Kefir shows promise for relieving constipation, though the evidence is still limited. A 12-week clinical trial found that daily kefir intake significantly reduced constipation compared to baseline, with some participants experiencing complete relief. The results were mixed, however: others in the same study saw no improvement at all.
What the Research Shows
The most direct evidence comes from a study published in the Japan Journal of Nursing Science, which tested freeze-dried kefir in 42 adults over 12 weeks. Participants took 2 grams of the kefir preparation with each meal, and researchers tracked bowel movement frequency, stool shape, and laxative use throughout the trial. Constipation decreased significantly compared to baseline measurements, and some participants were able to reduce their use of laxatives.
The catch is that responses varied widely. Some people saw complete relief while others saw none, which is consistent with how probiotics work in general. Your existing gut bacteria, diet, hydration levels, and the underlying cause of your constipation all influence whether adding fermented foods will make a difference.
The World Gastroenterology Organisation references several probiotic strains that have been studied for constipation in randomized controlled trials, including strains that naturally occur in fermented dairy products. One referenced study found that fermented milk reduced the incidence of hard or lumpy stools in a healthy population. But the organization stops short of specifically recommending kefir as a constipation treatment, noting that high-quality trials for this particular use are still needed.
How Kefir Works in Your Gut
Kefir is fermented using “grains” that contain dozens of bacterial and yeast species, far more than what you’d find in a typical yogurt. These microorganisms produce lactic acid and other compounds that lower the pH in your intestines, creating an environment where beneficial bacteria thrive. When those bacteria digest fiber in your colon, they produce short-chain fatty acids that stimulate the muscles lining your intestinal wall, encouraging them to move things along.
The fermentation process also partially breaks down lactose, which is why many people who are lactose intolerant can handle kefir better than regular milk. This matters for constipation because dairy products can worsen symptoms in people with lactose sensitivity. Kefir sidesteps that problem to some degree while still delivering its probiotic content.
How Much to Drink
Most guidance suggests one to three cups (8 to 24 ounces) of kefir per day. Starting with a single cup is a reasonable approach, especially if you’re not used to fermented foods. The bacteria and yeast in kefir can cause bloating and gas when your gut first adjusts, and jumping straight to large amounts increases the odds of that discomfort.
Based on the available research, expect to give it several weeks before drawing conclusions. The clinical trial that showed significant improvement ran for 12 weeks, so a few days of kefir is unlikely to tell you much. Consistency matters more than quantity.
Dairy Kefir vs. Water Kefir
If you’re choosing between traditional dairy kefir and water kefir (made with sugar water or fruit juice), the evidence favors dairy. A review of fermented foods and gastrointestinal health found that traditional milk kefir is the most widely studied fermented food for gut conditions, with randomized controlled trials backing its benefits for lactose digestion and as a supportive therapy during certain infections. Water kefir has not been studied to the same extent, and its microbial profile is different.
That said, water kefir still contains live microorganisms and is a reasonable option if you avoid dairy entirely. It just hasn’t been tested rigorously enough to say whether it offers the same digestive benefits.
Why It Works for Some People and Not Others
Constipation has many possible causes: low fiber intake, dehydration, lack of physical activity, medication side effects, stress, and conditions like irritable bowel syndrome. Kefir addresses only one piece of the puzzle by introducing beneficial microbes. If your constipation stems from something kefir can’t fix, like a medication you’re taking or chronically low fluid intake, probiotics alone probably won’t resolve it.
People who tend to respond best to probiotic interventions are those whose constipation is primarily functional, meaning there’s no structural or medication-related cause. If your gut microbiome is already diverse and healthy, adding more bacteria through kefir may not produce a noticeable change. If your microbiome is less balanced, perhaps after a course of antibiotics or a period of poor diet, the influx of new organisms has more room to make an impact.
Pairing kefir with adequate fiber (25 to 30 grams per day), plenty of water, and regular movement gives the probiotics the best chance of working. The bacteria in kefir feed on fiber, so without enough of it, they have less raw material to produce the short-chain fatty acids that stimulate gut motility.