Smoking affects sperm's ability to fertilize

February 1, 2006

Montreal--In case your patients need another reason to quit, recent research indicates that smoking tobacco reduces the ability of sperm to fertilize an egg. There appears to be a dose-response relationship, with heavy smokers at greater risk for more severe impairment of sperm, according to a study presented at the American Society for Reproductive Medicine annual meeting here. It remains to be seen whether quitting smoking or even cutting down will lead to improvements in sperm functions that support fertilization.

Montreal-In case your patients need another reason to quit, recent research indicates that smoking tobacco reduces the ability of sperm to fertilize an egg. There appears to be a dose-response relationship, with heavy smokers at greater risk for more severe impairment of sperm, according to a study presented at the American Society for Reproductive Medicine annual meeting here. It remains to be seen whether quitting smoking or even cutting down will lead to improvements in sperm functions that support fertilization.

Using the Hemizona Assay (HZA), which has been used to predict fertilization failure during in vitro fertilization, researchers from the University of Buffalo, NY, examined the semen of 20 men who were chronic smokers. The study required that they must be smoking more than four cigarettes a day for at least 2 years. The semen quality from two of the men, however, was too poor to perform HZA. As a result, the sperm from 18 of the experimental subjects, who had been smoking for a mean of nearly 16 years, was compared with the sperm from screened research donors, who served as controls.

The HZA involves cutting nonviable eggs in half via micromanipulation and using two matching zona pellucida halves in each experiment. Washed sperm from each smoker were incubated with one hemizona, while washed sperm from a control donor were incubated with the matching hemizona. After an incubation period of 2 to 3 hours, each hemizona was removed from the sperm and rinsed. The investigators then counted the number of sperm tightly bound to the outer surface of each hemizona, explained Lani Burkman, PhD, a research associate professor in obstetrics and gynecology and director of the section on andrology at the University of Buffalo.

"Tobacco smokers often have abnormally high levels of hyperactivation even before you wash them-before you capacitate the sperm. That's not good. Once a sperm has turned on its vigorous hyperactivated swimming, it cannot maintain hyperactivation very long. It will probably 'burn out' before it reaches the egg. Sperm are heavily affected by nicotine because they carry a cholinergic nicotinic receptor. That means that sperm respond to both acetylcholine and nicotine."

The number of patient sperm bound to one hemizona divided by the number of control sperm bound to the matching hemizona defines the Hemizona Index (HZI). Patients are considered to have failed the HZA when they have an HZI under 65, which indicates that the patient's sperm have egg-binding capacity that is less than 65% of the function seen in healthy, control sperm. An HZI below 36 would indicate a severe loss of fertilization capacity.

Among the 18 patients studied by Dr. Burkman and her team, 11 failed the HZA, with a mean HZI of 28. The remaining seven had an HZI within the normal range. Nine of the eleven patients who failed the HZA had a score less than 36, indicating a severe loss in fertilization capacity.

Heavier smoking, worse results

Heavier smoking was linked with poorer fertilization capacity. The investigators calculated a Smoking Load Index (SLI) by multiplying the number of cigarettes smoked per day by the number of years of continuous smoking. Among the seven smokers with a smaller SLI (16 to 180), 71% passed the HZA. In contrast, only 18% of the 11 smokers with a higher SLI (200 to 750) passed the HZA (p<.05).

"If a guy is absolutely addicted to nicotine, he had better go light if he wants to maintain decent fertility," Dr. Burkman said.

Light smokers, she said, smoked about 11 cigarettes per day, compared with 19 cigarettes per day for the heavy smoking group. It is possible that a further reduction to five or six cigarettes per day
would greatly improve their fertility, she speculated.