The Protein Deception
6/15/2018 - By Dr. Jodie Gruenstern, DVM, CVA
ZIWI’s consulting veterinarian, Jodie Gruenstern, DVM, CVA discusses why the designation of “high protein” can be over-rated, while the quality of protein in pet foods misunderstood and undervalued.
You are what you absorb, not what you eat.
The defining of quality protein depends on the species consuming it. Have you ever wondered how a large ruminant can eat grass all day, then produce a body full of high protein meat? An herbivore’s stomach, intestinal tract, and salivary enzymes are entirely different than those of a canine or feline carnivore in all respects. The plant-eating species digestive systems are designed to break down the proteins present in plants; the carnivores’ system is not. Therefore, what would be considered quality protein for one species is not so for another.
What is crude protein?
My clients have heard me say that you could put shoe leather in a bag and it would measure as high protein - although virtually indigestible! It is challenging to compare labels on pet food when all protein is not created equal. Pet food labels are required to list "crude protein" levels found in the food. Crude protein is technically defined as a measurement of all protein content. The most common way to calculate protein actually measures nitrogen - a waste product of protein - but gives no weight to the quality of the amino acids in the protein, nor to its digestibility - the ability of the particular species to breakdown and absorb the individual amino acids released from the protein molecule.
Protein Contains Amino Acids
Protein is made from combinations of 23 different amino acids. Some amino acids can be manufactured by your dog or your cat; others must be ingested and absorbed from foods consumed. For example, a feline can manufacture 12 of these amino acids from micronutrients, but the other 11 amino acids must be provided in the diet, so they are called essential. Arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine, and taurine are the essential amino acids for cats.
Animal-based protein sources like egg, meat, dairy, and fish contain more essential amino acids than do plant-based protein sources like soybeans or corn gluten meal. Products that rely on plants to provide most of their protein cannot provide all the essential amino acids cats need without supplementation - even if the food appears to be “high protein,” based on its guaranteed analysis.
Why do dogs and cats need quality meat protein vs. plant protein?
Let’s begin with the mouth. Carnivores don’t chew. Carnivore teeth are sharp and pointy for tearing, so carnivores rip and swallow. The teeth of ruminating herbivores (sheep, cattle, deer, goats, etc.) are flat and built to grind. The chewing process macerates plant material and mixes it with salivary enzymes for further breakdown. Herbivore saliva contains cellulase, which allows them to break down the cell walls of the plants which they consume (carnivores do not possess this enzyme). Additionally, some of these herbivores re-chew ingesta which has been swallowed (cud) for additional breakdown. Proteins must be broken down properly or they become allergens.
From the mouth, swallowed food enters the stomach which is typically full of acid. It is this acid which uncoils large protein molecules so that their individual amino acids may be presented properly to the upper small intestine for enzymatic breakdown. Next, the uncoiled proteins are acted upon by pancreatic enzymes, such as protease, which have traveled down the pancreatic duct to the upper small intestine where amino acid separation occurs. These amino acids and other nutrients travel through the intestines mixing with the gut flora where processing for absorption through the mucosal barrier and into the bloodstream occurs. The length of this journey varies among species. The character of the gut flora not only varies among species but also among individuals.
The proper array of amino acids must be absorbed into the circulation where they travel to the liver for additional processing and recognition. The liver is a complicated filter. It is a part of the body’s amazing detoxification system. If intact, large protein molecules make it to the liver, this wreaks havoc with the immune system. Unrecognized proteins behave as allergens or foreign invaders! Excess nitrogenous waste can build up in the bloodstream as NH4 (ammonia), and you can have one sick, nauseous, vomiting, itchy pet that is trying to cleanse impurities from the system via diarrhea, weepy eyes, or skin lesions.
Crude Protein vs. Quality Protein
Now you understand that a total measure of crude protein as stated on a pet food label does not tell us anything about the quality, safety, or digestibility of that protein. The quality of the protein is dependent upon the ability of the particular species to digest (breakdown and absorb) that particular protein. Lastly, when the protein is broken down, it must contain the array of amino acids that are essential to that species – known as a species-appropriate diet.
A smart pet parent must practice due diligence. Read ingredient labels. Research the reputation and integrity of the manufacturers of the pet food upon whom your pet’s life depends!