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In recent years, there has been growing interest in the influence of the gut microbiota on metabolic health, immunity, and the functioning of the nervous system. There is growing evidence that disturbances in the composition and diversity of the microbiota may play a significant role in the development of obesity, insulin resistance, metabolic syndrome, and non-alcoholic fatty liver disease (NAFLD) as early as childhood [1]. At the same time, we are observing a rapid increase in the incidence of these disorders – according to WHO data, the number of overweight or obese children has already exceeded 340 million worldwide. In Poland, this problem affects up to 20–30% of schoolchildren.
In this context, the question of the role of simple, natural dietary interventions returns, and fermented foods occupy a special place among them. These products, present in the culinary traditions of almost all cultures worldwide, are gaining new importance today as a potential tool to support the prevention and treatment of metabolic disorders by acting on the microbiota-gut-host axis [2, 3]. Moreover, some studies suggest that the positive effects may already start in foetal life: the consumption of a fermented diet by the mother may protect the offspring against intestinal inflammation and dysbiosis [1].
Fermentation is a process of microbiological transformations, which creates not only new flavours and food structures, but also numerous bioactive chemical compounds. Among them, short-chain fatty acids (SCFAs) are of particular importance, such as acetate, propionate and butyrate, as well as lactic acid, acetic acid, bioactive peptides, polyphenols and so-called postbiotics, metabolites of microorganisms that exhibit physiological effects even without the presence of live bacteria [4]. As has been shown, SCFAs affect the tightness of the intestinal barrier, the activity of T-regulatory lymphocytes, lipid and glucose metabolism, and can also perform signaling functions in the gut-brain axis through GPR41/GPR43 receptors located, among others, in the intestine, adipose tissue, and the nervous system. Importantly, fermented food works not only due to the presence of live probiotic bacteria – more and more attention is paid to postbiotics and their potential therapeutic application, especially in the context of inflammation and metabolic diseases [3]. At the same time, it is worth emphasising that different fermented products contain different microbiotic “signatures” and metabolite compositions, which may partially explain their diverse biological effects observed in studies. The composition of microorganisms and their metabolites depends on the type of substrate, strains used for fermentation, maturation time, as well as environmental conditions and storage method [2].
The gut microbiota plays a key role in regulating metabolic functions – it influences energy homeostasis, mucosal immunity, and intestinal barrier integrity. Its depletion and imbalance (dysbiosis) are associated with the development of obesity, insulin resistance, type 2 diabetes, and low-grade chronic inflammation. Mechanically, a disturbed microbiota can lead to reduced SCFA production, increased intestinal epithelial permeability, and translocation of lipopolysaccharides (LPS) into the general circulation, resulting in the so-called metabolic endotoxemia [1, 5]. Consumption of fermented foods can modulate the microbiota in many ways – by increasing the share of beneficial bacteria such as Akkermansia muciniphila, Bifidobacterium, Faecalibacterium prausnitzii, or Lactobacillus. Studies in animal models and humans have demonstrated that regular consumption of fermented foods leads to increased intestinal microbial diversity, an increase in the number of bacteria producing butyrate, and improvements in metabolic parameters, such as insulin sensitivity and triglyceride levels [6, 7]. Interestingly, changes in the microbiota can appear after just a few days of supplementation with fermented foods, and their metabolic effects can be long-lasting. Fermented plant products are also suggested to be more effective, as they contain prebiotic compounds and polyphenols that work synergistically with microorganisms [5]. Studies conducted in animal models have shown that metabolites such as lactic acid can act independently of the presence of live microorganisms – they affect the differentiation of T-regulatory lymphocytes (Treg), increase the production of mucins by goblet cells, and support the regeneration of the intestinal epithelium [6]. As a result, the intestinal barrier is strengthened, permeability is reduced, and translocation of inflammatory factors is limited, which reduces the level of so-called metabolic endotoxemia. At the same time, fermented foods support the colonisation of beneficial bacteria that compete with pathogens, stabilise local pH, and produce anti-inflammatory metabolites. Some data indicate that regular consumption of fermented foods leads to lasting changes in the composition and activity of the microbiota, especially in people with an initially depleted gut flora [5].
Importantly, the effects of fermented foods are not limited to the digestive tract. More and more studies indicate their impact on the gut-brain axis. In clinical studies, the consumption of fermented products was associated with improved mood, lower cortisol levels, and reduced anxiety symptoms, also in children and adolescents [5, 8]. It is suggested that the beneficial effects may result from, among others, the influence of SCFA on the production of neurotransmitters (e.g., GABA, serotonin), regulation of the reactivity of the HPA axis, and the expression of neuroimmune receptors in the central nervous system. These mechanisms may be of particular importance in the context of civilisation disorders of developmental age, such as ADHD, obesity with an anxiety component, or insulin resistance with sleep disorders.
Systematic reviews of clinical trials confirm the beneficial effects of fermented foods on selected metabolic parameters. A meta-analysis of 18 randomised controlled trials involving people with diabetes or prediabetes showed that regular consumption of fermented foods led to a significant reduction in fasting glycemia (on average 6.7 mg/dl), the insulin resistance index HOMA-IR (by 0.5), total and LDL cholesterol levels (by 9.4 and 8.1 mg/dl, respectively) and diastolic blood pressure (by an average of 3.3 mmHg) [9]. These benefits were observed regardless of the type of fermented product, although stronger effects were observed with the consumption of fermented milk drinks with added probiotics.
A review of fermented plant foods, such as kimchi, tempeh, sourdough from beetroot, fermented juices and teas, indicates their potential greater microbiological diversity and higher concentration of bioactive compounds – compared to fermented dairy products [10]. In 73% of the analysed studies involving adults, improvements in parameters such as glucose levels, insulin resistance, body weight, lipid profile and inflammatory markers were observed. Particularly beneficial effects were observed in groups with high baseline levels of inflammation or dysbiosis.
Although most of the data come from studies of adults, there are also some initial reports on the beneficial effects of fermented foods in the paediatric context. In observational studies, children who regularly eat fermented foods show greater diversity of the gut microbiota, lower levels of inflammatory cytokines, and a lower risk of central obesity [5, 8]. Dietary interventions using fermented milk drinks, natural yogurts, or pickles were well tolerated and acceptable even by children with picky eating habits. Introducing fermented foods to a child’s diet can be relatively simple, for example, adding sauerkraut or cucumbers to dinner, using kefir as a base for a fruit cocktail, or homemade beetroot sourdough as an alternative to sweetened drinks. Such changes can not only support metabolic health but also shape beneficial eating habits from an early age.
Challenges for further research include, among others, the high heterogeneity of fermented products (different composition of microorganisms, fermentation time, substrate carriers), difficulties in standardisation, as well as regional and cultural differences. There is also a lack of randomised studies with children, with an appropriate control group and a long-term follow-up period. In clinical trials, it is also difficult to fully blind the intervention - e.g. due to the distinct taste, texture, or smell of fermented foods. Nevertheless, the current data are encouraging enough that fermented foods should be considered as an element supporting metabolic health, both in the framework of primary prevention and as an adjunct to the treatment of metabolic disorders of developmental age. Fermented foods are an accessible, safe and culturally rooted source of microorganisms and their bioactive metabolites. Unlike probiotic supplements, which often contain a limited number of strains and require appropriate storage conditions, fermented foods can provide a rich, natural, and diverse environmental microbiome, along with a bioactive food matrix and colonisation-supporting compounds [3].
In everyday paediatric practice, encouraging the inclusion of natural fermented products in the diet – such as kefir, natural yoghurt, sauerkraut, kimchi, tempeh, or beetroot sourdough – can be a simple and effective step towards supporting the metabolic health of children and adolescents. These products are not only well tolerated but also often locally available, relatively cheap, and culturally accepted. These activities not only support the diversity of the intestinal microbiota, but also promote conscious dietary choices and contact with ‘living’, less processed foods, which may have long-term educational and health implications. In the era of the global increase in lifestyle diseases – such as obesity, type 2 diabetes, anxiety disorders, or depression – and the simultaneous loss of microbiota diversity (both intestinal and environmental), it is worth taking a fresh look at fermentation not only as a culinary technique, but also as a public health tool. Its role in the so-called nutritional psychiatry – as a potential element of prevention of mental and neurobehavioral disorders. As the literature indicates, it is not the capsule, but the pickle – local, seasonal, natural – that may prove to be one of the most accessible and effective ‘probiotics’ in clinical and home practice.
