Yeast: the evidence is about a capsule, not about your bread

Almost nothing on this page is about the yeast that raises bread. The strongest result here belongs to one named strain sold in capsules, Saccharomyces boulardii CNCM I-745, which shortened acute diarrhoea in children by about a day and a half across ten randomised trials. The review hands over no dose, and we are not going to invent one for a child, so if a number of capsules is what you came for, we do not have it. Read the next line of that review before you buy any. All ten trials came out of Chinese-language databases, and the authors screened 851 articles and found none of them anywhere else. A single body of literature agreeing with itself is weaker than the same number of trials from different places. The other supplement here is yeast beta-glucan, 900 milligrams a day through a winter in 299 people, and it did not change how many of them caught a cold. It made the first week of one slightly less unpleasant. As for nutritional yeast, the vitamin B12 in it is added by the manufacturer. Natural sources of B12 are animal foods, so an unfortified flake is not one of them, and for a vegan that fortification is the entire reason to buy the tub.

How much is actually in it

• The vitamin B12 in nutritional yeast is put there, and for a vegan that is the point of buying it. — vegans and vegetarians Evidence E source
  Natural food sources of vitamin B12 are limited to animal foods, and eating foods fortified with B12 such as fortified nutritional yeasts, along with supplements, can substantially reduce the risk of deficiency

What your body absorbs

• Three quarters of the folate in brewer's yeast reaches the blood, measured with the yeast itself labelled rather than a pill. — healthy volunteers (n = 6 enrolled, 5 assessed) Evidence B source
  Folate from brewer's yeast showed a mean bioavailability of 74% relative to the reference dose of 5-methyltetrahydrofolate
• The probiotic yeast survives stomach acid and antibiotics, which is why it is a yeast and not a bacterium. — published literature Evidence D source
  S. boulardii is described as the only probiotic yeast with established clinical validation for use in humans and livestock, with resistance to gastric acid and to antibiotics and broad-spectrum antimicrobial activity, alongside technical, regulatory and safety limits on its use in food

What it does to your health

• A probiotic yeast shortened children's acute diarrhoea by more than a day and a half across ten trials. — children with acute gastroenteritis in China receiving standard rehydration therapy (n = 10 randomized trials, 1125 participants) Evidence A source
  Duration of gastroenteritis fell (SMD -1.63 days, 95% CI -2.08 to -1.18), total effectiveness rating improved (RR 1.22, 1.16 to 1.28) and more children were cured (RR 1.47, 1.30 to 1.67), with reductions in TNF-alpha and IL-8
• Yeast beta-glucan did not stop people catching colds, and made the first week of one less unpleasant. — generally healthy men and women who reported at least three upper respiratory infections in the previous year (n = 299) Evidence B source
  70 placebo and 71 beta-glucan participants had at least one confirmed infection, so incidence was unchanged and global severity did not differ (p=0.53); severity of physical symptoms was lower on beta-glucan over days 1 to 2, 1 to 3, 1 to 4 and 1 to 7 (p values 0.02 to 0.05), with a rise in the joy subscore of a stress questionnaire and a fall in systolic blood pressure

Safety, limits and interactions

• Two thirds of reported bloodstream infections with baker's yeast happened in people who had been given it as a probiotic. — hospital patients with Saccharomyces in the blood, many of them in intensive care or fed through a tube or a line (n = 108 analysed cases) Evidence C source
  73 cases (67.6%) were in patients treated with S. boulardii probiotics and 35 (32.4%) were not; risk factors among the probiotic cases were intensive care stay (31.5%), parenteral or enteral feeding (29.6%), diarrhoea (21.3%) and diabetes (13.0%); all-cause mortality across the cohort was 36.1%

What people believe that the data does not support

• Nineteen health claims about yeast have been put to Europe and not one of them passed. — — Evidence E source
  The submission that beta-glucan from yeast acts as an immunomodulator and supports natural defences carries the status Non-authorised; of 19 register entries naming yeast, 0 are authorised, 18 refused and 1 withdrawn

What is still unknown

• Every trial behind that result sits in Chinese-language databases, and none of it shows up if you search in English. — — (n = 851 articles screened, 10 included) Evidence A source
  All ten eligible randomized trials came from Chinese databases and none were found in non-Chinese databases

The bottom line

There is a group this one is genuinely not for. The name in those case reports is Saccharomyces, which covers the yeast that raises bread and the probiotic strain alike, so the name alone tells you nothing about who is at risk. The patients do. Of 108 published cases of Saccharomyces in the bloodstream, 73 were in people who had been given S. boulardii as a probiotic. The risk factors around those cases were an intensive care stay, feeding through a tube or a line, diarrhoea and diabetes. Across the whole set 36.1 percent died of any cause, and the review does not put those deaths down to the yeast. They were hospital patients who were already gravely ill. The same species sits on the European list of organisms presumed safe for ordinary food use, so both of those things are true at once. If you or someone you care for is in intensive care or being fed through a line, a probiotic yeast is a decision for the treating team. The reason this organism gets used at all is that it survives what kills bacterial probiotics, stomach acid and antibiotics, which is also why it can reach places it should not. On the plainer question of yeast as food, one small study labelled the folate inside brewer's yeast with a heavy isotope and followed it into the blood. About 74 percent of it arrived, measured against a reference dose of 5-methyltetrahydrofolate. Five people were assessed, so that is a pilot and the authors call it one. And if you are wondering why none of this appears on any packet in Europe, 19 health claims about yeast have been submitted to the register and none were authorised. Eighteen were refused and one was withdrawn.

Sources

1. yst-r1-06 · agency fact sheet
   Consumption of foods fortified with vitamin B12 (such as fortified nutritional yeasts) as well as vitamin B12 supplements can substantially reduce the risk of deficiency [57].
   NIH Office of Dietary Supplements, Vitamin B12 fact sheet for health professionals, знімок 2026-09-21 · checked 2026-09-22
2. yst-r1-05 · randomized dual isotope human intervention study
   The main outcome of the study was that the yeast folates revealed for the five assessed volunteers a mean bioavailability of 74 % relative to 5-methyltetrahydrofolate as the reference dose.
   Mol Nutr Food Res, 2026 · checked 2026-09-22
3. yst-r1-07 · narrative and bibliometric review
   Saccharomyces boulardii is currently the only probiotic yeast with established clinical validation for use in both humans and livestock.
   Front Nutr, 2026 · checked 2026-09-22
4. yst-r1-01 · systematic review and meta-analysis of randomized trials
   S. boulardii CNCM I-745 was found to significantly reduce the duration of PAGE (SMD=-1.63 days, 95% CI -2.08, -1.18), improve the total effectiveness rating (RR=1.22, 95% CI 1.16, 1.28) and significantly more participants were cured (RR=1.47, 95% CI 1.30, 1.67).
   Front Cell Infect Microbiol, 2025 · checked 2026-09-22
5. yst-r1-03 · double-blind randomized placebo-controlled trial
   Subjects supplementing with yeast beta-glucan benefit by a reduced severity of physical URTI symptoms during the first week of an episode, even though the incidence and global severity of common colds could not be altered in comparison to placebo.
   J Am Coll Nutr, 2019 · checked 2026-09-22
6. yst-r1-04 · systematic review of case reports
   Saccharomyces fungemia is commonly seen in patients treated with S. boulardii probiotics (n = 73, 67.6%), and 35 (32.4%) patients did not receive probiotic therapy.
   Med Mycol, 2023 · checked 2026-09-22
7. yst-r1-08 · regulatory register entry
   POL-HC-6718 Beta-glucan of Saccharomyces cerevisiae Beta glucan from yeast as immunomodulators Beta glucan from yeast support of natural defences Non-authorised
   EU Register on nutrition and health claims, знімок 2026-09-21 · checked 2026-09-22
8. yst-r1-02 · systematic review and meta-analysis of randomized trials
   Of 851 articles screened, 10 RCTs (1125 participants) met the inclusion criteria, and none were found in non-Chinese databases.
   Front Cell Infect Microbiol, 2025 · checked 2026-09-22