What is it? Human growth hormone (hGH), produced by the pituitary gland, is essential for growth and development and helps spark the onset of puberty during adolescence. After age 30, hGH tends to decline, and one hypothesis asserts this leads to changes in body composition (loss of muscle and fat gain) associated with aging.

Evidence: A 1990 study of 21 older men, in the New England Journal of Medicine (NEJM), found that those who were given injections of hGH for six months had increased muscle, decreased fat, and higher bone density in the lumbar region, changes that were not seen in the control group. There were no assessments of strength, endurance, quality of life, or length of life, but this little study is the basis for false claims that growth hormone reverses aging.

Existing and future use: Investigators at the National Institute on Aging believe there may be a role for hGH injections combined with testosterone to increase strength and endurance in certain older men, but there are risks, and more research is needed. A 2002 study in the Journal of the American Medical Association on more than 120 men and women noted significant adverse effects in those given hGH, including a higher rate of diabetes and glucose intolerance. Disturbed by the number of commercial Web sites citing the 1990 study, Mary Lee Vance, MD, an endocrinologist at the University of Virginia Health Sciences Center in Charlottesville, presented more recent findings last year in NEJM: While hGH appears to increase muscle mass, similar results can be had by going to the gym -- for a lot less money.

What are they? The theory: The stem cells that give rise to eggs and sperm also control the rate of aging in adulthood.

Evidence: In Kenyon's lab, "proliferating germline stem cells" in C. elegans were found to be master control cells, affecting both reproduction and life span. One experiment, reported in Science in 2002, showed for the first time that stem cells can modify life span in C. elegans even as cells continue to age.

Current and future use: Far more research is needed before we'll know whether "master" stem cells in humans control adult life span. Last February, however, researchers in South Korea announced a stunning development: They successfully cloned a human embryo and extracted master stem cells (a process which destroys the embryo). This could increase stem-cell supply, and facilitate research into drugs for a wide variety of diseases (including Parkinson's and diabetes). However, cloning and stem-cell research face fierce political opposition here in the U.S.

What are they? As a cell ages, every time it divides, the telomeres at the ends of chromosomes get increasingly chopped off. Eventually, little is left, and the cell ceases to divide (a state called senescence). Certain cells -- egg cells, stem cells, and cancer cells -- keep the length of their telomeres, and escape senescence, through the action of an enzyme called telomerase.

Evidence: Several experiments in the '90s suggested that telomerase might slow aging in other types of cells. In 1995, scientists at Geron Corporation, a California pharmaceutical company, cloned the gene that turns on telomerase.

Current and future use: "Telomere shortening is certainly a piece of the aging puzzle, but it's not a likely avenue to slow aging," says Olshansky. Mice, for instance, are known to have naturally long telomeres, but short life spans. Geron has focused its research on inhibiting telomerase in cancer cells, to slow their ability to replicate.

Priscilla Grant is a contributing editor for MORE magazine.

Originally published in MORE magazine, May 2004.