What does “gut health” mean?

By Prof. Maria Marco PhD, University of California – Davis

Probiotics and prebiotics are frequently marketed to consumers for their capacity to improve or support gut health. Dietitian nutritionists responding to a survey ranked fermented foods as the top superfood for the past six years explaining gut health as a primary reason for their choice. But what is gut health exactly?

As it turns out, there is not a widely accepted definition of gut health. Dr. Stephan Bischoff at the University of Hohenheim, Germany, nicely summarized the situation in a perspective back in 2011. Using criteria from the World Health Organization, he proposed that gut health be defined as “a state of physical and mental well-being in the absence of gastrointestinal complaints that require the consultation of a doctor, in the absence of indications or risks of bowel disease, and in the absence of confirmed bowel disease”. The term gut health has since been increasingly used in scientific publications. However, is gut health really only the absence of complaints or indications, risk, or disease? Is gut health a condition that requires physical and mental well-being?

For the first question, it seems reasonable that gut health would refer to an absence of bowel diseases and acute or even mild symptoms localized to the digestive tract such as food intolerance, abdominal pain, nausea, flatulence, bloating, constipation, and diarrhea. The etiology of these presentations can be traced back to disruptions in the normal functioning of the gastrointestinal tract, including undesired dietary nutrient breakdown and absorption, pathogen introduction and colonization, and intestinal inflammation. However, recent studies of the intestinal environment, encompassing both the intestinal microbiome and mucosa, suggest that an absence of complaints or disease does not directly mean our gut is healthy. Mild mucosal inflammation, increased barrier permeability, or the presence of certain potentially undesirable intestinal microorganisms may confer no overt symptoms, yet still could signify the presence of an undesired or unhealthy intestinal state. The outcomes of that imperceptible unhealthy state may not be realized until years later with the development of intestinal disease or conditions at extraintestinal sites.

This latter point evokes the second question: Is gut health a condition that requires physical and mental well-being? The answer from popular media is – yes! Diseases and chronic conditions that are not overtly related to the gastrointestinal tract, such as allergy, arthritis, obesity, cancer, mood disorders and depression, are now considered by many to be traceable back to gut health. To that regard, it is now well-established scientifically that our gastrointestinal tract is indeed an important organ, housing the majority of our microbiome and mucosal immune system and pivotal for systemic metabolism and neurological signaling. However, I wonder if the term “gut health” is at all appropriate when implying such a broad range of whole-body responses? Could it be that “gut health” is seen as the root or origin of our overall health?

One way to reconcile this broad interpretation of gut health is to consider that “gut health” has become a simple way to explain, interpret, and understand how diets intersect with overall physical and mental well-being. Our daily lives are structured around mealtimes and the foods we eat don’t just provide nutrients, but also social interactions, and can be affected by our socioeconomic status among many other factors. We connect our gut with sensations felt when hungry, full, and after drinking an alcoholic or caffeinated beverage. The gut also connects to diet-based risks for the development of non-communicable diseases over our lifetimes. The quote “all diseases begin in the gut” attributed to Hippocrates still rings true after all the medical advancements over the past 2400 years.

So, since the term “gut health” has such a broad interpretation, we should be qualifying any statement that a biotic or fermented food supports “gut health” with an explanation for the specific feature(s) of gut health that are being improved with biotic use. Perhaps in the future, good gut health, and even good health generally, can be defined. Until then, we only appreciate how we are starting to get closer to understanding the true interconnectedness of the diet-gut-microbiome axis with our overall health and well-being.



Are probiotics effective in improving symptoms of constipation?

By Eirini Dimidi, PhD, Lecturer at King’s College London

Constipation is a common disorder that affects approximately 8% of the general population and is characterised by symptoms of infrequent or difficult bowel movements (1). People who suffer with constipation often report that it negatively affects their quality of life and the majority use some sort of treatment, such as fibre supplements and laxatives, to alleviate their symptoms (2). However, approximately half of those report they are not completely satisfied with the treatment options currently available to them, mainly due to lack of effectiveness in improving their symptoms (2).

Could probiotics offer an effective alternative way to treat constipation symptoms?

Our team at the Department of Nutritional Sciences at King’s College London has investigated the potential benefits of probiotic supplements in chronic constipation. We have extensively reviewed the available evidence on their mechanisms of action in affecting gut motility and their effectiveness in improving symptoms, and we have also conducted a randomised controlled trial of a novel probiotic in 75 people with chronic constipation (3-5).


Before looking at the evidence on the effectiveness of probiotics in constipation, it is easy to see that some people with constipation already choose to try probiotics for their gut health. A national UK survey of over 2,500 members of the public, which included people with and without constipation, showed that people with constipation have a 5.2 higher chance of currently using probiotics for gut health, compared to people who don’t suffer from it (3).

However, the majority of doctors do not recommend probiotics for the relief of constipation symptoms, nor do they believe there is enough evidence to support their use in this condition (3).

So, what is the current evidence on probiotics and constipation?


Probiotics may impact gut motility and constipation through several mechanisms of action. Depending on the strain, they may affect the number and composition of gut microbes, as well as the compounds they release. The gut microbiota and their released compounds can then interact with our immune and nervous system, with the latter being the primary regulator of gut motility, ultimately improving constipation symptoms. Therefore, there is a rationale to support a potential improvement in constipation. But is this supported by evidence from clinical trials?


A systematic review of the literature showed Bifidobacterium lactis strains appear to improve several symptoms of constipation, such as infrequent bowel movements and hard stools (4). At the same time, other probiotic species did not improve any symptoms. This is an important finding as it highlights that not all probiotics have the same effects in constipation, and that only certain probiotics may improve constipation. Therefore, people with constipation may only benefit from specific probiotic products – but which products would those be? Since the systematic review above showed that several B. lactis strains were effective, does this means that people with constipation may benefit from any B. lactis-containing product?

Unfortunately, it is a bit more complicated. Since the publication of the aforementioned review, new studies have been published showing that, while some probiotic products with B. lactis are effective, various other B. lactis probiotics do not impact constipation (5-6). This may be explained by strain-specific effects, but also other methodological differences among studies (e.g. probiotic dose).


Can we recommend probiotics for the management of constipation? At the moment, there is some low quality evidence to support the use of certain Bifidobacterium lactis strains to help manage symptoms of constipation. Further high-quality studies are needed to clarify which specific probiotic strains may be effective. However, given that there is some evidence in this area (albeit limited), along with the fact probiotics are safe for the general population to consume (unless clinically contraindicated), people with constipation could try a probiotic product of their choice for four weeks, should they wish to, bearing in mind the uncertainty in the evidence so far. But scientists continue to work to answer this question because the evidence is promising enough to warrant continued study of probiotics for constipation.


    1. Palsson, Gastroenterol 2020;158:1262-1273
    2. Johanson & Kralstein, Aliment Pharmacol Ther 2007;25(5):599-608
    3. Dimidi et al, Nutrition 2019;61:157-163
    4. Dimidi et al, Am J Clin Nutr 2014;100(4):1075-84
    5. Dimidi et al, Aliment Pharmacol Ther 2019;49:251-264
    6. Wang et al, Beneficial Microbes 2021;12:31-42