Yogurt, kefir, cheese, sauerkraut, kimchi, miso and sourdough bread all have one thing in common: fermentation.
Nutritionally, however, they can be very different foods.
Fermentation describes an important part of how a food is produced. It does not tell us everything about the nutritional quality or potential health effects of the final product.
Start with the food itself
Fermented dairy provides a good example.
Natural yogurt provides protein and calcium and may contain live bacterial cultures.
Greek yogurt is generally strained or concentrated and is typically higher in protein than standard natural yogurt.
Skyr is a cultured dairy product traditionally closer to a fresh cheese and is also typically high in protein.
Kefir is a fermented milk drink traditionally produced using a diverse community of bacteria and yeasts.
Cultured buttermilk — maślanka — is produced through bacterial fermentation.
Even within fermented dairy, these are therefore not identical foods.
The same principle applies beyond dairy.
Fermented vegetables can retain the fiber and nutrients of vegetables, while their salt content can vary substantially. Miso is also generally high in salt.
Sourdough bread may be produced from whole-grain or refined flour. The nutritional difference between these breads remains important even though both undergo fermentation.
What does long-term research tell us?
A 2026 meta-analysis examined 50 prospective cohort studies involving more than three million participants.
The most consistent associations with lower mortality were observed for fermented milk — a broad category that included yogurt and other fermented milk products.
When yogurt was analyzed separately, higher consumption was associated with approximately 7% lower all-cause mortality.
This does not mean that every fermented dairy product provides the same benefit.
The available evidence does not establish that natural yogurt, Greek yogurt, skyr, kefir or buttermilk has a superior effect on longevity.
Evidence for other fermented foods also varied considerably. Data for miso and fermented bread were limited, while fermented vegetables and fermented meats could not be adequately evaluated for mortality.
Fermentation is only one part of the picture
The original food and the final nutritional composition remain important.
Protein, fiber, fat, added sugar and salt can differ considerably between fermented products.
The microorganisms involved also differ, as does whether live cultures remain when the food is consumed.
This is why “fermented” should not be used as a health label on its own.
