The changing understanding of ageing. Part 3: Diseases of ageing.

This third and final paper in this series considers ageing mechanisms across species, with emphasis on conserved metabolic pathways that relate to disease. The growth hormone (GH)-insulin-like growth factor (IGF-1)-insulin axis continues as an example of how critical pathways might relate to longevity and senescence. Aligning theory, research outcomes and clinical investigations at the levels of the cell, organism and population, is suggested as a means by which to consider the many complexities of the ageing process in an orderly fashion. A contentious debate revolves around whether ageing is purely a combined effect of stochastic events on residual programming relating to reproductive robustness, or whether ageing itself is programmed by natural selection. Emerging data indicate that the influence of genetic programming on specific late-life diseases, and even individual tissue pathologies, will probably need to be reconsidered in the light of newer theoretical possibilities. In particular, the evidence that late life and its diseases are objects of considerable investment of energy challenges theory that couples longevity with reproduction. Furthermore, the author suggests that ageing may have evolved at least partly as a means of niche preservation for contemporaries and for progeny.