Fooding for Life

Not All Protein is Created Equal ..

The Function and Structure of Proteins
We are often reminded that some of the food fragments, “prescription” feed, non-species appropriate McKibble and McCan contains protein, and therefore, it’s appropriate. Reasoning is that it contains protein. Right?

Protein is Protein is Not?

(Co-author: Carla Rittonori, MSc Agricultural Science in Animal Science)

We are often reminded that some of the food fragments, “prescription” feed, non-species appropriate McKibble and McCan contains protein, and therefore, it’s appropriate. Reasoning is that it contains protein. Right?

Let’s start with what uncle Google suggest for a definition:


noun: protein; plural noun: proteins

any of a class of nitrogenous organic compounds which have large molecules composed of one or more long chains of amino acids and are an essential part of all living organisms, especially as structural components of body tissues such as muscle, hair, etc., and as enzymes and antibodies.

a protein found in wheat

So before we understand the primary building blocks of the two key constructs that we advocate, namely: (a) BIOLOGICALLY species APPROPRIATE and (b) FOOD, we want to draw your attention to the primary part of the definition – “any of a class of nitrogenous organic compounds which have large molecules composed of one or more long chains of amino acids.” Considering that proteins are the building blocks of life, and the reason we’re all here on this planet, you might find it weird to realize that not all proteins are created equal. We are often bombarded with the most dangerous chemicals, the carcinogenic this, the toxic that, the pathogenic so and so, but did you know that some of the proteins that are found in nature can be deadlier …

Our short list of killer proteins includes:

  • Ricin – readily available from castor beans, extremely potent toxin. Ricin prevents the synthesis of proteins by disrupting the ribosome, effectively inhibiting the activities of the human body on a cellular level ​1​ ​2​ .
  • Cholera toxin – extremely potent toxin. Yes, it’s a protein (see: Wikipedia);
  • Major Prion Protein – (also known as PrP) is already found throughout the human body, particularly within the nervous system. A slight change in the three-dimensional structure of the protein leads to a variety of debilitating and deadly diseases (see: Wikipedia);
  • Pertussis toxin – a multi-subunit protein, pertussis toxin shuts down communication between cells inside your body. Pertussis toxin is released by the bacteria Bordetella pertussis, with the protein interfering with the body’s immune system (see: Wikipedia);
  • Ectatomin – the primary toxic component of venom from Ectatomma tuberculatum ants (see: Wikipedia);
  • Conopeptides – A series of short molecules made up from the same amino acid building blocks as proteins, conopeptides originate from poisonous marine cone snails ​3​ ;
  • Abrin – the lesser known cousin of ricin, abrin is a protein found in seeds of the rosary pea (see: Wikipedia);
  • Verotoxin – this toxin interferes with the creation of proteins within small blood vessels in the kidneys and gastrointestinal tract (see: Wikipedia);
  • Tetanospasmin – this neurotoxin eventually causes muscle spasms, the telltale symptom of tetanus (see: Wikipedia);
  • Botulinum – botulinum toxin is the most toxic substance known to humankind and is also the protein behind the commercial product Botox! (see: Wikipedia)
  • And our “not so-“ favorite – snake venom! Snake venom contains mixture of bioactive proteins and polypeptides! (see: Wikipedia);

Not that we are eluding or implying that these nasties being used in any feed or food offerings for your fur kids. Just because the label states that it contains protein, don’t assume that the protein is biologically species appropriate or food-based.

Suppose we should have a disclaimer of sorts here. DISCLAIMER: This article was written to create awareness for our pet parents as part of our continued educational efforts that protein is protein … is not always biologically species appropriate or food-based. We are not inferring, eluding, implying or stating outright that any name brand McKibble or McCan contains any of these items on our list!

And our two key constructs? Firstly, it must be FOOD, not feed. Our opinion is that whole food synergy is the key to dietary vitality. Read our view on food synergy. Secondly, it must be BIOLOGICALLY species APPROPRIATE. Biologically appropriate, or, species appropriate food is FOOD appropriate for your fur kids’ (dog or cat) digestive tract.

As responsible pet parents, you would expect any quality pet food to meet this requirement. The definition has however recently been used to market types of dry feed diets for several “high quality” dry cat and dog foods. As we have eluded before, the pet food marketing engine is a powerful one, and to convince consumers that their feed is closer to your pet’s ancestors’ natural diet, and more beneficial for your pet than real food, included these terms as part of their marketing campaigns.

Which brings us back to our discussion on protein as a source of life. Real animal protein in the most digestible format. Our (pet parents and fur kids alike) bodies use proteins for many things, including repairing and building tissues, acting as enzymes, aiding the immune system, and serving as hormones . Each of these important functions requires a slightly different form of protein. In spite of their differences in structure, all proteins contain the same basic sub-components.

Read More: What are proteins and what do they do? (NIH)

Read More: Role of protein in the body, Science Learning Hub (Article)

Proteins are one of the four different types of macromolecules, in addition to carbohydrates, lipids, or fats, and nucleic acids, such as DNA and RNA. Macromolecules are large molecules that perform specialized functions inside living organisms. The structural arrangement of a protein molecule will differ in accordance with its function.

Technically, proteins are long chains of amino acids. Amino acids are the building blocks of protein. In other words, amino acids are like the links in a chain. The chain itself represents the protein molecule. Protein chains are then twisted and folded together in specific ways to create certain molecules.

Proteins are fascinating and a complex subject we hope you are gleaming by now. Let’s quickly explore the basics of these essential molecules.

  • Antibodies :- are specialized proteins involved in defending the body from antigens (foreign invaders). They can travel through the bloodstream and are utilized by the immune system to identify and defend against bacteria, viruses, and other foreign intruders. One way antibodies counteract antigens is by immobilizing them so they can be destroyed by white blood cells. Also the reason where vaccinosis are involved, as sometimes, depending on the specific vaccine and production process, the protein modules can bind with the anti-bodies, and then when injected, bind with the immune system. Hence, you often find that it’s chicken or beef related. Once the anti-body goes dormant, you find that the condition itself dissipate, simply because the protein molecule is no longer bonded with the immune system.
  • Contractile Proteins :- are responsible for muscle contraction and movement. Examples of these proteins include actin and myosin.
  • Enzymes :- are proteins that facilitate biochemical reactions. Bet you didn’t know that! They are often referred to as catalysts because they speed up chemical reactions. Enzymes include lactase and pepsin, which you might hear of often when learning about speciality diets or digestive medical conditions. For example, Lactase breaks down the sugar lactose found in milk. Pepsin is a digestive enzyme that works in the stomach to break down proteins in food.
  • Hormonal Proteins :- are messenger proteins which help to coordinate certain bodily activities. Examples include insulin, oxytocin, and somatotrophin. Insulin regulates glucose metabolism by controlling the blood-sugar concentration. Oxytocin stimulates contractions during childbirth. Somatotrophin is a growth hormone that stimulates protein production in muscle cells.
  • Structural Proteins :- are fibrous and stringy and because of this formation, they provide support for various body parts. Examples include keratin, collagen, and elastin. Keratins strengthen protective coverings such as skin, hair, quills, feathers, horns, and beaks. Collagens and elastin provide support for connective tissues such as tendons and ligaments.
  • Storage Proteins :- store amino acids for the body to use later. Examples include ovalbumin, which is found in egg whites, and casein, a milk-based protein. Ferritin is another protein that stores iron in the transport protein, haemoglobin.
  • Transport Proteins :- are carrier proteins which move molecules from one place to another around the body. Haemoglobin is one of these and is responsible for transporting oxygen through the blood via red blood cells. Cytochrome are another that operate in the electron transport chain as electron carrier proteins.

Let’s consider the building blocks of protein again – amino acids. There are 22 known amino acids (depending on references source), and they are classified as Essential (see: Wikipedia), Conditionally Essential, or Non-Essential. Essential amino acids are histidine (H), isoleucine (I), leucine (L), lysine (K), methionine (M), phenylalanine (F), threonine (T), tryptophan (W), valine (V). Conditionally Essential amino acids are arginine (R), cysteine (C), glutamine (Q), glycine (G), proline (P), tyrosine (Y). Non-essential amino acids are alanine (A), aspartic acid (D), asparagine (N), glutamic acid (E), serine (S), selenocysteine (U) and pyrrolysine (O).

Did you Know: There are over 10,000 different types of proteins in our human bodies. This number is misleading though, because all those different proteins are made up of the same 22 amino acids (some sources will say it is 20 amino acids). Depending on how the amino acids are linked, a different protein will be formed ​4​ .

Real Food for Cats and Dogs
Amino Acids as Building Blocks

(Image Credit:

Now, to ensure that a protein is digestible, it requires both the amino acids to be “balanced”, and the amino acids to be “available”. Meat is an illustration of high quality protein foodstuff; in general, UNADULTERATED meat contains high levels of HIGHLY digestible protein, as well an amino acid patterns that more closely resemble the actual animal amino acid requirements of our dogs and cats (lots of research on this topic available from the NRC) ​5​ .

Ever tried to run a diesel car on petrol? Epic failure! But they are both fuels, and so is paraffin! In context of food, protein is fuel, but you need the right protein (fuel) for the right function (engine), and when you “unroll” protein and expose the amino acids, this is where the rubber hits the road. Not all proteins are created equal, and for our pets to thrive, they need a whole toolbox full of amino acids, not just protein. Like burning fuel, in order for a specific amino acid to be completely available, the protein that contains it must first be digested and absorbed by the digestive system. This is what we refer to when we mention digestibility, bioavailability and species appropriate. The amino acids are then absorbed through the gastro-intestinal wall into the bloodstream to be utilized as needed by the body as building blocks, a source of energy or as a precursor (building blocks) for bioactive molecules (for instance thyroxine from tyrosine, niacin and serotonin from tryptophan).

Think of it this way – protein into the body via the digestive system is like a puzzle box with a whole bunch (22 odd) of Lego blocks inside that can make almost any shape, but with no instruction guides provided. To unlock the puzzle box, you need the smarts or code or key. Once unlocked (absorbed into the bloodstream), the Lego blocks can then be used to build toys, buildings, and all kinds of things that you can imagine.

Taurine (a “very essential” amino acid ​6​ ) is an amino-sulfonic acid, an acid containing an amino group. It is not an amino acid in the usual biochemical meaning of the term. It is a VERY important amino acid for heart, eye and central nervous system functions. Taurine is however not incorporated in proteins, whereas tryptophan is and is an important precursor for a neuro-transmitter called serotonin. Although taurine is one of the few amino acids not incorporated into proteins, taurine is one of the most abundant amino acids in the brain, retina, muscle tissue, and organs throughout a mammal’s body. It is free in meat in other words. Methionine and cysteine are precursors for taurine but cats and some dogs cannot synthesize (take the Lego blocks and build) taurine from these amino acids because the enzymes needed are minimally active in these pets ​7​ . Cats also do not have the ability to convert from glycine conjugate and an obligate taurocholate bile acid secretion. But taurine is not the only amino acid that cats cannot synthesize – they also cannot synthesize arginine ​8​ . Arginine is another essential amino acid in dogs and cats, but cats cannot synthesize enough amounts of ornithine or citrulline to be converted to arginine. Arginine deficiency would result in symptoms of hyperammonaemia (salivation, emesis, neurological abnormalities, ataxia, tetany, and coma).

All of life (not just mammals) utilize the same 22 recognized amino acids to build the proteins they express, but different species can synthesize various numbers of the 22 amino acids and must get the balance from their diets (for example, taurine and arginine). The amino acids we MUST ingest are called “essential amino acids”. These are the amino acids we and our pets cannot synthesize from raw materials.

For interest, the nine amino acids humans cannot synthesize are phenylalanine (F), valine (V), threonine (T), tryptophan (W), methionine (M), leucine (L), isoleucine (I), lysine (K) and histidine (H).

The ten essential amino acids for both dogs and cats include histidine (H), isoleucine (I), leucine (L), lysine (K), methionine (M), phenylalanine (F), threonine (T), tryptophan (W), and valine (V). Cats have an additional requirement for the essential amino acid taurine and arginine (R).

A complete protein (Wikipedia) or whole protein is a food source of protein that contains an adequate proportion of each of the nine essential amino acids necessary in the diet. Examples of single-source complete proteins are red meat, poultry, fish, eggs, milk, cheese, yogurt, soybeans* and quinoa*. (* these protein sources are of course not biologically, species appropriate for pets though. Some sources report soy protein as complete, however, two essential amino acids are only found in small amounts in soy, so, it isn’t comparable to animal protein ​9​ .

Real Food for Cats and Dogs
Amino Acids Illustrated

(Image credit Amino Acid Encyclopedia)

Egg protein has an amino acid score of 100%! That’s why egg protein is said to have the best amino acid balance of any protein. Wheat protein has an amino acid score of 50%, while milled rice has a score of 82%. In particular, wheat protein and milled rice have very little of the essential amino acid lysine.

To clarify – when we state, “a complete” or “whole protein”, we are not inferring that other protein sources, such as fruits, vegetables, grains, nuts and seeds, are not viable protein sources. As a matter of fact, there is really no “difference” between these proteins when you look at them. It’s a matter of perspective and how the amino acids are structured to build the protein that is important.

As we stated, animal sources of protein tend to deliver all the amino acids we (both pet parents and pets) need. Other protein sources may lack one or more essential amino acids, and therefore we don’t view them as “complete” or “wholeprotein. The more “complete” the protein, the more biologically, species appropriate the food source.

For a practical demonstration, let’s consider grains. Grains are naturally limiting in the amino acid lysine ​10​ , while legumes are naturally limiting in the amino acid methionine ​11​ . So as a standard, McKibble needs to add the extra lysine (l-lysine) as a synthetic to prevent deficiencies not only of lysine, but of all other amino acids. Because lysine is vitally important in forming all other amino acids (it’s a key building block or link in the chain), in other words, if lysine is deficient all the other amino acids are void (the chains cannot be formed) even if they are supplied in adequate amounts! So when we state “complete” or “whole”, lysine is one of the coolest building blocks in animal protein, as animal protein isn’t lysine deficient!

(Image Credit – Creativeconflictwisdom’s Blog)

In terms of biologically, species appropriate diets, let’s look towards tomorrow mornings’ bacon. When formulating diets for farm animals such as pigs in the agriculture industry, very young piglets are started on a diet very high in milk protein and fishmeal. It will surprise you to know that piglets at that age CANNOT digest plant proteins! The reality is that farmers cannot afford, or willing to pay for, fishmeal, as it is a very expensive source of protein. Instead, as the piglets grow older they are slowly introduced to soybean proteins, at a fraction of the cost of fishmeal protein, to get them used to the source and wean them of animal proteins. They still grow sufficiently fast enough on these non-species appropriate sources, and if the industry needs bigger pigs, they just engineer them to grow bigger. But for piglets to grow naturally, they actually need animal proteins.

In the end, we advocate that all food fed to your fur kids should be appropriate for your pet’s digestive tract, and indeed all the real meals we offer are suitable for a cat or dog’s digestive tract, whether or not labelled “Biologically Appropriate”. Our supreme catalogue only includes BIOLOGICALLY species APPROPRIATE meat-based raw frozen pet cuisine and whole bits from [our manufacturers] made from real complete or whole protein sources.

Our final thought on the topic is that you also realize that proteins are rarely found in isolation. They usually come with a wide variety of other nutrients! Foods that contain animal protein tend to be high in several nutrients that are often lacking in plant foods. These include:

  • Vitamin B12: is mainly found in fish, meat, poultry and dairy products. Many people who avoid animal foods are deficient ​12​ .
  • Vitamin D: is found in oily fish, eggs and dairy. Some plants contain it, but the type found in animal foods is better used by your body ​13​ .
  • DHA: Docosahexaenoic acid is an essential omega-3 fat found in fatty fish. It’s important for brain health and is hard to get from plant sources ​14​ .
  • Heme-iron: is predominantly found in meat, especially red meat. It is much better absorbed in the body than non-heme iron from plant foods.
  • Zinc: is mainly found in animal protein sources, such as beef, pork and lamb. It is also more easily absorbed and used from animal protein sources ​15​ .

Of course, there are also plenty of nutrients found in plants that are lacking in animal foods. Therefore, eating balanced amounts of both is the best way to get all the nutrients we as pet parents need.

Hopefully, by now you will realize that our fur kids’ intestinal tracks were not designed to break down the amounts of plant proteins added to McKibble and McCan, even if McCan is marginally better than McKibble. And even if they do manage to break down these plant proteins, remember that the amino acids in plants are not in the right proportions required to sustain all the important processes required for life. This means that your fur kids’ needs to work very hard to “repurpose and redistribute” the proportions of amino acids to something their bodies can use. Animal proteins on the other hand are already in the correct proportions, especially meat and eggs so your fur kids’ digestive tract finds it particularly easy to use these proteins, because it makes sense to them and we do not need to add artificial amino acid supplements, like L-lysine, to meet our pet’s dietary needs. It is therefore evident that we cannot, like McKibble, continue to rely on amino acid deficient, heated, pulverized, chemically treated and preserved plant proteins to supply a staggering 70% of the essential amino acids our fur kids require to lead healthy lives.

Just because the information on the bag states that it contains protein, remember that not all proteins are created equal.

Articles and Videos

Selected References and research.

References and Research

  1. 1.
    Sousa NL, Cabral GB, Vieira PM, Baldoni AB, Aragão FJL. Bio-detoxification of ricin in castor bean (Ricinus communis L.) seeds. Sci Rep. November 2017. doi:10.1038/s41598-017-15636-7
  2. 2.
    Tully R, Beevers H. Protein bodies of castor bean endosperm: isolation, fractionation, and the characterization of protein components. Plant Physiol. 1976;58(6):710-716. doi:10.1104/pp.58.6.710
  3. 3.
    Becker S, Terlau H. Toxins from cone snails: properties, applications and biotechnological production. Appl Microbiol Biotechnol. 2008;79(1):1-9. doi:10.1007/s00253-008-1385-6
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    Ponomarenko E, Poverennaya E, Ilgisonis E, et al. The Size of the Human Proteome: The Width and Depth. Int J Anal Chem. 2016;2016:7436849. doi:10.1155/2016/7436849
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    Campbell W, Barton M, Cyr-Campbell D, et al. Effects of an omnivorous diet compared with a lactoovovegetarian diet on resistance-training-induced changes in body composition and skeletal muscle in older men. Am J Clin Nutr. 1999;70(6):1032-1039. doi:10.1093/ajcn/70.6.1032
  6. 6.
    Ripps H, Shen W. Review: taurine: a “very essential” amino acid. Mol Vis. 2012;18:2673-2686.
  7. 7.
    Knopf K, Sturman JA, Armstrong M, Hayes KC. Taurine: An Essential Nutrient for the Cat. The Journal of Nutrition. May 1978:773-778. doi:10.1093/jn/108.5.773
  8. 8.
    Morris JG, Rogers QR. Arginine: An Essential Amino Acid for the Cat. The Journal of Nutrition. December 1978:1944-1953. doi:10.1093/jn/108.12.1944
  9. 9.
    Friedman M, Brandon D. Nutritional and health benefits of soy proteins. J Agric Food Chem. 2001;49(3):1069-1086. doi:10.1021/jf0009246
  10. 10.
    Ufaz S, Galili G. Improving the content of essential amino acids in crop plants: goals and opportunities. Plant Physiol. 2008;147(3):954-961. doi:10.1104/pp.108.118091
  11. 11.
    Galili G, Amir R. Fortifying plants with the essential amino acids lysine and methionine to improve nutritional quality. Plant Biotechnol J. November 2012:211-222. doi:10.1111/pbi.12025
  12. 12.
    Pawlak R, Lester S, Babatunde T. The prevalence of cobalamin deficiency among vegetarians assessed by serum vitamin B12: a review of literature. Eur J Clin Nutr. 2014;68(5):541-548. doi:10.1038/ejcn.2014.46
  13. 13.
    Romagnoli E, Mascia M, Cipriani C, et al. Short and long-term variations in serum calciotropic hormones after a single very large dose of ergocalciferol (vitamin D2) or cholecalciferol (vitamin D3) in the elderly. J Clin Endocrinol Metab. 2008;93(8):3015-3020. doi:10.1210/jc.2008-0350
  14. 14.
    Gebauer S, Psota T, Harris W, Kris-Etherton P. n-3 fatty acid dietary recommendations and food sources to achieve essentiality and cardiovascular benefits. Am J Clin Nutr. 2006;83(6 Suppl):1526S-1535S. doi:10.1093/ajcn/83.6.1526S
  15. 15.
    Cotton P, Subar A, Friday J, Cook A. Dietary sources of nutrients among US adults, 1994 to 1996. J Am Diet Assoc. 2004;104(6):921-930. doi:10.1016/j.jada.2004.03.019

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