Yeast

Yeast (Saccharomyces cerevisiae and Related Species) as a Dietary Supplement

1. Identity: Taxonomy, Chemical Names, Natural Source, and Common Forms

1.1 Taxonomic and Chemical Identity

Saccharomyces cerevisiae is the principal yeast used in modern fermentation processes, including winemaking, breadmaking, and brewing. Taxonomically, it belongs to the kingdom Fungi, phylum Ascomycota, class Saccharomycetes, order Saccharomycetales, and family Saccharomycetaceae. Its genus name derives from the Greek sakcharon (sugar) and mykes (fungus), and the species name cerevisiae from the Latin word for beer. The microorganism Saccharomyces cerevisiae is a well-known ingredient in food, particularly breadmaking. Employed for its role in bread dough rising, this single-celled microscopic fungus has an important nutritional interest, making it a good nutritional supplement.

A closely related and clinically important organism is Saccharomyces boulardii, a yeast used specifically as a probiotic. Saccharomyces boulardii was previously identified as a unique species of yeast, but is now believed to be a strain of Saccharomyces cerevisiae (baker's yeast).

1.2 Natural Source

The yeast Saccharomyces cerevisiae has been instrumental in the fermentation of foods and beverages for millennia. In addition to fermentations like wine, beer, cider, sake, and bread, S. cerevisiae has been isolated from environments ranging from soil and trees, to human clinical isolates. In the context of dietary supplements, yeast biomass is produced via two principal routes:

• Brewer's yeast: Brewer's yeast is a by-product of the beer-brewing process that is produced through the cultivation of Saccharomyces cerevisiae on malted barley. It is cultured on malted barley, which is why brewer's yeast has a characteristic bitter flavour.
• Nutritional yeast (primary-grown): Like brewer's yeast, nutritional yeast is produced via the fermentation of Saccharomyces cerevisiae, but instead of malted barley the yeast is grown on enriched, purified cane and beet molasses under carefully controlled conditions. Unlike brewer's yeast, which is a by-product of the brewing industry, nutritional yeast is classified as a primary-grown yeast.

1.3 Common Forms and Preparations

Nutritional and functional yeasts can be used in dietary supplements due to their nutrient-dense composition. They offer consumers a source of key nutrients, including fibers, vitamins, proteins, and minerals.

Active or living yeast is used to make bread rise; however, other yeasts such as inactive yeast can also be obtained from Saccharomyces cerevisiae, and can be used for other purposes including food supplements. Principal supplement preparations include:

• Deactivated/inactive dried yeast: Once the growing of the yeast is complete, the cultures are dried to render them inactive. This step prevents the yeast from reproducing or fermenting, and also concentrates the nutrients. This form is sold as powder, flakes, or tablet/capsule.
• Brewer's yeast tablets or powder: Once the fermentation process is complete the yeast is separated from the beer, dried via rollers, and then debittered for human consumption.
• Live probiotic yeast (S. boulardii): S. boulardii is a live yeast used extensively as a probiotic and often marketed as a dietary supplement.
• Yeast cell wall extracts (beta-glucan preparations): Isolated polysaccharide fractions, particularly beta-(1,3)/(1,6)-D-glucan, are produced from the yeast cell wall and marketed separately as immunomodulatory supplements.
• Chromium-enriched yeast: Yeast grown under conditions that incorporate organic chromium, yielding a more bioavailable form of the mineral than inorganic salts.

2. Traditional and Historical Use

2.1 Ancient Origins of Yeast Fermentation

The earliest known records of yeast-risen bread come from Ancient Egypt in 1300–1500 BCE and China in 500–300 BC. However, it is likely that organized reliance on organisms for fermentation is far older. From residue present inside one of the earliest known wine jars from Egypt, researchers extracted, amplified, and sequenced ribosomal DNA from S. cerevisiae. These results indicate that this organism was probably responsible for wine fermentation by at least 3150 B.C.

Archaeological evidence suggests that fermentation was practiced in ancient Egypt, Mesopotamia, China, and India, where fermented grains, fruits, and milk served both practical and ritual purposes. The ancient Egyptians, for instance, developed beer not only as a staple beverage but as a sacred elixir that symbolized the divine. In Mesopotamia, priests and scribes meticulously recorded the art of brewing, and fermentation soon spread to the surrounding regions as both a culinary staple and a cultural emblem.

2.2 Pre-Scientific Understanding

In antiquity, pure yeasts were unavailable due to a lack of understanding of microbiology necessary to produce them. Instead, mixtures of wild bacteria (especially Lactobacillus) and yeasts were used for brewing and leavening, resulting in acidic goods. However, empirical testing by beer makers around the 15th century led to the discovery that first boiling a wort containing hops gave a non-acidic beverage. The reason for this was unknown at the time, but boiling killed unwanted bacteria, and the hops contain natural chemicals that suppressed the regrowth of acid-making bacteria while the yeast flourished.

E. C. Hansen from Carlsberg Brewery (Denmark) established the basis for using selected yeast strains as starter cultures in brewing in 1883. He was the first to define ale brewing yeasts and to distinguish them from lager. Until this point, beer was traditionally obtained by the action of a mixture of microorganisms perpetuated from one batch to another.

2.3 Traditional Nutritional and Medicinal Use

Beyond fermentation, brewer's yeast accumulated as a by-product of beer production and was historically used as a food supplement in Europe and elsewhere. Saccharomyces cerevisiae has been demonstrated to exert health-promoting effects due to its rich vitamin (especially B-complex) and mineral content, as well as its role in the production of microbial proteins, beta-glucans and mannans. Hence, the intake of brewer's yeast as a nutritional supplement is popular among vegans and health-conscious people.

Yeast-derived food pastes represent traditional forms of culinary yeast supplementation. In Australia, a residue from the brewing process became Vegemite; in the United Kingdom a comparable product became known as Marmite — both thick, concentrated yeast extracts used as spreads and flavor agents, supplying significant B-vitamin content.

Brewer's yeast contains organic chromium with better absorption compared to inorganic chromium. Brewer's yeast has been used for decades in research evaluating the effect of chromium supplementation on fasting plasma glucose, lipid metabolism and blood pressure in diabetic patients. This use has roots in mid-20th century nutritional science, when chromium's role in carbohydrate metabolism first became apparent.

3. Key Constituents and Active Compounds

3.1 Macronutrient Composition

Yeast (Saccharomyces cerevisiae) is a rich source of protein, soluble fiber, and some minerals. Additionally, yeast is rich in fibers and proteins while having little to no fat or salt. Amino acid analysis shows that digestibility was higher (90%) for the phosphorylated protein concentrate form and lower (68%) for whole yeast cells.

3.2 Vitamins

Saccharomyces cerevisiae has demonstrated health-promoting effects due to its rich vitamin (especially B-complex) and mineral content. The B vitamins present include thiamine (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), pyridoxine (B6), and folate. The key nutritional difference between brewer's yeast and nutritional yeast is that brewer's yeast contains the trace mineral chromium but not vitamin B-12. Nutritional yeast can also be fortified with vitamin B-12, an important nutrient for vegans who are more susceptible to B-12 deficiency.

3.3 Minerals

Saccharomyces cerevisiae in particular is rich in zinc, phosphorus, iron, and magnesium — four minerals that are trace elements essential for the proper functioning of the human body. This yeast is high in B-complex vitamins, protein, chromium, and selenium.

3.4 Beta-Glucans (Cell Wall Polysaccharides)

Beta-glucans are a heterogeneous group of natural polysaccharides mostly investigated for their immunological effects. Due to the low systemic availability of oral preparations, it had been thought that only parenterally applied beta-glucans can modulate the immune system. However, several in vivo and in vitro investigations have revealed that orally applied beta-glucans also exert such effects. Various receptor interactions, explaining possible modes of action, have been detected. The effects mainly depend on the source and structure of the beta-glucans.

The manufacturing process and, hence, the isolation method impacts the structure of beta-glucans and consequently their effects on the immune system. The immune-modulating activity of different beta-glucans from the same source might differ considerably in level of purity, solubility, molecular mass, tertiary structure, degree of branching, polymer charge and solution conformation.

3.5 Chromium and the Glucose Tolerance Factor

Chromium deficiency leads to impaired glucose and lipid metabolism. Chromium supplementation in type II diabetic patients improves glucose and lipid profiles. Organic chromium, such as found in brewer's yeast, is much better absorbed than inorganic chromium. Chromium deficiency in experimental animals and in humans sustained by prolonged total parenteral nutrition has been shown to cause diabetes mellitus.

3.6 Mannan Oligosaccharides

Saccharomyces cerevisiae yeast-based supplements are rich in B vitamins, beta-glucan, mannan oligosaccharides and bioavailable chromium; these may impact breast milk production or composition. Mannan oligosaccharides (MOS) are derived from the outer cell wall of S. cerevisiae and have been studied for their prebiotic and immune-supportive properties, though most robust evidence for these effects in humans remains limited.

4. Scientific Evidence by Area of Use

4.1 Gastrointestinal Health and Probiotic Use (Saccharomyces boulardii)

4.1.1 Overview of S. boulardii as a Probiotic

Saccharomyces boulardii is the only yeast commonly used in clinical practice as a probiotic. Several clinical trials and experimental studies strongly suggest a place for S. boulardii as a biotherapeutic agent for the prevention and treatment of several gastrointestinal diseases. S. boulardii mediates responses resembling the protective effects of the normal healthy gut flora. The multiple mechanisms of action of S. boulardii and its properties may explain its efficacy and beneficial effects in acute and chronic gastrointestinal diseases that have been confirmed by clinical trials.

Mechanistically, S. boulardii mechanisms of action include regulation of intestinal microbial homeostasis, interference with the ability of pathogens to colonize and infect the mucosa, modulation of local and systemic immune responses, stabilization of the gastrointestinal barrier function, and induction of enzymatic activity favoring absorption and nutrition. More specifically, the exact mechanisms by which S. boulardii exerts its actions are multifactorial and include the interference with pathogen attachment, restoration of disrupted intestinal microflora, inactivation of toxins (including Vibrio cholerae, ETEC, Clostridioides difficile, etc.), antisecretory effects and immunomodulatory effects, both within the lumen and systemically.

4.1.2 Antibiotic-Associated Diarrhea

This is the best-supported indication for S. boulardii. Twenty-one randomized controlled trials (4,780 participants), among which 16 were new trials, met the inclusion criteria for an updated systematic review. Administration of S. boulardii compared with placebo or no treatment reduced the risk of antibiotic-associated diarrhea in patients treated with antibiotics from 18.7% to 8.5% (risk ratio 0.47; 95% CI: 0.38–0.57; number needed to treat: 10; 95% CI: 9–13).

In adults, S. boulardii can be strongly recommended for the prevention of antibiotic-associated diarrhea (AAD) and traveler's diarrhea. However, not all trials support uniform benefit: one randomized, double-masked, placebo-controlled trial found no evidence for an effect of S. boulardii in preventing AAD or Clostridium difficile-associated diarrhea (CDAD) in a population of hospitalized patients without particular risk factors apart from systemic antibiotic treatment. This underscores that evidence, while overall positive, remains somewhat heterogeneous across populations.

4.1.3 Acute Gastroenteritis in Children

A meta-analysis including 10 articles and a total of 1,282 children with acute gastroenteritis found that the use of S. boulardii could effectively shorten diarrhea duration (MD = 19.70, 95% CI: −24.87 to −14.52) and reduce the length of hospital stay (MD = −0.91, 95% CI: −1.28 to −0.54).

4.1.4 Traveler's Diarrhea, H. pylori, Inflammatory Bowel Disease

Randomized trials also support the use of this yeast probiotic for prevention of enteral nutrition-related diarrhea and reduction of Helicobacter pylori treatment-related symptoms. S. boulardii shows promise for the prevention of C. difficile disease recurrences; treatment of irritable bowel syndrome, acute adult diarrhea, Crohn's disease, giardiasis, and HIV-related diarrhea; but more supporting evidence is recommended for these indications.

For inflammatory bowel disease specifically, S. boulardii can be effective in inflammatory gastrointestinal diseases with diverse pathophysiology, such as Inflammatory Bowel Disease (IBD), and bacterially — or enterotoxin-mediated diarrhea and inflammation. Early research shows that adding S. boulardii to standard mesalamine therapy can reduce symptoms in people with mild-to-moderate ulcerative colitis. The evidence for IBD indications is preliminary and requires further well-powered trials.

4.1.5 Gut Dysbiosis and Microbiota Resilience

The probiotic S. boulardii CNCM I-745 notably counters antibiotic-associated perturbations and promotes a return to eubiosis by increasing microbial diversity, stimulating the microbial production of short-chain fatty acids (SCFAs), reinforcing the gut's barrier functions and diminishing local inflammation. Available clinical data support its safety and efficacy in the prevention and management of gastrointestinal disorders, particularly diarrhoeal conditions, and suggest a potential role in promoting microbiota resilience.

4.2 Immune System Modulation (Beta-Glucan)

4.2.1 Mechanisms

Beta-glucans are a heterogeneous group of natural polysaccharides mostly investigated for their immunological effects. Due to the low systemic availability of oral preparations, it had been thought that only parenterally applied beta-glucans can modulate the immune system. However, several in vivo and in vitro investigations have revealed that orally applied beta-glucans also exert such effects.

4.2.2 Human Clinical Trial Evidence for Respiratory Infections

A placebo-controlled, randomized, double-blind intervention study provides clinical evidence that supplementation with yeast (1,3)-(1,6)-beta-glucan helps to reduce the occurrence of symptomatic common cold infections by 25% as compared to placebo. In addition, consumption of yeast (1,3)-(1,6)-beta-glucan caused a milder progression of severe common cold episodes.

In a placebo-driven clinical trial in children, researchers focused on the effects of orally administered glucan in children with chronic respiratory problems. Measuring levels of albumin, lysozyme, and CRP in saliva of 40 children aged 8–12 years, they evaluated the effects of a 100 mg/day oral dose of glucan. They found a significant increase in changes in production of lysozyme and CRP in glucan-treated children. In addition, a strong improvement in general conditions was found. Short-term oral application of natural immunomodulator beta-glucan stimulated mucosal immunity of children with chronic respiratory problems.

In the meantime, several human clinical trials with dietary insoluble yeast beta-glucans have been performed. The results confirm the previous findings of in vivo studies. The results of all studies taken together clearly indicate that oral intake of insoluble yeast beta-glucans is safe and has an immune-strengthening effect.

Limitations: Although there are numerous in vitro and in vivo investigations, human clinical trials confirming the preclinical findings are rather scarce. Due to different natural sources of the beta-glucans, differences in application, differences in preparation and thus structural differences, the results obtained in the resulting clinical studies are non-homogeneous and often contradictory.

4.3 Blood Glucose Regulation and Diabetes (Chromium-Rich Brewer's Yeast)

4.3.1 Clinical Trials

The most historically robust clinical work on yeast supplementation for glucose metabolism involves chromium-rich brewer's yeast. Twenty-four volunteers, mean age 78, including eight mildly non-insulin-dependent diabetics, were randomly allocated to one of two groups and were fed daily for 8 weeks with 9 g of either chromium-rich brewers' yeast (experimental) or chromium-poor torula yeast (control). In the total experimental group (normals + diabetics) and in both the diabetic and nondiabetic experimental subgroups, glucose tolerance improved significantly and insulin output decreased after supplementation. Cholesterol and total lipids fell significantly after supplementation in the total experimental group. The cholesterol decrease was particularly marked in hypercholesterolemic subjects (cholesterol >300 mg/dL).

In a double-blind cross-over trial of 78 type 2 diabetes patients, subjects were given brewer's yeast (23.3 µg Cr/day) and CrCl3 (200 µg Cr/day) sequentially with placebo in between in a double-blind cross-over design of four stages, each lasting 8 weeks. Both supplements caused a significant decrease in the means of glucose (fasting and 2-hour post-glucose load), fructosamine and triglycerides. The means of HDL-cholesterol, and serum and urinary chromium were all increased. A higher percentage of subjects responded positively to brewer's yeast chromium, which was retained more by the body, with effects on fructosamine, triglycerides, and HDL-cholesterol maintained in some subjects when placebo followed it.

A separate study involved thirty-six subjects supplemented with 400 µg Cr/day as Cr-enriched yeast (n = 19) or placebo (n = 17) for 12 weeks in a randomized, double-blind study.

4.3.2 Meta-Analytic Evidence

A meta-analysis of chromium supplementation in diabetes found that compared to control, brewer's yeast showed a statistically significant decrease in fasting plasma glucose of −19.23 mg/dL (95% CI = −35.30 to −3.16, I² = 21%, n = 137). Chromium supplementation with brewer's yeast may provide marginal benefits in lowering fasting plasma glucose in patients with type 2 diabetes mellitus compared to placebo; however, it did not have any effect on HbA1c.

In a meta-analysis of brewer's yeast, doses of brewer's yeast with 10 µg/day chromium had larger net decreases in fasting glucose than lower doses. This supports the theory that type 2 diabetes patients may have a low level of chromium and an effective source for chromium repletion such as brewer's yeast may improve their carbohydrate tolerance. Therefore, patient selection may be an essential part in determining clinical response, as it was concluded that a clinical response may be more likely in insulin-resistant individuals with type 2 diabetes.

Overall evidence strength: Moderate. Multiple small to medium-sized RCTs demonstrate a statistically significant benefit on fasting glucose but not consistently on HbA1c. Results are most pronounced in populations with pre-existing chromium insufficiency or insulin resistance. Effect sizes are considered marginal in the context of standard diabetes management.

4.4 Blood Lipids and Cardiovascular Risk

Yeast-derived beta-glucan fiber has been examined for cholesterol-lowering effects. A study evaluated the effect on serum lipids of a yeast-derived beta-glucan fiber in 15 free-living, obese, hypercholesterolemic men; 15 g fiber/day was added to the diet for 8 weeks and then stopped for 4 weeks. The yeast-derived beta-glucan fiber significantly lowered total cholesterol concentrations and was well tolerated; HDL-cholesterol concentrations rose, but only 4 weeks after the fiber was stopped. LDL-cholesterol concentrations did decline by 8% at week 8 compared with baseline.

However, not all trials yield positive results. In a larger, well-controlled trial, a randomized double-blinded trial assessed the efficacy and safety of beta-glucan supplementation in reducing lipid levels. Subjects with LDL cholesterol levels of >3.37 mmol/L were randomly assigned to receive one of three daily doses of a tableted formulation of beta-glucan (1.5, 3, or 6 g) or placebo; a total of 263 subjects were enrolled. In subjects with LDL cholesterol levels of >3.37 mmol/L, a tablet formulation of beta-glucan was not effective in reducing LDL cholesterol concentration or other lipid subfractions when compared with a placebo.

It is important to note that most robust evidence for beta-glucan-related cholesterol reduction comes from studies of oat beta-glucan (a soluble form), not yeast-derived beta-glucan (an insoluble form). A comprehensive meta-analysis of 58 clinical trials and 3,974 subjects has shown that oat beta-glucan significantly affects the serum concentrations of LDL-C, non-HDL-C and apolipoprotein-B, concluding that the inclusion of oat-containing foods in the diet may be a valid strategy to prevent the onset of cardiovascular disease. The evidence for insoluble yeast-derived beta-glucan specifically in lipid management is considerably weaker and inconsistent.

4.5 Nutritional Support in Vegan and Vegetarian Diets

Nutritional yeast is grown specifically as a food supplement and, when fortified, is one of the only reliable plant-based sources of vitamin B12. Nutritional yeast is much higher in B vitamins than brewer's yeast, a nutrient that is important for vegetarians who are more prone to vitamin B-12 deficiency. This application is widely accepted from a nutritional standpoint but does not require clinical trials, as it simply represents a dietary source of established nutrients.

4.6 Lactation (Galactagogue Use)

Brewer's yeast is anecdotally used by breastfeeding women to increase milk supply. Saccharomyces cerevisiae yeast-based supplement is an inactive form of S. cerevisiae yeast either obtained from the fermentation process or grown on molasses. Dietary supplementation with S. cerevisiae yeast-based supplement during gestation and lactation significantly increases milk yield in ruminants. However, no human study has evaluated efficacy of this supplement as a galactagogue. The lack of consistent advice and the variable nutrient composition of commercial products has efficacy and safety implications. The evidence for this use is currently anecdotal only.

5. Body Systems and Health Areas

Based on the accumulated clinical and preclinical evidence, yeast supplements are associated with the following body systems and health areas:

• Gastrointestinal system: The most extensively studied area. S. boulardii has demonstrated evidence-based efficacy for AAD prevention, acute gastroenteritis, and traveler's diarrhea, with promising but preliminary data for IBD, IBS, H. pylori adjuvant therapy, and C. difficile prophylaxis. S. boulardii has demonstrated clinical and experimental effectiveness in gastrointestinal diseases with a predominant inflammatory component, indicating that this probiotic might interfere with cellular signaling pathways common in many inflammatory conditions.
• Immune system: Several human clinical trials with dietary insoluble yeast beta-glucans have been performed. The results confirm the previous findings of in vivo studies and clearly indicate that oral intake of insoluble yeast beta-glucans is safe and has an immune-strengthening effect.
• Endocrine/metabolic system (glucose and insulin regulation): Chromium-rich brewer's yeast has marginal but statistically significant evidence for improvement in fasting glucose in type 2 diabetic patients, particularly those with chromium insufficiency.
• Cardiovascular system: Evidence is mixed and inconsistent for yeast-specific beta-glucan in lipid management; however, a small study showed a statistically significant reduction in total cholesterol with yeast-derived beta-glucan fiber supplementation.
• Nutritional (B vitamins, minerals): Well established as a food/supplement source of B vitamins, protein, zinc, selenium, phosphorus, and chromium, with particular relevance for vegan and vegetarian dietary patterns.

6. Dosage Forms and Reported Dosages

The following dosages are drawn directly from cited studies and trials:

• Brewer's yeast (whole dried, chromium source): 9 g/day for 8 weeks (elderly subjects with and without mild non-insulin-dependent diabetes).
• Brewer's yeast (chromium-standardized tablets for type 2 diabetes): Brewer's yeast tablets containing 14.4 µg chromium per day, evaluated at 4 and 8 weeks.
• Brewer's yeast (double-blind cross-over diabetes trial): 23.3 µg Cr/day, in stages each lasting 8 weeks.
• Chromium-enriched yeast (randomized, double-blind): 400 µg Cr/day as Cr-enriched yeast for 12 weeks.
• Yeast beta-glucan (immune/common cold): Insoluble (1,3)-(1,6)-D-glucan preparations have been used in randomized trials at doses corresponding to those in the Yestimun® preparation evaluated in the multicenter study above.
• Yeast-derived beta-glucan fiber (cholesterol): 15 g fiber/day was added to the diet for 8 weeks.
• Beta-glucan tablet (lipid trial): Daily doses of 1.5, 3, or 6 g of a tableted formulation of beta-glucan for 12 weeks.
• Yeast-derived beta-glucan (oral pediatric glucan trial): 100 mg/day oral dose of glucan evaluated over 4 weeks in children aged 8–12 years.
• Saccharomyces boulardii (probiotic — vaginal microbiota study): S. cerevisiae CNCM I-3856 at 500 mg/day (2 capsules) or 1 g/day (2 capsules) for 4 weeks.
• S. boulardii (AAD prevention — recommended range): Clinicians should consider probiotic strains with well-established efficacy administered at dosages typically ranging from 10⁹ to 10¹⁰ colony-forming units (CFU) per day.

7. Safety Considerations and Interactions

7.1 General Safety Profile of Inactive/Nutritional Yeast

Inactive (deactivated) S. cerevisiae yeast, whether as nutritional yeast or brewer's yeast powder, is generally considered safe for the general healthy population when used in normal food supplement amounts. The results of clinical trials clearly indicate that oral intake of insoluble yeast beta-glucans is safe and has an immune-strengthening effect. Common minor adverse effects reported in S. boulardii trials include bloating, gas, and upset stomach.

7.2 Immunocompromised Patients and Risk of Fungemia

This is the most clinically significant safety concern associated with live yeast products. There is concern that critically ill people, people who have a weakened immune system, and people who are taking medicines that alter the immune system might have an increased risk for developing a yeast infection that spreads to the bloodstream and the rest of the body (fungemia) if they take Saccharomyces boulardii. Although Saccharomyces generally does not cause disease, there have been numerous cases of fungemia following its use, primarily in people with a weakened immune system.

In individuals with weakened immune systems (such as those with HIV/AIDS, cancer, or those taking immunosuppressive drugs), there is a risk of developing a fungal infection in the bloodstream (fungemia). This can be serious and requires immediate medical attention.

7.3 Central Venous Catheters

Patients with central venous catheters (a type of tube placed in a large vein) are at higher risk of developing a bloodstream infection from Saccharomyces boulardii.

7.4 Allergic Reactions

Yeast allergy is a documented contraindication. A published case report documented that a patient known to take S. boulardii as an antidiarrheal therapy developed an allergic reaction. A 60-year-old male patient was admitted to the hospital with itchy rash on both ankles within 1.5 hours after ingesting an S. boulardii 250 mg capsule. Although S. boulardii is known as a safe drug in the treatment of some gastrointestinal disorders, it cannot be referred to as completely reliable on the basis of allergic reactions. Previously, only one allergic reaction affecting the gastrointestinal system had been reported. People with yeast allergy can be allergic to products containing Saccharomyces boulardii and are best advised to avoid these products.

7.5 Interactions with Antifungal Medications

Antifungal medications have a minor interaction with S. boulardii. S. boulardii is a fungus, and medications for fungal infections help reduce fungus in and on the body. Consequently, antifungal drugs (such as fluconazole, itraconazole, or amphotericin B) may reduce the viability and efficacy of live S. boulardii probiotic preparations if taken concurrently.

7.6 Contamination Concerns

The safety of taking Saccharomyces cerevisiae yeast-based supplements during lactation is not well studied. Studies have reported contamination of these supplements with ochratoxin A (OTA) as well as minor side effects. Ochratoxin A is a mycotoxin produced by certain mold species and represents a relevant food safety concern in yeast products derived from agricultural substrates.

7.7 Pregnancy and Lactation

A doctor should be asked before using Saccharomyces boulardii products if pregnant or breastfeeding, as safety in these populations has not been adequately established through controlled studies.

7.8 Gout and High-Purine Diets

Yeast is a significant dietary source of purines, and individuals with gout or hyperuricemia are traditionally counseled to limit yeast-containing foods. This concern is documented in nutritional guidelines for gout management, though it primarily applies to regular dietary consumption rather than isolated beta-glucan or chromium supplements.

References

• Lahue C et al. History and Domestication of Saccharomyces cerevisiae in Bread Baking. Frontiers in Genetics, 2020. PMC7686800
• McGovern PE et al. Evidence for S. cerevisiae fermentation in ancient wine. J Archaeol Sci, 2003. PubMed 15008419
• Harasym J & Oledzki R. Dietary supplements based on the yeast biomass. ResearchGate, 2017
• Pacheco MTB et al. Yeast (Saccharomyces cerevisiae) protein concentrate: preparation, chemical composition, and nutritional and functional properties. J Agric Food Chem, 2005. PubMed 15884819
• Garber AK et al. Saccharomyces cerevisiae Yeast-Based Supplementation as a Galactagogue in Breastfeeding Women? A Review of Evidence from Animal and Human Studies. Nutrients, 2021. PMC7996189
• Stier H et al. Immune-modulatory effects of dietary Yeast Beta-1,3/1,6-D-glucan. Nutrition Journal, 2014. PMC4012169
• Vetvicka V et al. Placebo-driven clinical trials of yeast-derived beta-(1-3) glucan in children with chronic respiratory problems. J Am Nutr Assoc, 2014. PubMed 25332970
• Auinger A et al. Yeast (1,3)-(1,6)-beta-glucan helps to maintain the body's defence against pathogens: a double-blind, randomized, placebo-controlled, multicentric study in healthy subjects. Eur J Nutr, 2013. PMC3832763
• Offenbacher EG & Pi-Sunyer FX. Beneficial effect of chromium-rich yeast on glucose tolerance and blood lipids in elderly subjects. Diabetes, 1980. PubMed 7000589
• Ghosh D et al. The effects of inorganic chromium and brewer's yeast supplementation on glucose tolerance, serum lipids and drug dosage in individuals with type 2 diabetes. Saudi Med J, 2001. PubMed 11376359
• Asghari G et al. Brewer's Yeast Improves Glycemic Indices in Type 2 Diabetes Mellitus. Iran J Med Sci, 2013. PMC3843299
• Costello RB et al. Effect of chromium supplementation on glycated hemoglobin and fasting plasma glucose in patients with diabetes mellitus. PLoS ONE, 2015. PMC4430034
• Nicolosi RJ et al. Plasma lipid changes after supplementation with beta-glucan fiber from yeast. Am J Clin Nutr, 1999. PubMed 10426696
• Chan LWT et al. Supplementation with a β-glucan tablet has no effect on hyperlipidemia: a randomized, placebo-controlled clinical trial. Am J Clin Nutr, 2023.
• Kelesidis T & Pothoulakis C. Efficacy and safety of the probiotic Saccharomyces boulardii for the prevention and therapy of gastrointestinal disorders. Therap Adv Gastroenterol, 2012. PMC3296087
• Pothoulakis C. Review article: Anti-inflammatory mechanisms of action of Saccharomyces boulardii. Aliment Pharmacol Ther, 2009. PMC2761627
• McFarland LV. Systematic review and meta-analysis of Saccharomyces boulardii in adult patients. World J Gastroenterol, 2010. PMC2868213
• McFarland LV et al. Systematic review with meta-analysis: Saccharomyces boulardii in the prevention of antibiotic-associated diarrhoea. Aliment Pharmacol Ther, 2015.
• Zheng J et al. Effectiveness and Safety of Saccharomyces boulardii for the Treatment of Acute Gastroenteritis in the Pediatric Population: A Systematic Review and Meta-Analysis. Front Pediatr, 2022. PMC9514928
• Kulak Y et al. An unexpected allergic reaction with Saccharomyces boulardii: a case report. Turk J Med Sci, 2014. PMC4127795
• Narrative review on Saccharomyces boulardii CNCM I-745 for diarrhoeal conditions. PMC, 2025. PMC12844198
• Ehrhardt S et al. Saccharomyces boulardii to Prevent Antibiotic-Associated Diarrhea: A Randomized, Double-Masked, Placebo-Controlled Trial. Open Forum Infect Dis, 2016.
• Andjela L et al. Effects of fungal beta-glucans on health — a systematic review of randomized controlled trials. Food Funct, 2021.
• Bai FW et al. A short history of beer brewing: Alcoholic fermentation and yeast technology over time. EMBO Rep, 2022. PMC9724654