What are fermented foods?
Fermented foods are obtained through the action of microorganisms, namely bacteria, yeasts, and mycelial fungi, and their enzymes, in a process referred to as fermentation.
Microorganisms may be indigenously present on the food, or added as a starter culture, or they may be present in or on the ingredients and utensils, or in the environment. Basically, a suitable food, appropriate microorganism(s) and proper environmental conditions, such as temperature, pH, and moisture content must coexist in order to enable food fermentation.
During fermentation, carbohydrates and related compounds are partially oxidised and energy is released in the absence of any external electron acceptor. Factors like type of sugar, limited or unlimited nutrient and oxygen availability, presence of competitive microorganisms, and time influence the process.
Fermented foods have long been produced according to knowledge passed down from generation to generation and with no understanding of the potential role of the microorganism(s) involved in the process. However, the scientific and technological revolution in Western countries made fermentation turn from a household process to a controlled one, suitable for industrial scale production systems intended for the mass marketplace.
Nowadays, indigenous fermented foods such as dahi, bhalle, papad, idli, and dosa, made in rural and tribal areas by using local knowledge and locally available raw materials, coexist with industrially prepared fermented food products. It has been estimated that thousand different fermented foods and beverages are produced globally, using a wide range of different raw materials, microorganisms, and manufacturing techniques.
Over the last years, a renewed interest in fermented foods has been observed in Western countries largely driven by their supposed health benefits. To the best of our knowledge, several reviews emphasising health-benefits of fermented foods have been published, however none of them analysed and discussed the healthy components that form upon fermentation of the main food matrices (namely, milk, cereals and other grains, fruit and vegetables, meat and fish) and their possible effect on human health and well-being.
What are the benefits of fermented foods?
Fermented foods contain potentially probiotic microorganisms, such as lactic acid bacteria. In general, most fermented products have been found to contain at least 106 microbial cells per gram, with concentrations varying depending on several variables such as the product’s region, age and time at which the products are analysed/consumed.
The surrounding food matrix appears to play an important role in the survival of probiotic strains via its buffering and protective effect against gut conditions (e.g., low pH, bile acids). Indeed, a number of studies have shown that microorganisms from fermented foods can reach the gastrointestinal tract, this is likely to differ across products, and their presence in the gut appears to be transient.
Nonetheless, these microorganisms may still have the potential to exert a physiological benefit in the gut, through competition with pathogenic bacteria and the production of immune-regulatory and neurogenic fermentation by-products.
Secondly, fermentation-derived metabolites may exert health benefits. For example, lactic acid bacteria (relevant to both dairy and non-dairy fermented foods) generate bioactive peptides and polyamines with potential effects on cardiovascular, immune and metabolic health.
Thirdly, fermentation may convert certain compounds to biologically active metabolites. For example, lactic acid bacteria can convert phenolic compounds (such as flavonoids) to biologically active metabolites.
Fourthly, food components found in fermented foods, such as prebiotics and vitamins, may also exert health benefits.
Lastly, fermentation can reduce toxins and anti-nutrients, for example, fermentation of soybeans may reduce phytic acid concentrations, and sourdough fermentation can reduce the content of fermentable carbohydrates (e.g., fermentable oligosaccharides, disaccharides, monosaccharides and polyols, FODMAPs), which may increase the tolerance of these products in patients with functional bowel disorders such as irritable bowel syndrome.
A number of controlled human dietary studies have recently been undertaken. These studies include investigations that revealed strong associations between weight management and consumption of fermented dairy products, reduced risk of cardiovascular disease, type 2 diabetes, and mortality associated with consumption of yoghurt, and enhanced glucose metabolism and reduced muscle soreness following acute resistance exercise as a consequence of consuming fermented milk.
Consumption of kimchi was linked to anti-diabetic and anti-obesity effects , while consumption of different fermented foods was associated with alterations in mood and brain activity and in the gut microbiome.
However, reports have identified a lack of sufficient clinical trials, variation in the different fermented foods being investigated, and inconsistencies among ethnic groups in suggesting that more studies need to be undertaken in order to confirm the potential benefits of fermented food consumption.
Do Fermented Foods Modulate the Gut Microbiome?
The ability of fermented foods to modulate the gut microbiome has been documented by several groups with varying degrees of success.
A recent study evaluated the effect of general fermented plant intake on microbial and metabolomic differences in consumers versus non-consumers (nearly 7000 participants total), and determined that beta diversity was significantly different between consumers versus non-consumers.
In a similar fashion to the previous study, Wastyk and colleagues (2021) examined the effects of a diet rich in fermented foods (including fermented dairy products, vegetables and non-alcoholic beverages) on eighteen healthy adults over a period of seventeen weeks in parallel to a diet high in fibre.
The dietary intervention consisted of an initial four-week period whereby the quantity of fermented food in the diet was increased, followed by a six-week ‘maintenance’ period of very high fermented food intake, and closed with a ‘choice’ period of four weeks in which participants could maintain whatever level of fermented food intake they desired.
The fermented food-rich diet resulted in an increase in alpha diversity of the gut microbiome that was not observed with the fibre diet.
Interestingly, the increase in microbiome diversity remained during the designated ‘choice’ period despite intake being higher during the ‘maintenance’ period, with a strong relationship between time and diversity.
Do fermented foods impact gut microbiome?
The human gut microbiome has received much attention in recent years, with increasing evidence that it impacts both physical and mental health and that many metabolic disorders are associated with disruption to the gut microbiome.
Lifestyle, including diet, can impact the gut microbiome, and there is increasing interest in the potential of exploiting foods to positively modulate it.
Fermented foods offer an opportunity to positively impact the gut microbiome by either
(I) providing nutrients to promote or inhibit members of the gut microbiome.
(II) members of the food microbiome establishing residence in the gut and/or interacting with the resident gut microbiome.
However, few studies to date have specifically investigated the impact of fermented food consumption on the gut microbiome. Certain bioactive compounds produced by the microbes within food, including polyphenols and short chain fatty acids, can have beneficial effects when consumed.
Certain microbial strains found in food are capable of surviving digestion, and fermented foods can act as useful vehicles to carry probiotic strains safely into the gut. However, there are very few studies which focus on the potential of microbiota from fermented foods to impact the gut microbiome; one study investigating the consequences of yoghurt consumption observed higher levels of Lactobacillaceae associated with the gut contents.
In addition, most studies of the gut microbiome focus on the microbiota of the large intestine, due primarily to ease of sampling; however, it is likely that fermented foods have more influence on the microbiome in the small intestine, as this location exhibits a higher proportion of Lactobacillaceae, and other more aerotolerant bacteria often associated with fermented foods.
The possibility that fermented foods impact the gut microbiome is intriguing, and merits more study and additional efforts to include investigations of the small intestine.
Conclusion on fermented foods
There is only very limited evidence on the effectiveness of most fermented foods in gastrointestinal health, with the majority of studies being of low quality.
Kefir is the fermented food most commonly investigated in terms of its impact in gastrointestinal health, with evidence suggesting it may be beneficial for lactose malabsorption and H. pylori eradication.
No human studies have been conducted on the impact of kombucha, tempeh and kimchi in gastrointestinal health.
It is worth noting the difficulty in undertaking and replicating fermented food studies given the significant variability of cultures and ingredients present even within food categories, which may partly explain heterogeneous findings.
To conclude, there is insufficient evidence to determine the impact of fermented foods in gastrointestinal health and disease.