An individual's life expectancy is contributed to by the interaction of intrinsic (genetic and epigenetic) factors with extrinsic (environmental and life style) factors (Figure 1). In the world's more developed countries (MDCs) the life expectancy at birth in the 1900s was around 47 years. By the end of the twentieth century this rose to a mean of 78 and 76 years in western Europe and north America, respectively, with many individuals living much longer. This dramatic increase in average life expectancy has been largely due to improvements in environmental conditions such as nutrition, housing, sanitation, and medical and social services, and has resulted in a large increase in the number of older people around the world. This change in the age structure of society is compounded by the decreasing fertility levels in the world's populations leading to large gains in worldwide median population ages. Our aging populations have a growing number and proportion of older people and, importantly, a growing number and proportion of very elderly people.
Based on the current rates and trends in population growth it has been predicted that by the year 2025 the elderly population (aged 65 and above) in the world's MDCs will increase by more than 50%, and will more than double worldwide. The elderly population itself is aging with the very elderly (aged 80 and above) being the fastest growing section of the elderly population. This
' 'senescence' genes, genes coding ^ for components of biomolecule defense .---...systems, etc.____
Lifestyle factors: J \ Environment:
diet, housing, exercise, etc. I I exposure to chemicals, \ disease-causing organisms, etc.
Rate of aging
Figure 1 Interactive factors that contribute to the aging process. (Reproduced with permission from Barnett YA (1994) Nutrition and the ageing process. British Journal of Biomedical Sciences 51: 278-287.)
changing demographic picture will result in a large increased prevalence worldwide of long-term illness, disability, and the degenerative diseases associated with aging. These alterations in the proportions of the population of working age and those beyond working age will have a significant impact on the funding and costs of healthcare for all nations, making research into aging of critical international importance.
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