Basics of gerontology, demographic data - Molecular and Clinical Basics of Gerontology

1.1. Introduction, definitions

Gerontology (from Greek: Géron = “gray”, “old man”, logos = “study of”) is the study of the biological, psychological and social aspects of normal aging. It is distinct from geriatrics, which is the branch of medicine that studies the characteristic diseases of the elderly or age-related changes in diseases that already began in the young. Biogerontology is a sub-field of gerontology studying the biological processes of aging. It is composed of the interdisciplinary research on the causes, effects and mechanisms of biological aging, in order to achieve better understanding of human senescence. The huge increase in the elderly population in post-industrial Western nations has made biogerontology one of the most rapidly growing fields.

1.2. Population-wide aging

The worldwide prolongation of life expectancy has resulted in a rapid increase in the size of elderly populations (over the age of 65), both in absolute numbers and relative to the whole. Survival has increased with the passage of time since earlier historic periods (epochs) and shows larger improvements in more developed countries.

Age-specific mortality has decreased. However, the age maximum ever achieved by human beings has not changed. (Figure I.1-1 and Figure I.1-2)

Figure I.1-1: Survival curves for different populations

Figure I.1-2:Age-specific death rates of Swedish females from 1751 to 1950 and for 1988

Regional differences in mortality or in life expectancy at birth are generally determined by the combination of a huge number of different factors. These differences are known for many regions but are usually compared among nations (Figure I.1-3). One of the very few exceptions is Germany: here the survival conditions are not uniformly distributed over the whole national territory. The East-West differences are due to the special history of these two regions having belonged to completely different political and social regimes for several decades during the last century. The difference remains high despite the political reunion in 1990 (Figure I.1-4).

Figure I.1-3: Expected life-span at birth in different European states

Figure I.1-4:Regional pattern of life expectancy in Germany: East-West difference (2003)

In Europe the number of people over 65 years will increase from 15.5% in 2000 to 24.3% in 2030. These demographic changes have major implications for health care, the labor force, welfare, insurance, and pensions.

1.3. Chronological and biological age

“How old would you feel if you did not know how old you were?”

These two numbers are not necessarily the same. The functional-biological age is determined by physiology rather than chronology. Factors include changes in the physical structure of the body as well as changes in the performance of motor skills and sensory perception.

1.4. Etiology of aging: genetic mechanisms and environmental factors

Aging is a complex process that affects all living organisms. Animals living in the wild are less likely to live long enough to encounter aging, but interestingly all mammalian species (including humans) show very similar aging processes if kept under optimal conditions free from external risk factors like predators or famine. The aging process is multi-factorial, and no single factor has been identified which provides a satisfactory explanation of the phenomenon. During the process of aging, the organism accumulates damage to macromolecules of its own cells and tissues and to its organs. The maximum lifespan for humans is around 120 years, whereas the maximum lifespan of a mouse, commonly used in research as a model for aging, is about four years. Genetic differences between humans and mice that may account for these different rates of aging include efficiency of DNA repair, types and quantities of antioxidant enzymes, and different rates of free radical production.

Mutation rate in humans is about 1 per 10^7-11 base pair. Chromosome abnormalities, demethylation, as well as defects of protein synthesis also influence aging. The acceptable rate of mistakes (faulty aminoacids in the peptide chain) in protein synthesis is about 5/10.000 amino acids. Elongation factor-1 levels are also low in old populations, just as levels of some types of mRNA, e.g. mRNA for IL-1. The telomere is a region of repetitive DNA sequences at the end of chromosomes, which protects the end of the chromosome from damage during cell division. The telomere regions prevent the degradation of genes near the end of chromosomes by allowing for the inevitable shortening of the chromosome, which necessarily occurs during cell division. This telomere shortening mechanism normally allows cell lines only a fixed number of divisions. Animal studies suggest that this mechanism contributes to aging on a cellular level and sets a limit to lifespan. (It has been described that a certain cell type of a certain species is capable of only a certain number of cell divisions: Hayflick phenomenon.) Changing telomere lengths is usually associated with changing rate of senescence. This telomere shortening, however, might be a consequence of, and not a reason for aging.

Maximum lifespan of a species is determined by the rate of aging, inherent in its genes and also by environmental factors, i.e. high metabolic rate, free radical production, excessive caloric intake leading to high serum glucose level.