Fermenting at home: milk kefir, water kefir and S. boulardii soda

What lives in the grains, what studies show about kefir, how to do it safely, and why S. boulardii soda is an experiment.

How solid is the research? Plausible

Well established: kefir improves lactose digestion. For other effects the evidence is thin; 12 of 16 trials had a high risk of bias. For water kefir there is almost only lab knowledge, and for S. boulardii soda no studies at all.

Healthy state

Milk kefir is made from kefir grains: a community of lactic acid bacteria and yeasts embedded in a gel. Typical inhabitants are Lactobacillus kefiranofaciens, Lentilactobacillus kefiri, Lactococcus and Leuconostoc, plus various yeasts. The grains grow in milk and can be reused again and again. The bacteria turn lactose mainly into lactic acid, the yeasts into carbon dioxide and a little alcohol. That is why kefir tastes sour and fizzes slightly, and the milk curdles.

Kefir contains less lactose than milk, and its cultures help break it down. In 15 adults with lactose intolerance, kefir produced clearly less hydrogen in the breath than milk, a sign of less undigested lactose, and they tolerated it better. An important distinction: this applies to lactose intolerance, where the small intestine lacks the enzyme. In milk allergy the immune system reacts to milk protein, and kefir does not help.

Water kefir grains consist of a gel that lactic acid bacteria make from sugar. They contain lactic acid bacteria, yeasts such as Saccharomyces cerevisiae and acetic acid bacteria. Which species dominate depends on the origin of the grains, the sugar and the fruit. In the lab the sugar was fermented after 24 hours, mainly into lactic acid and alcohol, and the pH fell from 4.3 to 3.5 within three days.

The strong acidity and the large number of desirable microbes inhibit many unwanted germs. As a food safety rule of thumb, values below about pH 4.6 are a barrier for many dangerous germs, and kefir is usually well below that. This is why cleanly made kefir is generally safe for healthy people.

What happens to the cultures in the digestive tract: milk curdles in the stomach, which buffers the acid a little. Some of the microbes survive the passage, some do not. Like other ferment cultures they count as passers-through that do not stay. Their benefit lies more in what they already did in the jar: partly predigested lactose, made acid, produced flavour compounds. More on finished ferments is in the chapter on fermented foods.

Three homemade ferments, three very different levels of evidence.

Disrupted state: causes and effects

Kefir as a remedy: a systematic review of 16 Randomised trial A study in which chance decides who gets the treatment and who does not, for comparison. This makes results particularly reliable. found hints regarding caries bacteria in the mouth and as support during Helicobacter treatment, and only very weak ones for blood lipids and blood pressure. 12 of the trials had a high risk of bias.

Homemade means nothing is standardised. Species, germ counts, acidity and alcohol vary with time, temperature and sugar. Water kefir always contains some alcohol; alongside lactic acid it is its main product, and measurements mostly give about 0.5 to 2 percent by volume depending on sugar, fermentation time and temperature, sometimes more. This matters for children, in pregnancy and for anyone avoiding alcohol. Milk kefir contains traces too, usually far less.

If something goes wrong, fermented foods can contain mould toxins, bacterial toxins or pathogens, as well as Histamine A messenger substance of the body that is also found in aged and fermented foods. Too much can cause headache, flushing or diarrhoea. More in the chapter “Histamine intolerance: much searched, little proven” and other amines. Biogenic amines form when bacteria convert amino acids; the longer and warmer the fermentation, the more can accumulate. If you break down histamine poorly, you notice it as headache, palpitations or flushing (more in the chapter on the enzyme DAO (diamine oxidase) An enzyme in the gut wall that breaks down histamine from food before it reaches the blood. More in the chapter “DAO: the enzyme that intercepts histamine from food”). In 1995 the US CDC investigated severe illness in two people who drank home-brewed kombucha daily. A link was suspected but not proven.

Not everyone tolerates kefir. Milk kefir contains residual lactose, and water kefir with dried fruit a lot of fructose. Both are FODMAP Short for fermentable sugars and sugar alcohols that are poorly absorbed in the small intestine, such as fructans, lactose, fructose and sorbitol. More in the chapter “Doing low-FODMAP right” and can cause bloating with a sensitive gut or Small intestinal bacterial overgrowth (SIBO) Too many bacteria in the small intestine, where normally few live. Bloating and diarrhoea are typical; it is usually diagnosed with a breath test. More in the chapter “When the small intestine is too densely colonised”. And the sugar water has not fully disappeared: residual sugar remains, which matters in diabetes and for teeth.

S. boulardii soda: a popular online idea is to ferment the contents of S. boulardii capsules in sugar water or juice. There are no studies on this. The yeast multiplies like baker's yeast and makes alcohol and carbon dioxide. In lab experiments it grew and survived for weeks in fermented coffee. Nobody knows how many live yeasts end up in your own glass, and unwanted germs can grow along with it. The risk of yeast bloodstream infection in the severely ill remains.

Too long, too warm or not clean: pressure, alcohol and unwanted germs increase.

What often helps

Lifestyle

Diet, exercise, sleep, social contact and dealing with stress.

Work cleanly and throw away when in doubt Clean jars and spoons, fresh milk or freshly made sugar water. With fuzzy spots, pink or black patches or a foul smell, throw everything away, including the grains. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [1]
Ferment briefly rather than long Water kefir is ready after 24 to 48 hours. After that, mainly acidity and alcohol keep rising. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [2]
Beware of pressure in bottles In a second fermentation in closed bottles, carbon dioxide builds up, and bottles can burst. Only use bottles made for carbonation, with an intact seal; do not fill them to the top, keep the second fermentation short (one to two days) and then chill. Open them chilled with a cloth over the cap, and discard damaged bottles. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.
With lactose intolerance, try milk kefir Kefir is often tolerated better than milk. Start with small amounts. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [3]
Check acidity and smell Finished kefir smells fresh and sour, not rotten or sharply yeasty. If you want to know more precisely, measure the pH with test strips; finished kefir should be well below 4.6. This is a common food safety rule of thumb, not a study on kefir. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.
Let the grains rest If you do not want to drink it for a while, put the grains in the fridge with fresh milk or sugar water. Fermentation runs very slowly there. The grains stay healthy and no batch builds up to spoil. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.
With histamine problems, fresh and small If you react to fermented foods with headache or flushing, try, if at all, briefly fermented kefir in small amounts and do not let it stand long. More fermentation time means more amines. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [1]

Natural supplements

Supplements do not replace a medical check-up. Discuss dosage and interactions with your doctor or pharmacist.

S. boulardii soda only as an experiment It is no substitute for the capsules, for example during antibiotics, because the amount of live yeast is unknown. There are neither studies nor tested instructions. Not suitable with a central venous catheter, a weakened immune system, during pregnancy or for children. Experimental How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [4]

Medical treatments

Information, not a treatment recommendation. Decide together with a doctor. Approval and availability differ from country to country.

With weak immunity, skip homemade ferments During chemotherapy, after organ transplantation, with a central line or with severe immune deficiency, avoid uncontrolled live cultures and talk to your treatment team. In pregnancy, also discuss it with a doctor first. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [5][6]

From the community: what people report

Experiences and tips from people with the same problems that come up again and again, but for which there are no meaningful studies yet. They are explicitly not evidence.

Kefir and fermented foods with histamine complaints

Very many people with histamine or mast cell complaints report that kefir and other ferments trigger headaches, restlessness, insomnia, rashes or swelling in them. Recurring strategies are tiny amounts, only every few days, or leaving them out entirely; a few report that raw kefir is tolerated better than pasteurized. The same observation recurs in ME/CFS, mast cell and long Covid forums.

Evidence: first hints widespread Reported in: r/Kefir (Reddit community),r/HistamineIntolerance (Reddit community),r/Microbiome (Reddit community),r/MCAS (Reddit community)
"Die-off" when starting kefir

Many report feeling worse at first after starting homemade kefir – bloating, fatigue, light-headedness, skin rashes – and interpret this as "die-off". Almost always the same advice recurs: start smaller, increase more slowly, pause for a few days. Others clearly disagree and consider "die-off" in this context borrowed forum knowledge without evidence.

Evidence: no studies comes up again and again Reported in: r/Kefir (Reddit community),r/Microbiome (Reddit community),r/SIBO (Reddit community)
Homemade S. boulardii soda

Here and there patients report brewing their own "probiotic soda" with Saccharomyces boulardii and fruit juice, and describe it as light and easy to digest. There are no studies on this preparation; the yeast is studied in capsule form.

Evidence: no studies reported occasionally Reported in: r/Microbiome (Reddit community),r/CrohnsDisease (Reddit community)

Personal reports can mislead: what helps many people may be a placebo, and what helps one may harm another. Treat them as ideas to raise with your doctor or pharmacist – not as instructions.

In the mood for something completely different? Another one, at random

How we rate the evidence

Trust score 5 of 5:Established: Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies.
Trust score 4 of 5:Well supported: Several randomised studies in humans with largely consistent results.
Trust score 3 of 5:Promising: Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies.
Trust score 2 of 5:Plausible: Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms.
Trust score 1 of 5:Experimental: Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.

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