Like distance runners on a measured course, all of us will move through time in a roughly predictable
pattern.
     In the first stage of our lives, we develop and grow, reaching toward the top of physical vitality (energy).
     After we grow up, however, the body begins a process of gradually wearing out.
     A new awareness of physical fitness may help lengthen our years of health and vitality, yet nothing we
do will work to stop the unavoidable force of aging.
     Most of the changes of aging take place deep inside the body. The lungs become less able to take in
oxygen. Powerful muscles gradually lose their strength. The heart loses power and pumps less blood. Bones
grow easier to break.
     Finally, we meet a stress, a stress that is greater than our physical resistance. Often, it is only a minor
accident or chance infection (a disease caused by virus), but this time, it brings life to an end.
     In 1932, a classic experiment nearly doubled the lifetime of rats, simply by cutting back the calories (unit
for measuring the energy value of food) in their diet. The reason for the effect was then unknown.
     Today, at the university of California at Berkley, Dr. Paul Seagle has also greatly lengthened the normal
lifetime of rats. The result was achieved through a special protein (蛋白质) limited diet, which had a great
effect on the chemistry of the brain. Seagle showed that within the brain, specific chemicals control many
of the signals that influence aging. By changing that chemical balance, the clock of aging can be reset.
     For the first time, the mystery (something that is difficult to understand or to explain) of why we age is
 being seriously challenged. Scientists in many fields are now making striking and far-reaching discoveries.
     An average lifetime lasts 75 years, yet in each of us lies a potential for a longer life. If we could keep the
vitality and resistance to disease that we have at age twenty, we would live for 800 years.