Kefir vs yogurt: how they differ, and what the evidence says

In the United States, yogurt has a legal definition. FDA's standard of identity says it is made by fermenting pasteurized milk ingredients with a culture containing the lactic acid bacteria Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus FDA. Many brands add further strains.

Kefir is made differently. Milk is fermented by a kefir grain: a cluster of lactic acid bacteria and yeasts held together in a matrix of sugars and proteins Bourrie 2016. Because the grain carries yeasts as well as bacteria, kefir's mix of microbes is wider than yogurt's, and home-brewed kefir can vary from one batch to the next.

A review of kefir research lists many reported benefits, from cholesterol metabolism to allergyBourrie 2016. Our grades count only trials in people, and for kefir the clearest one is the lactose trial below.

The best-supported reason to choose either over plain milk is lactose. The European Food Safety Authority accepted that live yogurt cultures improve lactose digestion in people with lactose maldigestionEFSA 2010. The bacteria in yogurt carry their own lactase, the enzyme that breaks down lactose, and it survives the stomach well enough to work in the small intestine. Commercial yogurts differ little in this, because they all contain enough of the two standard cultures; flavored yogurts showed somewhat less lactase activity but were still well tolerated Savaiano 2014.

Kefir's evidence comes mainly from one randomized trial. Fifteen adults with lactose maldigestion were given meals containing 20 g of lactose as milk, plain or raspberry kefir, or plain or raspberry yogurt. Breath hydrogen, a sign of undigested lactose, was far higher after milk than after plain yogurt, plain kefir or flavored yogurt. Flavored kefir landed in between. All the yogurts and kefirs cut perceived gas by 54 to 71% compared with milk Hertzler 2003.

The grade on the cards above comes from the shared lactose row on our yogurt and kefir page. Most of its strength comes from the yogurt research. For kefir, the one trial is encouraging but small.

The most cited trial on fermented foods and the gut is a small Stanford study. Over 17 weeks, 18 healthy adults per group ate either a high-fiber diet or a high-fermented-food diet that included yogurt, kefir and fermented vegetables. The fermented-food diet steadily increased gut microbe diversity and lowered inflammatory markers; the high-fiber diet did not change diversity Wastyk 2021. Kefir and yogurt were eaten together, so the trial cannot say which did more.

Fermented foods can contain live microbes, some genetically similar to strains sold as probiotics, but only a limited number of clinical studies have tested fermented foods themselves Marco 2017. More on this is on the gut microbiome page.

NCCIH notes that the risk of harm from live microbes is greater in people with severe illnesses or compromised immune systems NCCIH. If that applies to you, ask your care team before adding live-culture foods. Choose pasteurized products; the yogurt standard requires the milk to be pasteurized before the culture is added FDA. Plain versions avoid the added sugar in many flavored cups, and in the lactose trial plain kefir also worked better than flavoredHertzler 2003.

Kefir is simple to brew at home from live grains. Our milk kefir recipe covers the steps and the food-safety basics, and the fermentation page covers clean equipment.