You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

All living cells require energy to function and in an animals body this energy is derived from nutrients in food

Complex living organisms such as humans, animals, birds, reptiles and fish consist of several organ systems, which are made up of tissues which consist of groups of cells. Some tissues and cells are very highly specialised in terms of the function that they they provide within the body, but all cells have a common need for energy. Ultimately energy sources used by the cells of the body come from nutrients in the food that the organism eats.

Energy is used for basic maintenance - the processes involved in living eg heart muscle beating, breathing, and other internal organ activity, as well as for work - movement and exercise. During certain life stage periods there are additional demands for energy , notably during the growth phase of young animals, and during pregnancy and lactation in females. Energy is also needed to overcome diseases and to repair tissues that have been damaged. Many major organ system diseases waste energy and greatly increase the animals requirement for energy intake - eg heart failure and cancer. Maintaining adequate energy intake can prove difficult if the animal doesn't feel like eating because of the illness, and a combination of increased energy use and decreased food intake results in rapid weight loss - a very important tell-tale sign of serious disease.

Of the six main classes of nutrient found in foods only three provide energy:

• fats (oils) - these are the "best" source of energy providing about 8.5 kcal Metabolisable Energy per gram
• carbohydrates - these provide about 3.5 kcal Metabolisable Energy per gram
• proteins - these provide about 3.5 kcal Metabolisable Energy per gram
• minerals - provide no energy
• vitamins - provide no energy
• water - provides no energy

Metabolisable Energy (ME) is the amount of energy provided by a nutrient after losses in faeces, urine and expired gases are taken into account. It can either be measured accurately (expensive) or approximated from standard formulae (this is usually what food manufacturers do).

Although they don't provide energy themselves vitamins and minerals are also important in the provision of energy at cellular level because they are required for many of the normal energy producing metabolic pathways that occur within the cells in the body.

Extracting energy from food requires several stages :

• Enough food has to be eaten to meet the animals requirements
• The food has to be digested - ie broken down into it's constituent nutrients
• The nutrients have to be absorbed into the body from the lumen of the gastrointestinal tract
• The nutrients may undergo chemical changes to become usable - for example carbohydrates are broken down to glucose which is used by many cells as an energy source.
• The nutrients have to be transported to the tissues - usually via the bloodstream
• The nutrients have to enter the cells and enter one of the metabolic pathways within cells to produce high energy compounds eg adenosine triphosphate (ATP)
• When needed high energy compounds are split to release energy within the cell (eg ATP is converted to ADP-adenosine diphosphate)
• Excess nutrients (fat or carbohydrate) can be stored for future use, as adipose fat or as glycogen in cells

If anything goes wrong in this sequence the cells may be deprived of energy - even though the animal is eating enough food. For example, diseases involving the gastrointestinal tract, the pancreas or the liver may prevent normal digestion and/or absorption of food ingredients from the gastrointestinal tract. This leads to nutritional deficiency, weight loss etc. In some forms of diabetes mellitus associated with resistance to the hormone insulin at tissue level, glucose that is in the bloodstream cannot move into cells (because the local effect of insulin is needed for this) so they are deprived of energy - even though there is plenty of glucose available for them. In this situation tissues such as the brain that rely on  glucose for energy cannot function properly.

If food supply is inadequate, or if an animal is unable or unwilling to eat it can utilise stored energy in tissues. This is not a problem for cells that use fat or carbohydrate for energy because the body has some energy stored as fat or as glycogen in cells. However, this can be a problem if the cells use protein as an energy source because there are no stored protein energy reserves and the animal will breakdown it's own body proteins to release energy. This is often seen in cats that are not eating properly, they will breakdown their own body muscles to produce energy and as a result they lose lean body mass becoming very thin and weak.

When animals are ill one of the first veterinary considerations is how to maintain energy intake in a form that the animal can use efficiently- bearing in mind the effects of the illness on food utilisation.

Excess energy intake results in increased storage (as fat mainly) and obesity.

Owners should weigh their animals periodically and any weight gain or loss should be reported to their veterinarian in case there is an underlying disease problem

.Updated October 2013