What fermentation is
Fermentation is a metabolic process in which microorganisms — bacteria, yeasts or fungi — convert sugars into other compounds, typically acids (lactic acid fermentation) or alcohol (alcoholic fermentation). The process preserves food, transforms its flavour and creates bioactive compounds with health effects. Yoghurt, kefir, cheese, sauerkraut, kimchi, miso, tempeh and kombucha are all fermented foods.
What the research shows
A landmark 2021 paper in Cell from Stanford researchers compared high-fibre and high-fermented-food diets over ten weeks in 36 adults. The fermented food group showed a significant increase in microbiome diversity — a marker generally associated with gut health — while the high-fibre group did not increase diversity. The fermented food group also showed decreases in multiple markers of immune activation.
Which fermented foods are best
The strongest evidence is for fermented dairy — yoghurt with live cultures, kefir — where the microbial content is relatively consistent and well-studied. Plant-based fermented foods like kimchi and sauerkraut have less clinical research but are rich in beneficial compounds. The key is variety and consistency — eating a range of fermented foods regularly rather than large amounts occasionally.