Context matters: fermented foods and the gut microbiota

The effects of a FF may depend on the conditions encountered after ingestion. Microorganisms from FFs enter an established gut microbial community, where resident microorganisms may support or limit their survival, persistence, and activity. Many ingested microorganisms are present only transiently (Derrien & van Hylckama Vlieg, 2015;Kim et al., 2026). Therefore, even when the same FF is administered, microbial and host responses may differ across recipient populations. Current evidence supports the plausibility of this hypothesis but does not directly demonstrate it. Baseline microbiome composition may influence the colonization and persistence of administered microorganisms, although this evidence comes from probiotic interventions (Goswami et al., 2026). Diet, lifestyle, physiological characteristics, and medication use are associated with gut microbiome composition (Asnicar et al., 2021;Forslund et al., 2021;Vujkovic-Cvijin et al., 2020). A metagenomic analysis of more than 2,500 food samples and 20,000 human microbiomes found that food-associated microorganisms contributed approximately 3% of the adult gut microbiome, with overlap patterns differing across ecological and cultural settings (Carlino et al., 2024). These findings do not demonstrate that recipient characteristics influence the colonization of microorganisms from FFs. However, they suggest that FF microorganisms may encounter different gut environments in different recipients. These environments could influence their persistence, interactions with resident microorganisms, metabolite production, and subsequent effects on microbial composition, microbial function, and host health outcomes. To interpret such differences, the FF intervention must be carefully characterized. Otherwise, apparent population differences could reflect variation in the product or its administration. From this perspective, the effect of a FF intervention should be understood as the result of interactions between the FF and the recipient gut ecosystem, rather than as a property of the product alone.The proposed framework identifies the main elements needed to compare responses to a FF across different populations (Table 1). The first step is to ensure that the FF intervention is standardized. This section does not aim to reiterate the importance of product characterization, as guidance from the European Food Safety Authority (EFSA) already addresses aspects such as microbial characterization, manufacturing consistency, stability, storage, and shelf life (EFSA NDA Panel et al., 2017). Rather, we emphasize that even small variations in FFs can alter responses to interventions. Proper FF characterization should include (Table 1, Column A summarizes the main elements): 1) the starter culture species and strain, as some probiotic strains can differ in survival and tolerance to environmental stress in vitro (Ansari et al., 2019;Arnold et al., 2018;Mazzantini et al., 2022); 2) the starting ingredient, as food matrix characteristics can influence the delivery, gastrointestinal survival, and potential persistence or recovery of microorganisms after consumption (Ba et al., 2021;Saxelin et al., 2010); 3) fermentation and storage conditions, as these can affect microbial growth (Jung et al., 2024); and 4) viable microbial count at consumption, dose, frequency, duration, and adherence (Hanifi et al., 2015;Hill et al., 2014;Taylor et al., 2020;Wastyk et al., 2021). Table 1. Proposed framework for evaluating whether a standardized FF intervention produces different microbial and host responses across populations. The framework first standardizes the FF intervention, then compares deliberately selected recipient populations using the same study procedures, and finally tests whether the effect of the FF differs between populations. Once the FF intervention has been standardized, studies could administer the same intervention and comparison food to individuals from at least two selected populations. The comparison food is needed to determine whether any observed effect is related to the FF rather than to other aspects of the participants' diet. If the aim is to study the overall effect of fermentation, the comparison food should be matched in ingredients, food matrix, and nutrient composition, differing primarily in the presence or absence of fermentation. If the aim is to examine the contribution of live microorganisms, an additional heat-treated product could be included. Studies should choose outcomes according to the hypothesis being investigated. These may include detection or persistence of ingested microorganisms, their metabolic activity, changes in gut microbiota composition and function, microbial metabolites, and relevant clinical or health outcomes. If microbial, metabolomic, or clinical responses differ between populations, researchers may then examine whether habitual diet, baseline microbiota, previous fermented-food intake, health status, medication use, or other measured characteristics help explain the difference. The proposed framework enables testing whether the effect of a standardized FF is reproduced across recipient populations and identifies the measurements needed to interpret a difference. Under this framework, inconsistent findings across FF studies should not automatically be interpreted as evidence that a FF is ineffective or that one study is incorrect. Cross-population differences should be assessed directly, for example using an appropriate treatment-by-population interaction or direct comparison of effect estimates. Variation in recipient characteristics may represent a biologically meaningful source of heterogeneity. Therefore, population context should be considered a potential effect modifier rather than simply unexplained variation. Not every FF study must be international. Trials conducted in one region can still compare individuals from the same broader population who follow different habitual diets, consume different FFs, or belong to different ethnic or socioeconomic groups. Well-characterized single-population trials remain useful and can support later replication.This Opinion also has implications for evidence synthesis. Although previous work has recognized that responses to FFs may vary according to population characteristics, habitual diet, health status, and gut microbiota (Humblot et al., 2025), future systematic reviews and meta-analyses should treat these factors as potential effect modifiers when interpreting heterogeneity across studies. This may help explain differences in study outcomes and identify the circumstances under which FFs are most likely to be effective. This framework has implications for translation into practice. Dietary recommendations based on FF interventions may require confirmation across diverse populations before broad implementation, particularly when evidence is derived from a limited geographic, cultural, or dietary context. Although FFs may offer health benefits, the magnitude and nature of those benefits may not be universally reproducible. Recognizing this possibility may improve study design and the interpretation of findings used to inform dietary guidance. This context-dependency is not unique to FFs. Similar microbiome-mediated variability has been described for herbal medicines, including Kampo and Traditional Chinese Medicine, where gut microbiota can transform compounds with limited bioavailability into more readily absorbed or pharmacologically active metabolites. Consequently, differences in gut microbiota composition and function may contribute to variation in pharmacokinetics, bioavailability, and therapeutic responses across individuals and populations (Ikarashi et al., 2025;Wang et al., 2019). These mechanisms differ from those involved in FF consumption. Nevertheless, they illustrate how variation in gut microbial ecology can influence biological responses to ingested products, suggesting caution when extrapolating findings from one population to another.Standardization allows researchers to determine whether an observed effect of a FF is reproduced across populations and to formally assess potential differences in response, but it does not establish that a FF has the same effect universally. If the FF product is not equally characterized and delivered across populations, an apparent difference in response may reflect differences in FF exposure rather conditions included in the study, but would not prove that the same than differences in recipient context. Considering recipient context may therefore reduce overgeneralization and premature translation of findings into broad public health recommendations. Evidence from one population should first support a claim about that population. Comparable effect estimates across recipient contexts, supported by appropriate direct comparisons, would strengthen the basis for broader dietary guidance. Evidence from direct comparisons that effects differ between populations would support recommendations adapted to local diets, health priorities, and food systems. The inclusion of underrepresented populations is therefore needed to improve geographical representation and to determine whether existing evidence applies to the populations for whom dietary recommendations are intended. This approach may improve food-based dietary guidelines by encouraging the evaluation of FFs within the dietary, cultural, and environmental contexts in which they are consumed. Rather than assuming that one fermented product represents all FFs, recommendations may be strengthened by generating evidence-based, population-specific guidance derived from diverse populations, food systems, and FF traditions. Standardization makes research comparisons meaningful, but it does not mean that, in real world settings, a standardized FF is always the most appropriate option. Traditional FFs may contain microbial strains and production characteristics not represented by standardized products. For example, lactic acid bacteria isolated from traditional African fermented cereals have shown in vitro inhibitory activity against pathogens, including Escherichia coli and Salmonella Typhi (Onipede & Fawole, 2017). These findings do not establish a clinical benefit, but they illustrate why the potential properties of locally consumed foods should not be assumed to be replicated by standardized products. Accordingly, studies of standardized and traditional FFs should be viewed as complementary rather than competing research approaches. Standardized products facilitate rigorous comparison across populations, whereas investigations of traditional FFs may identify culturally relevant microbial, metabolic, and dietary characteristics that would otherwise be overlooked. Standardized starter cultures may nevertheless be useful when adapted to local food systems and production conditions. The Yoba for Life project (a non-profit, multi-partner initiative promoting a probiotic starter culture for small-scale yoghurt production in parts of Africa) illustrates how fermentation technologies can be adapted to local conditions (Sybesma et al., 2025). This example supports the feasibility and potential value of locally adapted standardization, but it does not show that the same starter culture will produce equivalent effects in every population. The goal is to identify FF options that are safe, beneficial, affordable, culturally acceptable, and feasible in a particular setting.The effects of FFs observed in one population should not be assumed to apply universally. The same standardized FF may produce different responses when interacting with different recipient gut ecosystems. The framework proposed here provides a practical way to test this possibility by standardizing the FF intervention, measuring recipient characteristics, and comparing responses across populations. Incorporating population context as a testable variable in FF research enhances study design, refines data interpretation more robustly, and fosters evidence-based, communityspecific dietary guidelines.