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Brewers Yeast

- and why we perhaps shouldn't be feeding it anymore


Brewers Yeast has recently been at the mercy of new research and published studies, with the data showing the yeast involved isn’t equine gut-microbe appropriate, as in it’s the wrong type of microbe, aka an imposter.

If it’s any consolation I'm pouting a bit on this because until recently I was a huge BY fan, but the science appears to be unquestionable so, BY is now out in my book.

Basically, what the studies are saying is that, as per so many other dietary items we feed our horses - including our neon-green grass leaf-blades - it feeds the pro-inflammatory lactic-acid (LA) bacteria in the hindgut, which produce lactic-acid as their waste product, which can't be utilised by the horse, and also creates hindgut acidosis in an environment which should always be at a neutral pH.

So, despite my pout over losing BY, we can't ignore the fact that LA in turn lowers the gut pH, creating an acidic hindgut environment which leads to dysbiosis of the biome - a factor we need to avoid at all costs as hindgut acidosis is so prevalent these days.

Here's what apparently happens. The BY settles in the large intestine where it’s hot, humid, and airless, and it does the very job it was designed to do in the brewing industry – it converts sugar (from the plant starches) and water, to … alcohol! Seriously. This is a waste product of the hindgut’s normal energy metabolism when it digests sugar/starch.

This alcohol is then absorbed through the gut membrane into the body and, understandably, relaxes the horse, hence why it’s well known that a horse is more relaxed when they’re fed BY, which I completely get because let's face it, I’m a lot more relaxed after a couple of beers. 😉

And would you believe it, there’s actual current research into this in humans as we speak, with a whole series of studies and case reports on ‘Autobrewery's Syndrome’ (https://europepmc.org/article/nbk/nbk513346). The doctors that originally researched this in patients were laughed at, but today this is now considered to be an extremely under-diagnosed phenomenon. I’m just waiting for the published paper on ′'Blood alcohol levels in the horse after feeding'. Imagine that …

I know this sounds like an April Fool's joke but I'm seriously serious. Pulling this all together, BY has generally been fed as a prebiotic yeast, but it’s now confirmed as an inappropriate microbe of the biome structure in the horse’s gut system. In other words, we’ve added an unrecognised ‘invader’ into the biome when it should always be about re-establishing the species-appropriate cellulose-digesting microbes in the hindgut, the only source of this being long, stemmy, cellulose-rich hay, and not about feeding an unrecognised microbe for what we think is good measure.

Also, we used to think that BY added in a generous measure of the B-Vits, but again this has been thrown out. Provided the horse is fed species-appropriate fibre forage, i.e. hay, the microbiome produces its own B-vits, including B12 and B6 in the hindgut, and in the ‘active’ form that the equine body can utilise, not in a synthetic, unnatural form that we usually add in as a supplement. The same applies to vit.K and vit.C, the latter produced in the horse's liver.

So there we have it. BY is both inappropriate and potentially damaging to the equine physiological system.

For the record, as soon as I came across this I instantly removed it from my own horses' diets and interestingly, MacAttack - my most hindgut-challenged and very uncomfortable with it as in super-sensitive (which meant every time I reached under his belly for rug straps meant risking a severed arm), instantly softened and became hugely nicer to be around.

I can now get his sweet itch rug straps with no bother at all - literally no more cow-kicking which was previously a given - and he doesn't bite me at all now. Nothing else has changed so I can only put this down to removing the BY. He's literally turned from a bitey thug to a marshmallow softie who's happy to let me cuddle him. Happy days 😉


[1] Destrez A, Grimm P, Cézilly F, Julliand V. Changes of the hindgut microbiota due to high-starch diet can be associated with behavioral stress response in horses. Physiol Behav 2015; 149: 159–164.

[2] Fombelle A de, Julliand V, Drogoul C, Jacotot E. Feeding and microbial disorders in horses: 1-effects of an abrupt incorporation of two levels of barley in a hay diet on microbial profile and activities. J Equine Vet Sci 2001; 21 (9): 439–445.

[3] Grimm P, Philippeau C, Julliand V. Faecal parameters as biomarkers of the equine hindgut microbial ecosystem under dietary change. Animal 2017; 11 (7): 1136–1145.

[4] Julliand V, Fombelle A de, Drogoul C, Jacotot E. Feeding and microbial disorders in horses: Part 3 – Effects of three hay:grain ratios on microbial profile and activities. J Equine Vet Sci 2001; 21 (11): 543–546.

[5] Julliand V, Grimm P. The impact of diet on the hindgut microbiome. J Equine Vet Sci 2017; 52: 23–28.

[6] Kristoffersen C, Jensen RB, Avershina E, Austbø D, Tauson A-H, Rudi K. Diet-dependent modular dynamic interactions of the equine cecal microbiota. Microbes Environ 2016; 31 (4): 378–386.

[7] Langner K, Vervuert I. Impact of nutrition and probiotics on the equine microbiota: current scientific knowledge and legal regulations. Tierarztl Prax Ausg G Grosstiere Nutztiere. 2019 Feb;47(1):35-48

[8] Gottschalk C, Biermaier B, Gross M, Schwaiger K, Gareis M. Ochratoxin A in saccharomyces cerevisiae used as food supplement. Mycotoxin Res. 2016 Feb;32(1):1-5.

[9] Hoff RB, Molognoni L, Deolindo CTP, de Oliveira T, Mattos JLS, Oliveira LVA, Daguer H. J. Residues of antibiotics in yeasts from ethanol production: a possible contamination route for feedingstuffs. Environ Sci Health B. 2021 Feb 9:1-6.

[10] Garcia-Mazcorro JF, Rodriguez-Herrera MV, Marroquin-Cardona AG, Kawas JR. The health enhancer yeast Saccharomyces cerevisiae in two types of commercial products for animal nutrition. Lett Appl Microbiol. 2019 May;68(5):472-478.

[11] Garber A, Hastie P, Murray JA. Factors Influencing Equine Gut Microbiota: Current Knowledge. J Equine Vet Sci. 2020 May; 88:102943

[12] Gießelmann, Kathrin. Dtsch Arztebl 2019; 116(33-34)

[13] Medina, B., Girard,I.D.,Jacotot,E.,Julliand,V.,2002.Effect of a preparation of Saccharomyces cerevisiae on microbial profiles and fermentation patterns in the large intestine of horses fed a high fiber or a high starch diet.J.Anim.Sci.80, 2600–2609.

[14] Jouany,J.P.,Gobert,J.,Medina,B.,Bertin,G.,Julliand,V.,2008.Effect of live yeast culture supplementation on apparent digestibility and rate of passage in horses fed a high-fiber or high-starch diet.J.Anim.Sci.86,39–347.

[15] Jouany,J.P.,Medina,B.,Bertin,G.,Julliand,V.,2009.Effect of live yeast culture supplementation on hindgut microbial communities and their poly-saccharidase and glycoside hydrolase activities in horses fed a high-fiber or high-starch diet.J.Anim.Sci.87,2844–2852.

[16] Taran FMP, Gobesso AAO, Gonzaga IVF, Françoso R, Centini TN, Moreira CG, Silva LFP. Effects of different amounts of Saccharomyces cerevisiae supplementation on apparent digestibility and faecal parameters in horses fed high-roughage and high-concentrate diets. Livestock Science 2016; 186: 29–33.

[17] Painter K, Cordell BJ, Sticco KL. Auto-brewery Syndrome. In: StatPearls. StatPearls Publishing, Treasure Island (FL); 2020.

[18] Logan BK, Jones AW (July 2000). Endogenous ethanol 'auto-brewery syndrome' as a drunk-driving defence challenge. Medicine, Science, and the Law. 40 (3): 206–15.

[19] Cordell BJ, Kanodia A, Miller GK (January 2019). Case-Control Research Study of Auto-Brewery Syndrome. Global Advances in Health and Medicine. 8: 2164956119837566.

[20] Saverimuttu J, Malik F, Arulthasan M, Wickremesinghe P (October 2019). "A Case of Auto-brewery Syndrome Treated with Micafungin". Cureus. 11 (10): e5904

[21] Hafez EM, Hamad MA, Fouad M, Abdel-Lateff A (May 2017). "Auto-brewery syndrome: Ethanol pseudo-toxicity in diabetic and hepatic patients". Human & Experimental Toxicology. 36 (5): 445–450.

[22] Hammes-Schiffer S, Benkovic SJ (2006). Relating protein motion to catalysis. Annual Review of Biochemistry. 75: 519–41.

[23] Okoro VM, Mbajiorgu EF, Mbajiorgu CA (2019). Yeast (Saccharomyces cerevisiae) and its effect on production indices of livestock and poultry – review. Comparative Clinical Pathology. 28: 669-677.

[24] Ezema C, Eze DC (2012). Determination oft he effect of probiotic (Saccharomyces cerevisiae) on growth performance and hematological parameters of rabbits. Comparative Clinical Pathology. 21:73-76.

[25] Chaudhary LC, Singh R, Kamra DN, Pathak NN (1995). Effect of oral administration of yeast (Saccharomyces cerevisiae) on digestibility and growth performance of rabbits fed diets of different fibre content. World rabbit science 3(1): 15-18.

[26] Julliand V, Grimm P (2016). Horse Species Symposium: The microbiome oft he horse hindgut: History and current knowledge. Journal of Animal Science. 94(6): 2262-2274.

[27] Gomez A, Sharma AK, Grev A, Sheaffer C, Martinson K. (2021). The Horse Gut Microbiome Responds in a Highly Individualized Manner to Forage Lignification. Journal of Equine Veterinary Science. 96: 103306.

[28] Stothart MR, Greuel R, Gacriliuc S, Henry A., Wilson AJ, McLoughlin PD, Poissant J (2020). Bacterial dispersal and drift drive microbiome diversity patterns within a population of feral hindgut fermenters. Molecula Ecology. 30(2): 555-571.

[29] Kiros TG, Luise D, Derakhshani H, Petri R, Trevisi P, D’Inca R, Auclair E, van Kessel AG (2019). Effect of live yeast Saccharomyces cerevisae supplementation on the performance and cecum microbial profile of suckling piglets. PLoS ONE. 14(7): e0219557.

[30] Alugongo GM, Xiao J, Wu Z. et al. (2017) Review: Utilization of yeast of Saccharomyces cerevisiae origin in artificially raised calves. J Animal Sci Biotechnol 8(34).

[31] Welch BT, Coelho Prabhu N, Walkoff L, Trenkner SW (2016). Auto-brewery Syndrome in the Setting of Long-standing Crohn's Disease: A Case Report and Review of the Literature. J Crohns Colitis. 10(12):1448-1450.

[32] Cordell BJ, Kanodia A, Miller GK (2019). Case-Control Research Study of Auto-Brewery Syndrome. Glob Adv Health Med. 8:2164956119837566.

[33] Malik F, Wickremesinghe P, Saverimuttu J (2019). Case report and literature review of auto-brewery syndrome: probably an underdiagnosed medical condition. BMJ Open Gastroenterol. 6(1):e000325.

[34] Eaton KK, McLaren JH, Hunnisett A, Harris M (1993). Abnormal gut fermentation: Laboratory studies reveal deficiency of B vitamins, zinc, and magnesium. The Journal of Nutritional Biochemistry. 4(11): 635-638.