Age Retardation - Brutes or Angels: Human Possibility in the Age of Biotechnology - page 209

collagen. These cells died predictably after about fifty cell divisions in labo-
ratory culture, and the phenomenon became known as the hayflick limit.
There are some cells that ignore the hayflick limit and appear to con-
tinue dividing without ceasing. for example, cancer cells and some stem
cells disobey the hayflick limit. But most normal cells obey a hayflick limit
on their number of divisions. nobody yet knows what limits the number of
times a cell can divide. one possibility is the inexorable shortening of DnA
molecules at the ends of chromosomes (telomere shortening). in chapter 7
we saw that beginning life with abnormally short telomeres may have led
to premature aging in Dolly, the cloned lamb.
Hormones and aging. When levels of hormones such as estrogen and tes-
tosterone decline, physical and mental functions also decline. These ob-
servations led to hormone therapy to prevent bone loss by osteoporosis
and to maintain sexual functions in postmenopausal women. hormone re-
placement is uncommon in men, but a 1990 study showed that injections
of human growth hormone reversed some aging symptoms in men aged
sixty-one to eighty-one (rudman et al. 1990). effects of the therapy, ad-
ministered over six months, included increased lean body mass, decreased
fat tissue, increased lumbar-spine density, and increased systolic blood
pressure and fasting glucose levels, but the researchers did not assess ex-
ercise endurance, muscle strength, or quality of life. A later study in men
sixty-ive to eighty-two years old showed that strength training signifi-
cantly increases muscle strength and that growth hormone treatment did
not increase muscle strength beyond that achieved by exercise alone (Taaffe
et al. 1994). 4 Also, treating mice with a testosterone-related compound,
dehydroepiandrosterone (DheA), increased their life spans by as much as
40 percent.
Although hormone replacement/treatment does not retard aging in all
tissues and organ systems, and growth hormone therapy may not be risk-
free, some researchers believe that learning how hormones reverse some
symptoms of aging will give clues to natural causes of aging. 5 such infor-
mation could lead to new approaches to age retardation.
Caloric restriction and longevity. As early as 1935, research at Cornell Uni-