What are the Essential Amino Acids for Different Animal Diets?

If you are particularly knowledgeable about pet nutrition, you might have heard of amino acids, and their importance as part of a balanced pet diet. What are amino acids? What are they for? What amino acids are essential for healthy animals?

What Are Amino Acids and What Are They For?

Amino acids are a group of molecules made up of an amino group (nitrogen and hydrogen atoms) and a carboxyl acid group (carbon, oxygen and nitrogen atoms). Every amino acid has a unique side chain (“R”) that gives it properties and function. There are over 500 types of amino acid, but only a handful are relevant to pet nutrition. These are the twenty amino acids that make up proteins in all known organisms (as well as an additional two in certain organisms).

Proteins are essential building-blocks for life, carrying a huge number of functions ranging from being a component of muscle tissue, cell-to-cell communication as enzymes, control of body functions as hormones, carrying oxygen in red blood cells as haemoglobin, forming the cells of the immune system, and hundreds more. All proteins are made up of long chains of amino acids and other components. To form proteins, DNA strands are utilised, with individual sections of DNA coding for specific chains of amino acids in a process called translation. This is a very complex procedure. In short, if a protein is needed, the strand of DNA that codes for that protein will assemble chains of amino acids. The amino acid chain grows (often combining with other chains or molecules) until it forms the complete protein. Any changes in the DNA can result in an incorrect amino acid chain and a defective protein.

This means that without sufficient amino acids, an animal cannot create proteins essential for continued bodily functions, and disease can occur.

Where Do Amino Acids Come From?

We have now established what amino acids are and why they are important to the body, but how do living things get amino acids?

Amino acids can be synthesised in the body through various chemical reactions. However, not all organisms can create every amino acid. For example, humans cannot synthesise nine amino acids, making these ‘essential amino acids’ for humans. The amino acids that cannot be created by the body must be taken in through other means, usually diet. Different animals have different essential and non-essential amino acids, and even within humans it can vary (for example, premature infants require six additional amino acids as they cannot produce enough of their own). This means that almost all animals require amino acids in their diet, usually in the form of proteins from plant or animal products.

Certain foods are considered ‘complete’ protein sources, meaning they contain all essential proteins in high enough concentrations. ‘Incomplete’ protein sources do not contain all essential proteins, or contain them in insufficient amounts. Traditionally, animal products such as meat, fish, eggs and dairy were considered complete protein sources, while incomplete protein sources include nuts, seeds, beans and vegetables. This does not mean that vegetarian or plant-based diets necessarily result in amino acid deficiency, but care must be taken to ensure sufficient intake.

What Happens if an Organism Is Deficient in Amino Acids?

Because there are so many different functions amino acids have in the body, the effects of being deficient in any amino acid can vary wildly. Some of the most common include a weakened immune system, loss of muscle, reduced growth in young animals, digestive problems, skin disease, heart and lung disease, and even psychological and mental health issues. In extreme cases, deficiencies can be fatal.

What are the Essential Amino Acids for Different Animal Diets?

Dogs

Dogs require 10 essential amino acids in their diets to function properly, as they cannot synthesise these in their bodies. These are: arginine; histidine; isoleucine; leucine; lysine; methionine; phenylalanine; threonine; tryptophan; valine (with some debate over others such as aspartate, glutamate, and glutamine). Most branded dog food will contain the right concentrations of amino acids for the average dog, originating from both animal and plant protein sources. This means the average dog should not become protein deficient if their diet is appropriate. Deficiencies can occur in some cases, such as rapidly growing puppies on an inappropriate diet, older dogs with protein loss due to renal, gastrointestinal or neoplastic disease, low protein diets or those on a single protein source. Overall protein levels can be measured by a vet, and addressed if they are low.

However, a few years ago, vets started to notice more and more dogs were developing a heart condition called dilated cardiomyopathy (DCM), a disease where the heart muscle thins and becomes less effective. Curiously, many of these dogs were breeds not predisposed to DCM. It was noted that many of these dogs were being fed unusual diets, particularly grain-free diets. A large number of dogs were also deficient in taurine, normally a non-essential amino acid (though dogs are less effective at producing it). It was argued that a taurine deficiency may be leading to DCM in these dogs. Investigation is still ongoing, but pea- and pulse-based grain-free diets are frequently associated with the development of DCM. Thus, it is recommended to avoid these diets for now, especially in dogs at risk of or with DCM, and to discuss taurine supplementation with your vet if your dog needs this diet for a medical reason. Any dog that develops DCM should stop any pulse-based diets straight away.

It is also worth discussing other exotic diets, such as vegetarian, vegan or raw meat-only diets. We have written a number of articles about these diets before, but in relation to amino acids, these diets have a much higher risk of resulting in protein deficiency than a nutritionist approved commercial food, particularly if the diet is home cooked or made by someone with no nutritional training. We would generally not recommend these diets unless for a specific medical reason. Should you wish to feed your dog an exotic diet like those listed above, we strongly recommend speaking to an accredited veterinary nutritionist to ensure the diet contains all of the essential amino acids, as well as other nutrients, needed for your dog to remain healthy.

Cats

Like dogs, cats have essential amino acids, eleven of them, they must ingest to remain healthy, as their bodies does not produce it endogenously. These are: arginine; methionine; histidine; phenylalanine; isoleucine; threonine; leucine; tryptophan; lysine; valine; and taurine. Cats also differ from dogs in a number of ways that make amino acid intake and overall diet even more important to keeping them healthy. Cats have an overall higher protein and amino acid requirement than dogs and other animals, so their diets must be specially formulated (which is why cats should not eat dog food). Cats also have lower digestibility for lower-quality protein compared to dogs, meaning they receive less amino acids from the same amount of protein. However, for high-quality protein the values are similar, indicating cats thrive better on higher-quality diets (and need less of it, meaning less impact on your wallet!)

Two of the most important amino acids to cats are taurine and arginine. Taurine in particular is only found in animal protein (or synthetically produced by humans). Taurine deficiency can lead to DCM, as in dogs, as well as blindness and deafness. Arginine deficiency can lead to a dangerous condition called hyperammonia, where toxic products result in loss of appetite, vomiting, seizures and other neurological diseases.

Older cats also have slightly different requirements compared to adult cats (the same is true for older dogs but to a lesser extent). Many older cats suffer from the loss of muscle mass as they age, so ensuring they have a high-quality protein diet will maintain muscle for longer. Restricting protein is never recommended, though certain conditions, such as chronic kidney disease, may require specific diets to both maintain muscle mass and improve the course of disease.

The problem with plant-based cat food

Cats are considered obligate carnivores, meaning they must obtain all their nutrients from animal protein, unlike dogs that are facultative carnivores that can digest some amount of plant matter. Although cats can digest small amounts of plant matter, a cat on a plant-based diet would receive an overall lower amount of protein (and thus essential amino acids) and would likely be deficient in taurine. Furthermore, meat-based diets contain a low amount of glycogen (an energy-storage molecule) meaning cats must produce energy from amino acids to remain healthy – plant-based diets (although containing more glycogen) have less amino acids, meaning less energy production. Thus, non-meat based diets in cats are still not widely recommended, unless for a specific medical reason and guided closely by a veterinary nutritionist – supplementation would be essential.

While it is theoretically possible to produce a synthetic chemically enhanced vegan diet for a cat there are no rigorous scientific studies that have demonstrated the safety of such diets for long-term use. Most studies have instead relied upon owner-reported outcomes, a notoriously unreliable way of measuring health impacts, and the most recent despite the hype found only one statistically significant finding, that vegan fed cats had a higher risk of kidney disease.

Rabbits

Rabbits likely have similar amino acid requirements to dogs and cats, though their anatomy and how they get them is quite different. Just like dogs and cats, rabbits will get their amino acids from diet, primarily plants like grass in the wild, and others in pet rabbits. Lysine and methionine are common deficiencies if a rabbit is fed mainly on grains, so this should be avoided. Rabbits can also be supplemented with high-protein powders like soybean meal, sunflower meal, linseed meal and more.

As in other animals, some of the protein is broken down as normal in the rabbit’s stomach and intestines to be absorbed. But rabbits differ in that they have an extra part of their large intestine called the caecum (in humans, the appendix is the remnant of a caecum). The caecum is full of bacteria that digest food further, helping produce amino acids and other nutrients. These are formed into soft stools called caecotropes, which the rabbit will pass out and eat – all of the bacterial-origin nutrients can then be absorbed, the waste leaving the rabbit again as a normal firm stool. This process is called caecotrophy, and is essential for rabbit health. Ensuring your rabbit receives a good balanced diet high in fibre, and can easily reach their caecotropes will go a long way to maintaining their amino acid levels, and thus health.