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The Journal of Nutrition Vol. 128 No. 12 December 1998,
pp. 2627S-2631S
Waltham Centre for Pet Nutrition, Waltham-on-the-Wolds, Melton Mowbray, Leicestershire, UK
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ABSTRACT |
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Abstract
Introduction
References
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The drivers of the age-related decline in maintenance energy requirement in humans are primarily reduced physical activity and a decrease in basal metabolic rate, which is largely driven by changes in body composition. Most studies indicate that there is a significant loss of lean body mass and a concomitant increase in fat mass with advancing age. The causal factors appear to be changes in physical activity and a reduction in the activity of growth hormone. Sustained physical activity and/or administration of growth hormone have been shown to offset age-related changes in lean:fat ratios in humans and in rats. Very little information is available on dogs, but current data suggest that aging is accompanied by a decrease in lean:fat ratios. The rate and extent of change is similar to that observed in aging humans and it is assumed that the same causal factors are responsible. On this basis, it is likely that basal metabolic rate declines in older dogs. New evidence suggests that the situation is very different in cats, with no apparent change in lean:fat ratios with advancing age. This is probably related to constant activity levels throughout life and suggests that basal metabolic rate probably does not decrease as cats age. On the basis of this evidence, there is no reason to reduce energy provision to the majority of older cats.
KEY WORDS: cats · dogs · aging · body composition · body weight
There is considerable evidence in humans indicating an age-related decline in maintenance energy requirement (MER)2; the underlying reasons for this decline have been investigated in a number of studies (Fukagawa et al, 1990, Horber et al. 1996 Age-related changes in body composition have been widely reported for both men and women (Flynn et al. 1989
There is very little information on the relationship between age and body composition in dogs of any breed. Meyer and Stadtfeld (1980) There is often an assumption that senior cats have a tendency toward obesity and therefore energy intake should be restricted. Studies that have reported energy intakes across ages in cats have indicated that there is no age-related decline in energy intake (Burger 1994
INTRODUCTION
Abstract
Introduction
References
, Vaughan et al. 1991). It is apparent that a decline in physical activity is the most significant driver, usually accounting for >50% of the reduction in MER; however, the other components of the energy balance equation, basal metabolic rate (BMR) and thermic effect of food (TEF), also play a role (Roberts et al. 1995). Evidence to date suggests that TEF is relatively unimportant, accounting for <10% of the age-related change. Therefore the second biggest driver of the change is a decline in BMR, which has been widely reported (Calloway and Zanni 1980, Fukagawa et al. 1990, Keys et al. 1973). The most significant of the factors that control BMR is body composition; there are now a substantial number of studies demonstrating that there are dramatic age-related changes in body composition in a number of species. This paper reviews some of the studies that have contributed to our understanding of aging and body composition in humans, cats and dogs, and discusses how current knowledge can be used to ensure appropriate energy provision for senior cats and dogs.
HUMANS
, Forbes and Reina 1970, Tzankoff and Norris 1978). There appears to be a gradual reduction in lean body mass (LBM) and a corresponding increase in the proportion of fat mass (FM). Various techniques have been used to assess lean:fat ratios (LFR), but in every case the same age-related changes have been reported. Vaughan et al. (1991) used hydrodensitometry to investigate body composition in young and old female subjects and found that, although the elderly subjects had a significantly lower body weight, there was a highly significant decrease in fat-free mass in the older group, equivalent to a decrease of ~20% (P < 0.001). In a study recently reported by Welle et al. (1996), the body weight, LBM and FM as measured by creatinine clearance, for both young and elderly males and females, all of whom were healthy and normally active, were reported. The results show that in both sexes there was an age-associated decrease in LFR, with a more marked decrease in women. In men, the decrease in LBM was ~10% and in women it was ~7%. The use of dual-energy X-ray absorptiometry (DXA) to evaluate these changes has also been reported and again has demonstrated the same decline in LFR with age (Snead et al. 1993). Although the overall trend is consistent, the quantitative changes are more variable; nevertheless, it appears that between the third and eighth decade, there is a gradual decrease in LBM of ~15% (Evans 1995).
View this table:
Table 1.
Body composition in young and old men1
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Fig 1.
Relationship between age and percentage lean:fat ratios in adult Labrador retrievers.
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Fig 2.
Percentage of cats of different ages assessed as underweight or overweight by their owners. Reproduced with permission from Harper, 1996.
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Fig 3.
Prevalence of overweight cats among cats without serious illness by age. Reproduced with permission from Scarlett et al., 1994 .
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Fig 4.
Relationship between age and percentage lean:fat ratios in adult cats.
conducted a study to investigate whether physical activity alone could prevent an age-related decline in LFR in men. Three groups, a group of young, untrained, relatively inactive men (n = 14; mean age, 31.0 ± 2.1 y), a group of old, untrained men (n = 14; mean age, 68.6 ± 1.2 y) and a third group of old, physically active men who ran at least 30 km/wk (n = 14; mean age, 67.4 ± 1.2 y), were recruited to the study. Body composition was measured using DXA, and BMR was measured using indirect calorimetry. The results are presented in Table 1, and clearly show that the group of old men who regularly took strenuous physical exercise had LFR comparable to that of the young untrained men. One of the interesting findings in this study was that the resting metabolic rate declined relative to body weight with increasing age, independent of physical activity. Although this study indicates that physical exercise alone can prevent an age-related decline in LFR, it would have been useful to have had a fourth group of young physically active men to compare with. It is arguable that the old, trained group may have demonstrated a reduced LFR compared with a young, trained group. Thus, this study confirms that physical activity is a major factor in age-associated changes in LFR, but it does not exclude the possibility that GH may be equally important.
). Forty adult female Sprague-Dawley rats aged 14 mo, were allocated to control, ovine somatotropin administration [0.5 mg/(kg · d)], treadmill exercise (17 m/min for 1 h/d) or both treatments (n = 10 per group). After 4 mo of treatment, the results indicated that exercise could prevent any change in LFR compared with the control. However, the GH administration not only prevented an age-related increase in FM but appeared to increase LBM. Studies in which GH has been administered to aging humans have also indicated that nitrogen retention is improved and LBM increases in men, although LBM increases are less marked in women (Marcus et al. 1993).
). Physical activity, if sustained throughout adulthood, can maintain MER by two routes. First is the cost of energy expenditure due to physical activity per se and second is the elevated BMR that results from maintenance of LBM, directly resulting from physical activity.
DOGS
reviewed data on the body composition of adult dogs (breeds unspecified) and reported that the mean body fat content, as measured by proximate analysis, was typically 15-20% at 1 y but increased continually in successive years to reach 25-30% in dogs aged 8-10 y. In the same study, an age-related decline in total body protein was also reported, which indicates that dogs may exhibit the same age-related change in LFR as humans.
, Harper 1997). To confirm this hypothesis, a study was recently carried out at the Waltham Centre to investigate aging and body composition in adult Labradors. The body composition of 40 healthy Labradors, 5 entire males, 4 neutered males, 14 entire females and 17 neutered females, 2-13 y of age, was assessed by the use of DXA. Duplicate scans were carried out using a Hologic QDR 1000/W densitometer (Hologic, Waltham, MA); the mean values were used to compare LBM and FM in the dogs. The results indicated that the percentage of FM showed a positive linear correlation with age (P < 0.001, r2 = 0.50), and there was a corresponding negative linear correlation between age and percentage of LBM (P < 0.001, r2 = 0.52). Together these changes resulted in a highly significant negative linear correlation between age and LFR (r2 = 0.41, Fig. 1). Although this is a preliminary study and represents only one breed of dog, it is notable that exactly the same age-related decline in LFR was observed as has been reported in studies with humans and in the study reported by Meyer and Stadtfeld (1980). At this stage, it is not possible to explain the drivers of this observation; it is likely, however, that the same factors, i.e., lower physical activity and lower GH activity, are responsible. Certainly the older Labradors that participated in this study were far less active than the younger dogs, and it is reasonable to suppose that the reduced activity patterns of the older animals constituted a significant factor in the reduced LFR. The decline in LFR affects BMR, as discussed in the section on body composition in aging humans; this may be one of the underlying explanations for the observations that have been reported on age-related reductions in MER in dogs of many different breeds (Finke 1991, Kienzle and Rainbird 1991, Taylor et al. 1995). Given that sustained physical activity can, to some extent, offset age-related reductions in BMR, and thus MER, it is likely that dogs could maintain MER as long as they remained active. However, there is no doubt that many dogs choose to become less active with advancing age, and the probability of persuading them to behave otherwise is low (MacDougall and Barker 1984). One possible explanation for decreased exercise in older dogs is progressive sarcopenia, which may result in less physical strength and stamina. In practical terms, this probably means that many senior dogs are at risk of becoming overweight because TEE may decline without a concomitant decline in energy provision. This is borne out by a reported study on the prevalence of obesity in dogs in relation to age (Mason 1970). This survey of 1000 dogs at a veterinary clinic in the UK found that the prevalence of obesity in dogs <4y old was 16.5%, but was 35.5% in dogs aged 5-11 y and 40.5% in dogs >12 y old. A recent assessment of body condition scores of 3729 dogs seen at the Veterinary Hospital of the University of Pennsylvania confirms that aging in dogs is frequently associated with increased risk of obesity (Kronfeld et al. 1991). In the age range 7-9 y, 26.5% of the dogs were categorized as overweight, although it should be noted that 15.5% of senior dogs (>12 y) were considered to be underweight. Similar observations were noted by Armstrong and Lund (1996); in a study of more than 23,000 dogs in the U.S., they found that the proportion of overweight dogs peaked at around 9 y. Approximately 10% of dogs aged 12-14 y were categorized as thin; clearly such animals should be offered an appropriate ration to avoid risk of weight loss.
CATS
, Taylor et al. 1995). It therefore follows that if MER does decline, as is the case in most humans and dogs, there should be a large proportion of obese, old cats in the population. There are, however, few data to support this view; if we examine the evidence relating to body weight and age in cats, there are indications that, although obese cats do represent a proportion of the senior cat population, they tend to represent the minority (Scarlett et al. 1994). A recent survey carried out in the UK on 3108 adult cats (>1 y of age) showed that >70% of owners categorized the body weight of their senior cats as just right.3 The percentage of cats that were underweight increased with age, whereas the percentage of cats that were overweight decreased with age (Fig. 2). This is in line with our own findings at the Waltham Centre, where a comparison of the body weights of 191 healthy adult cats (72 neutered males, 52 neutered females and 67 entire females) ranging in age from 1 to 13 y showed no age-related trend in body weight. Generally the heaviest cats were neutered males and were in the age group broadly categorized as middle-aged (5-8 y). The main trend was a tendency for the cats 
11 y of age to exhibit lower body weights than the young adults. (Note that the distribution of males and females in this study was as indicated in Figure 4 and that the results were not biased by gender differences.) These findings parallel those reported by Scarlett et al. (1994). In a survey of >2000 cats presented in 31 veterinary hospitals in the Northeastern U.S., owners and veterinarians were asked to assess the body condition score of their cats. The results are presented in Figure 3 and indicate that most cats recognized as overweight were middle-aged (4-10 y). The proportion of cats recognized as overweight tended to increase until ~7 y of age after which it tended to decline, particularly in cats >10 y of age. It should be noted that owners may consider their pets to be "just right" when, in fact, they are overweight; thus the additional evaluation by veterinarians lends support to these data. Similar data have been reported by Armstrong and Lund (1996) who found that the proportion of overweight cats peaked at 7 y, whereas the prevalence of an underweight body condition was seen to rise sharply at 11 y.
followed up on the cats that had participated in the 1994 study and discovered that mortality was highest for those cats categorized as either cachectic or obese in middle age. The risk factors for early death in the middle-aged obese cats were not clear but, as would be expected, the risk of diseases including diabetes mellitus and hepatic lipidosis was elevated in these animals. These results suggest that the prevalence of "normal" and thin, old cats is driven at least in part by early death in the middle-aged obese cats. A second study carried out at the Waltham Centre (unpublished results) has provided a further explanation for the prevalence of old thin cats. In this study, the body weight records for 53 healthy old (>11 y) cats were traced back to birth for each animal. Every cat had participated in a range of studies and had therefore received a variety of diets in its lifetime, but weekly body weight records were available for every individual. A retrospective examination of the records indicated that significant body weight gain (>10%) was most common between the ages of 2 and 5 y. During this period the majority of cats (100% of males, 67% of females) had a significant and sustained increase in body weight. However, during middle age (5-8 y) only ~50% of the cats exhibited a significant increase in body weight; 35% maintained weight and 15% lost >10% body weight. From 8 y onward, 20% of the cats gained weight, 50% of the cats maintained weight and 30% lost weight. Thus, these longitudinal data indicate that, for many cats, weight loss or weight maintenance and not weight gain is a typical feature of old age. There are, of course, some old, obese cats but the longitudinal data indicate that cats that are old and obese have probably been obese all their lives. In answer to the question of why there are few obese, old cats it would appear that a significant proportion of the middle-aged obese cats die, and an equally significant proportion actually lose weight as they approach old age.
).There was no apparent relationship between increasing age and body composition in adult cats. To investigate this further, a Waltham Centre study assessed the body composition of 191 healthy adult cats (72 neutered males, 52 neutered females and 67 entire females) ranging in age from 1 to 13 y by using DXA. Duplicate scans were carried out using a Hologic QDR 1000/W densitometer (Hologic) and the mean values were used to compare LBM and FM in the cats. The results of the body composition measurements on this population of cats indicated no age-related change in either LBM or FM, and consequently no alteration in LFR (Fig. 4). Thus, the hypothesis that cats are relatively inactive throughout their adult lives, which means that LFR and BMR remain constant and there is no age-related decline in MER, is supported by all currently available evidence.
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FOOTNOTES |
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LITERATURE CITED |
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Abstract
Introduction
References
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