Not all fermented foods are a source of probiotics. Scientists from the Department of Nutritional Sciences at King’s College London have reviewed the impact of kefir and major non-dairy fermented foods on the gut microbiota and gastrointestinal health.

Fermented foods and beverages are produced via controlled microbial growth in the food material alongside the conversion of food components through enzymatic action.

Although fermented foods and beverages have been a major element in the human diet for thousands of years, we have only recently started to realize their potential benefits for health.

Historical reasons for food fermentation include the preservation of perishable foods that otherwise become unsafe for consumption and the development of organoleptic changes.

Although the effects on health of fermented dairy products (e.g., yoghurt and cheese) have been largely studied (with mainly epidemiological evidence), little is known about the science-based health benefits of other fermented foods.

Scientists from the Department of Nutritional Sciences at King’s College London (UK) have recently reviewed the impact of kefir and major non-dairy fermented foods on the gut microbiota and gastrointestinal health.

A review of the existing scientific literature showed that mechanisms of action that might explain presumed health benefits associated with fermented foods include:

The potential probiotic effect of their constituent microorganisms on the gut microbiota (e.g., competition with pathogenic bacteria).

The production of bioactive peptides and amines derived from fermentation that may exert health benefits.

The conversion of naturally present compounds in fermented foods into biologically active compounds.

The reduction of anti-nutrients (e.g., sourdough fermentation can reduce the content of fermentable carbohydrates, called FODMAPs, which might be a source of digestive discomfort in a subset of people).

However, clinical evidence that supports the effects of fermented foods such as kefir, sauerkraut, natto, and sourdough bread on gastrointestinal health and disease is scarce.

Traditional kefir (but not water kefir) is the most widely investigated fermented food with benefits for lactose malabsorption and Helicobacter pylori eradication supported by randomized controlled trials. However, the benefits of kefir in other gastrointestinal conditions such as functional constipation and inflammatory bowel diseases need to be explored in high quality trials.

Moreover, a few low quality and small human studies have tested the effects of sauerkraut and sourdough bread on gastrointestinal health, but it is not always clear that these effects are mediated by the gut microbiota.

In addition, only in vitro and/or animal studies support the impact of kombucha and fermented soybean products (tempeh, natto and miso) on gastrointestinal health. Thus, their effects in humans are largely unknown.

On the whole, there is limited clinical evidence for the effectiveness of kefir and the most popular non-dairy fermented foods. Traditional kefir is the fermented food with most human evidence for gastrointestinal conditions, including lactose malabsorption and as an adjunct therapy during treatment for H. pylori infections.

Some of the limitations acknowledged in human studies with fermented foods are the products’ heterogeneous microbial content and metabolic composition and the fact that, usually, methods of production are rarely reported. In addition, it is not possible to distinguish whether the benefits of fermented foods in health conditions are due to the microorganisms, byproducts or nutrients.

As Staudacher and Nevin highlight in an opinion article, issues such as optimum dose and safety—especially in populations such as the medically ill, the elderly, the immunocompromised, and pregnant women—need to be addressed before fermented foods and beverages are used for gastrointestinal conditions.

This review article belongs to the special issue “Food and Diet for Gut Function and Dysfunction” in the peer reviewed open access journal “Nutrients”. This issue was instigated by the European Society of Neurogastroenterology and Motility, guest edited by Profs Fernando Azpiroz and Paul Enck, and made possible through an unrestricted educational grant from Danone.

References:

Dimidi E, Cox SR, Rossi M, Whelan K. Fermented foods: Definitions and characteristics, impact on the gut microbiota and effects on gastrointestinal health and disease. Nutrients. 2019; 11(8). doi: 10.3390/nu11081806.

Staudacher HM, Nevin AM. Fermented foods: fad or favourable addition to the diet? Lancet Gastroenterol Hepatol. 2019; 4(1):19. doi: 10.1016/S2468-1253(18)30392-3.