Fermented foods: what yoghurt, kefir and kimchi actually do

Yoghurt, kimchi, sauerkraut, kefir — fermented foods have been hyped for years. What's behind it? What do human studies show — and where does the effect stop?

What fermentation means

Fermentation is one of humanity's oldest preservation mechanisms. Bacteria, yeasts or fungi break down carbohydrates, producing lactic acid, acetic acid or ethanol in the process. These breakdown products lower the pH of the food, inhibit mould and putrefactive bacteria, and alter flavour, texture and nutritional profile.

Living microorganisms — known as probiotics — survive in fermented foods. Yoghurt and kefir contain Lactobacillus and Bifidobacterium strains; kimchi and sauerkraut mainly Lactobacillus plantarum and related species; kombucha a mixture of bacteria and yeasts. Whether these microorganisms reach the digestive tract alive and in sufficient numbers depends on many factors — including stomach acid.

What studies show about effects on the microbiome

A 2021 study published in Cell by the research group of Justin Sonnenburg and Christopher Gardner (Stanford) provided one of the clearest answers to date. 36 participants ate either a high-fibre or a fermented diet for 10 weeks. The fermented food group subsequently showed measurably higher gut microbiome diversity and lower inflammatory markers (19 immune markers decreased, including interleukin-17).

At the same time, the study showed that the high-fibre group benefited less than expected — apparently because the participants' microbiome was not prepared to process the fibre optimally. This suggests that fermented foods may prime or expand the microbiome in a way that makes fibre more processable.

Yoghurt, kefir, kimchi — which is better?

Yoghurt is the most researched and shows effects in randomised studies on lactose intolerance (the live bacteria help break down lactose), on mild diarrhoea episodes during antibiotic treatment, and possibly on respiratory infections. Kefir contains a greater variety of microorganisms than yoghurt and has shown positive effects on blood pressure and blood sugar in smaller studies.

Fermented vegetables such as kimchi and sauerkraut deliver live bacteria and fibre in combination — an advantage over fermented dairy products, which contain no significant fibre. However, the sauerkraut must be unpasteurised: heat-treated sauerkraut from a jar no longer contains live cultures.

What to look for when choosing

Not every product labelled fermented still contains live cultures. Yoghurts that have undergone heat treatment after fermentation are probiotically inactive. Look for labels such as contains live cultures or live lactic acid bacteria. Kefir from the refrigerated section is generally active; kefir drinks from Tetra Pak often are not.

Home-fermented vegetables — cucumbers, sauerkraut, beetroot — are easy to make and have been shown to contain high counts of live bacteria. The basic principle: vegetables, salt, water, a sealed jar, room temperature. No complicated starters needed. Fermentation runs via the natural lactic acid bacteria present on the vegetables.

Limits of the hype

The evidence on fermented foods is promising but still young. Many studies are small, short and cannot be generalised, because the effects depend strongly on the individual microbiome. Probiotic supplements with isolated strains perform worse in clinical trials than expected — suggesting that the diversity of whole foods matters more than a single strain in high doses.

Fermented foods are not a cure-all. They can be part of a healthy diet, but they neither replace a high-fibre diet nor compensate for an otherwise one-sided way of eating. Anyone suffering from chronic gut conditions should introduce fermented foods slowly and consult a doctor if needed.

Track digestion and wellbeing with YUTHI

When you introduce fermented foods into your diet, it is worth observing changes in your wellbeing, digestion and energy. In the YUTHI app you can log these areas throughout the day and identify patterns. When you ate what and how you felt afterwards — this connection is subjective, but a valuable starting point for understanding your own body.

Sources

• Wastyk HC. et al. Gut-microbiota-targeted diets modulate human immune status. Cell. 2021;184(16):4137–4153.
• Marco ML. et al. Health benefits of fermented foods: microbiota and beyond. Curr Opin Biotechnol. 2017;44:94–102.
• Dimidi E. et al. Fermented foods: definitions and characteristics, impact on the gut microbiota and effects on gastrointestinal health and disease. Nutrients. 2019;11(8):1806.
• Hill C. et al. Expert consensus document: the International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nat Rev Gastroenterol Hepatol. 2014;11(8):506–514.