The importance of vegetable protein - Veterinary Practice
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The importance of vegetable protein

Lee Danks in this latest in a series of columns from Royal Canin focusing on innovations in small animal nutrition, looks at the growing role of vegetable proteins in veterinary diets.

PROTEINS are simply biological molecules consisting of chains of amino acids bound together to form peptides.

When these chains include 30 or more amino acids they’re termed polypeptides and as they grow larger in size they take on a folding three-dimensional structure which grants each protein its unique structural and functional characteristics.

When incorporated into our bodies, some proteins render structural support and others have a more functional role including catalysing metabolic reactions, replicating DNA, responding to internal and external stimuli, and transporting molecules from one location to another.

Nutritional requirements

Twenty-two different amino acids are required by dogs and cats to fulfil the functions required to sustain life. Of these 22 amino acids, 12 can be produced within the dog’s body, leaving 10 as nutritionally “essential”. In the cat, 11 amino acids are essential.

We need to provide these via our pet’s food and examples include arginine (which stimulates the immune system, induces growth hormone release and supports the liver), histidine (which releases histamines, is associated with pain control, and widens small blood vessels to stimulate stomach acid secretion), and methionine (which assists gall bladder functions, helps prevent fats deposits in the liver and balances the urinary tract pH).

Animal versus vegetable proteins

Due to the different amino acid mixes (or “profiles”) contained in different proteins, animal proteins were historically considered “complete proteins” for dogs and cats, while plant proteins were considered “incomplete”. This is because plant proteins such as grain, corn gluten, soybean meal or plant protein isolates by themselves do not contain all of the amino acids in the right proportions that a dog or cat needs.

Amino acids essential to dogs and cats often missing in plant proteins include arginine, taurine, methionine, lysine and tryptophan. As a result, it is imperative that the difference between the amino acid profiles of plant and animal proteins is considered when formulating diets.

It has been shown, for example, that whilst racing or working dogs may develop anaemia when fed unbalanced plant protein diets, they will remain in good health if a meat-free diet is correctly balanced. The pivotal difference here, of course, is in the expertise of the manufacturer. Nutritional balance rather than a fixation with ingredients is key.

Advantages of vegetable proteins

Protein digestibility depends on two major factors: their source and the processes by which they are incorporated into pet foods. Undercooked or overcooked proteins are reduced in digestibility but when correctly managed, vegetable proteins can be just as valuable to the pet as animal proteins.

Indeed, soy isolate, soy hydrolysate, corn gluten and wheat gluten are all purified sources of very highly digestible vegetable proteins. For example, wheat gluten has been shown to be more digestible (at 97-99%) than beef (80-97%), which means it is better tolerated by the gastro-intestinal tract and therefore a preferred ingredient for animals with GI disease.

This is because undigested protein can over-stimulate the GI immune system which may increase the risk of food intolerance and allergy. Undigested proteins may also promote the growth of bacteria that are believed harmful to the health of the colon and the host.

Finally, putrefaction products resulting from the fermentation of undigested proteins in the colon will induce poor digestive tolerance: namely a strong faecal odour, flatulence and diarrhoea.

In recent years, several plant proteins have been investigated for their potential inclusion in pet foods. Soybean, with a high protein content and an amino acid composition similar to that of meat, is actually less “incomplete” than first thought. Much research has therefore been conducted into the best method of processing for maximal digestibility.

This said, wheat gluten too has been found to be high in crude protein content and digestibility and so is increasingly being included in diets. More recently, maize gluten meal was also identified as a highly digestible plant protein suitable for use in canine diets.

The author of one report (Yamka et al, 2004) concluded that these plant protein sources represent practical alternatives to animal protein sources in extruded pet foods, as nutrient digestibility and faecal quality are similar to that of a meat diet.

There are certain clinical conditions in which patients respond positively to vegetable proteins. Humans suffering from liver disorders and susceptible to hepatic encephalopathy (HE) are fed vegetable or dairy proteins in preference to meat, fish or egg protein sources to control clinical signs and maintain body condition.

There is evidence that similar nutritional manipulations are beneficial in dogs with HE given the change in blood ammonia concentrations when moving from a meat to a vegetable-based diet.

Dogs and cats with other clinical conditions can also benefit from being fed vegetable proteins. Preliminary reports suggest that a soy hydrolysate and rice-based diet could significantly improve the clinical condition of dogs suffering both from EPI (exocrine pancreatic insufficiency) and skin disease.

In addition, the phosphorus-restriction, which is central to the life-sustaining features of a renal diet, centres on the use of vegetable ingredients as a source of nutrients.


Looking to the future, it is unlikely that animal proteins will be a sustainable source of protein – both in the farm feed and pet food industries. For this reason, investigations into the use of novel proteins (from new sources which may prove surprising to us at first) are currently being made.

The ability of animal-feed producers to deliver safe, quality product is being put to the test more than ever before and to be able to deliver consistently is key.

It goes without saying that all pet foods should be precisely formulated to meet the most exacting nutrient requirements of your patients, both healthy and ill.

They should be sufficiently palatable to ensure adequate intakes of all nutrients and not only maintain health when consumed, but well beyond the feeding bowl and into the future.


If a diet consisting of vegetable proteins meets all of these criteria, then it is a suitable diet for the pet irrespective of the preferences or motivations of the owner holding the lead or carrying the basket.

This fact – combined with considerations of sustainability and the growing evidence supporting the use of vegetable proteins for pets living with such conditions as renal, liver and GI disease – means that we are likely to see vegetable proteins become a common feature of pet foods in the future.

  • For further reading go to vet (or vetportal. for Ireland).

References and further reading

  • Biourge, V. and Fontaine, J. (2004) Exocrine pancreatic insufficiency (EPI) and adverse reaction to food: A positive response to a high fat, soy isolate hydrolysate based diet. J Nutr 134: 2,166-2,168.
  • Brown, W. Y. (2009) Nutritional and ethical issues regarding vegetarianism in the domestic dog. Accessed online 15/09/14:
  • Center, S. A. (1998) Nutritional support for dogs and cats with hepatobiliary disease. J Nutr 128 (12 Suppl): 2,733S-2,746S. FEDIAF (2012) Nutritional guidelines for complete and complementary pet food for dogs and cats. European Pet Food Industry Federation, Brussels.
  • Grandjean, D. and Butterwick, R. (eds) (2009) Waltham pocket book of essential nutrition for cats and dogs. Waltham, Leicestershire.
  • Yamka, R. M., Kitts, S. E., True, A. D. and Harmon, D. L. (2004) Evaluation of maize gluten meal as a protein source in canine foods. Animal Feed Science and Technology 116: 239-248.

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