Ames dwarf mice and Snell dwarf mice lack growth hormone (GH), prolactin (PRL), and thyroid-stimulating hormone (TSH), live much longer than their normal siblings, and exhibit many symptoms of delayed aging. "Laron dwarf mice," produced by targeted disruption of the GH receptor/GH-binding protein gene (GHR-KO mice), are GH resistant and also live much longer than normal animals from the same line. Isolated GH deficiency in "little" mice is similarly associated with increased life span, provided that obesity is prevented by reducing fat content in the diet.
Problems could be found in the fact that we very often look for one deciding, definitive reason for the process of aging. It is a sort of search for a big discovery, like a fountain of youth or such. More and more authors are trying to explain the unknowns in the understanding of these observations about aging by adding the statement that there are two subgroups in the general population. This acknowledgment of two subpopulations explains why there are numerous cases that cannot be explained, defined, or fitted in basic observations about caloric restrictions and the delay of reproduction.
A cost of reproduction, where lifespan and fecundity are negatively correlated, is of widespread occurrence. Mutations in insulin/IGF signaling (IIS) pathways and dietary restriction (DR) can extend lifespan in model organisms but do not always reduce fecundity, suggesting that the link between lifespan and fecundity is not inevitable. Understanding the molecular basis of the cost of reproduction will be informed by elucidation of the mechanisms by which DR and IIS affect these two traits.
The food restriction model for life extension is nearing "three-score and 10" years of age and remains in good shape, preserving much of the mystique of its youth. Although originally described for laboratory rodents, recent work shows that food restriction also appears to slow ageing processes in a range of other animal species, raising the question of whether this response represents some generalised evolutionary adaptation, perhaps a strategy to cope with periods of famine.
Caloric restriction (CR) extends maximum longevity and slows aging in mice, rats, and numerous non-mammalian taxa. The apparent generality of the longevity-increasing effects of CR has prompted speculation that similar results could be obtained in humans. Longevity, however, is not a trait that exists in a vacuum; it evolves as part of a life history and the physiological mechanisms that determine longevity are undoubtedly complex. Longevity is intertwined with reproduction and there is a cost to reproduction. The impact of this cost on longevity can be age-independent or age-dependent.
Laboratory study was conducted on the biology of the phorid fly Megaselia sacalaris. The influence of 3 constant temperatures and various larval densities was researched in the laboratory as these relate to mean life stage rate of development and survival. Flay development on a synthetic diet at 25 degrees C. 75=2% RH and 18 hr photophase was used as the standard. Mean times for development and survival were egg incubation 16.3 h (97.6%); larval period 7.3 days (97.6%); pupal period 9.8 days (95.8%, 91.6%); adult longevity 29.9 days, 24.8 days. Fecundity was 664.8 eggs.
The question has arisen in the literature as to whether dietary restriction (DR) will have a significant effect on human longevity. I initially use literature data to estimate the energy costs necessary to carry a human from conception to caloric self-sufficiency to be approximately 12.6 x 10(6)kcal, which amounts to approximately 25% of the the two parents' combined daily caloric intake for 20 years. Similar levels of financial costs are expended in developed societies. Thus, human reproductive costs are high enough to permit a DR response.
All scientists are acutely aware of the profound challenge that they face when communicating scientific findings to nonscientists, especially when great uncertainty is involved and when the topic is of personal interest to the general public. Simplification of the issues--sometimes extending to a degree of oversimplification--is a sad but generally recognized necessity.
The Journals of Gerontology. Series A, Biological Sciences and Medical Sciences
Dietary Restriction extends lifespan in a diverse range of animals, but this often comes at a cost to reproduction. While a number of molecular pathways integral to these relationships have been characterised, we still do not fully understand whether restriction of specific nutrients or calories is responsible. Two recent studies on insects have offered novel insights into this longstanding issue via the application of Nutritional Geometry. This technique promises to significantly advance our understanding of how nutrition influences reproduction and longevity.