In rats, for example, tumor growth is reduced by at least 30 percent, and testicular, breast, and other hormonally induced cancers are practically eliminated, says Ron Hart of the National Center for Toxicological Research in Arkansas. Caloric restriction appears to work not only by keeping down glucose and free radicals but also by fortifying the body's defenses. According to Hart, rats on caloric restriction have enzymes that are three to four times more active in attacking free radicals. The rodents also show increased levels of DNA repair enzymes, he says, and are at least five times better at disabling carcinogens like aflatoxin. Such dramatic improvements have inspired Hart and some of his colleagues to cut down on calories themselves, especially since they've observed that reducing caloric intake even by 12 percent yields some benefits.
Researchers are now anxious to determine the optimum caloric intake for healthy, long-lived humans.
Another clinical intervention is on the horizon as scientists divine the mysteries of hormones that maintain youth and health. Indeed, researchers have discovered that just as estrogen production drops during female menopause, many other hormones also decline with age. For instance, by age 65 at least 40 percent of men produce such low amounts of testosterone that their health may be compromised, says Fran Kaiser of St. Louis University's Medical School. She found that giving such men an injectable form of testosterone for three months increased their muscle strength and sex drive and countered their anemia. Scientists are now optimistic that they can restore youthful function in the elderly by replacing the hormones they lose with age.