UNDERSTANDING metabolism (as enzymes and some hormones) and




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
materials for growth,
repair or reproduction
substances necessary to
initiate or regulate the processes involved in the first two


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

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.






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. 

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. 

Amino acids may be classified as either essential or non-essential. 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.

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

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

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 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. 

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. 


Protein is an essential
component of the body: tissues and body fluids, hormones, enzymes,
The arrangement, sequence
and proportion of amino acids in each protein give it unique
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





Animal – dairy produce, meats, fish 

Plant – seed oils, nuts 

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. 

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 =

Alpha-linolenic acid (omega-3)
(18:3n3;18-carbon backbone, three double bonds, first at third carbon)

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. 


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

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

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. 

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
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. 


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





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. 

Animal: small amounts in animal flesh
(quickly lost after death); milk sugar (lactose)
Plants: fruit sugars, starches from
cereals and root vegetables

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

Not seen in dogs and cats 

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

Very high levels of dietary fibre can interfere with mineral absorption and
lead to excessive quantities of faeces and/or diarrhea. 


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 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 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. 

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 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. 

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.

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

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.