As the body ages, there is a gradual decrease in the mass and activity of muscular, nervous and other tissues. The accompanying deterioration in the functions of many of the vital organs may require adjustments to the diet, to accommodate the general slowing down of activity and the development of age-related diseases.

Studies of geriatric dogs have shown that the majority retain their ability to digest the essential nutrients (protein, fat, ash and energy) from food well into old age.^{5, 18, 19} However age-related changes in the requirements for energy and other nutrients may mean that modifications to the diet are beneficial for some older dogs.

Human studies have shown that the energy expenditure of the elderly is reduced for two main reasons. Firstly, their level of activity falls, although this varies between individuals. Secondly, there is a reduction in lean body mass and a subsequent decrease in metabolic rate. Therefore it is not unreasonable to assume that older dogs should be fed a lower energy requirement than the younger adult in order to reduce the risk of obesity. However in one study of 12 year-old dogs presented at a veterinary hospital 16% of dogs were underweight while only 5% were overweight.⁹

The average daily energy intake of dogs of various breeds tends to decline with increasing age, mirroring a fall in energy expenditure. ^7,8 A less energy-dense diet may be helpful for many older dogs, to ensure adequate intake of vitamins and minerals, which would otherwise fall with the declining food consumption. The smaller minority of obese older dogs require a diet with significant calorie restriction, such as PEDIGREE® PAL® Professional Formula Weight Control Diet.The statistics suggest that some older dogs would benefit from a more energy dense diet such as PEDIGREE PAL Professional Formula Conditioning Diet, while many older dogs will thrive on complete and balanced foods with an energy content designed for normal adult maintenance.

The quality and quantity of protein in the diet has received much attention in feeding of geriatric dogs. The protein requirements of elderly dogs need to be considered from two aspects. On one hand, older dogs may have a higher protein requirement than younger adult dogs. One study has shown that the optimal intake of dietary protein was greater for 12-13 year-old Beagles than for 9-12 month old Beagles, with repletion of tissue protein requiring 19% metabolisable energy from casein in the older dogs compared with 13% in the younger dogs.²⁰ On the other hand, excess dietary protein increases stress on renal function.

Renal disease is a common cause of illness and death in older animals, with chronic renal failure affecting 10% of all dogs over 15 years of age.¹⁶ The causes of progressive renal disease in dogs are as yet unknown.

Experiments in rats have shown that renal haemodynamics may be influenced by dietary protein intake. In rats, high protein intake is associated with glomerular capillary hypertension and hyperfiltration and promotes glomerular sclerosis.³ Restricting dietary protein intake reduces glomerular hyperfiltration in rats by a complex process involving glucagon, growth hormone, renal prostaglandins, the renin-angiotensin system, catecholamines and glomerulopressin, a hepatic-derived renal vasodilator. ¹¹

Morris (1960) ¹⁰ proposed that older dogs would benefit from lower protein diets, based on experimental studies in rats and the high incidence of kidney lesions in older dogs. The assumption was that low-protein diets retarded the progression of renal degeneration. This assumption was disproved, using partially nephrectomised dogs, which showed no uraemic signs and had reduced but stable renal function for 48 months.¹ These dogs did better on moderate-protein diets than on low-protein diets. There is no direct evidence that high protein intake damages canine kidneys or that reducing protein intake in dogs with renal dysfunction results in preservation of either renal structure or function. ^{1, 6, 12, 13, 17}

However there could be some side effects of high protein intake in dogs. These include ingestion of high levels of dietary phosphorus, the generation of metabolic acidosis and the imposition of an exidant or hypermetabolic stress on renal tubular cells. Dietary protein and phosphorus restriction are not synonymous, and within limits each can be separately controlled in a diet. (see Table 1 for the protein and phosphorus content of some PEDIGREE PAL diets)

Until the further evidence regarding the role of protein in progressive renal disease in dogs emerges, dietary protein restriction in dogs that are either non-azotaemic or mildly azotaemic cannot be based upon scientific rationale.^2,4 The current WALTHAM® recommendation is to feed older dogs moderate levels of protein of high biological value and high precaecal digestibility to reduce the formation of bacterial metabolites.

Because of the disadvantages associated with low-protein diets, such as risk of amino acid deficiency or hypoalbuminaemia ^14,15 it is recommended that a conservative approach is used when deciding on the critical point for their introduction in older dogs with chronic renal disease. This should be based on a complete assessment of the patient, including clinical status, degree of azotaemia and hyperphosphataemia and glomerular filtration rate.

The digestibility and absorption of dietary fat is not thought to be affected by age, however fatty acid metabolism may be less efficient in older dogs, so some older dogs may benefit from increased levels of polyunsaturated fatty acids in their diet.

Very little is known about the mineral requirements of old, healthy animals. It has been suggested that the intake of certain minerals, such as sodium and phosphorus, should be reduced due to their association with diseases of the older dog, such as those of cardiac and renal origin. However, there is little information to support the view that substantial reductions in intake are needed in the healthy individual.

It has been proposed that the minimum levels of some of the trace elements (copper, zinc, manganese and selenium) in the diet should be increased in older dogs. These minerals are involved in some of the free-radical scavenging enzyme systems and there is limited evidence to suggest that absorption may decline with age.

Finally, although there have been few studies on the vitamin requirements of elderly dogs, the particular importance of B vitamins and vitamin E has been cited in the scientific literature on various occasions. These vitamins are potent anit-oxidants, and are involved in detoxification processes that are believed to be increased in elderly animals. Also there is some evidence that excretion of intrinsic factor may decline with age, thereby decreasing the absorption of B vitamins.

In summary, no diet is ideal for all older dogs. Old dogs which are apparently healthy should have diets based on their individual needs, which will be related to their bodyweight, condition and physical activity. Dogs with age-related medical conditions may require special diets designed for the dietary management of their particular condition.

• The majority of older dogs retain their ability to digest the essential nutrients from food well into old age.
  
• Some older dogs require an adjustment to the energy density of their diet to compensate for changes in food intake or body condition.
  
• Moderate levels of high quality, highly digestible protein are indicated for healthy older dogs.
  
• Increased dietary levels of polyunsaturated fatty acids, some trace elements and vitamins B and E may be beneficial for senior dogs.
  

1. Bovée, K.C., Kronfeld, D.S., Ramberg, C.F. and Goldschmidt, M. (1979). Long term measurement of renal function in partially nephrectomized dogs fed 56, 27 or 19% protein. Invest Nephrol, 16, 378-384.
   
2. Bovée, K.C. (1991). Influence of dietary protein on renal function in dogs. J Nutr, 120, S128-S139.
   
3. Brazy, P.C. Stead, W.W. and Fitzwilliam, J.F. (1989). Progression of renal insufficiency: role of blood pressure, Kidney Int, 35, 670-674.
   
4. Brown, S. A., (1994). Canine Renal Disease, In The WALTHAM Book of Clinical Nutrition of the Dog and Cat, (ed. J.M. Wills and K.W. Simpson), pp.313-334. Pergamon, Oxford.
   
5. Buffington, C.A., Branam, J. E. and Dunn, G.C. (1989). Lack of effect of age on digestibility of protein, fat and dry matter in Beagle dogs, In Nutrition of the Dog and Cat, (ed. Burger, I.H. and Rivers, J.P.W.), pp.397. Cambridge University Press, Cambridge.
   
6. Finco, D.R., Brown, S.A. and Crowell, W.A. et al. (1992). Effects of dietary phosphorus and protein in dogs with chronic renal failure, Amer J Vet Res, 53, 2264-2271.
   
7. Finke, M.D. (1991). Evaluation of the energy requirements of adult kennel dogs, J Nutr, 121, S22-S28.
   
8. Kienzle, E. and Rainbird, A. (1991). Maintenance energy requirements of dogs: what is the correct value for the calculation of metabolic body weight in dogs?, J Nutr, 121, S39-S40.
   
9. Kronfeld, D.S., Donoghue, S. and Glickman, L.T. (1991). Body condition and energy intakes of dogs in a referral teaching hospital, J Nutr, 121, S157-S158.
   
10. Morris, M.L. (1960). Nutrition and Diet in Small Animal Medicine. Mark Morris Associates, Denver, Colorado.
    
11. O'Donnel, M.P., Kasiske, B.L., and Keane, W.F. (1987). Nonhemodynamic factors contribute to accelerated glomerular injury in nephrectomized rats fed a high protein diet, Kidney Int, 31, 390.
    
12. Polzin, D.J., Osborne, C.A. and Hayden, D.W. et al. (1983). Influence of reduced protein diets on morbidity, mortality and renal function in dogs with induced chronic renal failure, Amer J Vet Res, 45, 506-517.
    
13. Polzin, D.J., Leininger, J. R. and Osborne, C.A. et al. (1988). Development of renal lesions in dogs after 11/12 reduction in renal mass, Lab Invest, 58, 172-183.
    
14. Polzin, D.J., Osborne, C.A., Larry, G. and Adams (1991). Effect of modified protein diets in dogs and cats with chronic renal failure, J Nutr, 121, S140-S144.
    
15. Polzin, D.J., Osborne, C.A. and Lulich, J.P. (1991). Effects of dietary protein/phosphate restriction in normal dogs and dogs with chronic renal failure, J Sm Anim Pract, 32, 289-295.
    
16. Polzin, D.J., Osborne, C.A., Adams, L.G. and Lulich, J.F. (1992). Medical management of feline chronic renal failure, In Current Veterinary Therapy XI. (ed. R.W. Kirk), pp. 848-853. W.B. Saunders, Philadelphia.
    
17. Robertson, J. L., Goldschmidt, M.S. and Kronfeld, D.S. et al. (1986). Long-term responses to high dietary protein intake in dogs with 75% nephrectomy, Kidney Int, 29, 511-519.
    
18. Sheffy, B.E., Williams, A.J., Zimmer, J.F. and Ryan, G.D. (1985). Nutrition and metabolism of the geriatric dog, Cornell Vet, 75, 324-347.
    
19. Taylor, E.J., Adams, C.A. and Neville, R. (1985). Some nutritional aspects of ageing in dogs and cats, Proc. Nutrition Society, 54, 654-656.
    
20. Wannemacher, R.W. and McCoy, J. R. (1966). Determination of optimal dietary protein requirements of young and old dogs, J Nutr, 88, 66-74.