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Current status of research on probiotic and prebiotic mechanisms of action

By Mary Ellen Sanders, PhD, ISAPP Executive Science Officer

Human intervention studies in the fields of probiotics and prebiotics assess the health effects of these ingredients, whether it’s improving specific symptoms or preventing the occurrence of a health condition. Yet scientists in the field recognize the importance of learning the ‘chain of events’ by which probiotics and prebiotics are able to confer health benefits. Such mechanistic insights allow better probiotic selection and development of therapeutic approaches, as well as more precise dosing.

Mechanisms of action for probiotics and prebiotics are complex and often difficult to pinpoint, especially since any given health benefit may derive from multiple co-functioning mechanisms. However, scientists have made incremental gains in understanding these mechanisms. This scientific progress was covered in a recent webinar co-presented by ISAPP and ILSI-Europe, titled Understanding Prebiotic and Probiotic Mechanisms that Drive Health Benefits. Speakers for the webinar were:

  • Sarah Lebeer, University of Antwerp, Belgium
  • Colin Hill, University College Cork, Ireland
  • Karen Scott, University of Aberdeen, UK
  • Koen Venema, Maastricht University – campus Venlo, The Netherlands

The webinar was held live on September 17, 2020. Of the 499 webinar registrants, 357 attended the webinar live from 57 countries, from Australia to the US. ISAPP and ILSI-Europe hope the webinar will serve as a resource for people who want a rapid overview about mechanisms of action.

Watch the full webinar here, and read further for a summary of key points from these experts.

Prebiotic benefits and mechanisms of action

Prebiotics are defined as substrates that are “selectively utilized by host microorganisms conferring a health benefit”. ‘Utilization’ in the gut may involve crossfeeding, which means products produced by the first microbes degrading the prebiotic can then be used by different members of the host microbiota – so it may take a series of complex steps to get to a final health outcome. However, selective utilization and health benefit are always required for a substance to meet the definition of a prebiotic.

The health benefit of a prebiotic can be local (in the gut) or systemic. Locally, prebiotics can act via fecal bulking, as they are typically types of fiber. In addition, they can produce short-chain fatty acids (SCFAs), which reduce gut pH and thereby can discourage pathogenic and toxigenic activity of gut microbes, increase calcium ion absorption and provide energy for gut epithelial cells.

Systemic functions of prebiotic metabolism include them being used as substrates for microbes that produce or interact with host cells to produce molecules with neurochemical, metabolic or immune activity. Further, SCFAs can end up in the blood and can reach the liver, muscles and the brain. The SCFAs interact with specific host receptors and can lead to the release of satiety hormones or interact with receptors in the liver, adipose tissue and muscle tissue, leading to reduced inflammation. Prebiotics can also interact directly with immune cells.

Probiotic health effects and mechanisms of action

Health and disease are the end results of complex interactions on a molecular scale within a human or animal host.  Host molecules also interact with microbial molecules, including those molecules introduced with or produced by probiotics. Designing studies to discover probiotic mechanisms in human research is extremely challenging because both host and probiotic are very complex systems that most probably engage with one another on multiple levels. Probiotic molecules can have direct effects and downstream effects, and we are aware of only a few cases where a health effect can be tied to one specific probiotic molecule.

Probiotics can interact directly with the host, but also can act indirectly by influencing the microbiome. There may be many different mechanisms by which a given probiotic interacts with the host.

It is interesting to note that probiotics use some of the same types of mechanisms (pili, small molecule production, etc.) that are used by pathogens, microbes that have a detrimental effect on the host.  But these shared mechanisms are usually connected to surviving or colonising strategies, not those that cause damage to the host.

L. rhamnosus GG is a well-researched model probiotic, for which many mechanisms have been identified, including pili, immune modulators and lactic acid production, some mechanisms shared with other probiotic strains and species. Other studies have identified mechanisms for novel types of probiotics. For example, in mice and humans taking a strain of Akkermansia, heat killed cells had the same or even better effect on markers of metabolic health, which implies that the molecules (perhaps proteins in the bacteria, unaffected by heat treatment) are mediating the effect in this case.

See here to watch the webinar in full.

 

 

ISAPP partners with British Nutrition Foundation for fermented foods webinar

Did you miss the live webinar? Access the archived version here. Read the speaker Q&A here.

From sourdough starter tips to kombucha flavor combinations – if you’ve checked a social media feed lately, you’ll know how many people are sharing an interest in fermented foods as they self-isolate during the pandemic. And with this rise in popularity comes a host of questions about the practice and the science of fermented foods.

To meet the need for science-based information about fermented foods, ISAPP has partnered with the British Nutrition Foundation (BNF) on a free webinar titled ‘Fermented Food – Separating Hype from Facts.’ The BNF is a UK-based registered charity that brings evidence-based information on food and nutrition to all sectors, from academia to medicine.

The webinar, designed for practicing dietitians and nutrition-savvy members of the public, featured three leading scientific experts who explained the microbiology of fermented foods, the evidence for their health effects, and who might benefit from making these foods a regular part of the diet. Viewers will come away with a clear understanding of what fermented foods are and what evidence exists for their health benefits.

The webinar was held live on Wednesday, July 1, 2020 from 1pm-2pm (BST).

Webinar speakers & topics

 Understanding fermented foods: Dr. Robert Hutkins, University of Nebraska, USA

Exploring the evidence for effects of fermented foods on gastrointestinal health – how strong is it? Dr. Eirini Dimidi, Kings College London

What role can fermented foods have in our diet? A public health perspective, Anne de la Hunty, British Nutrition Foundation

For a quick primer on fermented foods, see the short ISAPP video here or the ISAPP infographic here.

Is probiotic colonization essential?

By Prof. Maria Marco, PhD, Department of Food Science & Technology, University of California, Davis

It is increasingly appreciated by consumers, physicians, and researchers alike that the human digestive tract is colonized by trillions of bacteria and many of those bacterial colonists have important roles in promoting human health. Because of this association between the gut microbiota and health, it seems appropriate to suggest that probiotics consumed in foods, beverages, or dietary supplements should also colonize the human digestive tract. But do probiotics really colonize? What is meant by the term “colonization” in the first place? If probiotics don’t colonize, does that mean that they are ineffective? In that case, should we be searching for new probiotic strains that have colonization potential?

My answer to the first question is no – probiotics generally do not colonize the digestive tract or other sites on the human body. Before leaping to conclusions on what this means for probiotic efficacy, “colonization” as defined here means the permanent, or at least long-term (weeks, months, or years) establishment at a specific body site. Colonization can also result in engraftment with consequential changes to the gut microbiota composition and function. For colonization to occur, the probiotic should multiply and form a stably replicating population. This outcome is distinct from a more transient, short-term (a few days to a week or so) persistence of a probiotic. For transient probiotics, it has been shown in numerous ways that they are metabolically active in the intestine and might even grow and divide. However, they are not expected to replicate to high numbers or displace members of the native gut microbiota.

Although some studies have shown that digestive tracts of infants can be colonized by probiotics (weeks to months), the intestinal persistence times of probiotic strains in children and adults is generally much shorter, lasting only few days. This difference is likely due to the resident gut microbiota that develops during infancy and tends to remain relatively stable throughout adulthood. Even with perturbations caused by antibiotics or foodborne illness, the gut microbiome tends to be resilient to the long-term establishment of exogenous bacterial strains. In instances where probiotic colonization or long-term persistence was found, colonization potential has been attributed more permissive gut microbiomes specific to certain individuals. In either case, for colonization to occur, any introduced probiotic has to overcome the significant ecological constraints inherent to existing, stable ecosystems.

Photo by http://benvandenbroecke.be/ Copyright, ISAPP 2019.

This leads to the next question: Can probiotics confer health benefits even if they do not colonize? My answer is definitely yes! Human studies on probiotics with positive outcomes have not relied on intestinal colonization by those microbes to cause an effect. Instead of colonizing, probiotics can alter the digestive tract in other ways such as by producing metabolites that modulate the activity of the gut microbiota or stimulate the intestinal epithelium directly. These effects could happen even on short-time scales, ranging from minutes to hours.

Should we be searching for new probiotic strains that have greater colonization potential? By extension of what we know about the resident human gut microbiota, it is increasingly attractive to identify bacteria that colonize the human digestive tract in the same way. In some situations, colonization might be preferred or even essential to impacting health, such as by engrafting a microbe that performs critical metabolic functions in the gut (e.g. break down complex carbohydrates). However, colonization also comes with risks of unintended consequences and the loss of ability to control the dose, frequency, and duration of exposure to that particular microbe.

Just as most pharmaceutical drugs have a transient impact on the human body, why should we expect more from probiotics? Many medications need to be taken life-long in order manage chronic conditions. Single or even repeated doses of any medication are similarly not expected to cure disease. Therefore, we should not assume a priori that any observed variations in probiotic efficacy are due to a lack of colonization. To the contrary, the consumption of probiotics could be sufficient for a ripple effect in the intestine, subtly altering the responses of the gut microbiome and intestinal epithelium in ways that are amplified throughout the body. Instead of aiming for engraftment directly or hand-wringing due to a lack of colonization, understanding the precise molecular interactions and cause/effect consequences of probiotic introduction will lead to a path that ultimately determines whether colonization is needed or just a distraction.

blog post resilience figure 1

Resilience as a measure of health: implications for health claims for foods

January 16, 2018. By Mary Ellen Sanders PhD, Sylvie Binda PhD, Seppo Salminen PhD, Karen Scott PhD

Demonstrating health benefits for healthy people is a challenge faced by those attempting to communicate claims on a health promoting food. Foods, in many global regulatory frameworks, are intended for the general population. Therefore, any benefits ascribed to them, the logic goes, must be demonstrated in the generally healthy population.

An old concept has new-found notoriety in the context of offering an approach for establishing health benefits for healthy people. It is the concept of resilience. In an ecological sense, resilience refers to the ability of an ecosystem to withstand perturbation and continue normal function, i.e. maintain homeostasis. In the context of human physiology, resilience enables a host to remain healthy even when exposed to a stress, or to recover from a stress faster. A variety of external challenges such as drugs, pathogens, emotional stress, poor diet among others, may perturb normal physiological function or disrupt the gut ecosystem. Individuals more able to maintain stability of physiological functions when exposed to such challenges would be healthier than those who cannot maintain stability.  Thus, a food would be considered to have a beneficial effect if it could increase the resilience of the consumer to a challenge.

This concept was described in an EFSA guidance document on biological relevance of data in scientific assessments:

“When subject to a disturbance, a biological system enters in a transient state: a process variable has been changed and the system has not yet reached steady state. Some systems, including humans, have the capacity to regulate their internal environment and to maintain a stable, relatively constant condition of properties; it is called ‘homeostatic capacity’. Resilience represents the amount of disturbance that can be absorbed by a system before the system changes or loses its normal function, or the time taken to return to a stable state, within the normal operation range following the disturbance…” [Reducing] “homeostatic capacity … might be detrimental, whereas increasing the capacity could be beneficial.”

This concept aligns with the definition of ‘health’, which includes the ability to adapt to the environment.

Resilience of gut microbiota

This concept of resilience can be applied to the human microbiota as an ecosystem. Once established in early childhood, our colonizing microbiota reaches a relatively stable state. Although brief fluctuations occur, especially in relation to daily diet and medicines used, the microbial ecosystem of a healthy adult provides relatively stable functionality.  Disruption of the microbiota by repeated stressors can be associated with poorer health. There seems to be a solid rationale that the ability of the colonizing microbiota to resist, or recover quickly from, perturbations reflects a person’s ability to remain healthy. The microbiota stability may be indicated in either populations of bacteria or their metabolic output.

Homeostasis and health: a statistical approach

“A statistical approach to measuring improved health was proposed by Dr. Dan Tancredi at the 2010 ISAPP meeting. It is reprinted here from: Sanders, et al. 2011. Health claims substantiation for probiotic and prebiotic products. Gut Microbes 2:3, 1-7.

An approach to measuring improved health may be to measure homeostasis, as suggested by D. Tancredi. From a statistical point of view, if an intervention were able to minimize the variation around the mean for a specific measure (even in the absence of changing the mean; Fig. 1), it could be a reflection of improved health, assuming a biological rationale exists that tighter control of the parameter is physiologically advantageous. In other words, lessening the fluctuation around an individual’s biomarker could be interpreted as contributing to improving health. This novel idea emphasizes the importance of homeostasis as a focus of studies on health, and provides a rationale based in solid statistical theory as a way to measure this.

One challenge to demonstrating the value of this approach is to identify appropriate biomarkers that could be studied. The following properties would be important to a relevant biomarker for homeostasis:

blog resilience figure one

  • maintaining moderate levels of the biomarker is associated with good health;
  • high or low values are associated with ill health;
  • biomarker levels in the same person can fluctuate over time; and
  • reducing the magnitude or duration of such fluctuations in healthy people is considered desirable (Fig. 2).

Such a biomarker could be an individual endpoint or be formed as a ratio of two other biomarkers, when maintaining the same relative amounts of the two component biomarkers would be desirable.

Assuming a biomarker with the above properties is available, it could be used as the outcome measure in a randomized controlled trial to provide evidence that the experimental food is able to improve the maintenance of health in humans. Statistically, the trial would be set up to address the hypothesis that the experimental substance is associated with lower variation in biomarker levels, compared to the control arm, in subjects who were healthy at baseline. Such a trial would be able to use information on within-person variations in biomarker levels, even those who did not become ill. Partly as a result of the more efficient use of study data, such a trial would require far fewer subjects than an intervention that instead addressed the hypothesis that treatment is associated with fewer healthy persons becoming ill.

A mounting understanding of the value of stability of the colonizing microbial communities makes this endpoint an attractive one to consider. Perturbation of gut microbiota is associated with intestinal dysfunction, as illustrated during antibiotic treatment. Specific probiotics have been shown to promote a quicker rebound from antibiotic-induced microbiota disruption, including a study on Lactobacillus rhamnosus GG (LGG) (Cox et al. 2000). This paper concludes ‘…that a key mechanism for the protective effect of LGG supplementation on the subsequent development of allergic disease is through the promotion of a stable, even and functionally redundant infant gastrointestinal community.’

However, it would be useful to define additional biomarkers that would be appropriate targets for this type of investigation.

In pediatric nutrition, the measurement of metabolic homeostasis has become a standard approach when developing infant formulas (Heird, 2005).  The concept of homeostasis as a model to distinguish between foods (including food supplements) and medicinal products was explored by the Council of Europe (2011), and is an interesting correlate to the above hypothesis.”

Conclusions

The recent recognition by EFSA that maintenance of homeostasis is a valid measure of health provides an opportunity to apply this concept to validate health benefits of specific foods and food ingredients. Stability of microbial populations, microbial metabolism or host physiological readouts could be measured to reflect the concept of resilience. Since there is no definitive composition of a ‘healthy human microbiota’, a more reasonable target for measuring positive impacts of a probiotic on the microbiota would be reflected not in absolute levels of specific microbes but in the ability of a specific probiotic or prebiotic to bolster the resilience of the microbiota.

 

References:

Council of Europe. Homeostasis, a model to distinguish between foods (including nutritional supplements) and medicinal products 2008; (Accessed February 24, 2011, at http://www.coe.int/t/e/social_cohesion/soc-sp/homeostasis%20%282%29.pdf ).

Cox MJ, Huang YJ, Fujimura KE, Liu JT, McKean M, Boushey HA, et al. Lactobacillus casei abundance is associated with profound shifts in theGunderson LH, 2000. Ecological resilience: in theory and application. Annual Review of Ecology and Systematics, 31, 425–439.

EFSA guidance document:  Guidance on the assessment of the biological relevance of data in scientific assessments; July 12, 2017; EFSA Journal 2017;15(8):4970

Heird WC. Biochemical homeostasis and body growth are reliable end points in clinical nutrition trials. Proceedings of the Nutrition Society 2005; 64:297-303.

Huber M, Knottnerus JA, Green L, van der Horst H, Jadad AR, Kromhout D, Leonard B, Lorig K, Loureiro MI, van der Meer JW, Schnabel P, Smith R, van Weel C, Smid H (2011). “How should we define health?” BMJ. 343:d4163.

Sanders, et al. 2011. Health claims substantiation for probiotic and prebiotic products. Gut Microbes 2:3, 1-7; May/June 2011