By  Dr Anton C Beynen*
Many dog and cat foods declare a yeast ingredient, most commonly brewer’s dried yeast, but also yeast extract, yeast cell walls, selenium yeast or yeast culture. Brewer’s yeast is a by-product from the brewing of beer or ale. Extracts and cell walls are yeast parts. Selenium yeast is grown in nutrient mixtures enriched with the trace element. Yeast culture is yeast and its growth medium.

Until the 1950s, whole dried yeast served as source of B-vitamins in experimental dog

foods with highly purified ingredients. By using that kind of foods, black-tongue disease in dogs was found to respond to consumption of brewer’s yeast (1), which led to the identification of vitamin B3. Yeast is lauded as B-vitamin powerhouse, but is as such needless for current petfoods that are effectively and profitably supplemented with pure B vitamins. Similar reasoning holds for selenium yeast (2).

Brewer’s yeast in petfood is rarely linked to health claims. Nutritional yeast supplements may promise flea control, healthy skin and coat, but do so without evidence.  About 1% of brewer’s yeast is often added to dry food for palatability, which requires securing for each application. MOS (mannan-oligosaccharides) from yeast cell walls is touted for gut and immune health, but dog studies are unsupportive (3). Purified yeast beta-glucans can stimulate immune responses, albeit without known impact on pets’ health maintenance (4).

Yeasts constitute a wide variety of single-celled organisms. Many are safe and useful such as the species employed in baking and production of beverage and fuel alcohol. Moderate yeast amounts in petfood seem harmless to dogs and cats. Curiously, one petfood brand carries “no added yeast” as label claim (5). As specific yeast species can cause skin infections in dogs, anti-yeast foods are being proposed, but their efficacy is unsubstantiated.

* Dr Anton C Beynen writes this exclusive column on dog and cat nutrition every month. He is affiliated with Vobra Special Petfoods.