10 Nutrition: Basics

General Nutrition Information

Nutrients are chemical substances obtained from food and are used to provide energy, to build structure (bone, muscle, etc.), and to regulate growth, maintenance, and repair. There is an optimal amount of intake of any given nutrient; if too little it taken in, the animal is deficient and if too much is taken in, toxicity may occur.

• Water – Water acts as a solvent for transport of dissolved substances through the body, it is required for hydrolysis reactions of other nutrients, it helps maintain normal body temperature, and it provides shape and resilience to the body. Young animals have a higher percentage of total body water. The body in animals ranges from 40 to 80% water. Water requirements vary with age, body surface area, ambient temperature, state of health, amount of exercise, and life stage (growth, maintenance, pregnancy, lactation, etc.). Water is taken in directly (drinking) and indirectly (eating and in some species, through the skin).

• Energy – Energy is not a nutrient but is what is generated as nutrients are broken down. The energy in food is chemical energy. The body converts chemical energy to mechanical, electrical, or heat energy. Gross energy (the full amount taken in) is higher than digestible energy (gross energy minus energy lost in feces), which is higher than metabolizable energy (digestible energy minus energy lost in urine and gases from the GI tract). Net energy is metabolizable energy minus the heat increment, which is the amount of energy lost in fermentation and in metabolic processes. Net energy is what is available to the animal to use and it is not used with the same efficiency for all processes.

Besides water, animals take in vitamins, minerals, fats, proteins, and carbohydrates. Fats, proteins, and carbohydrates are the nutrients that are broken down to provide energy.

• Fats – Fats are one type of lipid. Lipids include fats; sterols (for example, cholesterol); mono-, di-, and triglycerides; fat-soluble vitamins (A, D, E, K); and phospholipids. Lipids provide energy and are a structural component of cell membranes. Fatty acids are made from acetyl CoA and NADPH in the cytoplasm of the cell and are a linear chain of an even number of carbons with hydrogens along the length and on one end (the methyl end) and a -COOH group on the other (the -COOH is what makes it an acid). Fatty acids with no double bonds are “saturated”, those with one double bond are “monounsaturated” and those with multiple double bonds are “polyunsaturated”. Those fatty acids denoted as “omega 3” fatty acids have a double bond three carbons away from the methyl end and those denoted as “omega 6” fatty acids have a double bond six carbons away from the methyl end. Phospholipids make up the cell membrane, preferentially permit substances to cross the cell membrane, and when added to foods, act as an emulsifier. Cholesterol is a base lipid that is the precursor to many other substances in the body including bile acids in the GI tract, sex hormones, adrenal hormones, and vitamin D.

When ingested, fats remain separate from fluid components in the stomach until they are emulsified. Bile has affinity both for water and fat. Exposure to bile in the small intestine breaks up large fat globules, exposing more surface area to enzymes in the digestive fluid. Glycerol and small lipids move directly into the bloodstream. Large lipids combine with bile to form micelles that are water-soluble and move directly into cells. Most of the bile released into the small intestine is reabsorbed and sent back to the liver (= enterohepatic circulation of bile). Some bile moves through the GI tract and is excreted in feces.

The fat-soluble vitamins are A, D, E, and K. All fat-soluble vitamins are effectively stored in the body and so need not be taken in daily. Because they are stored, toxicity is possible if they are consumed in excess. Water-soluble vitamins are not stored and toxicity is rare. Vitamin A is also called retinol acetate and supports vision, cell differentiation, and reproduction and growth. It is available in plants as beta-carotene; not all species can readily convert beta-carotene to vitamin A. Vitamin D is also called calciferol. It can be synthesized by the body and functions in calcium metabolism. Synthesis is stimulated by exposure to sunlight; not all species readily can synthesize vitamin D even with sunlight exposure. Vitamin E is also called tocopherol. It is an anti-oxidant. Vitamin K is also called menadione. It is a component of the blood-clotting cascade.

The terms “fat” and “triglyceride” are synonymous, as fats are made up of three fatty acid chains and glycerol. Fats in the diet efficiently provide energy, transport and store fat-soluble vitamins, increase palatability and satiety, and reduce dustiness of feed. Fats produce energy by being broken down to acetyl CoA, which then enters the Krebs cycle (also called the citric acid cycle or tricarboxylic acid cycle [TCA]), where it is oxidized to produce CO₂ and ATP. As a general rule of thumb, lipids contain 2.25 times more gross energy than protein or carbohydrate (9 kcal/gm for fat versus 4 kcal/gm for protein and carbohydrate). Fats are considered energy-dense nutrients.

• Proteins – Proteins are made up of amino acids and are required in the diet both to provide essential aminoacids that cannot be synthesized by the body and
  to provide the nitrogen needed for other essential compounds such as heme, nucleic acids, and creatinine. Essential (also called indispensable) amino acids are those that cannot be produced by the body quickly enough to meet demands for normal growth; these vary by species. There generally are 10 essential amino acids (threonine, tryptophan, valine, arginine, histidine, lysine, phenyalanine, leucine, isoleucine, methionine) that can be remembered using these mnemonics – These Ten Valuable Amino acids Have Long Preserved Life In Man or PVT TIM HALL. Most true carnivores (for example cats), also require taurine so the mnemonics can be changed to – These Ten Valuable Amino acids Have Long Preserved Life In Mutts and Tigers or PVT TIM  T HALL. Conditionally essential amino acids are only required for specific life stages. Examples include an increased need for glutamine in animals with sepsis, and increased need for arginine in individuals with liver or kidney disease, or in burn patients. Limiting amino acids are those essential amino acids in a diet that are present in low enough quantities that their concentration controls rate of protein synthesis. Complete proteins are those food sources that provide all essential amino acids and usually readily support ongoing protein synthesis. These usually are from animal sources. Plant proteins are more likely to be incomplete and/or limiting. A prominent example is lysine, which is limiting in corn. Complementary proteins are two protein sources that are themselves incomplete but that together provide all essential amino acids.

In ruminants, protein must pass through the rumen to be absorbed. Protein broken down in the rumen is available to microbes for synthesis of high-quality proteins and the remainder passes into the small intestine. Factors that affect the extent of protein breakdown in the rumen include the chemical structure and solubility of the protein, how long it is retained in the rumen, the particle size, the rumen pH, and the stage of plant growth. The breakdown in the rumen is vital; microbes in the rumen contribute up to 70% of the protein available for absorption in the small intestine. In some specific circumstances, which will be described later, ruminants are fed protein that will not be broken down in the rumen, called bypass protein, specifically to ensure that there is a large concentration of protein available for absorption from the small intestine.

In monogastric animals, proteins are broken down by pepsin and HCl into large polypeptides. In the small intestine, pancreatic enzymes (trypsin, chymotrypsin) break polypeptides down further into small peptides and amino acids. Absorption across the cell wall is sodium-dependent and requires energy (active transport).

Absorbed small peptides and amino acids are reassembled into new proteins in the liver and other tissues. Amino acids become tissue proteins (muscle, liver, etc.); enzymes, albumin, hormones, and other nitrogen-containing compounds; and are broken apart to provide energy. There is little storage of excess amino acids. Tissue proteins and serum albumin are a store of amino acids if necessary. When amino acids are deaminated, ammonia is produced. Ammonia and carbon dioxide combine to form urea, which is excreted.

Protein digestion is intertwined with energy use. If the cells are starved for energy, the amine group is removed and excreted and the rest of the molecule is broken down for energy. If the cells have a surplus of energy, the amine group is again excreted and rest of the molecule is converted to glucose and fat, and stored. It is valuable to think about energy needs when feeding protein because if the body cannot use the protein, it will be excreted or “wasted”. Examples of circumstances where protein wasting occurs include lack of energy from other sources so the amino acids are used for energy only, if the diet supplies more protein than the body needs, if a single amino acid is provided in excess, or when the diet supplies low quality protein with too few essential amino acids. An example of a time when this information is valuable clinically is when weight loss is desired; an animal on an energy-restricted diet should get a generous amount of high-quality protein to help maintain lean body mass and ensure an appropriate intake of nitrogen intake to balance nitrogen use (= nitrogen balance).

• Carbohydrates – Carbohydrates are an important source of energy in all species and are the primary energy source in most plant products. At the molecular level, they are a “hydrate” of carbon, with C:H:O at a 1:2:1 ratio. Simple carbohydrates are monosaccharides (glucose, fructose), disaccharides, (sucrose, lactose), and oligosaccharides. Complex carbohydrates are polysaccharides (more than 9 CH₂O units; cellulose) and fiber. Mono- and disaccharides are readily absorbed to provide energy. Examples of polysaccharides include glycogen (storage form of energy in the body), starch (storage form of energy in plants), and fiber (structure in plants). Non-starch polysaccharides are not digestible by all mammals as they resist hydrolysis by digestive enzymes. Rumination (cattle, small ruminants) and hindgut fermentation (horses) are required for breakdown of these kinds of carbohydrates.

In ruminants, dietary carbohydrates from forages like hay are primarily structural carbohydrates, such as cellulose, pectins, hemicellulose, and lignins. Some are non-structural carbohydrates, such as sugars and starches. Carbohydrates undergo microbial fermentation to form volatile fatty acids; this is described in detail later in  these notes.

In monogastric animals, there is limited digestion of carbohydrates in the mouth and stomach. Breakdown of carbohydrates is mediated by pancreatic alpha-amylase and brush border enzymes in the small intestine and by fermentation of undigested and unabsorbed carbohydrates in the large intestine. Fiber is not digestible in small animals but serves other purposes such as preventing constipation and normalizing intestinal motility. Cats handle carbohydrates uniquely; this is described in detail later in these notes.

• Water-Soluble Vitamins and Minerals – These compounds are needed in minute quantities. Water-soluble vitamins are the B-vitamins and vitamin C. They are organic micronutrients, required in small amounts but essential. They are not stored in the body so deficiency states can occur more quickly than with fat-soluble vitamins. If deficiency does occur, often more than one body system will be affected. Toxicity rarely occurs. Water-soluble vitamins often act as coenzymes.

Vitamin C can be synthesized in many species. Primates and guinea pigs are two species that require a dietary source of vitamin C.

Minerals are inorganic elements in food. Macrominerals are those required in higher concentrations in the diet and include calcium, phosphorus, magnesium, potassium, and sodium. These are responsible for the structure of the bones and teeth, maintaining an action potential across cell membranes, fluid balance, acid-base balance, storage and transport of energy, and for acting as second messengers and co-factors. Microminerals often are associated with a specific enzyme, hormone, carrier protein, or vitamin, and include iron, copper, manganese, zinc, selenium, cobalt, and iodine. Other mineral compounds are classified as ultra-trace elements and are needed in very small amounts. Examples include molybdenum, fluorine, nickel, silicon, chromium, and vanadium.

    Large Animal Nutrition Basics

The Role of Nutrition in Large Animal Veterinary Practice

Some practitioners may choose not to include nutrition in the services they offer as a veterinarian. This is problematic because nutrition plays a key role in large animal veterinary care, especially in production medicine, and because avoiding nutrition issues removes your chance to help patients and clients, and is a loss of potential revenue. Feed is the largest input cost in animal production and what feed is provided varies with age, breed, production and gain needs, and pregnancy status of the animals, and season of the year, such that a given client may be responsible for a wide variety of different feeding regimens for the animals under their care. Feeding also is related to a significant number of large animal diseases, both directly (deficiencies and toxicities) and indirectly (through causing changes in metabolic and systemic disorders and in immune status and general health).

Nutrition comes to the fore for large animal veterinarians because:

• Nutrition is often an option on your list for differentials for the cause of disease.
• Nutrition may be a limiting factor in production or performance.
• Inadequate nutrition may be a factor in development of disease.
• Improper nutrition is an unnecessary cost of ownership / management of animals.

Our role occurs at several levels, from as simple as advising on questions of feeding to troubleshooting production problems to diagnosing disease to formulating rations to acting as a nutrition program advisor (closely linked in production agriculture with records management and analysis). Other nutritional resources with whom we may work include owners / farmers, feed company personnel, independent nutritionists, feed product companies, extension personnel, university experts, and agronomy experts who study crops and their nutrient content. In practice, you will have an opportunity to apply your knowledge of nutrition every day.

Foods and Feeds

There is no hard definition that distinguishes “food” from “feed” but in general, food = what is actually eaten, mostly by people and non-farm animals (dogs, gerbils, birds, lizards, etc.) and feed = what may be mixed to produce animal food and/or what is fed to farm animals (cows, horses, chickens, sheep, etc.). Feeds may be characterized by origin or major type of the feed (plant, animal, mineral, vitamin) or by how the feed is processed (dried; ground = pulverized to decrease particle size; pelleted = extruded under high pressure and steam and therefore partially cooked; flaked = rolled and flattened; steamed; cooked; extruded; ensiled = anaerobically fermented).

Animal Origin Feeds

• Meat
• Byproducts (protein sources from parts of the animal not used for human food)
• Meat and bone meal (cannot feed from ruminants because of concerns about bovine spongiform encephalopathy (BSE = mad cow disease))
• Blood meal (dried blood, great protein supplement)
• Feather meal (ground feathers, less digestible protein supplement)
• Milk and milk products (casein, whey, etc.)

Plant Origin Feeds

• Whole plants (corn, silage, alfalfa hay)
• Seeds (shell corn, wheat, oats, soybeans)
• Residues or byproducts (beet pulp = leftovers from sugar beet production, citrus pulp = leftovers from orange juice production, sweet corn stover, potato waste = leftovers from making French fries, almond hulls, cottonseed, distillers grains = leftovers from the bottom of the beer vat, brewers grains, straw)
• Extracted portion = soybean meal, corn starch, molasses, corn oil

How do we determine what is in a plant? A variety of extraction processes can be used. Ether extraction is used to identify the amount of fats / lipids. Nitrogen extraction is used as an estimate of crude protein – about 16% of protein is nitrogen. Ash is what is left after burning a plant and is the mineral content. Acid and detergent extractions are done to define the two forms of fiber, which are hemicellulose (neutral detergent fiber) and cellulose and lignin (acid detergent fiber). Subtracting these from total carbohydrates yields the non-fiber carbohydrates, which are sugars, starches, and pectins.

There are a variety of ways nutrients in feed can be analyzed. You can find much information about nutrients in tables. For some feeds this is fine – salt is salt – but for others, the feed itself must be tested (for example, forages).  Types of analysis include wet chemistry and near infrared analysis. Feed analysis is best used for directly measurable components (fiber, macronutrients, protein) and is poor for constituents with wide variation or error in measurement (microminerals). Energy and non-fiber carbohydrates can be assessed by calculation. Be cautious of feed tables that come pre-installed in computer programs.

Feed quality is dependent on the right feed component being used (be cautious of similar names), feed being collected at the proper stage of maturity and properly stored and processed, the bioavailability of the nutrients, and freedom from contamination, toxins, and micro-organisms. Feed delivery also can be a problem. Every single diet can be described as three diets – the one that is formulated on paper, the one that is delivered, and the one the animals eat. The goal is to make sure all three of these are the same for all animals intended to eat that diet.

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Formulating Rations

Formulation is only one step and is not the end point. The key value is added by monitoring the implementation and the results of the feeding program. When in doubt, look at the animals! It is important to complete all 6 steps described below when formulating a ration.

1. Describe the animal.
   • Species, breed, age, sex, production/rate of gain, stage of pregnancy/lactation, exercise/activity, health status, environmental conditions – all of these factors play a role. For example, ruminants who have not been weaned (lambs and calves who are still milk-fed) are not functioning as ruminants.
2. Describe the nutrient requirements.
   • This includes total food intake (usually best expressed on a dry matter basis [DMB]), energy (calories, essential fatty acids), protein (total and essential amino acids), fiber, macro- and microminerals, vitamins, and water. One must also consider how you feed animals in groups. If you feed the average animal in that group, you may be underfeeding high producing or fast growing individuals in the group. Diseased animals in that group (for example, those carrying parasites) also may require more nutrients. Feed intake is the limiting constraint to meeting nutrient needs in some animals because they cannot physically take in enough of a given ration to get all the nutrients available in that ration. This is particularly true of animals with high energy needs because of lactation, work, or life in harsh environmental conditions. This can be compensated for by adjusting animal comfort, access to feed (time, conditions, competition), physical forms of feeds, water content, palatability, and balance of feeds to enhance digestion.
     
     1. Energy
        • Energy cannot be measured directly in routine feed testing and is instead inferred from studies. In monogastric animals, energy may drive consumption; once the energy needs are met, the animal stops eating. If they do not, they become obese. If the diet is highly palatable, excess energy may be taken in. Energy density can be increased by using fats in the diet. Energy density can be decreased by increasing fiber, particularly in carnivores and omnivores.
     2. Protein
        • Digestibility and types of protein present play a role. “Bypass protein” in ruminants is dietary protein that, either by some means of alteration or because of the type of protein, is not broken down by microbes in the rumen. Essential amino acids are those that must be in the diet; these vary by species. For example, taurine and carnitine are essential amino acids in cats. Corn is lysine deficient so soybeans may be added in rations to provide lysine. Protein sources are processed by heating, grinding, extrusion, and cooking to increase digestibility. Protein sources may be contaminated (for example, Salmonella sp, bovine spongiform encephalopathy).
     3. Fiber and non-fiber carbohydrates
        • Concerns include digestibility, physical form, palatability, association with other nutrients (for example, nitrogen-containing nutrients to help break down carbohydrates). The more mature the plant, the less digestible the fiber. These include sugars, starches, pectin, hemicellulose, cellulose, and lignin.
     4. Macrominerals
        • These are ground and processed rock and have variable bioavailability. They have reliably measured levels in feeds but levels present do not guarantee digestive availability. They typically are balanced for minimum needs and are adjusted for specific conditions.
     5. Microminerals
        • These are unreliably measured in feeds and are typically added to satisfy complete requirements in food animal diets – this means a set amount is added regardless of how much may be in other components of the diet. They may interact with other microminerals (copper and molybdenum) or nutrients (selenium and vitamin E), or with macrominerals (most are divalent cations and so may alter each others’ absorption).
     6. Vitamins
        • Typically added to satisfy total needs – as above, vitamins are added without calculating vitamin content in all components of the diet. Need for water soluble vitamins varies with species and some have special needs – for example, guinea pigs, like humans, cannot synthesize vitamin C so it must be supplied in feed.
3. Describe the feedstuffs.
   • Type, form, nutrient content, quality, cost
4. Describe the feed delivery system.
   • Mix, order of mix, processing, feed delivery (amounts, timing delivery system), monitoring delivery and consumption
5. Describe the ration or diet.
   • Doing the arithmetic. Computers do a lot of it. Work from an existing ration. Be careful about trusting the computer (Is the program up to date? Are the feed tables right? Are the nutrient requirements right?).
     1. Simple calculations: Pearson square

This is straightforward math and is easy to calculate but is rarely used when formulating a complex ration.

A = % desired nutrient in first type of feed

B = % desired nutrient in second type of feed

C = % desired nutrient in formulation

D, E and F are calculated

How to Use the Pearson Square

• • 2. Cut and fit” formulation
       • Start by including some of the largest components (fiber in ruminants, protein in monogastrics). Add minerals and vitamins to meet needs. Fill remaining “intake space” with energy sources (carbohydrate, fats, fiber). Need to cut a little of something out of the ration to let something else fit in. Use trial and error until satisfied.
    3. Linear programming
       • “Least cost” diet formulations – This is performed after a ration is formulated as a way of trying to optimize some particular aspect, for example, cost, while meeting the nutrient requirements and pre-set constraints. This commonly is used in food animal formulation. Results depend on accuracy of the definition of constraints and of nutrient content, and on the prices of feed used. Understand the approach and nutrition first, then use the tool. This can be a dangerous black box for the unsophisticated.
    4. The final question
       • Would I really feed this ration to the animal being considered?

6. Describe the results.
   • Production – Growth – Body condition – Health – Fecal consistency – Behavior – Family living/economic outcomes – Family lifestyle; how hard is it to feed this diet? – Quality of food produced (wholesome, nutritious) – Environmental impact (for example, are we overfeeding something that will end up in the manure)

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    Pet Food and Small Animal Clinical Nutrition

What is the role of the veterinarian in small animal nutrition? The veterinarian provides nutrition counseling for healthy pets, for disease prevention, and for use of therapeutic diets. They are the link between the pet food manufacturer and the pet owner. You must also consider whether or not you will sell pet foods at your clinic and if so, how you will decide which you promote.

Clinical Goals

1. To provide optimal nutrition
2. To provide nutrition to support health and prevent disease
3. To help clients select pet foods
4. To address client misconceptions

Pet food industry in the United States – About 87% of dog and about 95% of cat owners in the United States feed at least 75% of their animal’s diet as commercial pet food. There are fewer than 100 pet food manufacturers and are about 5000 different pet food labels in the US.

You should expect yourself to be able to physically examine pet food, calculate food dosage, and interpret pet food labels. You should always take a diet history which should include, at a minimum, what type of food(s) they’re getting, how much they’re being fed, and how often they’re being fed.

Dogs and cats are fed a diet that is complete and balanced. Complete = all nutrients present and bioavailable. Balanced = feed to meet the pet’s energy requirement and requirements for non-energy nutrients will automatically be met. This is very different from our approach as humans where we eat a variety of foods and hope for balance in our diet over time.

Pet foods and pet food labeling are regulated by the American Association of Feed Control Officials (AAFCO). The front of the package will have a name for each product. The name tells you what likely percentage of a product is present.

A new trend is unique game meats – for example, buffalo – 100% buffalo = buffalo meat with water sufficient for processing – This is not a complete and balanced diet and the label will reflect that.

Consumers read the brand name, then the guaranteed nutrient analysis, then the description of the food, and finally, the weight. About 1/3 of consumers in one study understood how nutritional adequacy was determined in pet foods and 2/3 did not. It is valuable for us to know how to read the labels so we can educate clients.

The label information panel must contain the following things: nutritional statement (adequacy claim), basis of nutrition claim, ingredient list, guaranteed analysis, manufacturer or distributor name and address, feeding directions, and universal product code.

Nutritional Statement

Nutrition in a diet may be determined either by the formulation method or by a feeding trial. If the formulation method is used, the label will have the following statement: “Diet X” formulated to meet nutritional levels established by AAFCO. Dog/cat nutrient profile for…” followed by which life stage is represented (maintenance, gestation, lactation, growth, all life stages). No animal studies are done. Nutritional statement by the formulation method is either calculated from known composition of ingredients in the diet or is determined by laboratory analysis of the food.  Neither method considers digestibility or nutrient availability. Feeding trials are a superior way to determine nutritional adequacy. If a feeding trial is used, the label will have the following statement: “Animal feeding test using AAFCO procedures to substantiate that diet X provides complete and balanced nutrition for…” AAFCO feeding trials, for example for a maintenance food, must enroll 6-8 dogs, 1-6 years of age, with the food as the sole source of nutrition. The food is provided for a minimum time of 26 weeks, with a physical examination at beginning and end, body weight weekly, and labwork at the beginning, middle and end of the trial. All results must be normal for the food to “pass”.

Two other options exist for the nutritional statement. “This product is intended for intermittent or supplemental feeding only” is on things like therapeutic diets, which are intended to be used short-term, under a veterinarian’s supervision. No nutritional adequacy statement is needed on treats and other food products that are not intended to be fed as the sole diet.

Ingredient List

Ingredient versus nutrient – Nutrient = substance that must be consumed in the diet to provide a source of energy, substrate for growth or substance to regulate growth, metabolism, and energy production. Ingredient = the means to achieve nutritional and palatability goals – an ingredient may supply one nutrient, many nutrients, or none.

The ingredient list must show each ingredient, listed in descending order by weight and listed on “as is basis” (as it comes out of the bag or can).

Guaranteed Analysis = Proximate Analysis

Protein and fat are expressed as minimum amounts, and fiber and moisture are listed as maximum amounts. All are reported on an “as fed basis”. Any other nutrient information is optional. Amounts will be close but not accurate.

Comparison of products requires evaluation on a dry matter basis (DMB). This allows you to compare foods with widely varying moisture content, knowing that you are not being misled by weight of moisture. As a rule of thumb, canned foods are 75% water and dry foods are about 10% water.

Dry matter (%) = 100 – moisture

DMB (%) = nutrient (%) divided by dry matter (%)

Example:

A food is 12% moisture and 17% protein. What is percentage protein on a dry matter basis? Dry matter = 100-12 = 88. Percentage protein on a dry matter basis is 17 divided by 88 = 19.3%.

Feeding Guidelines

The general recommendation is to start at the low end of the recommended amount listed on the bag and to monitor body condition to ensure the animal is not being underfed or overfed.

Manufacturer or Distributor Name and Address

This permits the consumer / veterinarian to contact the company for accurate information that may not be readily accessed from the label and to bring forward concerns about the product.

Universal Product Code

This is a unique identifier of a batch of pet food and is valuable in face of a recall or other concern.