For any individual, energy requirements may stay the same, increase or decrease with advancing age. There are few studies looking at the energy needs of large numbers of old cats and dogs.

With advancing age a fall in basal metabolic rate has been recorded in humans and experimental animals. This is thought to be due to a change in the ratio between lean body mass and fat, there being an increasing tendency to lay down body fat with advancing age. There are several possible explanations for this trend:

(1) reduced thyroid hormone activity (secretion or receptor response)
(2) other hormonal effects, e.g. sex hormones, catecholamines.

Similar effects may also be seen in dogs and cats. Certainly there is an increased incidence of obesity in dogs with increasing age (Edney & Smith, 1986).

Energy requirements may also be reduced if an individual is doing less exercise due to changed behavioural patterns or secondary to other problems, e.g. an orthopaedic problem such as degenerative joint disease or osteoarthritis.

Older animals with reduced energy requirements should have their energy intake reduced otherwise obesity may result. Regular weighing of older animals should be recommended to detect any trend towards weight gain.

Obesity should be regarded as a serious problem in older cats and dogs. Obese animals have reduced glucose tolerance and hyperinsulinaemia (Mattheeuws et al. 1984a; Mattheeuws et al. 1984b) even in the absence of frank evidence of diabetes mellitus. Gross obesity can significantly reduce cardiovascular and respiratory function and also exacerbates numerous other problems such as skin disorders, and orthopaedic problems. Obesity in cats is a risk factor for the development of hepatic lipidosis and in cats and dogs it is a risk factor for the development of diabetes mellitus.

Most major organ system diseases seen in older animals (e.g. cardiac disease, renal disease, hepatic disease and neoplasia) result in catabolism and weight loss. This is particularly important in cats which, because of their high protein-calorie requirement rapidly break down their own body muscle and other available proteins in the presence of inadequate protein intake or excessive energy utilisation. Almost all chronic diseases in the cat result in significant weight loss or even cachexia.

When energy intake does not meet requirements additional energy should be provided and the selection of energy source (fat, protein or carbohydrates) will depend upon the underlying clinical status of the animal. In cats protein is a major provider of energy because of their obligate carnivorous nature, however fat provides 2.25 times more energy than either protein or carbohydrate and so this will often be the high energy source of choice for both cats and dogs. Carbohydrate will be used when high protein or fat intake is contraindicated in the individual because of the presence of impaired organ function or disease (e.g. renal failure, hyperlipidaemia).