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What is the current state of research?

The horse community is increasingly recognising the importance of intestinal health. More horses suffer from modern diseases like colic, laminitis, EMS, and insulin resistance.
Fortunately, there’s growing research on the horse’s microbiome and intestinal well-being.

The feed impacts the horse’s microbiome

The equine microbiome is highly intricate and affected by various factors, including the fiber composition of the feed and social interactions within the group [27], [28]. It has long been known that a diet rich in starch shifts the intestinal bacteria towards starch-utilising, lactic acid-producing bacteria [1] [2] [3] [4] displacing the fiber-digesting bacteria [2] [3] [4] [5] [6] that the horse requires for energy production from hay.
Saccharomyces cerevisiae is not detectable in the large intestine of horses that have never been given brewer’s yeast, suggesting that it is not necessary for digestion in the healthy horse intestine [7].

Brewer’s yeast survives the horse’s gastrointestinal transit

Probiotic feeds, including brewer’s yeast, need to reach and settle in the large intestine to have a lasting effect beyond administration [7]. As far back as 2002, studies by Medina et al. and later by Jouany et al. showed that Saccharomyces cerevisae can arrive alive in the caecum and colon, which is the horse’s large intestine [13] [14] [15]. This contradicts the common belief that Saccharomyces cerevisisae is completely destroyed in the horse’s stomach.

Live yeasts permitted as feedstuffs for horses under the German feed act

Two types of brewer’s yeast products are commonly used in supplements for horses: one is live yeast or YeaSacc (a term derived from the words “yeast” and “Saccharomyces“), and the other is a fermentation product. Both contain live yeast cultures, but the fermentation product has significantly fewer live yeast cells (on average 4.67 x 10³ colony forming units) compared to live yeast (on average 1.21 x 10⁹ colony forming units) [10]. It is concerning that there are no studies on the recommended feeding amounts of live yeast for horses [10].

Saccharomyces cerevisiae promotes weight gain in livestock feeding

The overall research on brewer’s yeast (Saccharomyces cerevisiae) is quite inconsistent, with more studies focused on its use in farm animals compared to horses.
In farm animals, brewer’s yeast is commonly used to accelerate weight gain during fattening [23].This effect is particularly pronounced when compared to control groups, especially in stressed and disease-prone animals [30]. The exact mechanism behind why brewer’s yeast leads to more significant weight gain in stressed animals compared to non-stressed ones is still unknown. Studies in pigs have shown that feeding brewer’s yeast results in faster weight gain, but it also leads to a significant decrease in the diversity of the intestinal microbiome [29]. In rabbits, whose digestive system is similar to that of horses, feeding brewer’s yeast also leads to significantly higher weight gain and an increase in blood parameters indicating chronic inflammation, all other factors remaining equal [24]. [24]. Obesity has become a prevalent issue in our horse stables, and while it has multiple causes, the practice of feeding fattening feed to horses should be questioned in this context.

Feeding strach has a greater impact on fibre digestibility than brewer’s yeast.

While some studies suggest improved fibre digestibility in horses, other research presents a different view. Many studies claiming increased fibre digestibility involve starch feeding to a degree not commonly found in normal horse diets [7]. However, the changes observed in horses fed high carbohydrate and brewer’s yeast did not differ significantly from the control group without brewer’s yeast and starch feeding, indicating that the observed changes returned to normal values [16]. In rabbits, the combination of starch-rich feeds with brewer’s yeast leads to significant changes compared to the control group [24], but no notable improvements in digestibility were observed with brewer’s yeast in a purely fibre-based diet [25].
In foals, the use of probiotics led to more frequent visits to the vet, as the probiotics likely caused overgrowth of the foals’ intestinal flora [7].

“Postbiotics” – Fermentation products of Saccharomyces cerevisiae

A recent addition to the series of pro- and prebiotics is the concept of postbiotics,
which involves using fermentation products of brewer’s yeast in feeding. These products are claimed to stabilise the intestinal environment during periods of stress or enhance the effects of vaccinations. However, closer examination of the studies reveals that the horses were fed large amounts of concentrate feed, and there was no proper control group without “postbiotics” for comparison, only with pure hay feeding. The results of the two experimental groups (both with high amounts of concentrated feed, one with postbiotics and one without) showed that the microbiome balanced out 72 hours after the stress stimulus in both groups [35]. Therefore, the postbiotics showed no significant effect, even with a diet that is not suitable for the species.
Moreover, such set-up studies cannot be applied to recreational horses, as the microbiome may react differently to stress when horses are fed species-appropriate diets without excessive amounts of concentrates. Previous research has demonstrated that the introduction of starch into the large intestine when feeding large amounts of concentrated feed has negative effects on the microbiome, destabilising it, as evidenced by numerous studies.

The equine microbiome remains largely a mystery

To truly understand the effects of brewer’s yeast on the equine microbiome, comprehensive studies need to examine the entire microbiome, including fungi, protozoa, and viruses [11], rather than just the most common bacterial strains. Moreover, most horses used in microbiome studies belong to research institutions and may have undergone various (feeding) studies, making it challenging to predict the long-term consequences of feeding brewer’s yeast to horses, especially since research in this area is still in its early stages [26].

Brewer’s yeast (Saccharomyces cerevisiae) converts sugar into alcohol

Yeast has the ability to convert glucose into alcohol and CO2, with the amount of sugar affecting the production of alcohol and CO2. This activity requires zinc as a substrate [22], which is consequently removed from the food bolus.

In humans, a well-known condition called auto-brewery syndrome exists. It can be triggered by various yeasts from the Candida and Saccharomyces families, leading to the metabolism of carbohydrates into alcohol within the affected patients’ digestive tracts.Consequently, these patients experience elevated blood alcohol levels solely from carbohydrate intake, even without direct alcohol consumption. Moreover, individuals with diabetes mellitus, obesity, or liver disease may experience even higher blood alcohol levels [17] [21]. Stress and prolonged breaks between meals can also contribute to significant increases in blood alcohol levels in affected individuals [33]. Possible factors contributing to the development of this syndrome include frequent or prolonged use of antibiotics, a high-carbohydrate diet, diabetes mellitus, and a possible genetic predisposition of reduced liver enzyme activity[18] [19] [20]. Auto-brewery syndrome may also be linked to chronic intestinal mucosal inflammation, such as in Crohn’s disease, and it can occur in patients without any concurrent or previous diseases [31] [32]. The syndrome can lead to deficiencies in B vitamins, zinc, and magnesium in humans [34]. Despite brewer’s yeast being promoted as a source of B vitamins, it can have the opposite effect when colonising the intestine. To date, no studies have examined auto-brewery syndrome in horses or other animal species. In human medicine, it is believed that many more people suffer from auto-brewery syndrome than are diagnosed, as these diseases often go unrecognised by general practitioners [33].

Probiotics are controversial in human medicine

The use of probiotics to support the intestinal flora after antibiotic administration is controversial in human medicine. Some studies have even shown that probiotics can delay, rather than support, the restoration of a healthy intestinal flora. In patients with acute pancreatitis, the mortality rate increased from 6% to 16% when probiotics were given. Prophylactic use of probiotics is also questioned due to insufficient studies [12].

Similar to human medicine, there are few well-defined therapeutic indications for the use of probiotics in horses. Even studies on acute enterocolitis or salmonella excretion in horses have produced conflicting results [7], making the overall efficacy of probiotics uncertain in this context as well.

The use of probiotics such as brewer’s yeast in horses must be questioned critically from a scientific point of view

The results indicate that the use of probiotics such as Saccharomyces cerevisiae in horses requires further research. Preventive administration without a therapeutic indication or expert consultation is not recommended. In the studies published so far, no long-term trials with brewer’s yeast feeding have been undertaken or epigenetic changes determined, meaning that no data is available at all on the effects of long-term administration over months or years. Many horse feed products contain brewer’s yeast without the owner’s awareness, and multiple feeds may be given simultaneously.

Most studies on feeding Saccharomyces cerevisiae to horses primarily focus on improved fibre digestibility when large amounts of starch are given simultaneously, rather than addressing the more valuable goal of enhancing the horse’s health or performance. This would however be an enticing reason for horse owners to consider feeding brewer’s yeast.

While increased fibre digestibility might be advantageous in farm animals to boost yield, it becomes questionable for most recreational horses that already face challenges with obesity and related health issues. Moreover, the fact that brewer’s yeast is used as a fattening agent in livestock feed contradicts its administration, especially considering the prevalence of overweight horses when fed hay alone. Additionally, there’s a risk that brewer’s yeast may settle in the horse’s intestine, leading to a condition similar to auto-brewery syndrome in humans, along with resulting deficiencies in zinc and B vitamins. This demands critical consideration, as many recreational horses exhibit zinc deficiencies in their blood work despite being given ample commercial mineral feed.

Furthermore, it’s important to note that several publications have already reported on the contamination of brewer’s yeast with antibiotics and mycotoxins as a consequence of the production process [8] [9]. These contaminants have significant and negative effects on the horse’s intestinal microbiome.


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