Fermented foods are often discussed as though they form one naturally healthy category.
They do not.
Even within fermented dairy, there are important differences between natural yogurt, Greek yogurt, skyr, kefir and buttermilk.
They may look similar in the supermarket refrigerator, but they are produced differently and can differ considerably in protein, fat, sugar, microbial cultures and consistency.
What do we mean by fermented dairy?
Natural yogurt is milk fermented with traditional yogurt cultures. It is usually the simplest reference point when we talk about yogurt.
Greek yogurt is typically yogurt that has been strained or otherwise concentrated. It is thicker and generally contains more protein than standard natural yogurt, although its fat content varies considerably between products.
Skyr is an Icelandic cultured dairy product. It is traditionally closer to a fresh cultured cheese than a conventional yogurt, although it is commonly sold and eaten like yogurt. It is typically high in protein.
Kefir is a fermented milk drink produced using a more complex community of microorganisms, traditionally including both bacteria and yeasts.
Buttermilk — maślanka in Polish — is different again. Traditional buttermilk was the liquid remaining after butter was churned. Most buttermilk sold today is cultured milk produced through bacterial fermentation.
These products should therefore not automatically be treated as nutritionally or microbiologically identical.
What does the long-term evidence tell us?
A large 2026 analysis examined 50 prospective studies involving more than three million people.
The most consistent associations with lower mortality were found for the broad category of fermented milk.
When yogurt was analyzed separately, higher consumption was associated with lower all-cause mortality. However, associations with cardiovascular and overall cancer mortality were not statistically significant.
This distinction matters.
The study does not establish that Greek yogurt, skyr, kefir or buttermilk individually provide the same mortality benefit. Evidence for specific fermented dairy products remains much less uniform than the broad term fermented milk might suggest.
The product still matters
When choosing fermented dairy, fermentation is only part of the picture.
Protein content, added sugar, fat composition, ingredients and whether live cultures remain in the final product can all differ.
A plain natural yogurt, a high-protein skyr and a sweetened flavored yogurt should therefore not be considered nutritionally equivalent simply because all belong to the wider fermented dairy category.
