Yogurt and kefir: lactose, gut health and what the evidence says

Yogurt is milk fermented by two bacteria, Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, which turn some of the milk sugar (lactose) into lactic acid and thicken it. Kefir is a thinner, tangier drink fermented by "kefir grains", a mixed community of bacteria and yeasts, so it usually carries a wider range of microbes.

For any of the effects below, the cultures need to be alive. Look for "live and active cultures" on the label. Yogurt that is heat-treated after fermentation, and many sweetened desserts sold as yogurt, carry few or none.

People with low levels of the enzyme lactase can get gas, bloating and loose stools from milk. The bacteria in live yogurt carry their own lactase. They survive the stomach well enough to keep breaking down lactose in the small intestine, which is the mechanism behind the EFSA opinion EFSA 2010; Savaiano 2014.

In a controlled trial in adults with lactose maldigestion, plain yogurt and plain kefir both produced a fraction of the breath hydrogen that milk did (a marker of undigested lactose), and cut reported flatulence by roughly half or more Hertzler 2003.

Across three large US cohorts with more than 15,000 new cases of type 2 diabetes, total dairy was not linked to risk, but yogurt was: each daily serving tracked with a lower risk, and a pooled analysis of 14 cohorts agreed Chen 2014. People who eat yogurt daily may differ in other ways, so this cannot show cause and effect.

In 2024 the FDA said it would not object to a qualified health claim on yogurt labels about type 2 diabetes risk, worded to make clear the evidence is limited FDA 2024. That is a labeling decision, not a finding that yogurt protects anyone.

Probiotics are the most studied use here. A 2019 Cochrane review of 33 trials in children found antibiotic-associated diarrhea in 8% of those given probiotics versus 19% of controls, with higher doses (5 billion or more organisms a day) doing better Guo 2019. NCCIH notes a benefit for young and middle-aged adults but not clearly in older people NCCIH.

Those were specific strains in measured doses. A pot of yogurt contains different strains in unknown numbers, so it is reasonable food during antibiotics but should not be relied on in place of what your clinician advises. Some antibiotics also bind calcium, so timing matters (see the interaction note below).