By Panayiotis M. Zavos
While the majority of Americans recognize that cigarette smoking is a health hazard, many are unaware of how varied its adverse health consequences are—despite the vastness of the scientific evidence of such consequences and of their remarkable diversity. Among the least publicized risks of cigarette smoking are its possible effects on reproductive processes and human sexual behavior.
Approximately 30 percent of Americans of reproductive age smoke cigarettes. Cigarette smoking affects reproductive processes in both males and females. In women, it can adversely affect the menstrual cycle, the function of the fallopian tubes (ducts through which ova [eggs] pass from the ovaries to the uterus), and the viability of ova. As for men, cigarette smoking can decrease both the quality and quantity of spermatozoa and can cause erectile dysfunction (a consistent inability to sustain a penile erection sufficient for sexual intercourse).
Effects of Smoking on Fertility in Women
Cigarette smoking can diminish the ability to become pregnant and to bear a full-term child. Women who smoke cigarettes are three times likelier than nonsmoking women to have difficulty conceiving; and the higher the average number of cigarettes a woman of reproductive age smokes, the smaller her chances are of becoming pregnant. Such smokers are also less likely to benefit from treatments for infertility than are their nonsmoking peers. Although there is no definite consensus on whether cigarette smoking is a direct cause of infertility, research has consistently shown that cigarette smokers have a higher risk of being unable to conceive than do nonsmokers. Perhaps lifestyle factors among cigarette smokers other than smoking account for this elevated risk. For example, the number of sexual partners of adolescent and/or unwed smokers may exceed those of corresponding nonsmokers, and having many sexual partners increases one's risk of contracting sexually transmitted diseases, which in most instances can cause infertility.
Cigarette smoking alters bodily concentrations of several hormones, including estrogen. Such changes can disrupt ovulation and, consequently, menstruation; and menstrual irregularity reduces the chances of becoming pregnant. Women who smoke cigarettes tend to have amenorrhea (abnormal stoppage of menses) more often than do nonsmoking women. They are also likelier to have menstrual irregularity in middle age. Menopause tends to begin earlier in smokers. Thus, the reproductive window is longer in nonsmoking women. Clinical reports and evidence from animal experiments suggest that nicotine may be principally responsible for menstrual effects of smoking.
Abnormalities in the function of the fallopian tubes—where conception normally occurs—have been found in cigarette smokers. Cigarette smoking increases concentrations of the hormones epinephrine and vasopressin. Such an increase can accelerate the passage of ova through the fallopian tubes, and early entrance of a fertilized ovum (embryo) into the uterus can increase the probability of a miscarriage.
In one large study, researchers found that ectopic pregnancy—extra-uterine implantation and development of a fertilized ovum—is 2-4 times likelier among women who smoke cigarettes than among those who do not smoke. The fallopian tubes are the most common sites of ectopic pregnancy. If it is not detected early, ectopic pregnancy can result in fallopian-tube rupture and even death.
Evidence from numerous studies suggests that cigarette smoking adversely affects the immune system. Some scientists have speculated that such changes may result in infertility through an increase in the frequency of fallopian-tube infection.
Evidence from research on rats and mice suggests that cigarette smoking can affect the quality and functionality of ova. Some of this evidence suggests that nicotine can prevent formation of cortical granules. These are specialized vesicles near the surface of an ovum that fuse and, upon fertilization of the ovum, release their contents and thus prevent passage into the ovum of more than one sperm cell. Such passage, termed "poly-spermy," renders the embryo incapable of becoming a fetus. The incidence of polyspermy is larger among cigarette smokers than among nonsmokers.
Scientists hypothesize, largely on the basis of animal studies, that cigarette smoking diminishes the number and/or viability of ova. Such an effect may result in premature meno-pause.
A spontaneous abortion, or miscarriage, is a spontaneous, unintentional, and usually early termination of pregnancy, with expulsion from the uterus of a nonviable or lifeless fetus. Women who smoke cigarettes have a greater risk of spontaneous abortion than wo-men who do not smoke.
Of the ap-proximately 140,000 cases of miscarriage per year in the United States, approximately 19,000 have been attributed to cigarette smoking. The fetuses of nonsmokers who miscarry are likelier to have abnormalities—a common cause of spontaneous abortion—than are those of cigarette smokers who miscarry. This finding suggests that cigarette smoking can account for spontaneous abortion independently of fetal abnormalities. Smok-ing can nonetheless cause birth defects and neonatal underweight.
Effects of Smoking on Fertility in Men
While research on the reproductive effects of smoking has focused on women, evidence has increasingly suggested that cigarette smoking adversely affects the male reproductive system as well. Cigarette smoking damages sperm, affects hormone concentrations in men, and reduces blood flow to the penis. This reduction in blood flow can result in erectile dysfunction, or impotence. Recent experimental findings have also linked cigarette smoking in men with low sex drive and low sexual satisfaction.
Several chemicals found in the semen of cigarette smokers are components of cigarette smoke or are metabolic by-products of such components. Some of these chemicals—nicotine, for instance—are found at higher concentrations in semen than in blood. This suggests that some components of cigarette smoke tend to build up in the semen of cigarette smokers.
The ejaculate of cigarette smokers is less than that of their nonsmoking counter-parts. This difference is more pronounced with heavy smokers. Scientists hypothesize that the effects of nicotine on the nervous system, or possibly the low testosterone concentrations that characterize cigarette smokers, account for the low volume of ejaculate. Although low ejaculate volume does not alone impede reproduction, it may be a sign of other reproductive irregularities.
Smokers' sperm density (the concentration of spermatozoa in semen) has been estimated at 22-57 percent less than that of nonsmokers. Such decreases in sperm density can cause infertility in men.
Sperm motility—the ability of spermatozoa to propel themselves through the female reproductive tract—is critical to conception. Smokers' sperm cells, on average, are 20 percent less motile than nonsmokers' sperm cells.
Some evidence suggests that men who smoke cigarettes are likelier to generate sperm of abnormal shape than are men who do not smoke. Sperm malformation may contribute to the occurrence of spontaneous abortion and birth defects.
Cigarette smoking raises the concentration in semen of white blood cells—whose function is to counteract infection—without infection. This increase tends to diminish the ability of the smoker's sperm to enter and unite with the ovum.
Research findings suggest that cigarette smoking may result in damage to the seminiferous tubules. These tubules, which make up the bulk of the testes, are the sites of sperm production. Damage to them can cause diminishment of the production and quality of sperm.
Establishing the apparent relationship of smoking to decreases in the quantity and quality of sperm would require much more research.
Cigarette smoking significantly alters concentrations of the sex hormone testosterone, growth hormone, and other hormones in men. Male cigarette smokers have less testosterone than do male nonsmokers. Men with low testosterone concentrations have a lower sex drive and lower sperm counts than do those with normal or high concentrations of the hormone.
In numerous studies cigarette smoking has been linked to a doubling of the incidence of erectile dysfunction. Smoking diminishes blood flow both in the heart and in the penis. This decrease can result in a type of erectile dysfunction termed "vascular impotence." In one study, 97 percent of those patients diagnosed with vascular impotence were cigarette smokers. In another study, 87 percent of those patients with this condition smoked cigarettes. Heavy smoking has been connected to a high risk of impotence.
Frequency of sex and sexual satisfaction
How cigarette smoking affects sex drive and sexual satisfaction was investigated in a 1999 study of 290 couples undergoing treatment for infertility. None of the female subjects smoked. On avarage the cigarette-smoking subjects stated that they had sex about six times a month, while on average the nonsmokers said that their lovemaking was about twice as frequent. Moreover, nonsmoking couples described their sexual satisfaction more approvingly than did those couples in which the male smoked.
The Bottom Line
Specialists who treat infertility often omit that cigarette smoking can significantly impair both male and female fertility in varied ways. But scientific evidence has clearly implicated cigarette smoking with infertility. Further research should better define their relationship.
ACSH scientific advisor Panayiotis M. Zavos, Ed.S., Ph.D., is Professor of ReproDuctive Physiology and Andrology at the University of Kentucky and Director of the Andrology Institute of America (www.aia-zavos.com), in Lexington, Kentucky.
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