Our carnivore companions are designed to eat a diet composed of animal sourced proteins. Protein plays an important part in our carnivore companion’s diet and is a nutrient that needs to be properly understood to ensure the health and wellbeing of our companions. There seems to be a lot of myths and misinformation surrounding the topic so it's time to set the story straight.
Why are Pet Food Companies using Plant Proteins and Why are Pet Owners Buying it?!
Most reasons for pet owners to believe companion animals should have plenty of plant matter in the diet is partially marketing but also perceived health value. As humans, fruits and veggies are part of our regular diet and many of us know one or two key benefits to specific fruits and vegetables. We assume the same applies to our companions. There is actually no quality research that supports the idea raw animal proteins have detrimental effects on long term health. -Cost is another factor. Fruits, veggies and grains are significantly less expensive than meat. -The concern with “high protein” diets is often perpetuated by veterinarians and other animal “professionals.” -Concerns with how food animals are treated and raised. -That plant matter is required as a quality fiber source (1).
-The effects on the environment that raising livestock has. While out of the scope of this article, the effect of livestock on the environment is actually low on the scale of climate change and environmental impact compared to such things as fossil fuels and from other man made activities (2).
Why do Professionals Misunderstand Protein?
Companion animal professionals also contribute to the problem. Misinformation in relation to numerous illnesses and diseases and outdated studies can impact the advice veterinarians give to pet owners. Lack of proper education on nutrition in Veterinary School.
Only having a basic knowledge and assuming the face value of the marketing of pet food often from pet food representatives or those with vested interest. Human error with the use of animal based proteins amongst the general population.
Importance of Protein
While some functions in the body are more demanding of protein supply than others, such as pregnancy/gestation, growth and development, other processes require more rapid replenishment of protein. Maintaining the skin and coat is one such example (3). However in general, protein is essential for most biological processes and the general health and wellbeing of our carnivore companions. We can provide proof by taxonomy naming as well as through anatomy and physiology, but what is it about protein that is so important?
Many organisms can make all 20 of the amino acids that the body requires but cats and dogs cannot. They must get them from their diet. This is because specific enzymes are available to create these amino acids. In order to save valuable energy, if the amino acids are available around them, they will consume them directly instead. If they are unable to obtain these amino acids, instead the body consumes its own reserves which results in muscle wasting, protein starvation and many biological processes that cannot be maintained for the animal to function properly.
What is Protein?
Proteins are made up of a string of amino acids or polypeptides that act as a code to build complex structures, cells and perform a plethora of functions within the body. These amino acids can come in many forms creating proteins of different sizes, shapes and structures that all dictate their activity in the body (4). Proteins are not as stable as one would think. They only exist for a period of time. Some can last for a few minutes while others can last for a few years however more often they only exist for 1-2 days in animal cells for example. This is one of many reasons why our carnivore companions need regular sources of quality, bioavailable protein. If misfolded or degraded in another way such as via the cooking (5) process, they are destroyed much quicker (6). Keep in mind that other factors such as the digestibility of proteins and amino acid profile will also impact the rate of digestion, absorption and utilization.
Protein serves many functions which have been encoded into them and can be seen in action in vitro (outside the body and in lab settings) , in vivo (in the body), and in silico (via simulation) (7).
How does the Body use Protein?
When protein is consumed, enzymes, the stomach acid and the small intestine break down the proteins into their amino acid base units and distribute them for use throughout the body via the blood. At these locations they recombine with other amino acids and proteins to perform their roles and functions (8).
Roles of Proteins
Act as Enzymes (9) that help cause over 4000 reactions in the body (10)
Cell Signaling (11)
Provide Essential Amino Acids that cannot be made by the body (12)
Component of viruses and bacteria (13, 14)
Component of haemoglobin which helps in transporting oxygen (15) Creation and maintenance of the Genome, DNA and RNA (16)
Hormone Activity (17, 18)
Maintain skin health (19) Components of neurotransmitters that send signals to the brain, regulate mood etc. (20)
Functions of Proteins
Aid in digestion and metabolic use (22)
Immunity including antibodies (23, 24)
Cell Shape (25, 26)
Structural function for muscles (15), connective tissues, cartilage, nail/hooves, hair/fur
Interact with DNA (replication, repair etc.), proteins fats and carbohydrates (12, 15, 27, 28)
Permeability of cell membranes (29)
Reproduction such as formation of sperm (30, 31)
Quality source of useable energy
Repair Damage (32, 33)
Regulate Blood Sugar (34, 35)
Amino Acids Make Up Proteins
Amino Acids are like the letters of an alphabet that make up words (proteins). There are 21 different amino acids. A portion of these are essential for cats (11) and dogs (10) and required to be supplied by the diet (3).
Essential Amino Acids
Essential Amino Acids cannot be made in the body by the cat or dog without assistance, instead they need to be obtained through diet in order to properly maintain the need for it in the body to function and maintain health. Essential Amino acids need to regularly be consumed as they don’t last very long. They are not stored by the body and they are in constant use for various roles. There are 21 amino acids, cats can only synthesize 11. Essential amino acids for cats include Arginine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Taurine, Threonine, Tryptophan, and Valine. Dogs on the other hand require all the same except Taurine to be supplied in the diet.
Methionine and Cysteine
Found primarily in fish and eggs, Methionine and Cysteine (derived from Methionine) is essential for the creation of keratin, a protein found in fur and used to maintain the health of the skin. A deficiency is often evident when the skin and coat are dry and fur is missing (3).
Found primarily in hard working muscle meats like the heart, thigh and small whole prey, Taurine is essential in cats who cannot make the amino acid unlike dogs.
It is responsible for maintaining proper calcium levels in cells and the heart as well as the need for reproduction, proper eye function and hearing.
Common signs of Taurine deficiency include blindness, problems with growth and development, birth defects and lowered immune system (3).
Found primarily in organ meats and the gelatin of bone broth, Arginine is important for the relaxation of blood vessels and facilitation of hormone release.
Common signs of Arginine deficiency can be as minor as excessive drooling and as severe as ammonia intoxication and death. Cataracts and muscle tremors are also known to develop (3).
Primarily found in muscle meats, Lysine is important for protein synthesis. It is one of the major limiting essential amino acids which means not only is it the first to stop protein synthesis, it is most likely to be the first to be deficient.
Common signs of Lysine deficiency include weight loss. While an overdose will cause an Arginine deficiency (3).
Phenylalanine and Tyrosine
These amino acids found in all proteins, are responsible for the pigments that make up the color of an animal’s fur. Phenylalanine is also important for proper thyroid function. Tyrosine is made from Phenylalanine but the unique aspect of this is if there is enough Tyrosine, Phenylalanine can be used for other vital functions. In addition to fur and eye color, Tyrosine also is required for a functioning brain and for reproductive function. Therefore deficiencies will cause neurological problems, weight loss and changes in fur pigmentation (3).
Leucine, Isoleucine and Valine
Primarily found in muscle meats of poultry, beef and lamb these amino acids are important for protein synthesis.
Common signs of a deficiency include loss of weight and lethargic behavior. Specifically Isoleucine can cause paw lesions, rough coat texture and unstable movements (3).
Primarily found in meat and blood, Histidine has an important structural function for protein as well as a precursor to other compounds in the body.
Common signs of a deficiency include weight and appetite loss as well as cataracts specifically in cats (3).
Primarily sourced in most proteins. Threonine is a precursor for many metabolic functions.
Common signs of a deficiency include weight and appetite loss and specifically in cats, problems with the nervous system (3).
Primarily found in poultry and fish, Tryptophan is important for hormone production such as melatonin and serotonin. These are important hormones for sleep cycles, regulating food intake and pain.
Common signs of a deficiency include weight and appetite loss (3).
Limiting Amino Acids
An amino acid is defined as limiting if the amino acid is needed by the body and is in the lowest amount (36). This means that if one amino acid is deficient, even if the other amino acids are present, protein synthesis will not be able to occur (37, 38), they do not make up for what is lacking (39). Without protein synthesis this can and will
Affect Brain Development (40, 41)
Inhibit the Immune System
Affect gut health and permeability
Inhibit the structure and function of organs such as the kidneys (42) Result in Poor skin and hair
Failure to thrive
Cause Muscle wasting (43, 44)
Cause Nervousness and anxiety (45)
Cause Exhaustion (45)
Again it's important to note, the absence of any of the essential amino acids from the diet stops the synthesis of essential proteins. Under these conditions, the animal then breaks down its own body tissue to provide the required amino acids, seriously compromising health. Protein Structure
Proteins are not just a straight, linear assembly as seen in a primary structure, they have 3D structures as well (46). There are five types. Aside from the primary there are secondary structures that are flat sheets or are in helices which look like a corkscrew (47). The tertiary structure is what creates a folded protein structure and then defines the function and role of the protein. Quaternary structures are made up of several protein structures that all work together as one (48). Finally you have a Quinary structure. They are an additional layer of surface structures which aid in positive and productive interactions with other cells (49, 50) kind of like little ID cards.
Heat and Protein Structure
Obviously the processing of commercial food involves cooking but many feed cooked vegetables to help with digestibility of plant proteins (51, 52). Heat treated plant proteins have been known to increase digestibility by 18% however while helping to solve one problem we create another which is denaturing proteins, and destroying or altering fats, enzymes (53), vitamins and minerals in
the process. Denaturation can actually be measured using various methods such as Dual-polarization Interferometry (54) and Circular Dichroism (55).
It seems quite counter productive in the short and long run to cook something to increase digestibility only to lose most of its nutrients especially where you can simply feed bioavailable foods that are easy to utilize.
In their native state a protein is made up of several protein structures connected by several weak
bonds and folded correctly to perform its ideal function (56). When denatured the weak bonds are altered or broken. The result is not only a reduction or even complete loss in protein function but also cell death (57). Classic and visual examples of the alteration of protein is when egg whites turn from liquidy and clear to solid and white (58) or meat turning from a softer consistency to a firm consistency.
Heat can mutate proteins as well (59), which not only will alter the structure of the protein, unfold it and degrade it, but mutations can occur which also affect the function of the protein including enzyme activity (60) making it harder to perform proper function and less identifiable by the body. This can lead to allergies, food intolerances, diarrhea, vomiting and other digestive issues.
Other denaturing elements include heavy metals. They are known to disrupt the activity and stability of proteins. This is a concern because metal contaminants in food as well as vaccines used in food stock animals and even our own companions can cause serious health problems and alter the function of regular cellular activity (61). This can be linked to digestive issues, allergies, autoimmune disease and other diseases that affect the regular function of the body. Unfortunately we are still lacking information as to how the processing of proteins affects the immune system including the effects it has on food intolerances and allergies (62).
Chemicals can also denature and alter proteins in numerous ways.
This can include affecting net charge which influences solubility (63), activity (64), pH (65), cross linking of proteins (66) and the functionality (64) of the protein as well as stability of enzymes (67) responsible for breaking down RNA (68).
Side Chain Modifications
Side Chain Modifiers tend to be chemicals that alter the structure of amino acids preventing proper interactions and function. In addition it can produce toxic by-products. The problem with Side Chain Modifications not only makes proteins nutritionally dead, but the by-products that are produced can cause serious problems with the kidneys (69).
Now that we know how proteins are created, how structure determines function and how heat and
other additives can denature proteins, how do we know what is a good source of protein?
There are several criteria to evaluate the quality of a protein source. 1. Amount of protein in a food
2. Amount of Essential Amino Acids
3. How well the protein is digested and amino acids are absorbed
This is typically measured by analyzing the amino acid content of fecal matter (70).
It should be noted that digestibility is unique to each food item. It can not be translated or generalized to other food and anti-nutrients and other factors must be accounted for that affect digestion (70).
While the pet food industry is not allowed to make claims of quality of their food, there are very scientific methods and systems in place for determining the quality of various protein sources. The first way to determine quality is based on the essential amino acid profile in regards to what our companion animals need and the body's ability to digest and utilize the protein (71).
Low quality proteins such as those in fruits, veggies and grains are harder to digest resulting (72) in digestive problems, pancreatitis and other symptoms that show the body is being taxed. Due to the difficulty in digesting plant matter, often the food stuff ferments in the gut leading to flatulence and odorous feces. Gas, indigestion or other digestive discomfort is an indicator of slow digestion. Low quality proteins can be measured by the levels of essential amino acids found in a food, the fewer essential amino acids the lower quality the protein. A complete protein is one that contains all the essential amino acids (73). Based on this criteria plant proteins are much lower quality than animal based proteins (74, 75).
Other methods of determining quality of protein include: Biological Value
This value is how much of the protein is absorbed into the body and how quickly it can be used in protein synthesis. It however assumes that the protein item is the only source of nitrogen (76) and does not account for digestibility or how well the protein is absorbed. In addition the result is affected by how the protein item is processed as well.
Net Protein Utilization or NPU identifies how much protein is consumed and remains in the body. It also maintains a scale from 0-100 with 100 being able to use all the nitrogen present and made into protein (77).
Protein Efficiency Ratio or PER is a value derived from the total energy received from a protein. Essentially it evaluates weight gained by the consumer (77) and resulting growth.
Protein Digestibility-Corrected Amino Acid Score or PDCAAS.
This is a method developed by FAO Expert Consultation on Dietary Protein Quality Evaluation in Human Nutrition (78) for evaluating the proteins ability to meet the body's amino acid requirements (78) and fecal digestibility which determines nutrient absorption. A score of 100 can meet all diet needs.
Digestible Indispensable Amino Acid Score or DIAAS
This method replaced PDCAAS in 2011 because it accounts for the digestibility of every single amino acid. This is important to know because it also accounts for any processing that has been applied to the ingredient including heating as well as anti-nutrients (79). A DIAAS score is based on a 100 point scale. If the protein ranks at 100 then the protein is of the same quality as the reference protein used.
Slow and Fast Proteins
Proteins can be classified as slow or fast proteins. This helps describe why proteins digest at different rates and affect the proper and complete absorption of other nutrients.
Slow proteins often include sources from carbohydrates, and are absorbed and utilized over 4 or more hours. While they do not contribute as much to protein synthesis, they do prevent protein breakdown within the body up to about 30%. We see that these become limiting proteins as they slowly digest.
Fast proteins are those that are absorbed and utilized in 1-2 hours. These proteins are best for protein synthesis, tissue repair and other processes in the body that consume and use protein quickly.
The rate of fast and slow proteins is important for understanding the rate of protein synthesis in the body affecting metabolism, protein utilization (80) and hormonal activity (81).
Plant vs. Animal Proteins
There is much debate that plant and animal proteins are no different from each other and that you can get the same amount of protein in a plant food item as an animal food item. There are some very key differences between animal and plant proteins which are especially important to note for our carnivore companions who thrive on an animal based diet.
Structurally plant proteins contain fibers that make it harder for digestive enzymes to access protein, decreasing the digestibility of plant based proteins (82).
Plant and Animal proteins have different protein structures. We won't get too sciencey here but just know that ratios of protein shapes vary in animal versus plant proteins (83, 84). Plants are higher in β-sheet conformation but lower in α-helix. The higher β-sheet conformation is what makes these proteins harder to digest.
In order to consume the same amount of essential amino acids, your companion would need to eat 20-50% more plant proteins than animal protein (79). This is why when feeding a kibble, canned diet or other diet with a higher plant matter content you will need to feed more than feeding a raw species appropriate diet of meat, organs and bones.
As our companions age they tend to have a reduction in appetite so it's essential that their diet is high quality readily available protein in smaller nutrient dense amounts.
A fundamental difference between animal and plant proteins is that the plant proteins are mostly storage proteins with large and compact structures that are hard to break down.
10-40% of the dry weight of the seed for example can be composed of storage proteins (85).
Whereas animal protein builds muscle (86, 87, 88) and contributes greatly to protein synthesis, plant proteins result in more oxidation and lower muscle gain (89) than protein synthesis (90).
Collagen is only found in animal proteins and has important functions to the maintenance and repair of skin, bone, cartilage, tendon and blood vessels.
Animal proteins taste better which is due to their “gellin, emulsification and foaming” functions (91).
Meat includes three primary types of muscle (92) which are unique in structure, aiding in taste and texture, making it hard to replicate in vegetable based meats.
Animal Proteins are also a better source of Vitamin B12, D, omega 3, Iron, Zinc and Vitamin K2 as well as all essential amino acids. As mentioned many organisms often can make all 20 of the amino acids but cats and dogs cannot. They must get them from their diet and several of them cannot be found in any plant matter including fruits, veggies or grains.
Plant and animal proteins contain varying amino acid chains and fold differently depending on its natural environment (94). While animal and plant proteins loosely contain the same amino acids, animal and plant proteins contain amino acids in different configurations and sequences creating many more complex differences including digestibility, functionality, structure and amino acid profile all affecting its usability, bioavailability and bioactivity by our companion carnivores (94).
When comparing proteins often you will see grams of protein per serving versus calories. You will not see however the proportion of essential amino acids and thus quality of protein factored in.
Commonly you will see that broccoli is compared to animal protein saying that it is higher in protein than the animal source. Let’s compare values to an egg (a food item used often to explain a complete protein)
In 100 g of Broccoli there is 2.8 g of protein and 82 mg per calorie, compared to an egg that has 12.5 g of protein in 100 g and 4 mg per calorie. It may seem like there are more grams of protein per calorie however the ratio of essential amino acids in each of these are unaddressed. In reality, your companion would need to consume 3 kg of broccoli to have a complete amino acid profile and 6 kg to get enough calories (95) equal to the egg (96, 97).
The reasons illustrated above also provide evidence that it is inappropriate and detrimental to feed a purely vegetarian or vegan diet to our carnivore companions. Sole consumption of plant matter is not only species inappropriate but is a sure way to become deficient and even lose essential amino acids (98).
Generally speaking the following percentages are found in various protein sources (not accounting for essential amino acid content).
Another common misconception is that a raw diet is too high in protein. Despite popular belief meat and organs are not 100% protein.
Muscle Meat is on average 22% protein and if organs are mixed in the percent is actually lower (99). Insects are on average 19-24% (100) protein whereas legumes and soybeans are 20-36% and 35-40% respectively (101).
Animal sourced proteins such as chicken, pork, rabbit and beef among others are fractionally made up of protein as one can see from the chart above. Protein, specifically animal sourced proteins are essential in our carnivore companions' diets whether cat, dogs or ferret. Everything about it from the source, structure, make up and function is what makes our companions thrive and live the healthiest and fullest life. As we can see there are many myths and misconceptions when it comes to plant and animal protein but its clear as day, animal based protein is what our companions thrive on and can properly digest and utilize for their every day function anatomically, physiologically, dietary and more. The proof is in the food bowl (and what comes out the other end).
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