Natural probiotics through food: what do the large clinical trials show?

Natural probiotics through food: what does the evidence show?

---

Abstract

Fermented foods such as yoghurt, kefir, kombucha, sauerkraut and kimchi have become very popular as a natural source of beneficial microorganisms. But fermented is not the same as probiotic: for a product to be called probiotic, it must contain live, identified microorganisms in a documented beneficial amount. This article reviews what we actually know about the most common fermented foods, and where the evidence is weak or lacking.

---

Introduction

Before industrialisation and the invention of dietary supplements, humans obtained part of their bacterial intake through fermentation - an ancient preservation method in which bacteria and yeasts break down carbohydrates in food.

When we eat fermented food, we ingest microorganisms in their natural food matrix. But far from all fermented products contain live bacteria in the finished product (pasteurisation and heat treatment can kill them), and even when the bacteria are alive, it is not a given that they are documented as beneficial or able to survive the passage through stomach acid. See also our article on probiotics in capsule form and the overview of gut microbiome, probiotics and IBS.

---

The most common fermented foods

1. Kefir

Kefir is a sour drink made by fermenting milk with kefir grains (a culture of bacteria and yeast).

• What does it contain? Typically a composition of Lactobacillus and Leuconostoc species, Acetobacter and yeasts. The exact composition varies considerably between products and home-made cultures.
• Evidence: Some smaller studies point to a positive effect on the gut's microbial composition and certain inflammation markers, but results are inconsistent, and large, well-designed trials confirming clinical effects such as lower cholesterol are lacking.

2. Traditional yoghurt and cultured dairy products

It is important to distinguish between pasteurised industrial yoghurt, where any bacteria have been killed by heat, and yoghurt with live, active cultures.

• What does it contain? As standard, Lactobacillus bulgaricus and Streptococcus thermophilus; some products are enriched with Lactobacillus acidophilus and Bifidobacterium.
• Evidence: Yoghurt with live cultures contains less lactose and appears to be better tolerated by some people with lactose intolerance. Observational studies have found an association between regular yoghurt intake and lower risk of type 2 diabetes, but association is not the same as causation, and there is not sufficient basis for calling yoghurt an established way to improve insulin sensitivity or blood pressure.

3. Kimchi and sauerkraut (fermented vegetables)

Kimchi and sauerkraut are produced by wild fermentation, in which the vegetables' own bacteria - typically Leuconostoc, Lactobacillus plantarum and Lactobacillus sakei - mature the food.

• Evidence: A few controlled studies have examined associations between kimchi intake and markers such as body weight or blood pressure in selected populations, but results are not consistent enough to conclude a general, established effect on insulin sensitivity, blood pressure or cholesterol. Kimchi and sauerkraut are also often high in salt and in certain FODMAP groups, which can cause symptoms in sensitive individuals.

4. Kombucha

Kombucha is a fermented tea, brewed with a SCOBY (a culture of bacteria and yeast).

• What does it contain? Primarily acetic acid bacteria, lactic acid bacteria, yeast strains and antioxidants from the tea. Kombucha can contain traces of alcohol and sugar, which vary between products and batches.
• Evidence: Evidence is mostly from laboratory and animal studies, with a few small human studies. Large, controlled trials are lacking before specific health claims can be confirmed in humans.

---

Important caveats

Fermented is not the same as probiotic

A fermented product does not necessarily contain live bacteria at the time of consumption, and even when it does, the bacterial strains are rarely identified or documented in the amount actually present in the product. The term "probiotic" should be reserved for products with documented, live and identified content.

Fermented is not automatically low-FODMAP

The fermentation process can in some cases reduce the content of certain FODMAPs, but this is far from true of all products. Many fermented foods can still contain significant amounts of FODMAPs, lactose or other substances that cause symptoms in people with IBS.

Content varies considerably

Sugar, alcohol, histamine, lactose and FODMAP content in fermented foods varies widely between products, brands and production methods. People with histamine intolerance, lactose intolerance or IBS should be aware of this variation and assess their tolerance individually.

---

Conclusion

Fermented foods can be part of a varied diet, but they are not a documented treatment for IBS, IBD or other conditions, and they should not be described as the most effective or natural route to probiotics in general. The effect depends on the specific product, its content of live and identified microorganisms, and individual tolerance.

---

Assessment and treatment at Kirurgen.dk

We work up gastrointestinal symptoms with colonoscopy, gastroscopy, coeliac screening and faecal calprotectin. See also: Probiotics in capsule form, Gut microbiome, probiotics and IBS, Chronic diarrhoea, Chronic constipation, SIBO, Low-FODMAP diet for IBS.

---

References

1. American Gastroenterological Association (AGA). AGA Clinical Practice Guidelines on the Role of Probiotics in the Management of Gastrointestinal Disorders. Su GL et al. Gastroenterology 2020;159:697-705.
2. American College of Gastroenterology (ACG). ACG Clinical Guideline: Management of Irritable Bowel Syndrome. 2021.
3. British Society of Gastroenterology (BSG). BSG Guidelines on the Management of Irritable Bowel Syndrome. 2021.
4. World Gastroenterology Organisation (WGO). WGO Global Guidelines: Probiotics and Prebiotics. 2023.