Yeast is big business these days. From probiotics to prebiotics to postbiotics, the animal feed yeast market was valued at USD 1.91 billion in 2023 and is expected to continue growing.
Yeasts can be a good source of essential nutrients like vitamins, minerals and amino acids, but they can also provide bioactive compounds like β-glucans, chitin, and mannan oligosaccharides (MOS). Some yeasts are highly palatable to both dogs and cats; others, less so. But what are yeasts? And when is it appropriate to give supplements containing yeast to animals?
To keep it simple, yeasts are microscopic fungi. They have a nuclear membrane and cell walls, but unlike plants, they contain no chloroplasts (and so, they aren’t green). Pet products contain either active or inactive yeast. Active yeast (live yeast cells) is often used as a probiotic, whereas inactive dry yeasts can be whole inactive yeast (like Brewer’s yeast), or yeast fractions (like yeast extract, or yeast cell wall extract).
Let’s have a look at some of the different options found in diets and supplements for dogs and cats:
1) Active probiotic yeast e.g. S. cerevisiae var. boulardii
Saccharomyces boulardii is a tropical yeast first isolated from lychee and mangosteen fruit peel in 1923 by French scientist Henri Boulard. It grows at 37 °C and is non-pathogenic and non-colonizing. There is research that demonstrates that it may be effective at preventing antibiotic-associated diarrhoea, improving symptoms of acute diarrhoea, treating Clostridium perfringens infections, and treating Clostridioides difficile infections. As a strain it has very different properties to other yeasts in the same S. cerevisiae species.
There is some evidence that it may improve clinical signs in dogs and cats with chronic enteropathies, and even reduce stress in dogs. In cats, S. boulardii may increase microbiome-derived short-chain fatty acid production, reducing inflammatory conditions, and improving antioxidant status. It is a very popular ingredient in gastrointestinal supplements for both dogs and cats, either as a single ingredient, or in combination with prebiotics and/or postbiotics. It is generally considered very safe, however there are increasing reports of fungemia (systemic fungal infection) in humans with gastrointestinal disease, associated with S. boulardii use. This would imply the need for some caution when making recommendations for dogs and cats with disease affecting gastrointestinal tract integrity.
2) Inactive whole cell yeast e.g. brewer’s yeast, nutritional yeast
Brewer's dried yeast is a common ingredient in diets and supplements for dogs and cats. It is also classified as Saccharomyces cerevisiae, and is produced as a by-product from the brewing of beer and ale. It is a good source of protein and some essential nutrients, and has been considered as an alternate protein source for diets for dogs and cats. It is highly digestible and shows good palatability.
Nutritional yeast is also a whole-cell inactive yeast. It is a significant source of some B-complex vitamins and contains trace amounts of several other vitamins and minerals. Sometimes nutritional yeast is fortified with vitamin B12 and/or iron. It is sold as fine yellow flakes or a powder, and has a 'nutty' or 'cheesy' savoury flavour. It can be popular with some dogs and cats, and so can be a useful meal topper.
4) Autolysed yeast
Autolysis is the process of self-digestion, in which the cell is destroyed through the action of its own digestive enzymes (it digests itself!) Yeast autolysates are produced in a similar way to yeast extracts, but the cell walls are not removed in the process.
Autolysates combine the positive attributes of both yeast extracts and yeast cell wall extracts – improved palatability and some functional benefits. Autolysed yeast is commonly sold as an ingredient for diets, but not often as a standalone supplement.
5) Yeast extract
Yeast extracts are produced in three steps: fermentation (growing the yeast), disruption (breaking of the cells), and separation (to keep the contents of the yeast cells, without the cell walls). They are rich in glutamic acid, nucleotides, and peptides. Diets containing yeast extract have been shown to be more palatable for dogs, and also to increase fermentation by gut bacteria, producing beneficial compounds.
For all the Australians out there, the main ingredient of Vegemite is yeast extract, which contains a high concentration of glutamic acid, the source of Vegemite's delicious umami flavour!
Not surprisingly, yeast extracts are often used in pet food as a palatability enhancer. Many commercial options are available, for example: “Ohly-GO® KTD is a yeast extract derived from Saccharomyces cerevisiae, providing particularly strong roasted notes, as well as free amino acids and (small) peptides. It's especially palatable for carnivorous animals and has shown great results in cat food.” Yeast extracts are typically added to diets at levels between 0.1 and 0.5%.
6) Yeast cell wall extract
Yeast cell walls contain two main polysaccharides: mannan-oligosaccharides (MOS) and β-Glucans. They are produced by centrifuging autolysed or hydrolysed yeast, to separate the wall components. Numerous studies in different species have shown that yeast cell wall extracts may improve animal health and growth performance.
Yeast cell wall extracts are considered prebiotics, as MOS and β -glucans have been shown to increase beneficial bacterial species and short-chain fatty acid production in the gut. MOS is also capable of binding pathogenic bacteria, helping to avoid colonization and multiplication to disease-causing levels.
7) Yeast fermentate or yeast culture
(Also known as Saccharomyces cerevisiae fermentation product. For example, EpiCor®, Trumune®)
Yeast fermentates or cultures include both heat-inactivated S. cerevisiae, as well as the fermentation ‘broth’ that was utilized for growth. So, they actually contain some residual live yeast cells, dead cells, and yeast cell wall fragments, fermentation metabolites, and residual growth medium. They are considered ‘postbiotics’, and contain different bioactive compounds that may have beneficial health effects.
8) Selenium yeast
Selenium yeast typically contains 0.2-0.26% selenium. It is made by fermenting S. cerevisiae in a selenium rich medium (sodium selenite). It is predominantly used as a livestock supplement, but is occasionally found in pet food too. There is considerable variability in products described as selenium yeast and the selenium compounds found within. The yeast converts selenium to selenomethionine, however other compounds are also produced.
Selenium yeast is currently used as a source of selenium in some diets and supplements for dogs and cats. There is some evidence that selenium from selenium yeast is more bioavailable, and more effective at increasing the expression of antioxidant enzymes.
Are there any concerns with giving my pet a yeast supplement?
Let’s get this one out of the way straight away. Giving your pet yeast-derived supplements (probiotics or otherwise) will not increase the risk of ear infections or other skin disease. The yeast that can sometimes cause otitis externa (Malassezia spp.) is not the same yeast added to pet supplements. So, no need to worry about that.
Despite the fact that yeast-containing additives and feed ingredients have been widely used for decades, there are no standard analytical methods to quantify yeast and their biologically important components. Therefore, quality control and consistency can be lacking. Unfortunately, it can be difficult to know if supplements contain what they are supposed to. This is perhaps more relevant for S. boulardii probiotics, and for yeast cell wall extracts. Purchasing from reputable suppliers is a good first step.
How can I add yeast to my pet’s diet?
Nutritional yeast can be used as a palatable meal topper (try a light sprinkle), to encourage eating. So can Brewer’s yeast. Be aware that both of these products are high in protein and relatively high in phosphorus. Yeast extract spreads (sodium-reduced, if possible) can also actually be useful - you can mix a small amount (e.g. one quarter to one half of a teaspoon, depending on meal size) into your dog's food, to add some more flavour and aroma. This can be especially helpful for dogs transitioning to lower protein diets, like some diets for liver disease.
In consultation with your veterinarian, S. boulardii probiotic supplements can be trialled for some dogs with acute or chronic gastrointestinal signs. Human products are fine to use; popular brands include ‘Jarrow Formulas S. boulardii + MOS’, or 'Now Foods S. boulardii (also contains FOS)'. We recommend beginning with 2 capsules per day (10 billion CFU) and monitor signs.
Please do be certain to visit your pet's veterinarian before attempting any treatment for gastrointestinal signs, as there are many potential causes that need to be considered.
If you have any questions, concerns, comments or feedback: info@vngpets.com
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