19 JANUARY 2006
hear
some women talk, you'd think testosterone makes men stupid. The male sex hormone
gets
blamed for all sorts of interpersonal conflicts, violent crimes
and other aggressive excesses.
To the contrary, though, biomedical science suggests that testosterone makes you smart -- or at least that reduced testosterone levels contribute to diminished intellectual power, particularly in the elderly.
That's probably not the first diminished thing that comes to mind when thinking
about lower levels of testosterone, but it's nevertheless a major concern of
aging men and women alike. Age-related decline in cognitive ability may have
a lot to do with a lack of testosterone, a belief that has fostered the growth
of a testosterone supplement industry.
Sadly, for those senior citizens seeking to regain the peak of their power (mental, that is), the commercial hype (as usual) exceeds the science.
"The effects of testosterone on cognition are not nearly as dramatic as the advertisements for the products suggest," writes behavioral neuroscientist Jeri Janowsky of the Oregon Health and Science University.
Testosterone pills or injections do not magically restore youthful vigor and intellect, but can be helpful in some situations, Janowsky observes in a paper to be published in Trends in Cognitive Sciences. "Testosterone's role as a cognitive modulator is complex," she writes, "but . . . it could be particularly important for maintaining cognition in aging."
Just how important remains unknown, though, as the complexities of testosterone chemistry have not yet been thoroughly studied, especially in normal healthy people.
"New formulations of testosterone make testosterone replacement in men and women a reality, but without a full understanding of its effects on the brain and almost no data on its effects on cognition in women," Janowsky notes.
Testosterone chemistry begins with cholesterol, which is transformed into several steroid hormones by bodily chemistry. Testosterone is one of the links in that chain of chemical reactions. It is produced mainly in the sex organs and adrenal glands, but there is evidence that it can also be produced in brain cells. While testosterone is widely known as the hormone embodying maleness, women's bodies also make it (though not nearly as much of it as men's).
It's a raw material for making estradiol, the most important of the class of female hormones known as estrogens. And it's also a raw material for making other male hormones (known as androgens). In both men and women, testosterone levels decline with age. Otherwise healthy men in their 70s typically have 40 percent less testosterone then men in their 20s, Janowsky points out. In women receiving hormone replacement therapy after menopause, testosterone levels drop drastically.
Numerous studies in animals and people suggest that testosterone can affect various learning and memory skills. In rats and monkeys, eliminating testosterone (by removing the gonads) causes a substantial reduction in connections between brain cells. Testosterone replacement restores the connections to normal levels.
But the animal studies are not directly comparable to people studies. Animals in such studies are typically adults, but not very old, and testosterone is reduced to virtually nothing by gonad removal. Human research is generally done on elderly men with merely reduced testosterone, or on men with diseases reducing testosterone levels. Very few studies have been done on the mental effects of testosterone in women.
Furthermore, it's not obvious how testosterone exerts its effects on mental ability. Influences on cognitive skill could be caused by testosterone directly, or by either the male or female hormones that it produces. Without knowing which, it's hard to say just what will happen if you add extra testosterone to your body.
In older men, for instance, testosterone supplements seem to improve some sorts of memory. But verbal memory (such as remembering the content of a story) improves only if the testosterone can be converted to estradiol. So it seems that improving verbal memory depends on estradiol, not testosterone itself.
Other studies suggest that lower testosterone levels are linked to a higher risk
of Alzheimer's disease, possibly because less testosterone allows higher deposits
of beta amyloid protein, Alzheimer's molecular hallmark. In some cases testosterone
supplements for patients in the early stages of Alzheimer's seems to help their
memory, but not all studies agree.
In any event, total testosterone levels can be misleading. To trigger its effects in the brain, testosterone must stimulate other molecules designed to sense its presence. But a chemical known as SHBG can attach itself to testosterone and prevent contact with the sensor molecules. Your body could be flooded with testosterone, but if SHBG levels are too high (and they increase with age), your cells wouldn't respond.
All in all, it seems that boosting testosterone probably doesn't do your brain much good if you're young and healthy to begin with, but it might help protect the brain against some of the declines brought on by aging. It's likely that testosterone may end up aiding some mental functions but not others, and its effects are likely to differ in men and women.
If all this seems hopelessly complicated, it's because complicated chemistry is essential for a sophisticated brain. And it shouldn't be surprising that sex hormones are involved -- after all, sex is complicated, too, and also requires chemistry.
E-mail: tsiegfried@nasw.org
