Introduction

Improved veterinary care, nutrition and lifestyle have collectively improved the life expectancy of dogs and cats over the last 20 years1 2. Nutrition and appropriate nutritional management have been shown to alleviate, postpone, slow down and even prevent diseases commonly associated to the process of aging, allowing a better quality of life in older pets 3 4 5 6. Veterinary practitioners should thus assess the diets and dietary management of the aging pet and make recommendations based on the latest scientific knowledge.

Many age-related feline diseases (e.g., chronic kidney disease, osteoarthritis, diabetes mellitus, hyperthyroidism) are more frequently observed after ten years of age, and with most of these diseases (and especially in cats) obvious clinical signs occur rather late in the pathophysiological process. Diets for “healthy” senior cats should therefore integrate aspects that will help prevent diseases associated with aging in their formulation. Cats tend to live longer than dogs and it is not uncommon for veterinarians to see cats that are more than 15 years of age. However, as with humans, the genetic and environmental background of each animal means that the chronological and physiological ages do not always match. The nutritional recommendations for the older healthy cat must therefore take into account the specific requirements of the feline species and the impact of aging on physiology and metabolism, and include dietary strategies that could help prevent or slow down disease processes commonly associated with aging (Table 1) 1 7.

Although this paper will focus on the nutritional needs of the healthy aging cat, longevity requires a healthy start from an early age. Both nutritional deficiencies and excesses will significantly affect lifespan, and it is important to maintain optimal body condition in cats for their entire life 9 10. This might be a challenge, especially following neutering, but feeding diets formulated for neutered cats (e.g., less energy-dense) and adjusting the ration for the body condition have clearly been shown to contribute to achieving life-long optimal body condition (Figure 1) 11.

The most precisely formulated diet will only help an aged cat if it is willing to eat it. Special care should thus be taken to ensure maximum palatability by a combination of quality ingredients, and careful selection of aroma, kibble size and the texture of the diet. Note that older cats are at increased risk of suffering from diseases that cause polyuria/polydipsia and are more likely to suffer from lameness, and it is therefore essential to provide them with easy access to fresh water (Figure 2). 

Consequences of aging on dietary requirements

Energy

As with other species, aging in cats is associated with a decrease in lean body mass and an increase in fat mass1 12. Since lean mass is the metabolically active tissue, energy expenditure and energy needs should thus decrease with age, but this is not true in all cats, especially those above 12 years of age. There is evidence that as many as one-third of older cats have a decreased ability to digest food, especially fat and protein 1. This is supported by a lower prevalence of obesity in cats above 10 years of age (Figure 3) 9 13. Therefore not all senior diets should have reduced energy content; energy density and calorie allowance should be adjusted to maintain optimal body condition. As a starting point recommendations are available (Figure 1) 7 9 as follows;

• Adult cat (moderate activity – all cats, whether neutered or entire) = 77,6 x BW ^0,711 KcalME/kg
• Adult intact cat and mature cat (high activity) = 93 x BW ^0,711 KcalME/kg
• Overweight cat = 62 x BW ^0,711 KcalME/kg

As individual and seasonal variability can be high in cats, calorie allowance should then be adapted to the body condition of each patient. Life expectancy is lower in cats that are below or above optimal body condition 10. Poor body condition can be an early indicator of a clinical problem that will not allow compensating episodes of fasting commonly associated with disease. As well as promoting lameness and encouraging skin problems (because the cat does not groom) obesity promotes insulin resistance (Figure 4) 9 and consequently diabetes mellitus is a frequent complication of feline obesity. There are some controversies regarding the use of carbohydrates as an energy source in feline diets; cats have evolved as true carnivores and as such, when compared to dogs or humans, have a lower ability to digest and metabolize carbohydrates 14 15. However, cats are very good at using large amounts of well-cooked carbohydrates in their diet and obesity, rather than high carbohydrate consumption per se, promotes insulin resistance14 15 16 17. In cats that are overweight and/or have diabetes mellitus, reduction of dietary carbohydrates, reduction of energy intake and increased dietary protein can collectively improve both the body condition and rate of remission, and allow reduction in the insulin dose 17.

In addition to being an energy source, fats also provide essential fatty acids 7. There are two families of essential fatty acids, omega-6 (vegetable oil) and omega-3 (soybean, flaxseed and fish oils) and it is important to balance them. Early signs of essential fatty acid deficiencies are a dry, unkempt, dull hair coat. There is good evidence in other species (and some suggestions in cats) that long-chain omega-3 fatty acids (EPA and DHA) from fish oil contribute to a healthy immune system and cognitive function (e.g., learning, memory); they may also reduce inflammatory processes (such as those observed in osteoarthritis and dermatitis), slow the progression of renal disease, and could be beneficial in preventing diabetes mellitus and cancer 6 15 18.

Shorter-chain omega-3 fatty acids (α-linolenic acid) such as found in flaxseed oil, cannot be converted to EPA and DHA in cats (and there is a very poor conversion efficiency in dogs) 7. Gamma-linolenic acid (borage oil, primrose oil), although an omega-6 fatty acid, will also have anti-inflammatory benefits through the prostaglandin-1 pathway.

Protein

In the past, it has been commonly recommended to restrict protein in older cats based on the (false) belief that this would help to protect kidney function. However research shows that protein restriction is unnecessary in older cats, even with early renal disease 18 19, and restriction in dietary phosphorus and supplementation with EPA/DHA have been shown to be the most important factors in slowing the progression of renal disease 18. In dogs, protein requirements have been found to increase with age 1; similar data are missing in cats but because of their true carnivorous nature, dietary protein requirements are much higher in cats of all ages 7. Protein quality must also be taken into account. Poor protein digestibility will promote proteolytic colonic flora (the so-called “bad” flora, Clostridium perfringens etc.), softer feces and fecal odor. Contrary to common belief, some vegetable proteins, such as wheat gluten, corn gluten, soy protein isolate and hydrolysate, are very digestible, and their amino-acid composition is easily balanced by careful choice of ingredients; they are also low in phosphorus, and are therefore the proteins of choice in diets for senior cats.

Dietary fiber

Dietary fibers are essential for the health of the gastrointestinal tract 7. They contribute to the regulation of transit, although excessive amounts (> 25%) of insoluble (non-fermentable) fibers may promote constipation in the aged cat. Dietary fibers will contribute to satiety and reduce energy intake in those cats that tend to be overweight (Figure 5) 20. Fermentable fibers, also called prebiotics, such as beet pulp, chicory pulp, and fructo-oligosaccharides promote a healthy flora and the production of short-chain fatty acids, especially butyrate, an important energy source for the colonocytes. A recent study indicated that a combination of prebiotic fiber, linoleic acid, EPA/DHA and antioxidants could promote longevity in aging cats by better preserving their body weight, lean body mass, red blood cell count and skin thickness 6. Note that the level of dietary fiber will vary considerably depending on the target energy density of the diet (15-90 g/1000 Kcal ME).

Minerals

As phosphorus restriction has been shown to slow the progression of renal disease, and the prevalence of renal disease is very high in older cats, it is often recommended to reduce the level of phosphorus to the minimum recommended levels (1.25-2.0 g/1000 Kcal) in diets for senior cats 1 18. The calcium/phosphorus ratio should be balanced accordingly. It has been suggested that levels of dietary sodium (2.5-3.0 g Na/1000 Kcal) that stimulate diuresis will promote the progression of renal disease 21 but literature published in peer-reviewed journals does not support this claim; of five scientific papers looking at the association between sodium intake and progression of renal disease in cats and dogs, four failed to find any association 21 22 23 24 25and the claim that foods high in NaCl are associated with a progressive decline in renal function 26 has been disputed. Hypokalemia is common in older cats, especially in those with renal disease, and dietary levels of potassium should therefore not be too near the minimum recommendations, especially if the diet is acidifying. Calcium oxalate (CaOx) stones are more common in older cats, but the pathophysiology of CaOx is still poorly understood 27. Urinary pH is a poor predictor of CaOx urine saturation 27 28, but promoting urine dilution by feeding moist food and/or increasing dietary sodium (2.5-3.0 g/1000 Kcal) is an efficient way to reduce CaOx saturation 27.

Vitamins

Oxidative damage plays an important role in many diseases associated with aging such as arthritis, cancers, cardiovascular and neurological diseases. Considerable evidence in humans and animals suggest antioxidants may offer some protection against oxidative stress and normal aging processes 1 6. Studies on antioxidants in cats have reported beneficial effects on markers of oxidative status 29 and it is thus reasonable to recommend increased amounts of antioxidant nutrients. Supplementation with a combination of antioxidants (e.g., Vitamins C and E, taurine, lutein, polyphenols) is preferred, as they act in different areas of the cell and have a synergistic action. As they are necessary for the synthesis of key enzymes and substrates in the antioxidant pathways, it would seem prudent to provide adequate protein and trace-element intakes in diets for aged cats.

Other nutrients

Several non-essential nutrients have been recommended in the diets of aged cats, for example L-carnitine for its role in fat oxidation. Osteoarthritis is very common in older cats, but the clinical signs are poorly recognized by owners 30, and whilst the owner may not seek veterinary advice, dietary supplementation may well be beneficial for the cat – a recent double-blind randomized clinical study showed the efficacy of a combination of green-lipped mussel, glucosamine, chondroitin sulphate plus EPA/DHA on the perceived and objective activity of cats with osteoarthritis 31.

The nutritional recommendations of the aged healthy cat must therefore take into account the specific requirements of the feline species, to promote maintenance of an optimal body condition and adjust and/or provide nutrients as necessary that will help prevent or slow down the progression of diseases associated with aging.