Sperm from nearly two-thirds of the chronic smokers in the study failed a special test that measures the ability of sperm to fertilize an egg.
On average, those men showed a 75 percent decline in fertilizing capacity when compared to nonsmokers.
" Like other cells in the body, human sperm carry a receptor for nicotine, which means they recognize and respond to nicotine," said Lani Burkman at the University at Buffalo School of Medicine and Biomedical Sciences. " This happens because nicotine from tobacco mimics one of the most important neurochemicals produced in the body.
" Using sperm of nonsmokers, we reported previously that the addition of nicotine changed three sperm functions required to fertilize an egg.
" In this study, we examined whether sperm from chronic tobacco smokers are defective in binding to the zona, the cover surrounding an egg," said Burkman. " Our results could mean that heavy smoking overloads the nicotine receptor in human sperm and in the testes, leading to a decline in fertilizing potential."
The study involved 18 men who reported smoking at least four cigarettes a day, every day, for more than two years. On average, these men had smoked for about 15 years. Their sperm function was compared to that of non-smoking donors who served as controls and whose fertilizing capacity had been confirmed.
Using a test called the Hemizona Assay developed by Burkman, the researchers cut a zona in half, placing one half with a smoker's sperm and the matching half with control sperm. After two to three hours of incubation, researchers counted the number of sperm attached tightly to the outside of each half.
The number of attached sperm from the smoker was compared to the control number, which gave a ratio or index. The Hemizona Assay has been shown to predict fertilization failure during in vitro fertilization.
" To fail, the index must be less than 65, meaning that the smoker's sperm had less than 65 percent of the fertilizing capacity found in the donor," Burkman said. " n index below 36 identifies a severe loss in fertilizing capacity."
Results showed that the sperm from almost two-thirds of the smokers failed the test, while the remainder showed normal function. Almost all the smokers whose sperm failed the test had an index of 36 or less, with an average of 25.
" None of these men had a zero fertilizing potential," said Burkman, " but the results mean that their sperm had only 25 percent of the fertilizing function found in nonsmoking men. The data also showed that the men who failed were smoking about twice as many cigarettes per day, an average of 19 per day, compared to the smokers who passed the assay."
As another way to understand the impact of smoking, the researchers calculated a "smoking load" for each smoker by multiplying the number of cigarettes smoked per day by the number of years smoked. The load varied from 16 to 750 for the 18 men.
Results showed that the men who smoked fewer cigarettes for fewer years had smaller smoking loads, ranging from 16 to 200. In this group, 71 percent passed the Hemizona Assay, indicating normal fertility. The remaining men had a high smoking load, and only 18 percent passed the assay.
" Specialized testing clearly reveals a significant drop in fertility potential for men who are heavy tobacco smokers," said Burkman. " Smoking men also should be aware that smoking can damage their sperm DNA, passing on faulty DNA to their baby. Concerned smokers should quit or be tested in a local andrology laboratory."
Burkman added that other scientists have shown a similar decline in fertility among women who are heavy smokers.
Source: University at Buffalo, 2005