EXTERNAL ENVIRONMENTAL AGENTS OF AGING
25.8.1 Ionizing Radiation
The shortening of life caused by ionizing radiation (e.g., X-rays) has been determined for many species, including mice, rats, hamsters, guinea pigs, and dogs.
The occurrence of some diseases, such as leukemia, may increase disproportionately after irradiation, with the degree of increase influenced by age and sex. The permanent nature of radiation damage is shown by the comparison of life spans of irradiated and control populations. An irradiated population dies out like a chronologically older unirradiated population. Members of a population given a single dose of X-rays or gamma rays in early adult life die of the same diseases that afflict the unirradiated control population, but they die months or even years earlier.Continuous irradiation throughout life at low dose rates speeds the mortality process. Studies of animals and of cells grown in culture suggest that large doses of radiation kill by producing deleterious rearrangements of chromosomes in the proliferative cell population. Such aberrations also increase with age, but they seem to be less important in the natural aging process. At low radiation doses, chromosome aberrations become relatively less important than other effects, and the primary radiation damage in these conditions may bear a closer relation to the aging lesion. Natural radioactivity in the body, arising mostly from radioactive potassium and radium, and natural background irradiation, from Earth and from cosmic rays, are not major contributors to the aging process, even in the long-lived human species. They are responsible, however, for a small percentage of cancer incidence.
25.8.2 Temperature
Flour beetles, fruit flies, fishes, and other poikilother- mic (temperature-variable) organisms live longer at the lower range of environmental temperature. An organism’s life span is dependent on some critical substance that is exhausted more rapidly at higher temperature.
An important factor that has not yet been adequately taken into account is the relation of metabolic efficiency to temperature. The energy cost of the biosynthetic processes studied has been discovered to be minimal at an intermediate temperature in the range to which the species is adapted and to increase at higher or lower temperatures. A related phenomenon holds for longevity; the number of calories expended by fruit flies per lifetime is maximal at an intermediate temperature, so the rate of aging per calorie is minimal at that temperature. Thermal denaturation of proteins is predominately a disruption of the folding of molecules, which requires the breaking of numbers of low-energy bonds. Mutations may arise to a significant degree from thermal denaturation.Research has suggested that humans might live longer if their core body temperatures were lower, since in shorter- lived species there is a relationship between high metabolism, which increases core temperature, and short life span. In a study of mice engineered to have a lower-than-normal core body temperature, a reduction of about 0.5 °C (0.9 °F) was associated with a roughly 20 percent increase in life span.
25.8.3 Physical Wear of Nonrenewable Structures
One of an animal’s most important assets is its chewing apparatus, including jaws and teeth. Adaptation to tooth rate of wear is especially important for animals that consume large quantities of grass and herbage. Such adaptations include higher tooth crowns (hypsodonty), larger grinding area, and longer tooth growth period. Tooth wear may be limiting for survival in adverse environments, but, on the whole, it is not an important life-limiting characteristic. The same can be said for other external organs subject to physical wear.
25.8.4 Infectious Disease and Nutrition
The populations in poor environments, characterized by high rates of infectious disease and poor nutrition, have higher death rates than populations in good environments at all ages, yet there is no positive evidence that disadvantaged populations experience a higher rate of aging. Rats kept on diets restricted in calories live longer and have lower cancer incidence than do rats that are allowed to eat at will. Maximum longevity, however, is achieved at a nutritional level that keeps the animal sexually immature and below normal weight.
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