Older
men with higher levels of free, or unbound, testosterone circulating
in their bloodstreams have better visual and verbal recall and perform
spatial tasks more adeptly than their peers, according to investigators
at the National Institute on Aging (NIA). The study* identifies a potential
biological factor that one day could be used to protect against decline
in memory and other cognitive skills in later life.
"Although we can't firmly establish a causal relationship
without further study, this finding suggests that there may be hormonal
modulation of cognitive abilities as people get older. Clearly, having
higher levels of circulating free testosterone is associated with a
reduced risk of certain types of memory loss,” said Susan Resnick,
Ph.D., an investigator in the NIA's Laboratory of Personality and
Cognition, and corresponding author of the study, published in the November
2002 issue of The
Journal of Clinical Endocrinology & Metabolism.
Dr.
Resnick, Scott Moffat, Ph.D., and their colleagues evaluated the testosterone
levels of 407 men, ages 50 and older, who participated in the Baltimore
Longitudinal Study of Aging (BLSA)**. The investigators then correlated
free and total testosterone levels—measured over an average of
10 years—with the men's performances on memory and other
cognitive tests.
In the
body, testosterone tends to bind with sex hormone binding gobulin
(SHBG). But some testosterone remains freely circulating in the bloodstream.
Unlike the SHBG-bound form of the hormone, free testosterone can circulate
into the brain and affect nerve cells, Dr. Resnick said. Total testosterone
is a measure of both free and SHBG-bound forms of the hormone. Of
these measures, only free testosterone was significantly associated
with higher scores on verbal and visual memory tests, such as recalling
word lists and drawing a recently seen image. Free testosterone was
also linked to high scores on a spatial test in which participants
were asked to determine if rotated shapes were the same or different.
"Based
on our results, testosterone levels are associated with selective
and very specific effects on some aspects of cognition, including
memory,” Dr. Resnick said.
In men, testosterone is produced in the testes, the reproductive glands
that also produce sperm. As men age, their testes often produce somewhat
less testosterone than they did during adolescence and early adulthood,
when production of this hormone peaks.
Based on BLSA data, as many as 68 percent of men older than 70 have
low levels of free testosterone. But while prescription testosterone
replacement therapy is available, it may not be advisable for most
older men because many effects of hormone supplementation remain unclear.
It is not yet known, for instance, if testosterone replacement increases
the risk of prostate cancer, the second leading cause of cancer death
among men. In addition, studies suggest that supplementation might
trigger excessive red blood cell production in some men. This side
effect can thicken blood and increase a man's risk of stroke.
"We still have much to learn,” Dr. Resnick said. "Until
we know much more about the fundamental effects of sex hormones on
the aging brain and other parts of the body, testosterone supplementation
is not a prudent choice for older men seeking to improve their memory
and cognitive performance.”
The National Institute on Aging, one of 27 Institutes and Centers
that constitute the National Institutes of Health, leads Federal efforts
to support and conduct basic, clinical, epidemiological, and social
research on aging and the special needs of older people. For more
information about the NIA, visit the website at http://www.nia.nih.gov.
* S.D.
Moffat, A.B. Zonderman, E.J. Metter, M.R. Blackman, S.M. Harman, and
S.M. Resnick, "Longitudinal
Assessment of Serum Free Testosterone Concentration Predicts Memory
Performance and Cognitive Status in Elderly Men,” The
Journal of Clinical Endocrinology & Metabolism, 2002; 87 (11):
5001-5007.
**Launched
in 1958, the BLSA is America's longest running scientific examination
of human aging. Volunteers receive comprehensive medical, physiological
and neuropsychological evaluations every two years at the NIA Gerontology
Research Center in Baltimore. The BLSA has measured testosterone levels
in its male participants since 1963.
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