UNDERSTANDING metabolism (as enzymes and some hormones) and

UNDERSTANDINGTHE BASICS OF PET NUTRITION – MODULE – 1 Nutrients   Like all living creatures, companion animals require food to stay alive and healthy. Food may be defined as any solid or liquid which, when ingested, can supply any or all the following: energy-giving materials from which the body can produce movement, heat or other forms of energy materials for growth, repair or reproduction substances necessary to initiate or regulate the processes involved in the first two categories   The components of food which have these functions are called nutrients and the foods or food mixtures which are actually eaten are referred to as the diet. Any nutrient which is required by the animal and cannot be synthesized in the body is called an essential nutrient and a dietary source must be provided.  If any essential nutrient is lacking or present in insufficient quantity in the diet, then the diet, as a whole, must be considered inadequate. Long term feeding of a nutritionally inadequate diet can result in sub-optimal performance or overt disease.

  In this first learning module, we will examine each of the basic nutrients in turn, and looking at their structure and functions in the body.                               Protein   WHAT IS PROTEIN? Proteins are very large molecules made up of hundreds of simple, single units called amino acids, bound together by peptide bonds. A wide variety of different proteins are found in nature, with each made up of strings of hundreds or thousands of amino acids, like the beads in a necklace. There are only about 20 amino acids typically found in proteins, but these may be arranged in any combination to give an almost infinite variety of proteins, each with its own characteristic properties.  FUNCTIONS All animals need protein in their diet. Proteins are essential components of all living cells where they have several important functions including regulation of metabolism (as enzymes and some hormones) and a structural role in cell walls and muscle fiber. Protein is continually being lost in faeces, hair, skin and sweat, so there is a constant turnover of protein in the body, even in adults.

Of course a growing body needs large amounts of protein for building new tissues.  Additional protein is needed during periods of growth, pregnancy, lactation and for repair of damaged tissue, such as wound healing. Protein is essential for the body’s defenses against disease, including the formation of antibodies. Proteins are also a source of energy in the diet.  Cat’s and dog’s coat is made primarily of protein.

Protein is required for the normal growth of hair and epidermal cells, for skin pigmentation and for sebum production. In the dog, this may account for over one quarter of the daily protein requirement.  ESSENTIAL AND NON-ESSENTIAL AMINO ACIDS Amino acids may be classified as either essential or non-essential.

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Essential amino acids cannot be synthesised by the body in sufficient amounts and must, therefore, be provided in the diet.  Non-essential amino acids are equally important as components of body proteins, but they can be synthesised from excesses of certain other dietary amino acids or other sources of dietary nitrogen.  The amino acid profile of a protein determines the proportion of essential and non-essential amino acids. Animal proteins generally have a more balanced amino acid profile, with a greater proportion of essential amino acids, and better digestibility than plant proteins. As a general rule, the more egg, fish, poultry and meat protein a food contains, the better it meets the animal’s needs for amino acids. This does not mean that pets should be fed entirely on meat, milk and eggs, but the diet should be carefully balanced with amino acids if cereals form a large part of the diet.

PROTEIN QUALITY Amino Acid Profile + Digestibility = Protein Quality  Not all the nutrients in food can be digested and absorbed, so the amount of protein an animal needs in its diet also depends on how easily it is digested by the animal. Digestibility is a measure of the how efficiently the nutrients in a food are digested and absorbed into the body.  The digestibility of proteins varies from 50-95%.

This means that between 5 and 50% of protein in food remains undigested and is not available to the animal. Plant proteins generally have lower digestibility than animal proteins. The protein in high quality pet foods usually has a digestibility of over 75%. However over-processing of prepared pet foods can reduce their digestibility.  THE PROTEIN CYCLE When foods containing protein are eaten and digested, the amino acid necklace is progressively cut into smaller pieces by specific digestive enzymes in the gut, until eventually the whole structure has been dismantled either into single beads or pairs of beads called peptides. Protein can only be absorbed from the gut into the bloodstream in this simple form.  Once absorbed into the bloodstream, most of the beads are taken in by the body’s cells and reassembled in a different order to build the protein structures which the body needs eg. hair protein or muscle tissue.

Therefore after a dog digests a meal containing beef protein, the components of the protein are pulled completely apart and rebuilt into new dog proteins, leaving no trace of the original beef protein in the dog’s body.  THE UREA CYCLE Excess protein is not stored in the body. Instead the left-over protein is used to produce energy in a system known as the Urea Cycle.

This process creates a waste product called urea, which must be eliminated from the body via the kidneys. Unlike fats and carbohydrates, proteins contain nitrogen molecules, and urea is one of the few safe forms in which nitrogen can be eliminated from the body.  PROTEIN DEFICIENCY Protein deficiency can result from either insufficient protein in the diet or from a shortage of particular amino acids.

Signs of protein deficiency include poor growth or weight loss, rough and dull hair coat, anorexia, increased susceptibility to disease, muscle wasting and emaciation, oedema and finally death. Deficiency of a single essential amino acid results in anorexia and subsequent negative nitrogen balance.  PROTEIN EXCESS Dietary protein in excess of the body’s requirements is not laid down as muscle but is, instead, converted to fat and stored as adipose tissue (fat). Feeding excess protein is a relatively inefficient and expensive source of energy in the diet.

  KEY POINTS Protein is an essential component of the body: tissues and body fluids, hormones, enzymes, antibodies The arrangement, sequence and proportion of amino acids in each protein give it unique properties There are about 20 different types of amino acids, of which 10 are essential, since they cannot be made in sufficient quantity in the body Protein quality is a function of the protein source and its digestibility Excess protein is not stored, but broken down to produce energy in the urea cycle Protein deficiency causes poor growth, lack of appetite, loss of coat condition and impaired immune function   Fats   SOURCES Animal – dairy produce, meats, fish  Plant – seed oils, nuts  STRUCTURE Fats consist largely of mixtures of triglycerides. Each triglyceride is made up of a backbone of glycerol, to which three fatty acids are attached. The differences between one fat and another are mostly the result of the different fatty acids in each.  Fatty acids can be saturated i.e. contain no double bonds, or unsaturated with one or more double bonds. Polyunsaturated fatty acids contain two or more double bonds in their hydrocarbon backbone.  ESSENTIAL FATTY ACIDS There are two main families of polyunsaturated fatty acids: omega-6, and omega-3.

Some fatty acids contain double-bonds that cannot be made by animals, and therefore must be supplied in the diet. They are termed essential fatty acids. The longer chain polyunsaturated fatty acids can be made in the body through progressive elongation and desaturation of these fatty acids.  Figure 1.

 Structure of omega-3 and omega-6 fatty acids Linoleic acid (omega-6) (18:2n6; 18-carbon backbone, two double bonds, first at sixth carbon) CH3-CH2-CH2-CH2-CH2-CH = CH-CH2-CH = CH-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH Alpha-linolenic acid (omega-3) (18:3n3;18-carbon backbone, three double bonds, first at third carbon) CH3-CH2-CH = CH-CH2-CH = CH-CH2-CH = CH-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH Essential fatty acid (EFA) requirements in dogs can be met by linoleic acid (omega-6). Cats, on the other hand, lack a key enzyme needed for the production of the longer chain omega-6 fatty acids from linoleic acid, and therefore need arachidonic acid in their diet as well.  FUNCTIONS OF FAT Key role in the absorption, transport and storage of the fat soluble vitamins (vitamins A, D, E and K) Energy source (containing 2½ times more energy per gram than either proteins or carbohydrates) Provide essential fatty acids, necessary for cell membranes, kidney function and reproduction Increase palatability of foods, particularly dry complete products DEFICIENCY Essential fatty acid deficiency may occur in animals eating diets low in fat or poor-quality commercial dry food for long periods. On rare occasions, animals develop fatty acid deficiency in association with liver disease, biliary disease and chronic pancreatitis or malabsorption problems.  Signs of EFA deficiency in dogs and cats include dull, scurfy coat, fatty liver, anaemia and impaired fertility.

Changes in the lipid film of the skin can alter the normal bacterial flora of the skin, and predispose the animal to secondary bacterial infection. This condition is known as fat deficiency seborrhea.  EXCESS Too much fat in the diet can result in an excess calorie intake.

In the long term, this can lead to obesity and/or growth abnormalities in young growing animals. As animals usually eat to satisfy their energy requirements, a high fat diet may not be balanced with respect to other essential nutrients.  Diets high in polyunsaturated fatty acids can become rancid through oxidation. Inadequate amounts of antioxidant in dry foods or prolonged storage of food, especially at high temperatures, may cause the fat in the food to become rancid.

As fats are oxidized, the essential fatty acids are destroyed, as are vitamin D, vitamin E and biotin. In general, dry foods should be kept at room temperature, out of direct sunlight, in non-lipid-permeable or non-absorbing containers and not stored open for longer than a month.  KEY POINTS Fats provide a concentrated energy source Dogs need the essential fatty acid, linoleic acid Cats also require arachidonic acid in their diet Long term EFA deficiency can cause skin lesions, poor coat condition and reproductive failure    Fibre   STRUCTURE Carbohydrates are found in nature as either polysaccharides (starch and cellulose) or sugars. Sugars are simple molecules, found as single units (monosaccharide, e.g. glucose) or in pairs (disaccharides, e.

g. sucrose).  Polysaccharides such as cellulose and starches are long chains of these sugars.

Before carbohydrates are digested and absorbed from the gut into the blood stream, these chains have to be broken down into the component simple sugars by the digestive enzymes.  Starches are readily broken down into their component sugars in the digestive tract, whereas cellulose, because of the way the sugars are linked (see figure), is not easily digested by single-stomached animals. Most of the carbohydrate fed to dogs and cats is in the form of starches, derived mainly from grains.  The term “dietary fibre” describes a group of complex carbohydrates, usually plant-derived, that resist digestion by mammalian enzymes. Common sources of dietary fibre include the plant cell walls of vegetables, fruits and cereals and the outer protective layer of seeds. Dietary fibre is a diverse group of substances, which includes cellulose and other complex carbohydrates such as fructose and oligosaccharides.  Dietary fibre reaches the colon relatively unchanged, where it provides a useful substrate for bacterial fermentation.

It can have a beneficial effect on both colonic health and faeces quality.  SOURCES Animal: small amounts in animal flesh (quickly lost after death); milk sugar (lactose) Plants: fruit sugars, starches from cereals and root vegetables FUNCTIONS Carbohydrate is a cheap energy source yielding approximately 3.5 kcal/g. Carbohydrate is not considered to be an essential nutrient for cats and dogs, as these animals are able to maintain blood sugar levels by making glucose in the body, mainly from amino acids.  Strictly speaking dietary fibre is not classed as an essential nutrient.

However we know that dietary fibre has an important role in the health and function of the large bowel. The digestive system of dogs, and to a lesser extent cats, benefits from a diet containing a moderate amount of dietary fibre.  DEFICIENCY Not seen in dogs and cats  EXCESS Dogs and cats can utilise high levels of carbohydrate provided that: 1. Other nutrients are present in appropriate amounts 2. Vegetables and cereals are cooked or treated to allow starch digestion In some dogs and cats, lactose causes diarrhea. This is known as lactose intolerance, and occurs when the animal does not produce sufficient lactase (the lactose digesting enzyme).

  Uncooked cereal or potato starch can cause diarrhea because it is not fully digested. Very high levels of dietary fibre can interfere with mineral absorption and lead to excessive quantities of faeces and/or diarrhea.  KEY POINTS Carbohydrates are made of sugar molecules, found singly or in pairs (simple sugars) or in long chains (starches and other complex carbohydrates) Plant materials which resist digestive enzymes are classed as dietary fibre Carbohydrates are a good energy source for dogs and cats, but are not essential Poorly digested carbohydrates can cause diarrhea    Vitamins   VITAMINS Vitamins are organic compounds that are essential for life and are required only in small amounts in the diet. The most important fact to know about vitamins is that there are two groups: fat soluble (A, D, E and K) and the water soluble ones, (B group and C). Unlike humans, dogs and cats can make vitamin C (ascorbic acid) from glucose, and do not require vitamin C in their diet.  Fat soluble vitamins require fat for their absorption, for their utilisation and storage.

Once in the body, unused fat soluble vitamins just stay there, so they are potentially quite dangerous if over-supplemented. For instance, hypervitaminos is A is used to be quite common in cats. It was usually created by well meaning people who only wanted to feed the best and overfed their cats with vitamin A-rich foods like liver.

  MINERALS Minerals are classed as macro minerals or micro minerals depending on their concentration in the body. Macro minerals are usually measured in milligram quantities and include calcium, phosphorus, sodium, potassium and magnesium, while trace elements or micro minerals are usually measured in microgram quantities and include iron, copper, zinc, manganese and iodine.  Minerals have many important functions in the body: 1. Structural (e.g. calcium and phosphorus in the skeleton.

2. Maintenance of fluid balance (e.g.

sodium and potassium) 3. Regulation of metabolism through enzyme function.  It is vital to remember that the balance of minerals in the diet is just as important as the absolute quantity.

For example, the correct ratio of calcium to phosphorus should be maintained ideally at 1.2:1. These two elements are closely linked nutritionally and problems can arise if this balance is disturbed, as occurs for instance when puppies are over-supplemented. For instance the digestive availability of zinc is reduced by if you put high levels of calcium in the food, and the animal will become zinc deficient, just because you have upset the balance of minerals in the body.  NUTRITIONAL ADEQUACY If any essential nutrient is lacking or present in insufficient quantity in the diet, then the diet, as a whole, must be considered inadequate. Long term feeding of a nutritionally inadequate diet can result in sub-optimal performance or overt disease.  The graph on the below represents the intake of a typical nutrient on the horizontal axis, against a measurement of health on the vertical axis.

  For each nutrient, there is a minimum requirement below which there are signs of a deficiency and health deteriorates. For most, there is a maximum tolerated level, above which signs of toxicity develop.  For most nutrients, there is is a range of acceptable intake, as long as the animal is fed a level within this range, health remains at an optimum. This range is narrow for some nutrients and broad for others.  In the first learning module, we will examine each of the basic nutrients in turn, and looking at their structure and functions in the body.  WATER Water is the single most important nutrient needed to sustain normal function of all living cells.

The body of most mammals is composed of about 73% water, and body water must be maintained within narrow limits for survival. To achieve this, all losses of water from the body must be offset by an equal intake of water.  Water balance in the body is represented by the following diagram. On the intake side there is water being drunk, there’s water coming in with the food, and there is metabolic water.

These inputs are balanced by losses through urine, faeces, lungs, skin and milk. Remember, a lactating Labrador bitch may make up to three and half litres of milk per day.  What is metabolic water? When carbohydrates, proteins and fats are broken down in the body they release a certain quantity of water. For example when 100g of carbohydrate is broken down in the body, 55g of water is produced. But with fat there is 107g of water produced. So fat is actually “creating” more water in the body than the weight of fat that the dog or cat took in.  Now that’s particularly useful to the cat – which evolved as a desert dwelling animal.

If it eats a high fat diet it needs to drink less, because it’s producing water all the time as a result of normal metabolism.  FUNCTIONS OF WATER 1.     Transport The low viscosity of water allows it to be moved around the body easily. Many materials are soluble in water and can be transported in solution in the gut, and in the bloodstream. 2.     Body Temperature Regulation Water has a high specific heat capacity which makes it an ideal medium to remove heat from hard working organs. The high heat of evaporation makes it a good way of losing latent heat from the body.

3.     Digestion The addition of water to protein, fat and carbohydrate allows digestion by hydrolysis. Even without digestive enzymes, water would eventually “digest” these complex molecules. KEY POINTS Vitamins divided into two groups, fat soluble and water soluble. Unlike humans, dogs and cats can make their own vitamin C. Minerals can be divided into two groups depending on their concentration in the body. The balance of minerals in the diet is just as important as the overall quantity.

Water is needed for transport, temperature regulation and digestion. Metabolic water is produced during metabolism of carbohydrates, proteins and fats You’ve reached the end of the first course module about the nutrients. If you would like to check that you have understood the key points so far.