Women who smoke are likely to undergo menopause at a younger age than nonsmokers, according to the findings of two studies involving more than 3,500 middle-aged women in seven countries.

In fact, the studies showed, the more a woman smokes, the earlier her menopause is likely to occur. The average age for menopause in women who have quit smoking was found to fall between that of light smokers (half a pack a day) and that of women who never smoked.

Most women in developed countries stop menstruating between the ages of 44 and 54, with the average age around 50. in one of the studies, the researchers found that at ages 48 to 49, a woman who smokes a pack or more a day is nearly twice as likely to be past menopause as a woman who never smoked. 

At ages 50 to 51, 79 per cent of the women who smoked a pack or more a day were past menopause, as against 56 per cent of those who never smoked.

This "striking association" between smoking and the onset of menopause was accidentally discovered as part of a continuing international research project, the Boston Collaborative Drug Surveillance Program. The researchers, Dr. Hershel Jick and Jane Porter of Boston University School of Medicine and : Dr. Alan S. Morrison of the Harvard School of Public Health, said they noticed the relationship while exploring the link between smoking and heart disease.

A closer look at the data suggested to the researchers that smoking might actually precipitate earlier menopause. At each age, the women in the studies who were smokers were more likely to be past menopause than those who had never smoked, and the heavy smokers were more likely to be past menopause than the light smokers. The association between smoking and earlier menopause was similar in all the countries examined.

The researchers said they could discern no other factors to account for the finding. Writing in the current issue of the British journal The Lancet, the researchers suggested two possible mechanisms for the link between smoking and menopause. One is the effect of nicotine on the central nervous system, possibly resulting in changes in the secretion of hormones involved in the menopause. The second is the effect of cigarette smoke on certain enzymes that may in turn influence the way the body handles the sex hormones.

The researchers said the effect of smoking on the onset of menopause might account for the association previously found between menopause and heart disease. Women past menopause have been shown to have a higher rate of coronary heart disease than premenopausal women of the same age. But since smoking is known to increase a  person's chance of developing heart disease and has now been show to accelerate the onset of menopause, smoking rather than menopause itself may be the real explanation for the heart disease statistics,, the Boston researchers suggested.

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