Fermented foods are celebrated for their bold flavours and potential health benefits, but how much of this is supported by science? This blog unpacks the latest evidence, examining their role in health and wellbeing. From dairy options like live yoghurt to non-dairy choices such as kombucha, we explore the differences between fermented foods and probiotics, highlighting strain-specific effects. Discover what research reveals about their impact on gut health, blood sugar, and even mental health, providing healthcare professionals with insights to guide patients and clients.
By Sophie Bell, Registered Dietitian, Science Communications Manager at Danone
Background
Whilst they have been part of our diet for thousands of years, interest in fermented foods has increased significantly in recent years. From kombucha in cans to kimchi on burgers, it seems this food trend is here to stay. Fermented foods often have important traditional and cultural origins, or may be eaten primarily for their taste, however for many it is the purported health benefits of fermented foods that drive them to include kefir or sauerkraut on their weekly shopping list. But are fermented foods actually as beneficial for our health as mainstream and social media would have us believe? Grab yourself a mug of miso soup or a pot of live yoghurt as we dive into what the latest evidence shows about fermented foods, and see which fermented foods are the cream of the crop when it comes to health credentials.
Research interest in fermented foods
Fermented foods are a growing area of research interest. Since 2020, over 2,000 journal articles have been published on the topic.1 While many of these articles are reviews, new intervention studies and meta-analyses continue to explore the broad range of conditions and health outcomes where fermented foods may play a role. Fermented dairy products, namely live yoghurt, kefir, fermented milk drinks, and even cheese to a lesser degree, dominate the research. The vast majority of new research focuses on these products, and so perhaps unsurprisingly, the majority of positive outcomes and beneficial effects have been observed in fermented dairy. However, a small number of studies have also looked at the potential health effects of consumption of non-dairy fermented foods. In this evidence update we’ll review some of the key studies exploring fermented foods of all kinds, focusing mainly on those published in the last 5 years.
Before we begin, it’s important to be clear on what exactly fermented foods are, as well as what they are not. Fermented foods are defined as “foods or beverages produced through controlled microbial growth, and the conversion of food components through enzymatic action.2 Fermented foods and probiotics are sometimes used interchangeably, however they are very much distinct concepts. The term ‘probiotic’ was defined in 2001 by an Expert Consultation of the Food and Agricultural Organization of the United Nations and the World Health Organization (FAO/WHO) as “Live microorganisms that, when administered in adequate amounts, confer a health benefit on the host.3 The action of probiotic microorganisms is highly strain specific, meaning any evidenced benefits relate only to that individual microbial strain. Therefore only fermented foods containing specific strains demonstrated to confer a health benefit, may be considered probiotic, with specific benefits linked to the individual strain.
An overview of the evidence for non-dairy fermented foods
While a very wide variety of foods can potentially be fermented, sauerkraut and kimchi (cabbage/vegetables), miso, tempeh and natto (soybean) and kombucha (sweetened tea) are probably the most widely consumed non-dairy fermented foods globally.
Currently, there are no published randomised controlled trials (RCTs) investigating the impact of tempeh, and only one RCT examining consumption of miso. Kondo et al4 examined the effect of a blend of two specific varieties of miso on blood pressure in individuals with high-normal blood pressure or hypertension. They found significant differences in nighttime blood pressure profiles between the miso and control groups, in favour of the miso group. There is also no recent RCT data examining the potential health effects of natto or kimchi, however some studies from the early 2000’s did identify significant changes to the gut microbial landscape, including the levels of beneficial bacteria such as Bifidobacteria and Lactobacillus5,6, as well as an increase in short chain fatty acid (SCFA) production in one of the studies7, following consumption of kimchi and natto. A more recent study examined the impact of sauerkraut intake on irritable bowel syndrome (IBS) with a control arm of pasteurised sauerkraut (heat treatment through pasteurisation deactivates live cultures or probiotics)8. Significantly lower IBS Severity Scoring System scores were observed in both groups, with no difference between groups, suggesting either a placebo effect, or that the attributes of the cabbage itself, such as fibres, or compounds produced by the fermentation process, aside from the cultures themselves (referred to as postbiotics), were responsible for the benefit. This study highlights a key consideration for fermented foods. Most fermented foods will typically provide a source of macro and micronutrients, as well as potentially phytonutrients, fibres or other beneficial food compounds, depending on the substrate. Unlike probiotic supplements, with fermented foods the whole food matrix can play a role in the beneficial health effects seen9.
Previously, in vitro studies of kombucha demonstrated that the low pH resulting from the high concentration of acetic acid produced during fermentation can prevent the growth of pathogenic bacteria such as H.pylori, E. coli, Salmonella and Campylobacter10and animal studies demonstrated effects on blood glycaemia, oxidative stress, diabetes-induced weight loss, hypercholesterolaemia and gastric ulcers.11 More recently however the first RCT of kombucha intake in humans was published, demonstrating that consumption of kombucha following a standardised high Glycaemic Index meal resulted in significantly reduced glycaemic and insulin responses compared to soda water, suggesting kombucha may have a role in reducing postprandial glycaemic response.12
Fermented dairy evidence update
Unlike other fermented foods, a wealth of research exists on fermented dairy, demonstrating beneficial effects across a broad range of health outcomes. Positive effects on different facets of gut health are probably the obvious benefits which come to mind, though recent research has demonstrated effects on health outcomes reaching far beyond the gut, as we shall review here.
As already discussed, probiotics function in a highly strain-specific nature. As such, different strains can support opposing health outcomes. One area where this can be seen is in two of the most common gut health complaints, diarrhoea and constipation. When the appropriate strains are utilised, probiotics can help support both these contrasting health conditions.
Olayanju et al conducted a systematic review examining the role of fermented foods in reducing diarrhoea in children.13 Seven RCTs were included, and in this case all bar one used fermented dairy, with one using a fermented corn drink. A meta-analysis showed that compared to control, consumption of fermented foods significantly reduced mean duration of diarrhoea (−0.61 days), and length of hospitalisation (−0.35 days), but not mean daily frequency of stools. While statistically significant, the clinical significance of these findings is somewhat limited. As can be observed in meta-analyses that cover a broad range of different fermented foods (including different kinds of fermented dairy), in addition to the heterogeneity of dose and duration often observed, each food contains very different microbial strains in varying amounts, with some being significantly less effective for the condition in question than others, which will naturally impact any pooled benefit observed. Within these meta-analyses however individual studies show promising results, for example, within the Olayanju et al meta-analysis a 6 month long study of nursery school aged children comparing yoghurt containing Lactobacillus casei CNCM I- 1518 with jellied milk found the severity of diarrhoea over the six-month study was significantly decreased (4.3 days) in the L. casei group compared with the jellied milk group (8.0 days). This finding, both statistically and clinically significant, demonstrates the importance of strain specificity when considering the probiotic action of fermented foods. Examining a broad range of different fermented foods containing different microbial strains can impact the likelihood of finding a pooled benefit.
On the other end of the gut health concern spectrum, constipation is a common complaint that can impact both health and quality of life. Aruja et al conducted a meta-analysis investigating the effects of Bifidobacterium animalis subspecies lactis on gastrointestinal symptoms, with a focus on indicators of constipation.14 Unlike many of the other meta-analyses in this area, Aruja and colleagues focused solely on strains of Bifidobacterium animalis subsp. lactis (B. lactis), however they included both probiotic supplements as well as fermented milk and yoghurts containing the relevant strains. A number of different strains were included, with the well-researched strain Bifidobacterium animalis subsp. lactis CNCM I-2494 making up a quarter of the studies. Overall, supplementation with B. lactis increased defecation frequency. Sub-group analysis found a decrease in colon-transit time and improvement in stool consistency in some groups. While overall positive effects were found, varying degrees of effect were observed between studies, which may be related to the dose, duration, or importantly the different specific strains of B. lactis used. Owing to the strain-specific effect of probiotics, even different strains of B. lactis can result in different outcomes. These results build on findings from an earlier meta-analysis which found that B. lactis improved whole gut transit time, stool frequency and stool consistency15. Furthermore, RCTs in live yoghurt containing Bifidobacterium animalis subsp. lactis CNCM I-2494 have previously found potential improvements in outcomes such as GI transit, abdominal distension and other symptoms in adults with constipation-predominant IBS.16,17
While much of the focus of probiotic and fermented foods research focuses on supporting the management of specific clinical conditions, fermented foods also have a role in optimising wellness in those without defined conditions, for example in the support of mild, but very common gut health complaints. In 2019 a post-hoc pooled analysis of two previous studies revealed that fermented milk containing Bifidobacterium animalis subsp. lactis CNCM I-2494 resulted in observed improvements in common symptoms such as bloating, abdominal discomfort, flatulence and borborygmi (stomach rumbling).18 Two previous meta-analyses across a total of 958 participants and 4 RCTs found similar results for the same strain, with daily consumption associated with a consistent and significant improvement of outcomes related to GI discomfort and symptoms.19,20
Beyond the gut
In recent years, much of the focus of probiotic and fermented foods research has now been moving away from attention solely on gut health, and looking at the broader impact of fermented foods on whole body health outcomes.
Inflammation
SaeidiFard, Djafarian and Shab-Bidar looked at the effect of a broad range of fermented foods on inflammatory markers. 21While most studies included were fermented dairy, a small number of studies looking at fermented dietary fibre, wine, bread, black tea, olive, and rice bran were also included. Overall, intake of fermented foods did not improve serum C-reactive protein (CRP) or interleukin (IL)-6, though a reduction in tumor necrosis factor (TNF)- α was observed, indicating a potential anti-inflammatory action. No analysis was conducted to identify the impact of different types of fermented food, which may be particularly relevant in this instance, as many of the fermented foods included, such as bread and wine, do not typically contain live microbes due to their processing.
Diabetes
A meta-analysis of 15 studies, with a total population of close to half a million, found decreased diabetes risk was associated with a higher intake of fermented dairy foods.22 Yoghurt intake was found to have a dose-dependent effect, with subgroup analysis showing higher yogurt consumption was associated with the greatest decrease in diabetes risk. In fact, the evidence for an effect of yoghurt consumption on diabetes risk is so compelling that the FDA in the US recently permitted a qualified health claim that “Eating yogurt regularly, at least 2 cups (3 servings) per week, may reduce the risk of type 2 diabetes according to limited scientific evidence”.23 Some recent evidence also suggests that for those with type 2 diabetes, yoghurt consumption may have beneficial effects on lipid profiles and gycaemic control, however further research is needed in this area24. Another meta-analysis looking specifically at kefir consumption found kefir intake significantly reduced fasting insulin levels, however no effects were observed on the other cardio-metabolic risk factors explored.25
The gut-brain axis
The gut-brain axis is an exciting area of gut health research, with a growing number of studies exploring how modulation of the gut microbiota landscape may impact different aspects of mental health. A 2023 meta-analysis looked at the impact of fermented dairy food consumption on depressive symptoms26. They found evidence of significantly decreased depression risk associated with fermented dairy foods intake, in particular cheese and yoghurt. Potential mechanisms of action proposed by the authors included reduced gut inflammation, decreased intestinal permeability or a reduced stress response via the hypothalamic–pituitary–adrenal axis. Marx et al conducted a meta-analysis on the impact of fermented foods on cognitive outcomes.27 The majority of the 6 studies utilised fermented dairy products, however fermented soybean and fermented kelp extract studies were also included. No statistically significant differences in global cognition were observed overall compared to controls, however a trend towards improvements was observed, suggesting further, larger scale studies in this area are required.
Mortality
Considering the broad range of different facets of health that consumption of fermented dairy has been linked to, it is unsurprising that a 2022 meta-analysis by Tutunchi et al examined the impact of yoghurt consumption on all cause mortality, as well as cardiovascular disease (CVD) and cancer mortality specifically.28 High intake of yogurt compared with low intake was significantly associated with a lower risk of death from all causes. There was also evidence of a dose-response, with each additional serving of yogurt significantly associated with a reduced risk of all cause mortality and CVD mortality, up to an average intake of 0.5 servings per day, with no additional benefit observed for intakes above this. No association with cancer mortality was observed.
Looking to the future
As we can see recent evidence for non-dairy fermented foods is very limited. This does not mean that other fermented foods do not have a notable health benefit, just that little research has been done in these areas to date. Furthermore, as we have repeatedly seen, probiotics work in a highly strain specific manner. So while many fermented foods may potentially be probiotic candidates, pooling results for a broad variety of fermented foods, containing numerous different species let alone strains, may not be the most effective method for identifying probiotic effects. Furthermore, if the strains contained within the foods have not been properly characterised, any specific health benefits they support may not be related to the outcomes explored in the study. Ongoing research shall hopefully continue to uncover the links between different fermented foods and their health effects. Fermented dairy is already supported by a wealth of evidence demonstrating meaningful health effects across a broad range of clinical areas. As consumer interest in gut health and fermented foods continues to grow, research interest in this area is also likely to continue, providing healthcare professionals with relevant and up to date information on the benefits of fermented foods to assist in providing recommendations to patients and clients.
Key take aways
- Despite their existence for thousands of years, fermented foods are currently experiencing a surge in popularity, both with consumers and researchers
- Current evidence points to a broad range of potential health benefits from a spectrum of fermented foods, however the evidence base for non-dairy fermented foods is still largely emerging
- Substantial evidence exists for the positive impacts of fermented dairy across a range of health outcomes, from supporting the management of gut disorders to cardiometabolic diseases and cognitive effects via the gut-brain axis
- As new research emerges we can continue to improve our understanding of the strain-specific action of fermented foods so that healthcare professionals can provide guidance to patients and clients on their use to support health and wellbeing
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