Protein, Fat, and Carbohydrates:
What are they, and why do animals need to eat them?
by Mark E. Rogers
PROTEINS
General Considerations-
Proteins are the most numerous, complex, and important of all the nutritive biomolecules animals consume. Accounting for 35% of the dry weight of cells, they constitute the majority of macromolecules contained in all living cells and occur in all cells, and all parts of cells. They are the most diverse biomolecules in both structural variation and function. The ultimate expression of the genetic information contained in DNA results in protein synthesis. To fully appreciate the importance of protein synthesis (making proteins) it should be pointed out that up to 90% of the energy utilized by cells is strictly for this function! Despite their importance and diversity, proteins are probably the most poorly understood and under appreciated of all the biomolecules we consume as food and use to sustain life functions.
All proteins are made up of just 20 different amino acids. Perhaps our misunderstanding and under appreciation of proteins comes from not truly understanding what they are, their structure, and the fate of the proteins animals eat. Amino acids are organic chemicals with the basic structure shown in Figure 1 below. They all have a carboxylic acid or carboxyl group, an amino group, and a variable “R” group. The “R” group is what gives each amino acid it’s unique structure and properties, and gives proteins their limitless diversity. The “essential” amino acids vary by species, they are the amino acids an organism cannot make itself and, therefore, must consume in the diet.
Proteins are long chains ( mixed polymers) of amino acids arranged in specific and unique sequences. Because the amino acids can be joined (bonded) together in an infinite number of possible mixed combinations or sequences, the total number of unique proteins possible is limitless! To appreciate the magnitude of possible unique structures and, therefore, unique functions of proteins, one must take into account several features of proteins. They can range in size from hundreds to tens of thousands of individual amino acids in unique sequences. These long chains of amino acids are not just linear chains. The chains fold over one another looping back and forth in unique patterns. As they do this, they take on secondary structure in the form of sheets, helices, and random loops. They bond across these secondary structures from one part of the chain to another forming a third level of structure and fixing them together in discreet 3-dimensional shapes and structures. A simple illustration of this complexity (not showing individual amino acids just the gross structural concepts) is shown in Figure 2.
Finally, one or more of the bulk structures shown above can aggregate and bond together forming even larger and more complex structures. From the set of 20 amino acids, trillions of unique proteins can be constructed, each with a unique purpose! To appreciate the complexity of the structure and function of proteins the reader is encouraged to do a “Google” or similar search on protein structures and protein function.
There are two general classes of proteins, fibrous and globular. Fibrous proteins have fairly simple, regular, and repeating patterns whose primary role is structural in nature. Examples include, hair, fur, feathers, some connective tissues, etc. Globular proteins have more complex structures and functions. These include, but are by no means limited to; enzymes, antibodies, transport molecules such as the oxygen carriers hemoglobin and myoglobin, hormones exemplified by insulin, cell signaling molecules, pigments, rhodopsin for vision, cell structure components, proteins for energy utilization and the list goes on!
The importance of the exact, intact 3-dimensional structure of proteins cannot be overemphasized. It is part of what makes every protein unique and is crucial to its function. Changing even one amino acid can render a protein inactive at best, toxic at worst, or somewhere in between. For example, the difference between healthy red blood cells and “sickle cells” that cause the painful and often fatal disease known as sickle cell anemia, is the result of a single change in an amino acid among thousands in the oxygen carrying protein hemoglobin! The extremely complex 3-D structure of proteins is not only crucial to its function but is, also, in most cases quite fragile. External forces such as, heat, acid, alkali, oxygen, and light can all damage proteins and render them, again, either inactive or possibly toxic. Enzymes, for example, are rendered inactive by heating (cooking) above certain temperatures or by the action of digestive fluids in the mouth, stomach, or intestines. It is awe inspiring to realize that tens of thousands of individual amino acids must all be bonded (attached) to one another in a specific order, that they must be arranged in a precise 3-D structure, interact with one another perfectly, so that a protein can be exactly what nature intended it to be!
Proteins as Nutrients-
As a nutrient, proteins are unique when compared to fats and carbohydrates in that they are not stored for later use. While animals have millions of different proteins throughout their bodies, none of these are a storage reservoir of amino acids or “protein pieces” for later use. All proteins have specific functions and none of them are storage. This means when proteins are consumed as food, whatever is not used within approximately 24 hours is excreted through the feces and urine. Therefore, protein should be consumed everyday. Further, proteins are constantly being “used up” and remade by an animal based on its minute-by-minute needs. The “remnants” of the used proteins, the amino acids, are not stored away as building blocks for later use to make new proteins. They are excreted as waste products. Any condition such as growth, muscle building for strength or endurance, tissue repair after injury or surgery, or recovery from illness, etc., all require greater quantities of dietary protein.
The primary reason an animal eats protein, is to maintain a steady supply of amino acids. This is especially true of the “essential amino acids” that an animal needs as building blocks for making the proteins required to sustain life and grow. So what happens when we eat proteins?
When a protein food source is eaten, it is digested or “chopped up” into its individual amino acids. They are absorbed through the small intestine into the bloodstream, then travel to the liver. From the liver, they are transported as “free” amino acids to the various cells where they are required for use in new protein synthesis. The proteins an animal needs are assembled from individual amino acids, one at a time, at truly amazing speeds, up to 100 in 5s! The amino acids must be free of one another to be of any use. In healthy animals, amino acid chains (peptides or polypeptides) of two or more amino acids that are not part of the animal’s normal biochemistry should not be circulating in the bloodstream or any where else outside the digestive tract. Such polypeptides can be very harmful to an animal. This point cannot be overstated! When a protein (large complex structures of hundreds to tens of thousands of amino acids) is eaten by a healthy animal it IS broken down into its individual amino acid components before they are absorbed into the bloodstream for use to build new proteins. Large non-endogenous proteins do not, and should never be, absorbed by a healthy animal! Whatever amino acids are not used in a relatively short time, 24 hours, are excreted in the feces and urine and are not stored.
Other useful features of amino acids is they can be “burned” as a source of energy for an animal, though they are less efficient then fat molecules, ~4kcal/g protein vs ~ 9-10 kcal/g fat. The metabolites of certain amino acids are also used by animals, especially carnivores like cats and dogs, to make the important sugar glucose that animals need. This last point is important because it drives home the fact that dogs and cats do not need to consume carbohydrates (sugars) in their diet for energy or as a source of the important sugar, glucose.
Proteins as allergens and toxins-
Whole meat and some vegetable are readily digested and utilized by healthy animals. These include the food-appropriate proteins from animal sources (flesh, skin, organs, some connective tissues, muscle, viscera, i.e., meat) and vegetables (the vegetable proteins). Inappropriate proteins include hair, feathers, some connective tissues, leathers, etc. The latter proteins are highly resistant to digestion and have questionable value as a food source. Some “healthy” sources of easily digested protein can be rendered “unhealthy” by excessive heating (cooking), light, or exposure to oxygen. Because cooking and other forces can alter the 3-D structure of proteins they can make them less digestible and cause health issues when used as a food source.
Some proteins are very toxic. These include snake and spider venoms, and the proteins released by certain bacteria, fungi, mold, and some plants. Some of them can enter the body through oral ingestion, or are released in the digestive tract after ingestion. Others, such as venoms, illustrate how intact proteins are poisonous in the bloodstream. Some of the proteins release by bacteria in the digestive tract are resistant to digestive action and cause GI distress or toxicity. In a healthy animal, partially digested proteins are not absorbed into the bloodstream or even the inner layers of the mucosal membrane of the GI tract. They are excreted in the feces. Animals that have internal inflammation as the result of a reaction to environmental allergens or diseases that result in an inflammatory response can absorb partially digested proteins into the mucosal membrane and cause further problems like intolerance or allergy. This is most often the cause of food intolerance or allergy.
In healthy animals, partially digested proteins are excreted in the feces. However, any degree of inflammation of the mucosal membrane of the GI tract or any other disfunction of the amazing digestive tract can permit partially digested proteins to absorb into the inner portions of the GI membrane. This can result in either removal or further response including intolerance (most common) or allergy (uncommon). The higher the quality of ingested proteins the higher the digestibility and, therefore, the lower the incidence of intolerance or allergic response. Similarly, the healthier the animal, the lower the incidence of intolerance or allergy. For an allergy to occur, an animal must be exposed to the allergen (protein) at least twice. This exposure must generally be inside, not at the surface, of the GI tract membrane. This precludes a healthy membrane if allergic response occurs from GI tract exposure. Inflammation that results from non-food sourced allergies can cause the GI tract membrane to be more permeable to partially digested proteins. Treating the primary cause of the inflammation when possible in addition to feeding higher quality, more digestible proteins will lessen further irritation and deleterious health conditions. The least amount of processing possible contributes to protein digestibility.
Protein Requirements-
As mentioned previously, proteins are not stored for later use. Proteins are constantly being made and destroyed within the cells of every healthy animal. The used up or “destroyed” proteins are not recycled as sources of usable amino acids for new protein synthesis. The newly forming proteins require a constant supply of free amino acids supplied from the digestion of food proteins. An animal in a growth or repair phase requires a greater supply of free amino acids then a mature, sedentary animal. Older animals, though less active, also require large amounts of protein to maintain their lean muscle mass.
Proteins are the “universal” food. They supply the necessary amino acids for new protein synthesis, they are a source of energy and some amino acid metabolites are precursors for the synthesis of fats and glucose (the most important sugar). Deprivation of food source proteins can cause damage more quickly then any other food ingredient. If an adequate supply of free amino acids is not maintained an animal will eventually begin to digest its own proteins, for example muscle, to supply the necessary raw materials for the most critical proteins required for basic survival. This is the beginning of starvation. At this point, the digestion or destruction of internal proteins becomes a food source. Although there are minimum requirements for protein intake in both cats and dogs, these are just minimums to ensure they survive not thrive. There is NO maximum for protein intake. That is correct. NO MAXIMUM! Provided an animal has an adequate intake of water, the excess protein consumed in a diet is harmlessly passed through the feces and urine, as are the waste products of amino acid metabolism. The notion that too much protein is harmful for an animal is a myth whose origin cannot be traced, but continues to be perpetuated by those who wish to feed foods high in unnecessary carbohydrates and lower in meat and other high quality proteins. Carbohydrates are sugar! See accompanying section on carbohydrates. Carbohydrates are unnecessary in a balanced carnivore diet. There is NO SUCH THING AS TOO MUCH PROTEIN! Quality proteins can never be supplied in overabundance. Quality however, is essential. Highly digestible forms of animal protein (skin, organs, muscle, connective tissue, viscera, etc.) and quality vegetable proteins that are not excessively processed are all good sources of proteins for cats and dogs. Remember the amazing complexity of proteins is not only necessary to sustain an animals life, as food it is also a source of those tiny little precursors to proteins - amino acids!
FATS
General Considerations-
Fats or fatty acids are an important and diverse class of biomolecules. As a food ingredient they are an efficient source of energy, ~9-10 kcal/g approximately twice that of carbohydrates (sugars) and protein. Fats are the most biologically appropriate source of energy for dogs and cats. The glycerol portion of ingested fatty acids (see below) is also an excellent precursor for the synthesis of the important sugar glucose. The four general types of fatty acids found in food are saturated fatty acids (FAs), monounsaturated fatty acids (MUFAs), polyunsaturated fatty acids (PUFAs) and small chain fatty acids (SCFAs). Essential fatty acids (EFAs) are a subclass of PUFAs and are “essential” because animals cannot make them and must therefore consume them in their diet. Fatty acids derive their name from the fact they have a carboxylic acid group at one end and a hydrocarbon chain of variable length at the other. The Hydrocarbon chain is what makes them “fatty” or fat like and makes them very insoluble in water. The longer the chain the less water soluble. The basic structure of a fatty acid with important features indicated is shown in Figure 3.
Saturated fats are an important source of energy and used for insulation. Monounsaturated fats are generally considered healthier than saturated fats because they are more easily metabolized. They are used as highly efficient sources of energy, insulating fats, and as structural components of various parts of cells. SCFAs are given the least attention. They are efficiently produced by the action of the healthy microorganisms (bacteria that live in the large intestine) on soluble and insoluble fiber sources, the so-called prebiotics. “Prebiotics” is a nice way of saying bacteria food. Fermentation of fibers such as beet pulp, rice bran, oat bran, inulin, etc., produces SCFAs. The SCFAs serve two general purposes. They act as an energy source and by feeding and sustaining the billions of bacteria in the large intestine they help keep the mucosal membrane of this crucial component of the GI tract healthy. They are also found in small quantities in certain foods. PUFAs have multiple cis double bonds in the hydrocarbon chain portion of the FA structure. EFAs are a subgroup of PUFAs . These are the ω-3 and ω-6 (omega-3 & omega-6) fatty acids and will be discussed in greater detail below. See Figure 4.
PUFAs have numerous critical biological functions. They, like all FAs, are an efficient source of energy, but, more importantly, they have a role in vision, membrane structure and function, the nervous system, cell signaling, regulation of gene expression, fat soluble vitamin absorption and utilization, the immune system, and more.
Fats as a source of energy-
Dogs and cats derive the majority of their energy from fats, 70 - 90%, not carbohydrates. Animals have a limited storage capacity for carbohydrates as an energy source. Fats have unlimited storage capacity. Fats also provide steady energy supplies where carbohydrates are quick to burn and quick to be depleted. Although muscle cells in animals use glucose as an energy source from glycogen (the storage form of glucose in animals), its storage capacity is very limited. Recall 1gram of fat contains more then twice the energy as 1 gram of sugar. Glycogen depletion is also very detrimental to the health of an animal. To maintain glycogen reserves for emergency energy production and the unique functions only glucose can accomplish, dogs and cats should eat more fat NOT more sugar. Excess sugars are not stored as glycogen but rather are converted irreversibly to storage fats. When fats are consumed in a normal diet, they are in the form of triglycerides. That is three fat molecules attached to one glycerin molecule. The glycerin can be used by animals to produce glucose via gluconeogenesis ( literally making new glucose). SeeFigure 5.
Fats also require less water for storage then glucose. Glycogen requires large amounts of water for stabilization. The more stored glucose, the more water weight that is carried. Overweight animals fed high carb diets that are low in fat and low in protein (the vast majority of weight loss diets for cats and dogs are of this type), may lose weight but they do not form lean muscle tissue. The weight they retain is often water weight and “flab”. The high carb diets also lead to many other health issues including diabetes, tooth decay, and organ degeneration.
Fats, not carbohydrates, are the appropriate energy source for carnivores. This is most clearly demonstrated by the fact dogs most efficiently obtain their energy requirements through the aerobic metabolism (use of oxygen for metabolism) of fat and not the anaerobic oxidation (absence of oxygen) of carbohydrates. While carb loading in people has some merit before strenuous exercise, it has been shown to be very unhealthy in working dogs and results in more frequent and severe injuries. The burning sensation a person feels when pushing their limit in exercise is partially from a build up of lactate in their muscle. This is the result of anaerobic burning of energy sources and is also called an oxygen debt. It is cleared after a rest period. For working dogs (and horses incidentally) this lactate build up is a source of dangerous cramping and “ceasing”.
Fat is “burned” in animals in the mitochondria. Mitochondria are an organelle found in cells and can be thought of as our cellular furnaces or engines. A diet rich in healthy fats stimulates the production of more mitochondria, resulting in a higher concentration of mitochondria per cell. This is especially true of muscle cells. A measure of an animals fitness is a measurement called VO2 max. VO2 max is a measure of how efficiently an animal uses oxygen especially during strenuous exercise. Feeding “pet dogs” a diet rich in healthy fat sources dramatically increases the VO2 max, and their values approach those of hard racing sled dogs. This indicates diet, not selective breeding, is responsible for the legendary stamina of these animals. Burning fat increases the utilization of oxygen and increases stamina. Burning carbs during heavy exercise produces an “oxygen debt” as described above.
Another interesting feature of large volumes of oxygen use through fat metabolism is that it increases heavy breathing in dogs. This in turn helps to moisten the nasal passages and has been linked to an increase in the sensitivity of dogs sense of smell.
Essential Fatty Acids (EFAs) ω-3 and ω-6 fatty acids-
Essential fatty acids probably get more media attention then any other food ingredient. Much like anything that gets this much attention, a great deal of what is said is, at best, confusing, at worst, misleading and wrong. Amazing benefits of various sources of EFAs abound. Cryptic warnings of deficiencies are just as prevalent. Unfortunately, with all this attention the amount of misinformation, opinions, and confusion, are often difficult to wade through. To a chemist, EFAs are simple little molecules. To a living organism, they are, as the name suggests, essential. The totality of their biochemistry, importance, structure and function relationships, etc. could constitute an entire life’s work for a scientist! Searching the scientific literature for just one year’s worth of publications on the subject could take a trained professional many months. That being said, we will do our best to provide some useful, clear information on the subject, and provide the interested reader a starting point for further investigation.
A very brief chemistry lesson is in order here to help understand EFAs and how the ω-3 and ω-6 fatty acids are different. This may be skipped if you like. Although humans and other mammals can make saturated FAs and some MUFAs from some carbohydrates and amino acids, they cannot make the necessary insertion of a cis double bond at either the 3 or 6 positions. This is what makes the omega - 3 and omega - 6 fatty acids essential. The parent ω-3 FA is alpha linolenic acid (ALA, 18:3n-3) The first part (18:3) tells the reader that ALA is an 18-carbon fatty acid with three double bonds, while the second part (n-3) tells the reader that the first double bond is in the n-3 position, which defines it as an omega-3 fatty acid. The parent ω-6 FA is linoleic acid (LA, 18:2n-6), the numeric nomenclature follows that of the ω-3 series. Figure 6.
Humans and other mammals can synthesize longer (20 carbons and greater) EFAs from these two parent compounds, assuming their are in good health and not under extreme stress. One exception is cats. They cannot make the important ω-6 FA arachidonic acid (AA, 20:4n-6) and must have it in their diet. Most pet foods use fat sources that contain sufficient AA for cats. The approximate ratio of ω-6:ω-3 FAs for dogs and cats should be 5:1. In humans it is approximately 1:1 though our modern diets are in excess of 10:1! Supplementation of EFAs should not be necessary for dogs or cats under normal health conditions provided they are fed quality foods. Lactating or pregnant bitches and queens can be supplemented for good measure as well as animals under extreme duress like heavy exercise or recovery from injury or illness.
Good sources of EFAs include, but are not limited to, seed oils like flax seed, canola oil, etc., and animal sources such as beef tallow, fish oil, chicken fat, krill oil, etc. The actual chemical structure and function of EFAs is identical, no matter the source. Different ratios are found in different sources and the interested reader is encouraged to find reputable third party sources for this information. Minimal processing of any EFA source is always best. Since one of the critical functions of EFAs in cells is as an antioxidant, they are prone to oxidative degradation. Therefore, they should be preserved with appropriate antioxidants to prevent spoilage (oxidation). Typically, fats are preserved with mixed tocopherols (vitamin E), an antioxidant itself.
Fats are eaten as triglycerides. In the stomach and small intestine, they must be partially cleaved from the glycerine molecule to allow for absorption. This is done with the help of pancreatic enzymes (lipase's) and bile salts. They are then absorbed through the intestinal lining with the help of highly specialized proteins, reassembled, and sent on their way to where they are needed. Their ultimate fate is complex and varied and beyond the scope of this discussion. Fats of all types are a necessary and a critical nutrient.
Essential fatty acids, though required, should not be consumed in huge quantities hoping for some magic benefit. Many wild health benefits are touted for these simple and poorly understood little molecules. Some are; “more EFAs will make you smarter, wiser, faster. Your heart, the muscle, will last forever. You won’t have heart attacks, strokes, high blood pressure or high cholesterol, or other illnesses. Your dogs and cats will all have luxurious coats and perfect skin and they will all win BIS as will all their puppies and kittens, and they’ll all be the sex and color you want!” Obviously, these were all said in jest and wildly exaggerated. The point is that the media and unscrupulous marketing companies will tell you these things, to one degree or another, so you will buy their products. In the end, make sure your animal is eating a balanced, varied diet. It should consist of quality fresh ingredients and pet foods and ingredients made by specialists in pet food manufacturing and nutrition. Preferably those who are pet nutrition and health specialists, not marketing companies, mega-conglomerates or self proclaimed experts.
It is hoped that this brief introduction to fats, including EFAs, has helped the reader understand what fats are and their role in nutrition. This was not meant to be a complete treatise on fat structure, function, or sourcing. The biochemistry of fats is complex and the interested reader is encouraged to pursue their interest in scientific, peer reviewed literature articles and texts on the biochemistry of fats.
CARBOHYDRATES
General considerations-
Carbohydrates are the most abundant biomolecules on earth. They are also called sugars, saccharides, polysaccharides, starch, and fiber. They all mean the same thing and are, in essence, the same, just having different molecular structures. Their primary function is to provide short term energy for herbivores, omnivores, and other organisms. Glucose a very simple sugar and is the only energy source for the brain. Recall that glucose can be made from the glycerine portion of fats and the metabolites of some amino acids. Other important functions of sugars include a source of carbon for the synthesis of more complex molecules, structural components of all cells, the cell wall of bacteria, the exoskeleton of insects, crustaceans and similar organisms, components of connective tissues, transport molecules, components of DNA, RNA, and complex proteins, etc. All sugars have a basic type of structure. The word carbohydrate literally means hydrated carbon, or water on carbon. Sugars are made of the elements hydrogen, oxygen, and carbon. They can form long complex chains, bind to fats, proteins and small molecules. Some simple sugars that illustrate the basic structural features are shown below. See Figure 7.
The most abundant sugar is glucose. It is the primary sugar burned for energy. It has three major polymeric forms. One is starch which is the storage form of glucose in plants that can be used by animals for energy. The second is glycogen, the storage form of glucose in animals. Lastly, there is cellulose which is a form of glucose polymer used as a structural component of plants for instance bark, grass, celery, etc. This form of glucose is not digestible by animals but can be digested to release free glucose by bacteria, fungi, and other organisms. Ruminants such as cows have large populations of these bacteria in their stomachs which allows them to derive energy from grass, hay, straw, etc. In herbivores and omnivores, starch begins to be digested in the mouth by enzymes called salivary amylases. Dogs, cats, and other carnivores lack this enzyme. Digestion of starch and other sugars in dogs and cats must be done in the small intestine with enzymes released by the pancreas. The forms of glucose are schematically represented in figure 8. Note the subtle structural differences that result in dramatic biochemical differences.
Sugars can be easily recognized by their names. All sugars of greater then three carbons end in “ose”. The most common simple sugars are lactose, fructose, glucose, sucrose. Simple sugars can be one or two molecules of sugar. For instance, a simple disaccharide (two sugars together) is sucrose. This is made up of one glucose and one fructose and is what we usually call “table sugar”. Lactose, found in dairy products, is one glucose and one galactose. All food sugars come in essentially two forms, simple and complex. Some simple sugars are listed above. Complex sugars are also called polysaccharides, meaning multiple sugars. These include starch, glycogen, cellulose, chitin, pectins, etc.
Sugars in food-
In cats and dogs, simple sugars and starch are not well tolerated. This is especially true for cats. In the wild, dogs are opportunistic feeders and will eat nearly anything. However, anatomically and biochemically they are carnivores and have evolved to eat primarily meat. Diets high in carbohydrates cause both species problems ranging from digestive distress and diarrhea to diabetes, pancreatitis, tooth decay, IBD, etc. Lactose in milk is especially problematic in weanling kittens and puppies and poorly tolerated in adult animals. Dogs and cats, unlike herbivores and omnivores, do not have the enzymes in their saliva to begin the digestion of starches. Most digestion occurs in the small intestine with the aid of pancreatic enzymes. Once the sugars have diffused into the bloodstream through the intestinal lining the pancreas must excrete the hormone insulin to remove the sugar from the bloodstream and facilitate its storage in the liver and muscle cells. This doubly taxes the pancreas. Additionally, this process of absorption into the blood stream followed by storage causes a sharp rise then decline in blood sugar levels. This can alternately give the animal a “sugar rush” and feeling of energy and being satisfied followed by a “crash” and rapid feeling of listlessness and hunger.
Diets high in grains, e.g., corn, rice, wheat, oats, etc. or grain substitutes, e.g., potatoes, sweet potatoes, tapioca, lentils, peas, sorghum, millet, flours, etc. are all high in starch. Some have very high glycemic indices (cause a very rapid blood sugar rise) some have low glycemic indices. However, they are all sources of dietary sugars. Any amount of these food ingredients is not healthy for a cat and high amounts are bad for dogs. Cats and dogs have NO dietary requirement for carbohydrates. Just because they can eat them does not mean they should eat them. Humans can eat fast food everyday. We all know we should not eat this way everyday. Pet foods are fed the same everyday and the animal has no choice but to eat what they are given or die. There is a big difference between survive and thrive. Many petfoods contain greater than 30% carbohydrates. That’s 10 pounds of sugar in a 30 pound bag, or more! Since the requirement for glucose in dogs and cats can be met through synthesis from protein and fat it seems prudent to limit the amount of sugars they eat. There is no upper limit for protein consumption and depending on activity level 70% of caloric intake can come from fat. It seems logical that a food high in protein and with a fat content appropriate for activity level would be the best food for a cat or dog. These would be the ingredients found in meat, the ingredient in a carnivore’s diet.
Some carbohydrates do have benefits for dogs and likely for cats. These are the moderately fermentable to moderately high fermentable dietary fibers, e.g., rice bran, oat bran, beet pulp, inulin, frutooligosaccharides, etc. In small quantities these can greatly benefit an animals digestive system, boost the immune system, and provide a source of energy. These ingredients are often called prebiotics.
A prebiotic is a nice word for bacteria food. In the large intestine of healthy animals live billions of bacteria. Prebiotics are food for these bacteria. As the prebiotics are eaten by the healthy bacteria in the large intestine they keep the populations high. These healthy bacteria help fight off invasive pathogenic bacteria and other organisms. The bacteria also produce small chain fatty acids (SCFAs) from “eating” prebiotics which serve to keep the mucosal lining of the large intestine healthy and provide an additional source of energy for the animal as do any fat. Some dietary fibers, e.g., psyllium, help absorb water and can regulate the hardness or softness of the stool thereby controlling diarrhea and constipation.
Proteins, fats, and carbohydrates are the macronutrients found in food. The micronutrients are vitamins, minerals, antioxidants, minor phytonutrients (plant products not falling into the other classes), etc. This discussion has been a brief introduction to the structure and function of the macronutrients. Vitamins and minerals are discussed in another paper listed in the references below. Interested readers are strongly encouraged to pursue their interest in scientifically sound, peer reviewed, textbooks and journal articles, not in myth and opinion laden chat rooms, message boards, etc., and manuscripts and other publications by self appointed experts. This discussion was meant to be an introduction to macronutrients based on known science and not a comprehensive treatise. We sincerely hope it will help guide you to a better understanding of how to feed your beloved dogs and cats. Again, please pursue your questions and interests in the scientific literature, and not on the internet.
General References
Principals of Biochemistry, 2nd ed., Lehninger, A.L., Nelson, D.L., Cox, M.M. Worth New York NY, 1993.
National Research Council of the National Academies The Nutritional Requirements of Dogs and Cats, National Academies Press, 2006.
Small Animal Clinical Nutrition, Hand, M.S., Thatcher, C.D., Remillard, R.L., Roudebush, P., Novotny, B.J., Eds., Mark Morris Institute, 5th Edition, 2010.
Reynolds A.J., Taylor, C.R., Hoppler, H., Weibel, E., Weyand, P., Roberts, R., Reinhart, G.A., The effect of diet on sled dog performance, oxidative capacity, skeletal muscle microstructure, and muscle glycogen metabolism. In: Carey, D.P., Norton, S.A., Bosler, S.M., eds. Recent Advances in Canine Feline Nutritional Research: Proceedings of the 1996 Iams International Nutritional Symposium. Wilmington, OH: Orange Frazer Press, 1996; 181-198.
Hincliff, K.W., Reinhart, G.A., et. al., Metabolizable energy intake and sustained energy expenditure in Alaskan sled dogs during heavy exertion in the cold. Am. J. Vet Res 1997; 58: 1457-1462.
McNamara, J.H., Nutrition for working military dogs under stress. Vet Med Sm Anim Clin 1972; 67:615-623.
Bergstom, J., Hermanson, L., et al., Diet, muscle glycogen, and physical performance. Acta. Physiol Scand 1967; 71: 140-150.
Theriault, D.G., Beller, G.A., et. al., Metabolic responses to exhaustive exercise in racing sled dogs fed diets containing medium, low, or zero carbohydrate. J Lipid Res 1973; 14:54-61.
Reynolds, A.J., Fuherer, L., Dunlap, H.L., et. al., Effect of diet and training on muscle glycogen storage and utilization in sled dogs. J Appl Physiol 1995; 79: 1601-1607.
Downey, R.L., Kronfield, D.S., et. al., Diet of beagles affects stamina. J Am Hosp Assoc 1980; 16: 273-277.
Reynolds, A., Hoppler, H., Reinhart, G., et. al., Sled dog endurance: A result of high fat diet or selective breeding? FASB J 1995; 9:A996.
Kronfield, D.S., Diet and performance of racing sled dogs. J Am Vet Assoc 1973; 162:470-473.
Other useful information can be found in various articles in our nutrition series:
The ABCs of Vitamins A,B,C… An Explanation of the Vitamin Mineral Pack Found in Most pet Foods
So What Are Enzymes Anyway?
Grain Free Pet Food - To Feed Or Not To Feed - That is THE Question
Adverse Reactions To Food In Dogs And Cats
Myth Busters
These can be found in previous Editions of THE GREEN DOG, At www.greendogpromotions.com, or on our Facebook Page “Dog Food Demo Girl and Biscuit Boy” Notes section. They are also available upon request at greendogpromotions@comcast.net
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