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Probiotics 101: Everything You Need to Know

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  • The human microbiome contains trillions of microorganisms that carry out life-sustaining functions we cannot perform ourselves
  • Probiotics can restore intestinal balance and support the gut microbiome through a number of beneficial mechanisms
  • The effects of different probiotic products vary according to CFU, the number and types of strains they contain, and their probiotic mechanisms

Gut health: The role of the microbiome 

The human body plays host to roughly 40 trillion bacterial cells and other microscopic organisms—otherwise known as the human microbiome.1 Fortunately, most of these microorganisms do not cause disease, and are actually quite useful! Different populations of these microbes occupy our skin, nose, mouth, and urogenital tract; however, the population that plays the most significant role in our overall health and wellness is the one living in our gastrointestinal tract, or gut.2

Humans rely on the population of non-pathogenic microbes inhabiting the gut (also known as the gut microbiota) to perform important human functions that we cannot perform ourselves.3 For example, intestinal microbes help:

  • synthesize small amounts of essential vitamins
  • digest dietary fiber and convert it into nutrients for colon cells, 
  • protect against foreign pathogens,
  • promote the maturation of immune cells,
  • teach the immune system how to recognize and attack harmful bacteria.3

So essential are these microbes and their genetic material—known collectively as the gut microbiome—that scientists often refer to them as the “forgotten organ” or “second brain”.4

When populated by a large and diverse range of beneficial intestinal microbes, the gut microbiome is in balance, and able to function optimally.5 Conversely, when the diversity and/or abundance of intestinal microbes is low, the gut microbiome is in a state of dysbiosis, and susceptible to a number of suboptimal health outcomes.6

Unfortunately, common factors including stress, poor diet, lack of sleep, aging, environmental toxins, and use of antibiotics can result in the loss of microbial abundance and diversity, putting you at risk for dysbiosis.710 Although the best way to prevent or improve symptoms of dysbiosis is to avoid risk factors such as a poor diet and stress, there are certain factors (such as aging or infection) that you can’t avoid. This is where probiotics may be able to help.

What are probiotics and what do they do?  

Probiotics are live bacteria and yeasts that, when consumed in adequate amounts, provide benefits for immune and digestive health.11. They can be found in natural sources, such as fermented and cultured foods (e.g., yogurt, kefir, buttermilk, kimchi, sauerkraut), or supplemental forms such as liquids, powders and capsules. Although many different types of bacteria can be classified as probiotics, the majority fall into one of two broad species: lactobacillus and bifidobacterium.

Probiotics may help support a healthy microbiome through their ability to:

  • increase the number of healthy microbes in the gut,12
  • stimulate immune functions and responses,12
  • enhance the integrity of the mucosal barrier,13
  • compete with pathogenic bacteria for nutrients,12
  • block adhesion sites for pathogenic bacteria,14 and 
  • secrete anti-bacterial molecules (e.g., hydrogen peroxide, organic acids).15

In light of their ability to repopulate the gut with friendly bacteria and inhibit the colonization of bad bacteria, probiotics are often recommended for people following a course of antibiotics—which unfortunately wipe out the good bacteria with the bad.16

Things to consider when selecting a probiotic supplement

For those of you considering whether and how to add probiotics to your daily routine, it bears mentioning that a healthy diet featuring probiotic-rich foods is generally preferable to supplements. For this reason, we encourage those of you interested in taking a probiotic supplement to first look at your diet, and evaluate whether modifications can be made there to enhance gut health.(For more information, see our article “Is it better to eat probiotic foods or take a probiotic supplement?”)

Although it’s tempting to think of all probiotics as one in the same, the efficacy of different probiotic foods and supplements can vary considerably. In the following sections, we discuss some common questions and considerations when deciding on a probiotic. While most of these considerations apply specifically to probiotic supplements, considerations for food sources can be found here

CFU: How much is enough?

CFU stands for colony forming units, and is used to estimate the number of bacterial or fungal cells capable of dividing and forming colonies in a sample. Basically, the greater the number of viable cells in the sample, the higher the CFU.  Despite the fact that most of the clinical research on probiotics has been performed using doses of 10-20 billion CFU per day, a number of companies now offer high-dose probiotics featuring CFUs of 50, 75, and even 100+ billion.17,18

Although CFUs this high may be advisable for specific conditions (as indicated by a physician), most people do not need doses this high, and would likely benefit from a CFU mimicking the amount of bacteria consumed in a diverse and healthy diet.19 All things considered, a daily dose of 10-15 billion CFU is recommended for individuals seeking daily immune and digestive support. 

It is also worth pointing out that just because a product says it has 50 billion CFU does not mean you will actually receive and/or benefit from that amount. 

In order for probiotic bacteria to take effect, they must be able to survive passage through the gastrointestinal tract and adhere to the mucus surrounding the intestinal lining. However, because these microorganisms are sensitive to light, temperature, moisture, stomach acids, and bile—maintaining their viability long enough to successfully colonize the gut can be extremely difficult.20

Thus, in addition to CFU, you should pay attention to the science behind the product (i.e., whether the strains are research-backed for stability and efficacy) and how the probiotics are being protected from degradation (e.g., form of delivery, packaging). 

Does type of strain matter? 

Just like no two Homo sapiens are exactly alike, no two strains from the same bacterial species are exactly alike. Consequently, the probiotic capacities and mechanisms of individual strains are subject to variability. For example, while some strains may be especially resistant to acid and bile, others may boast impressive adhesion capabilities. And importantly, research finds that certain strains are effective for managing specific health conditions, and that specific strain combinations can provide synergistic benefits beyond the effects of each probiotic alone.21

Basically, your ability to benefit from a probiotic supplement will depend on the survivability and probiotic mechanisms of the strains it contains. This means that simply picking the probiotic with the highest CFU or the best price point isn’t the best approach.  Instead, look for a probiotic containing strains with evidence-based benefits for the outcome you are seeking. 

Regardless of your purpose for taking probiotics—daily support, help with specific gastrointestinal symptoms, help with a specific health condition—speaking with a health professional about strains that have proven efficacious in human clinical trials is highly recommended.21

The following table provides the names of evidence-based probiotic species and species combination—and importantly, the level of scientific evidence supporting their benefits for various bodily systems.

Level of EvidenceSupports Health ofProbiotic Combinations
Probiotic Species
Gastrointestinal tract during and/or after antibiotics2224Bifidobacterium species 
Lactobacillus species 
Streptococcus species
Saccharomyces boulardii
Lactobacillus rhamnosus   
Streptococcus thermophilus
Bifidobacterium breve
Bifidobacterium infantis
Bifidobacterium longum
Bifidobacterium lactis
HighStomach and small intestine2529Lactobacillus species         
Bifidobacterium species
Streptococcus species         
Saccharomyces species
Bacillus species 
Enterococcus species              
Lactobacillus acidophilus
Bifidobacterium animalis
HighColon3032Bifidobacterium species         
Lactobacillus species 
Streptococcus species            
Lactococcus species
Lactobacillus acidophilus
Lactobacillus rhamnosus   
Lactobacillus paracasei
Lactobacillus plantarum
Lactobacillus casei         
Bifidobacterium animalis
Bifidobacterium longum
Bifidobacterium lactis
Bifidobacterium bifidum
Bifidobacterium breve
Bifidobacterium infantis
Streptococcus thermophilus
Streptococcus salivarus
Bacillus lactis 
Lactococcus lactis              
MediumImmune system33,34Bifidobacterium species         
Lactobacillus species 
Streptococcus species            
Lactobacillus paracasei 
Lactobacillus ramnosus 
Lactobacillus acidophilus 
Lactobacillus sporogenes
Bifidobacterium animalis
Bifidobacterium lactis
MediumGlucose metabolism3539Bifidobacterium species 
Lactobacillus species                        
Lactobacillus casei 
Lactobacillus ramnosus 
Lactobacillus acidophilus 
Lactobacillus sporogenes
Bifidobacterium bifidum
Bifidobacterium lactis
Bifidobacterium breve
Bifidobacterium longum
Streptococcus thermophilus
LowLipid (Fat) metabolism40,41Lactobacillus acidophilus
LowBlood vessels42Lactobacillus species                
LowExcessive crying in infants43Lactobacillus reuteri

How many strains should I look for?

People interested in probiotics often ask how many strains a probiotic should have. Is single-strain the best? Is 4 or 5 strains ideal? Or should you just go all in with the 32-strain mega-probiotic? The short answer to these questions is, “we don’t really know”. Theoretically speaking, the advantage to a multi-strain blend is that it has the potential to introduce a broader range of probiotic effects and mechanisms than a probiotic with fewer strains.44However, that isn’t always the case, and for some conditions, a single-strain may actually be more effective.45

Instead of picking the supplement with the most strains, look for a supplement offering  research-backed strains and strain combinations. After all, a supplement with 30 different strains and 50 billion CFU isn’t going to do you much good if its full of non-viable bacteria that can’t colonize the gut, and offers few probiotic mechanisms. 

Are probiotics safe? 

Probiotics are “generally recognized as safe” by the FDA.46 Some studies report minor gastrointestinal side effects, including loose stools, flatulence, and bloating; however, these symptoms are usually mild and temporary.47 These side effects are also less likely to occur when probiotics are taken with food. 

Although probiotics are generally safe, there is an inherent risk of infection and sepsis associated with introducing bacteria into the body. For this reason, individuals with severely compromised immune systems, preterm infants, and individuals using intravenous medical devices should refrain from taking probiotics, unless under the care of a health professional.48,49 Regardless of your health status, speaking with your doctor before starting a probiotic regimen is highly recommended.

Concluding thoughts

As advancements in microbial research continue to expand our understanding of the gut microbiota’s role in human health, one thing has become increasingly clear—these tiny microbes impact our health and physiology in a big way. So do yourself a favor and support microbial health in return. Ways to support a healthy microbiome include: eating a balanced and probiotic-rich diet, nourishing probiotic bacteria with prebiotics, limiting sugar consumption, avoiding antibiotics when possible, and taking a probiotic supplement with research-backed strains and CFU.  

Gina Jaeger, PhD is a Developmental Specialist and Lead Research Writer for Nordic Naturals. She holds a doctorate in Human Development, and has published several research articles on children's cognitive development. Gina enjoys studying and educating others on strategies for optimizing health and wellness throughout the lifespan.

Dysbiosis: The state of having an imbalance in the microbial communities on or inside the body.

Non-pathogenic: Not capable of causing disease.

Sepsis: Life-threatening condition that arises when the body's response to infection causes harm to its own tissues and organs.

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