Fermented Foods for Your Gut Microbiota

There are many dietary factors that can influence your gut microbiota – the community of trillions of microorganisms residing in your intestine that take part in many important functions such as digestion and defence, making them vital to your health and wellbeing.  

Fermented foods have been gaining in popularity in recent years due to their potential health benefits related to gut health – but what is research showing?  Read on to find out more!

What are fermented foods?

First off, let’s look at what are fermented foods by looking at the definition:

Fermented foods and beverages are, “Foods made through desired microbial growth and enzymatic conversions of food components” (M. Marco et al. 2021)

Fermented foods can include foods such as:

• Dairy products – kefir, yogurt

• Soy products – miso, natto

• Tea – kombucha

• Vegetables – kimchi, sauerkraut

• Others such as sourdough bread, beer, wine, chocolate and coffee

While live microbes carry out fermentation, not all foods that undergo fermentation will have live microbes in the food you consume, as these foods will then undergo processes that remove or destroy the live microbes such as baking, roasting, heating, pasteurization or filtration.  So, fermented foods that do not contain live microbes when you consume them include sourdough bread, most beers, wine, chocolate, coffee and pasteurized or cooked vegetables such as sauerkraut.

Fermented foods have been around for thousands of years.  Historically, fermentation was used as a method of food preservation and to improve taste or texture, however, in recent years fermented foods have been gaining in popularity due to potential benefits related to health.

What is research showing regarding fermented foods?

Though there are many in vitro and animal model studies investigating the health benefits of some fermented foods for gut health; there is limited clinical evidence in humans of the health benefits from the array of fermented foods, with the exception of yogurt and other cultured dairy products.

Last year, a study from Stanford School of Medicine, was one of the first to show a diet rich in fermented foods, such as kefir, yogurt, kombucha, kimchi and fermented vegetables can improve gut health, and may also benefit immune health in healthy adults – although it was a small study.

The study, published in July 2021 in the journal Cell, conducted a 17-week randomized, prospective study among 36 healthy adults.  The subjects were randomly assigned to one of two groups: people eating a high-fibre diet or people eating a high-fermented-food diet for 10 weeks.

The researchers collected and analyzed blood and stool samples before the trial, during the 10-week diet period and during a four-week period after the trial when they could eat whatever they would like.

The researchers found the two diets resulted in different effects on the gut microbiota and immune system.

High fermented foods diet

Those in the high-fermented-food diet group, ate foods such as kefir, yogurt, fermented cottage cheese, kimchi, kombucha and fermented vegetables.  Participants increased their fermented food intake from an average of half of a serving per day at baseline to an average of six servings per day during the maintenance phase.  One serving of fermented foods was defined as 6 ounces of kombucha, yogurt, kefir, buttermilk, kvass; ¼ cup kimchi, sauerkraut, or other fermented veggies; or 2 ounces vegetable brine drink.

Researchers found the high-fermented foods diet resulted in an increase in overall gut microbial diversity and the more servings consumed, the stronger the results.  As well, the researchers found four types of immune cells were lower in the fermented-food group, as were the levels of various inflammatory proteins measured in the blood samples, including ones related to Type 2 diabetes and rheumatoid arthritis.

High-fibre diet

Those in the high-fibre diet group, ate foods such as legumes, nuts, seeds, whole grains, fruits and vegetables.  Participants increased their daily fibre intake from an average of 21.5 grams at baseline to 45 grams during the maintenance phase.  To put this fibre intake into perspective, Health Canada recommends Canadian adult women get 25 grams of fibre per day and adult men get 38 grams of fibre per day.  Most Canadians are only getting about half that much, so the 45 grams per day achieved in the study is substantial. 

The researchers found the microbial diversity remained stable and none of the inflammatory proteins decreased in the high-fibre diet.

While the study authors hypothesized that increasing dietary fibre consumption would lead to an increase in microbiota diversity, they were surprised there was not a greater microbiota response given the substantial increase in fibre consumption.  The researches also explained they found increased fibre consumption led to more carbohydrates in the stool samples, suggesting insufficient fibre breakdown by gut microbes.  Therefore, the researchers wondered if the intervention was too short and explained it’s possible a longer intervention would have allowed for the microbiota to adequately adapt to the increase in fibre consumption.

When looking at these results, it’s important to keep in mind limitations of this study such as the small sample size, short study time period and there was no control group to compare the dietary interventions to.  Nonetheless, this study contributes valuable information regarding how different diets can impact the gut microbiota and immune function.  

Why does gut microbiota diversity matter?

One factor thought to contribute to a healthy gut microbiota is diversity, which is a measure of the different types and amounts of bacterial species.

Past research has shown diet can shape your gut microbiota and influence its diversity.  Whole food, plant-rich, high-fibre diets support the microbial ecosystem, enhancing the diversity of gut microbes and have been linked with better health.  In contrast, diets high in ultra-processed food, low-fibre, very high fat, especially saturated fat have been linked with low gut microbiota diversity and certain digestive and immunological diseases in some people.

How fermented foods may benefit health

Researchers have uncovered several mechanisms to help explain how fermented foods may benefit health such as:

• Many, but not all fermented foods contain beneficial and potentially probiotic microorganisms (live ‘good’ microbes that can provide certain health benefits when taken in adequate amounts).  These microbes can reach the gut, increasing the diversity of the gut microbiota and provide specific health benefits.

• Fermentation also modifies and the improves the nutritional profile of the food:
  • Fermenting produces vitamins and new compounds that benefit health and may also convert food compounds into biologically active molecules
  • Fermentation increases the digestibility of foods.  For example, yogurt and kefir contain bacteria that breaks down lactose, resulting in improved lactose digestion and tolerance.
  • Fermentation also increases nutrient bioavailability, making various nutrients more available for the body to absorb
  • Fermentation can reduce anti-nutrients in raw foods.  For example, fermentation of soybeans can reduce phytic acid (also known as phytate) concentrations.  Phytic acid can inhibit the absorption of minerals such as iron, zinc and calcium.

• Exactly how fermented foods may modify the immune system is still under investigation.  More research is this area is required; however, it’s hypothesized it may be the due to the combination of the compounds present in the starting materials, those produced during fermentation and the microorganisms.  With approximately 70% of the immune system located in the gut, this will be an interesting area to follow!