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Biology of Ageing
In humans, it is thought that the accumulation of random molecular damage in the macromolecules of all cells is the major determinant of lifespan, and this results in age-dependent defects in cellular function and reduced function at a tissue level which manifest as age-related frailty, disability and disease(1). Many factors, both endogenous and exogenous, are considered to contribute to the cellular and tissue damage that contribute to ageing process. The development of interventions to prolong lifespan and/or halt the onset of age-related diseases is dependent on advancing knowledge and understanding of the fundamental biology of ageing.
Factors influencing ageing in the companion animal
Reproductive status has been shown to affect life span in pets.
There is evidence that sterilisation is associated with increased lifespan in dogs and cats. Sterilised dogs are less likely to die from infectious disease, trauma, vascular disease and degen-erative disease and more likely to die from neoplasia and immune mediated disease. The nature of this association and potential mechanism is not understood; it could be mediated through a direct physiological pathway, or indirectly through behavioural changes, or a combination of both. Body size in dogs is another factor known to influence lifespan. Dogs are unusual because the relationship between lifespan and body size is different from that of the interspecies association(7), where generally larger animals live longer. In dogs, small breeds are expected to live much longer (from 10 to 14 years) than large and giant breeds (from 5 to 8 years). A comparison of longevity of pet dogs and humans showed that at any chronological age, larger breed dogs are older in human year equivalents compared with their smaller counter-parts(9), dispelling the myth that 1 human year is equivalent to 7 dog years. A recent study investigated different models of ageing to see if they could identify attributes of the ageing process that could explain why large and giant breeds have a ‘shorter’ lifespan. They found no effect of breed size on baseline mortality, early life mortality or the age at which mortality started to increase (senescence). However, there was a clear positive relationship between the absolute rate of ageing and body size, indicating that large and giant breed dogs simply age faster than their smaller counterparts. There is a growing body of evidence on the influence of diet on longevity and lifespan. Dietary energy restriction has been extensively studied in rodent models and has been shown to have a generally positive impact on lifespan. However, there is limited evidence that this effect translates to humans(1). One study has investigated the effect of energy restriction in dogs. A total of forty-eight Labrador retrievers from seven different litters were pair-matched (sex and weight) and assigned to one of two groups. One group (over fed) was initially fed ad libitum, although this was later restricted to avoid excessive weight gain, and the other group was restricted to 75 % intake of their littermates (restricted). The study followed dogs over two decades and reported a significant increase in median lifespan (13·2 v. 11 years) in the ‘restricted’ group.
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