder and the most common form of eczema. This dermatitis affects at least 1% to 3% of adults globally, translating to some 70 million to 210 million adults worldwide.

However, this disorder is more common in children and affects at most 20% worldwide. 

Clinical trials and observational studies have demonstrated that probiotics taken during pregnancy and infancy may reduce the risk of atopic dermatitis or control this disease early in childhood.

Current studies report that taking probiotics during pregnancy for two weeks to 7 months and during infancy for two months to 13 months could prevent the development of atopic dermatitis.

In children aged six months to 9 years, treatment with mixed or single strains of probiotics may also help prevent atopic dermatitis. 

The following probiotics were found to be effective in preventing atopic dermatitis: 

  • Lactobacillus 
  • Bifidobacterium 
  • Propionibacterium 
Inflammatory bowel disease 

Inflammatory bowel disease is a chronic inflammatory disease (IBD) that includes Crohn’s disease and ulcerative colitis. 

The exact cause of IBD is currently unknown but is believed to be a combination of genetic and environmental factors. Oral steroids and other treatments are available to manage IBD. However, no cure exists as of today. 

Animal model studies have shown that probiotics might have beneficial effects against IBD. However, human studies or clinical trials do not offer conclusive results on whether probiotics can cure IBD or lead to sustained remissions for IBD.

For example, a systematic review [9] published in 2020 demonstrated no conclusive findings that mixed probiotic strains could result in IBD remission.

This review included 12 clinical trials that recruited 689 adults and children with Crohn’s disease. It also examined another 17 clinical trials that examined probiotics in maintaining remission in 1,674 adults and children with ulcerative colitis.

All the participants were treated with probiotics for several months. However, the authors could not draw a definitive conclusion that probiotics can induce or maintain remission from ulcerative colitis and Crohn’s disease. 

It should be noted that the systematic review included clinical trials with different formulations of probiotic supplementations and different treatment lengths.

Apart from differences in treatment duration, there were also differences in patient populations. 


Obesity is a chronic condition that affects millions of people worldwide and a significant risk factor for

  • Type 2 diabetes
  • Cardiovascular diseases
  • Certain forms of cancer

Treating obesity remains a priority for many nations, including the US and the UK, with an obesity epidemic. 

Probiotics have been touted to have a beneficial effect against obesity. Remember that the gut’s microbiota has an essential role in extracting energy and nutrients from our food.

Animal model studies that include mice suggest that the gut’s microbiota affect the expenditure or use and storage of energy. However, it is still being determined if these observations in animal studies are also seen in human studies. 

Clinical trials have been done to assess the effects of probiotics in reducing obesity. In a study [10] published in the British Journal of Nutrition, 210 healthy adults were recruited to examine if probiotic treatment results in decreased visceral or stomach fats.

These healthy volunteers aged 35 to 60 years old agreed to participate in the 12-week clinical trial. Those in the experimental group consumed 200 grams per day of fermented milk that contains Lactobacillus gasseri. 

The first two groups received 107 CFU and 106 CFU of the bacteria, respectively. Findings showed that those in the first two groups had an 8.5% and 8.2% mean visceral fat reduction, respectively, compared to the control group.

Compared to the placebo or control group, these two groups exhibited a significant decrease in waist and hip circumference, body mass index and body fat mass. 

Probiotics: Benefits to obesity


Probiotics are sold in colony-forming units (CFU). These units indicate the number of microorganisms that are viable or alive. Most probiotics supplements are sold between 1 billion to 10 billion CFU per ml or dose.

Some supplements contain more than 50 billion CFU per mil. However, the products’ health effects are not necessarily improved with higher CFU counts.

To date, the Food and Drug Administration in the US does not have recommended dosages for probiotics. However, supplements that contain 1 billion to 10 billion CFU per ml are generally safe and do not lead to any side effects or adverse events. 

In summary, probiotics are now recognized to play crucial roles in maintaining gut health and improving overall immunity.

The benefits of probiotics extend to mental health as these microorganisms produce serotonin, dopamine and norepinephrine, neurotransmitters that enhance mood and prevent depression and anxiety.

Besides improving mood, recent studies indicate probiotics can enhance sleep quality

Recent clinical trials show promising results when probiotics are used to manage obesity and chronic conditions such as irritable bowel syndrome.

However, there are conflicting results on whether probiotics can induce and sustain remission for inflammatory bowel diseases such as Crohn’s disease and ulcerative colitis in children and adults. 

Despite the many benefits of probiotics, some people may develop allergies to their components when marketed as supplements.

There are also isolated incidents of skin rashes and allergic reactions. Some may develop diarrhea, especially when using probiotics for the first time. 

Finally, always consult your doctor when planning to take probiotics to ensure the safe use of these supplements. 

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The information included in this article is for informational purposes only. The purpose of this webpage is to promote broad consumer understanding and knowledge of various health topics. It is not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of your physician or other qualified health care provider with any questions you may have regarding a medical condition or treatment and before undertaking a new health care regimen, and never disregard professional medical advice or delay in seeking it because of something you have read on this website.