189 HOW TO LIVE TO BE 100 Since aging and lifespan are broadly determined by the genetic plan of a species, attention has been directed to the possibilities of their modification by altering the environment. The biologist Jacques Loeb showed early in the 20th century that the life span of the fruit fly was halved by every 10-degree rise in temperature. This led to impractical speculations about prolonging the human life span by experimenting with various degrees of cooling the body. Fairly severe restriction of caloric intake in the laboratory rat can more than double its life span, chiefly by prolonging the period of immaturity. Caloric restriction is so far the only factor shown to have a major effect on aging and lifespan. Unfortunately, food restriction has less effect on species other than rodents. It has not been shown that undernourished human populations live longer, but vitamin deficiency, disease, and poor medical care found in such groups complicate the analysis. In geriatric medicine, the hope is to eliminate the disease processes that prevent human beings from living to the end of their natural lifespan.
