The Western diet, which is characterised by high consumption of processed foods – high in sugars, salt, and fat – and little consumption of fresh and fibrous vegetables and fruits, has been associated with numerous gastrointestinal problems [1]. Many of these problems are accompanied by symptoms such as bloating, constipation, or diarrhoea. Unfortunately, in many cases, these symptoms cannot always be pinpointed to a specific disease (such as inflammatory bowel disease (IBD)), but rather are classified as irritable bowel syndrome (IBS). With no treatment or cure for IBS, one of the few things doctors can recommend to these people is to find a solution in their diet, either by changing it or by adding supplements, such as probiotics. These days, “probiotic” products can easily be found, whether in milk, yoghurt, cereals, or even added to your favourite chocolate bar. The popularity of probiotics has grown exponentially in recent years, but what do we actually know about the health benefits? This article aims to summarise the current understanding of gut probiotics and will provide more information about how and when to use them.

The human gut holds more bacteria than the human body holds cells [2]. In an ideal world, these trillion microorganisms live in harmony in our intestinal tract and provide us with a large range of health benefits. Having more “good” bacteria in your gut can inhibit the growth of “bad” microorganisms by improving the transit time of food passing through or by lowering the gut pH [3]. Unfortunately, the harmony of our gut microbiome is in many cases far from perfect. Recent studies have clarified that many gastrointestinal disorders are characterised by dysbiosis in the gut microbiota. This means that the composition and activity of certain bacteria species is overruling other – perhaps more beneficial – species.

It is now well known that the microbiota composition can change upon dietary alterations. Gut bacteria adjust their metabolism according to the nutrient supply. On one hand, excess sugar, fat, and red meats have been shown to be detrimental to our gut microbiome. Research into this type of diet states that toxin producing bacteria become more abundant [4, 5], associating these diets with chronic inflammation [6, 7]. On the other hand, an increased consumption of fibre is associated with positive effects on gut function [8, 9]. These fibres serve as a food source for the gut’s own microorganisms and are used to form beneficial substances [10]. As an example, fresh fruits and vegetables contain dietary fibres, such as inulin and pectin. These fibres cannot be absorbed in the small intestine, but rather arrive intact in the large intestine (home to most gut bacteria), where they feed the microbiome. Our gut bacteria can convert these fibres to, for example, short-chain fatty acids, a bioactive metabolite which has been shown to hold anti-inflammatory, antitumorigenic, and antimicrobial effects [11]. These fibres are defined as prebiotics and differ from probiotics. To sum up this difference: prebiotics are food for our already inhabitant microbiome, whereas probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host [12]. These microorganisms occur naturally in fermented food, such as yoghurt, where they ferment lactose into lactic acid. Kefir is another fermented dietary product that besides containing more probiotic bacterial strains, also contains yeast. Two yeast species, Kluyveromyces lactis and Saccharomyces unisporu, isolated from the kefir grain, were successful in inhibiting the growth of Salmonella during an in vitro study [13]. 

Having said that, it is important to mention that there are many different subsets of bacterial species (i.e. bacterial strains). Those have species- or strain-specific mechanisms which all differ slightly from one another. Probiotic supplements come in various forms such as capsules, powders, liquids, and more, encompassing a diverse range of strains and dosages. Frequently, these supplements consist of a blend of live microorganisms rather than individual strains. The efficacy of numerous commercially available “probiotic” products remains unexplored in scientific research, making it challenging for individuals without a background in probiotic studies to recognise which products are supported by scientific evidence. Moreover, probiotics are not regulated by the Food and Drug Administration (FDA) yet [14], which means that companies do not have to prove that the supplements work before selling them. 

The importance of a healthy gut microbiome is reiterated repeatedly by many researchers. The National Institutes of Health (NIH) allocated 7.5% ($3.1 billion) of its annual budget to gastrointestinal research in 2020 [14]. In 2018, 40 million people in the US alone, were affected with gastrointestinal diseases, leading to health care expenditures of a total of $119.6 billion [15]. Interestingly, a striking increase in gastrointestinal disorders has been reported due to the COVID-19 pandemic. Whereas some studies show a direct correlation between COVID-19 infection and gut health [16], other researchers state that quarantine and stressful periods are the origin of this increase [17, 18]. Due to the increase in gastrointestinal disorders and a better understanding of the importance our gut bacteria play in our health, research in probiotics has thrived over the last few decades.

A significant portion of our understanding regarding the mechanisms of probiotics stems from research conducted using animals (in vivo), or artificial experimental setups (in vitro and ex vivo models) [19]. Many of the success stories are therefore based on the workings of probiotics in rodents or on cells in a Petri dish. As with any study that uses animal models, the findings might not be fully translatable to the human situation. What must be kept in mind too is that the benefits of probiotics may diminish once their consumption is discontinued. For people suffering from an inherent dysbiosis in their microbiome, the positive effects might only be transient [20]. Lastly, a standardised method of studying probiotics has not been established yet, making it challenging to compare results from different studies [21, 22].

Nevertheless, there are many studies in the field that have shown positive effects of probiotics. A big meta-analysis of pooled studies has led researchers to the conclusion that probiotics are mostly safe to use and can be beneficial for certain ailments such as antibiotic-associated diarrhoea, irritable bowel syndrome, and bacterial-related gut infections [23]. However, the available evidence for e.g., Crohn’s disease or traveller’s diarrhoea, is lacking regarding its efficacy [23, 24]. Similarly, the beneficial effects are also questionable for healthy individuals, who already harbour a harmonised gut microbiota and would not need the extra support offered by probiotics. 

As the human gut microbiota is complex and highly individualised, what works for one person may not work for another. Therefore, it is important to note that probiotics are not a panacea for all health issues. Lastly, there could be potential adverse consequences by taking too many or the wrong probiotics for example, infections, production of harmful compounds by the microorganisms, and transfer of antibiotic resistance genes from probiotic microorganisms to other microbes within the gut [25]. Thus, the use of probiotics should be based on individual needs and under the guidance of healthcare professionals.