The Journal of Nutrition Vol. 128 No. 12 December 1998, pp. 2623S-2626S

Changing Perspectives on Aging and Energy Requirements: Aging and Energy Intakes in Humans, Dogs and Cats¹

E. Jean Harper

Waltham Centre for Pet Nutrition, Waltham-on-the-Wolds, Melton Mowbray, Leicestershire, UK

	ABSTRACT

Abstract Introduction References

A series of cross-sectional and longitudinal studies conducted in humans has shown that aging is associated with a gradual decline in the maintenance energy requirement. Generally, this is equivalent to a total decrement of 20% of young adult maintenance energy requirements and is a result of a decrease in both physical activity and basal metabolic rate. Relatively few such studies have been conducted in dogs, but the results have been consistent. It appears that maintenance energy requirements decline by ~20%, and it is assumed that the causal factors are the same as those for humans. The situation appears to be somewhat different in cats, with evidence to date indicating that maintenance energy requirements remain constant throughout adult life. Why cats should be different from other species is not clear, but it is hypothesized that relative inactivity is typical of most cats' behavior, such that there is no obvious age-related change. In terms of feeding regimens for senior cats and dogs, it is appropriate to decrease energy provision for senior dogs by ~20%, whereas the energy provision for senior cats should not be decreased.

	INTRODUCTION

Abstract Introduction References

One of the most intriguing areas of research in cat and dog nutrition is the effect of aging on nutritional requirements. Improved veterinary care, nutrition and husbandry have collectively delivered an increase in the life expectancy of companion animals, and understanding of normal age-related physiologic changes in both cats and dogs is gradually becoming apparent. However, what of the most fundamental nutritional question, the relationship between age and energy requirements? There is a considerable amount of literature on the relationship between age and maintenance energy requirement (MER)² in adult humans, with a number of studies reported in the last four decades (Keys et al. 1973, McGandy et al. 1966, Sawaya et al. 1996, Vaughan et al. 1991). MER includes resting energy expenditure (REE) together with the thermic effect of food (TEF) and normal activity. It is only in recent years that equivalent publications on cats and dogs have begun to appear in the literature and add to our knowledge base. In this paper, the key findings from the human literature are reviewed along with the studies on cats and dogs that report information relating to age and MER. This paper does not set out to explain the findings; the second paper in this series (Harper 1998) reviews the possible mechanisms responsible for the observations reported to date.

View larger version (12K): [in this window] [in a new window]   Fig 1. Maintenance energy intakes for senior dogs based on predicted metabolizable energy (PME) intakes. Reproduced with permission from Harper, 1997.

View larger version (21K): [in this window] [in a new window]   Fig 2. Relationship between age and daily metabolizable energy (ME) intake in Border collies.

View larger version (18K): [in this window] [in a new window]   Fig 3. Daily digestible energy (DE) intakes in cats of different ages. Reproduced with permission from The Nutrition Society, Taylor et al. 1995.

	HUMANS

One of the first studies to investigate the relationship between age and MER in humans was the Baltimore Longitudinal Study of Aging (McGandy et al. 1966). In that study, the food intake of 252 male subjects 23 -99 y of age, all of comparable health, educational and economic status, was assessed. Typical energy intakes were 11.24 MJ/d for the men aged 23-34 y and 8.75 MJ/d for the men aged 75-99 y. This average age-related difference of 2.5 MJ/d was driven by both a lower basal metabolic rate (BMR), which constituted about one third of the difference, but more significantly by reduced physical activity, which accounted for the remaining two thirds (1.6 MJ/d). A series of follow-up studies over a 15-y period with these men confirmed that, as expected, the decline in energy intake was gradual and was equivalent to a reduction in daily MER of 52 kJ/y of age. A subsequent longitudinal study has suggested that the decline may be somewhat greater at 73 kJ/(d · y) for females and 94 kJ/(d · y) for males (Garry et al. 1989). Although the Baltimore study provided valuable data, there were a number of confounding factors that raised questions about the interpretation of the results. Not least of these was the fact that the youngest men tended to be taller and heavier and therefore possibly had a higher MER because of their physical stature rather than their age per se. Subsequent studies have attempted to correct for such factors and have also employed a series of techniques to evaluate both MER and total energy expenditure (TEE) to provide more robust data.

With the use of a respiratory chamber, Vaughan et al. (1991) measured the 24-h energy expenditure (24EE) of a group of young (33 male; 31 female; mean age, 24.0 ± 3.6 y) and elderly (17 male; 21 female; mean age, 71.2 ± 6.1 y) subjects. The results indicated a 24EE of 9.01 ± 1.47 MJ for the young group and a 24EE of 7.81 ± 1.19 MJ for the elderly subjects. This difference (~13%) is somewhat less than has been observed in other studies (Keys et al. 1973, McGandy et al. 1966) but supports the hypothesis that MER declines significantly with increasing age. The TEE comprises a number of components, namely, REE, TEF and energy expenditure due to physical activity. The design of this study allowed an evaluation of the contribution of both BMR and the TEF on TEE, and found that there was no age-related change in the TEF.

Reilly et al. (1993) evaluated the MER of 11 healthy elderly women (mean age, 73 ± 3 y) utilizing the doubly labeled water technique to measure TEE, and indirect calorimetry to measure BMR.The women recruited to this study were all active and were therefore not considered to be generally representative of the elderly in their community. The mean TEE was 9.21 ± 1.48 MJ/d which was considerably higher than anticipated, and almost certainly reflects the elevated physical activity levels. What is interesting about this particular study is that the TEE measured was not different from that of young healthy women, and thus MER did not decline as has been observed in other studies. These results imply that an age-related decline in MER is not an inevitable consequence of aging but is a result of reduced activity levels in later life. Furthermore, if activity levels are sustained, BMR, the most significant driver of TEE after activity, may be increased. One criticism of this study is that the participants may have been very physically active as young adults, and that their TEE measured at 9.21 MJ/d was indeed lower than it had been previously. Unfortunately cross-sectional studies always lend themselves to this type of criticism, which is why the longitudinal studies have proved to be particularly valuable (Garry et al. 1989, McGandy et al. 1996).

Roberts et al. (1995) compared measured metabolizable energy (ME) intake in two groups of healthy men, young (n = 17, mean age, 22.7 ± 0.6 y) and old (n = 18, mean age, 68.0 ± 1.5 y). TEE was determined using the doubly labeled water technique over a 10-d period in which the subjects were provided with all of their food. The values for ME intakes were calculated by analysis of the gross energy (GE) content of aliquots of food and by using standard assumptions of energy losses of urinary nitrogen and stools. The mean measured ME intake was 14.48 ± 0.65 MJ/d for the group of young men and 11.26 ± 0.54 MJ/d for the older group. The corresponding TEE was 14.07 ± 0.54 and 11.05 ± 0.39 MJ/d for the young and old groups, respectively. When the TEE of the two groups in Roberts' study was broken down into these components, differences in REE represented 36% of the decrease in TEE, whereas reduced physical activity represented 54% of the observed difference. These results represent a difference of ~22% for MER in young and old men with ~12% of this accounted for by age-related differences in physical activity. These results suggest that, despite the criticisms of the Baltimore Study, the conclusions reached were subsequently confirmed in that MER of adult males was again shown to differ by ~22% over the course of a lifetime, driven predominantly by differences in levels of physical activity.

Sawaya et al. (1996) utilized food intake assessment to measure daily energy intake and the doubly labeled water technique to measure TEE in two groups of women. Data from 24-h recalls indicated that the younger group (n = 10; mean age, 25.2 ± 1.1 y) had a mean energy intake of 8.51 ± 2.01 MJ/d compared with 5.66 ± 1.36 MJ/d for the older women (n = 10; mean age, 74.0 ± 4.4). The TEE was calculated to be 9.82 ± 1.74 and 7.52 ± 0.88 MJ/d for the two groups, respectively. Despite the slight discrepancy in the data obtained from the two methods, in both cases, a significant age-related difference in MER was observed. The doubly labeled water technique provided results that correspond closely with those found in men, i.e., an age-related decline in MER of ~22%. However, the 24-h recall data suggested a much greater decrement, of ~33%, although the validity of this is disputed by the authors. It is believed that poor recall gave an underestimation of food intake in the older group of women.

From the few studies reported, it appears that both measured energy intake and TEE decline steadily as adults age and that this is largely driven by changes in physical activity, and, to a lesser extent, by changes in BMR. There appears to be little age-related change in TEF. As would be expected from studies using different techniques and different populations, the results are not always consistent but it seems that, typically, a decline in TEE of ~20% is seen in adults over the course of a lifetime. If, however, activity levels are sustained throughout adulthood, there are some indications that TEE may also be sustained. It should be noted, however, that attempts to increase TEE in elderly subjects by subjecting them to an exercise program have failed (Poehlman 1992). Although training of 11elderly volunteers resulted in a 9% increase in VO_{2 max}, there was no measurable change in TEE because of a compensatory decline in physical activity during the remainder of the day. This poses the question of whether an age-related decline in TEE is a normal consequence of aging, one controlled by physiologic factors rather than psychological or social drivers.

	DOGS

Relatively few studies have been conducted on the relationship between aging and energy requirement in dogs; of those that have been done, the simplest technique (i.e., measurement of food intake) has been the only reported method. Nonetheless, these studies have provided some interesting data that indicate some remarkable parallels compared with the results from studies on humans. Finke (1991) studied a group of kennel dogs comprising Beagles (n = 6), Labrador Retrievers (n = 9) and Siberian Huskies (n = 6), aged between 1 and 10 y. Daily food intakes were recorded over a 54-wk period and ME intake thus calculated. The ME content of the diet had previously been estimated using a group of Beagles on a full balance study. The results of the study indicated an age-related decline in MER that was equivalent to ~24% and was best represented by the following equation: where BW is the body weight of the dogs. Kienzle and Rainbird (1991) reported a study in which the digestible energy (DE) intakes of young (1-2 y), middle-aged (3-7 y) and old (>7 y) adult Beagles and Labradors were compared. Between the ages of 2 and 7 y, there was a decline of ~2.5% in DE intake (kJ/kg BW^0.75); dogs >7 y had a DE intake ~19% lower than that of the youngest group.

Taylor et al. (1995) reported a study carried out with 20 adult dogs (9 neutered females, 11 neutered males) comprising Cairn Terriers, Labrador Retrievers, Dachshunds and West Highland White Terriers. Dogs were in either the young age group (<6 y, n 0) or the senior group (>8 y, n = 10 y). All dogs were maintained on a canned diet for 16 wk; the diet was offered according to body weight and the ration adjusted to ensure that body weight was maintained within 2%. Overall, the ME requirement for body weight maintenance of the older group was significantly lower than that in the young group (P < 0.05). On average, the senior dogs required ~90 kJ/(kg BW^0.75 · d) less than the young group, which is equivalent to an 18% reduction in daily energy intake.

A more recent study conducted at the WALTHAM Centre for Pet Nutrition measured ME intakes of 32 senior (>7 y) dogs (Harper 1997). Data were obtained from Cairn Terriers, Dachshunds, Beagles, Labradors, West Highland White Terriers and Newfoundlands, all of which had maintained body weight on a known energy allowance for at least a 6-wk period. All of the dogs were maintained under similar environmental conditions including activity levels, and the data were all collected during the same week. The data were used to plot a power equation to determine daily ME requirement (r2 = 76.3, Fig. 1). The equation that best describes this regression analysis is 380BW^0.75 (kJ). This equation seems to be the best predictor for the majority of senior dogs although clearly some animals may differ from this, depending on their individual characteristics. Thus, in comparison to the MER of moderately active adult dogs, which can be expressed as 460BW^0.75 (kJ), there is an approximate reduction of 18% for the senior animals (Burger 1995).

Despite the relatively limited number of reported studies on aging and energy requirements in dogs, it is apparent that, in those breeds studied, the MER declines by ~20%. This is consistent with the observations made in aging humans and is all the more remarkable given the tremendous breed differences in conformation that dogs exhibit. It can only be assumed that TEE also declines as dogs age, because there have been no reported studies in which TEE has been measured in relation to age. However, it is characteristic of the majority of senior dogs that they appear to sleep more, prefer shorter walks and are generally less active, all of which are consistent with a decline in TEE (Sheffy and Williams 1981). It would be worthwhile to investigate whether dogs that remain active throughout their lifetimes maintain a steady TEE and, hence, MER. Some preliminary data from a study carried out at the Waltham Center compared daily ME intakes in working Border Collies (n = 19; age range, 1-12 y) and pet Border Collies (n = 18; age range, 1-12 y). The results are presented in Figure 2, and indicate that the working dogs had consistent ME intakes across all ages; these ranged from 450 to 850 kJ/(kg BW · d), whereas the pet dogs displayed an age-related decline in ME intake similar to that reported by Finke (1991). These data therefore support the hypothesis that dogs that remain active throughout their lifetimes may not display an age-related decline in MER. Of course, breed characteristics may influence this to some extent in that it may be relatively straightforwad to maintain activity levels in a Border Collie, whereas other breeds may prefer to become less active with age.

	CATS

There have been few studies on MER and aging in dogs, but even fewer have been reported on MER and aging in cats. Anantharaman-Barr et al. (1991) presented the results of a digestibility study with young (n = 7; age 1 y) , middle-aged (n = 8; age 3-5 y) and old (n = 7; age >10 y) cats. Although the focus of the presentation was the digestibility data, the mean food intakes for each group of cats were also presented. The typical intakes were 328, 343 and 379 g/(cat · d), for the three age groups, respectively, although it was not reported whether these were significantly different. Subsequently, Burger (1994) reported measurements of the DE intakes of 203 adult British domestic short-haired cats aged between 1 and 11 y. Individual data had been collected for 3 wk in cats allowed ad libitum access to a range of dry and canned foods and, over this relatively short time period, indicated no evidence of an age-related change in daily DE intake. The DE intakes ranged from 200 to 500 kJ/(kg BW · d) across all age groups. Following this, a study was published in which the DE intake of 56 adult British domestic short-haired cats (20 neutered males, 36 entire females, age range 18 mo-13 y) were measured over a 9-wk period (Taylor et al. 1995). When DE intakes were measured and compared by using simple linear regression, there was no significant age-related effect (P < 0.05), Figure 3. This is in contrast to the decline that is so characteristic of humans and dogs. To date, these studies are the only ones that have attempted to evaluate the effect of age on ME intakes in cats but, given the population size in each case, it is unlikely that they misrepresent the situation. It is fascinating to speculate the reasons why humans and dogs do consistently display an age-related decline of ~20% in MER, whereas cats apparently exhibit no age-related change at all. The studies with humans indicate that the single biggest driver of the decrement is reduced physical activity with age; the evidence available indicates that the same driver may be responsible for the measured changes in dogs. Given this information, it may therefore be apparent why cats are different. The behavior of cats is such that throughout their lifetimes they are relatively inactive; compared with most other mammalian species, they spend a considerable proportion of their day sleeping (Panaman 1981). There are no data to support the statement that aging in cats is not accompanied by a significant alteration in activity patterns, but it cannot be disputed that, on the basis of activity levels alone, it is extremely difficult to distinguish a young adult cat from an old adult cat. This is in sharp contrast to other species, particularly dogs, which, in most cases, do display markedly reduced activity patterns with age. When the contrasting behaviors of the two species are compared, it is a logical step to suggest that one might anticipate age-related changes in MER in dogs but not in cats; this has certainly been borne out by the studies reported to date. This intriguing finding challenges many assumptions about the dietary requirements of senior cats, not least that energy requirements are reduced (Lewis et al. 1987). It could be that the assumption that older cats have a reduced MER is unfounded and, in the context of our understanding of cat behavior and the drivers of age-related changes in MER, quite inappropriate. The evidence currently suggests that, as the majority of dogs age, their MER gradually declines such that elderly dogs require ~20% less energy compared with younger dogs, whereas the energy requirement of adult cats remains relatively constant throughout life.

	FOOTNOTES

	LITERATURE CITED

Abstract Introduction References

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