The idea that aging should be linked to energy expenditure has a long history that can be traced to the late 1800s and the industrial revolution. Machines that are run fast wear out more quickly, so the notion was born that humans and animals might experience similar fates: the faster they live (expressed as greater energy expenditure), the sooner they die. Evidence supporting the "rate-of-living" theory was gleaned from the scaling of resting metabolism and life span as functions of body mass. The product of these factors yields a mass-invariant term, equivalent to the "amount of living." There are at least four problems with this evidence, which are summarized and reviewed in this communication: 1) life span is a poor measure of aging, 2) resting metabolism is a poor measure of energy expenditure, 3) the effects are confounded by body mass and 4) the comparisons made are not phylogenetically independent. We demonstrate that there is a poor association between resting metabolic rate (RMR) and daily energy expenditure (DEE) measured using the doubly labeled water (DLW) method at the level of species. Nevertheless, the scaling relation between DEE and body mass still has the same scaling exponent as the RMR and body mass relationship. Thus, if we use DEE rather than RMR in the analysis, the rate-of-living ideas are still supported. Data for 13 species of small mammal were obtained, where energy demands by DLW and longevity were reliably known. In these species, there was a strong negative relationship between residual longevity and residual DEE, both with the effects of body mass removed (r(2) = 0.763, F = 32.1, P < 0.001). Hence, the association of energy demands and life span is not attributed to the confounding effects of body size. We subjected these latter data to an analysis that extracts phylogenetically independent contrasts, and the relationship remained significant (r(2) = 0.815, F = 39.74, P < 0.001). Small mammals that live fast really do die young. However, there are very large differences between species in the amounts of living that each enjoy and these disparities are even greater when other taxa are included in the comparisons. Such differences are incompatible with the "rate-of-living" theory. However, the link between energetics and aging across species is reconcilable within the framework of the "free-radical damage hypothesis" and the "disposable soma hypothesis." Within species one might anticipate the rate-of-living model would be more appropriate. We reviewed data generated from three different sources to evaluate whether this were so, studies in which metabolic rate is experimentally increased and impacts on life span followed, studies of caloric restriction and studies where links between natural variation in metabolism and life span are sought. This review reveals that there might be contrasting effects of resting and nonresting energy expenditure on aging, with increases in the former being protective and increases in the latter being harmful.

The influence of feeding regimen on body condition in the cat was studied in a sample of the UK domestic cat population (n = 136). Data were collected by interviewing cat owners and assessing body condition of cats in the owners' homes. Four main factors were identified which were related to body condition. These were, in descending order of significance: neuter status, age, frequency of treat feeding, and ad libitum feeding. Feeding regimen as a risk factor in feline obesity is discussed.

SummaryWe measured body composition and resting metabolic rates (RMR) of three dog breeds (Papillons, mean body mass 3.0 kg ( n = 35), Labrador retrievers, mean body mass 29.8 kg ( n = 35) and Great Danes, mean body mass 62.8 kg ( n = 35)) that varied between 0.6 and 14.3 years of age. In Papillons, lean body mass (LBM) increased with age but fat mass (FBM) was constant; in Labradors, both LBM and FBM were constant with age, and in Great Danes, FBM increased with age but LBM was constant. FBM averaged 14.8% and 15.7% of body mass in Papillons and Labradors, respectively. Great Danes were leaner and averaged only 10.5% FBM. Pooling the data for all individuals, the RMR was significantly and positively associated with LBM and FBM and negatively associated with age. Once these factors had been taken into account there was still a significant breed effect on RMR, which was significantly lower in Labradors than in the other two breeds. Using the predictive multiple regression equation for RMR and the temporal trends in body composition, we modelled the expenditure of energy (at rest) over the first 8 years of life, and over the entire lifespan for each breed. Over the first 8 years of life the average expenditure of energy per kg LBM were 0.985, 0.675 and 0.662 GJ for Papillons, Labradors and Great Danes, respectively. This energy expenditure was almost 60% greater for the smallest compared with the largest breed. On average, however, the life expectancy for the smallest breed was a further 6 years (i.e. 14 years in total), whereas for the largest breed it was only another 6 months (i.e. 8.5 years in total).Total lifetime expenditure of energy at rest per kg LBM averaged 1.584, 0.918 and 0.691 GJ for Papillons, Labradors and Great Danes, respectively. In Labradors, total daily energy expenditure, measured by the doubly labelled water method in eight animals, was only 16% greater than the observed RMR. High energy expenditure in dogs appears positively linked to increased life expectancy, contrary to the finding across mammal species and within exotherms, yet resembling observations in other intraspecific studies. These contrasting correlations suggest that metabolism is affecting life expectancy in different ways at these different levels of enquiry.

The effects of ovariohysterectomy on bodyweight, composition and condition score were evaluated in 49 cats that were fed ad libitum and 11 cats that had their food allowances controlled with the aim of maintaining a stable bodyweight. In cats fed ad libitum, bodyweight increased by an average of 31 per cent in the 12 months following ovariohysterectomy compared with 3.1 per cent over the 12 months before surgery, and this was largely due to increased body fat content. There was no difference in weight gain between cats fed dry or canned foods, but weight gain was inversely related to age and bodyweight at the time of neutering. Mean bodyweight increased by 7.5 per cent in the controlled feeding group, compared with 3.6 per cent over the 12 months before surgery, and individual bodyweights were maintained to within 10 per cent of pre-neutering values in nine cats. The other two cats experienced substantial weight gain (+20 per cent and +36 per cent), despite being fed only 40 kcal/kg/day. There were no significant changes in body composition of cats with controlled dietary allowances and their condition scores were significantly lower than those of cats fed ad libitum. The results confirm a link between ovariohysterectomy and the development of obesity in cats with free access to food, and indicate that substantial reductions in energy intake are required to prevent weight gain in such cats.

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