(HealthScoutNews) -- The hormone that makes a man a man may also protect him from having a stroke if he doesn't smoke.

New research being presented today at the American Academy of Neurology's annual meeting in Denver claims that the higher a nonsmoking man's testosterone levels soar, the less likely he is to have a stroke.

Conversely, the study also found a woman's natural levels of estrogen, the female equivalent of testosterone, seem to have no impact on her risk of stroke.

The study, conducted by doctors from Erasmus Medical Center in Rotterdam, The Netherlands, was among the first to examine the role of natural levels of estrogen in women and testosterone in men, and their influence on stroke.

For New York neurologist Dr. Keith A. Siller, the finding is important because it underscores the role that hormones play in vascular events. He's not certain, however, that the answer lies in estrogen and testosterone alone.

"I think this study is interesting in that it draws a correlation between testosterone and stroke, which in turn underscores the idea that hormone levels do play a role," says Siller, an assistant professor of clinical neurology at New York University School of Medicine. "But I think it is likely that hormone levels may be the marker for stroke risk, rather than the risk itself."

Siller adds he can't agree with the finding that natural estrogen levels have no impact on stroke in women. "I don't believe it would be accurate to say, based on this one study alone, that estrogen has no effect on a woman's risk of stroke, because I think we have some evidence to the contrary," he says.

Interestingly, while the study showed elevated levels of testosterone did have protective effects on men, that protection was lost if the men smoked -- something that Siller says is not hard to understand.

"Smoking is a risk factor for stroke, and it seems clear that whatever protection a man might get from testosterone, it's countered by the increased risk related to the smoking," he says. "The protection is just lost."

Dr. Dan Fisher, a cardiologist, says the finding is too new to draw any definitive conclusions.

"Before this study, no one really looked at the influence of endogenous hormones on stroke risk, so in this respect it is an interesting finding," says Fisher, an assistant professor of cardiology at New York University School of Medicine. "It could open some new doors and take us in a new direction, in terms of further research and treatment possibilities."

While the study found no influence of natural estrogen levels on a woman's stroke risk, Fisher says that estrogen replacement therapy, as well as birth control pills containing estrogen, have been found to influence the risk, particularly in women who smoke.

"It's possible the body has some kind of built-in regulating system that controls the effects of natural estrogen -- something we might also be able to accomplish with hormone replacement as we learn more about the subtleties of dosing," Fisher says.

The controversial new study involved a population of 6,732 men and women, all over age 55, none of whom had strokes. None had used hormone replacement of any kind. All had baseline blood tests for hormone levels registered between 1990 and 1993.

The health of the men and women, particularly concerning stroke, was followed until January 1998. At that point, researchers documented 217 strokes -- 97 in men, 120 in women.

The researchers then compared the original hormone levels in the blood of the stroke victims to those of 1,372 participants who remained healthy. More specifically, they looked at total levels, as well as free circulating levels of both testosterone and estrogen, in both men and women, separately.

To reduce the influence of known risk factors for stroke, the researchers controlled for smoking, diabetes, history of cardiovascular disease, body weight and disability.

While estrogen levels did not appear to play any role in the risk of stroke in either men or women, testosterone was an influencing factor in men.

Researchers found those men who had the highest levels of testosterone also had the lowest risk of stroke. However, once smoking became a factor, the protective effects of testosterone were wiped out.

What To Do

To learn more about the risk of stroke, visit the National Stroke Association or the National Institute of Neurological Disorders and Stroke.

For additional information on testosterone deficiency, try the Urology Forum.

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