Some of the targeted genes have homologues across species and in some cases have been associated with human longevity.
In his book How and Why We Age, Hayflick says that caloric restriction may not be effective in humans, citing data from the Baltimore Longitudinal Study of Aging which shows that being thin does not favour longevity.
Similarly, it is sometimes claimed that moderate obesity in later life may improve survival, but newer research has identified confounding factors such as weight loss due to terminal disease.
Older adults, however, may not suffer depression as much as younger adults, and were paradoxically found to have improved mood despite declining physical health.
A distinction can be made between "proximal ageing" (age-based effects that come about because of factors in the recent past) and "distal ageing" (age-based differences that can be traced to a cause in a person's early life, such as childhood poliomyelitis).
Less still is known of mammalian ageing, in part due to the much longer lives of even small mammals such as the mouse (around 3 years).
A model organism for studying of ageing is the nematode C. Thanks to its short lifespan of 2–3 weeks, our ability to easily perform genetic manipulations or to suppress gene activity with RNA interference, or other factors.Programmed ageing should not be confused with programmed cell death (apoptosis).In 1934, it was discovered that calorie restriction can extend lifespan by 50% in rats and this has motivated research into delaying and preventing ageing.In contrast, many species can be considered immortal: for example, bacteria fission to produce daughter cells, strawberry plants grow runners to produce clones of themselves, and animals in the genus Hydra have a regenerative ability by which they avoid dying of old age.Early life forms on Earth, starting at least 3.7 billion years ago, which occurred with the emergence of the fungal/animal kingdoms approximately a billion years ago, and the evolution of seed-producing plants 320 million years ago.In a detailed review, Lopez-Otin and colleagues (2013), who discuss ageing through the lens of the damage theory, propose nine metabolic "hallmarks" of ageing in various organisms but especially mammals: The rate of ageing varies substantially across different species, and this, to a large extent, is genetically based.For example, numerous perennial plants ranging from strawberries and potatoes to willow trees typically produce clones of themselves by vegetative reproduction and are thus potentially immortal, while annual plants such as wheat and watermelons die each year and reproduce by sexual reproduction.fall into two main categories, programmed and damage-related.Programmed factors follow a biological timetable, perhaps one that might be a continuation of the one that regulates childhood growth and development.Once these factors are accounted for, the optimal body weight above age 65 corresponds to a leaner body mass index of 23 to 27.Alternatively, the benefits of dietary restriction can also be found by changing the macro nutrient profile to reduce protein intake without any changes to calorie level, resulting in similar increases in longevity.