The finding comes from a study of sperm from 53 heavy smokers and 63 nonsmokers among male partners of couples seeking help for infertility.
Previous studies show that men who smoke are less fertile than men who don't smoke. Now a research team led by Mohamad Eid Hammadeh, PhD, professor of obstetrics and gynecology a the University of the Saarland, Homburg/Saar, Germany, have learned why.
Human sperm cells carry two tiny, highly charged proteins called protamine 1 and protamine 2. Nature keeps them in a perfectly balanced one-to-one relationship. But in smokers, Hammadeh and colleagues find, sperm cells carry too little protamine 2. This imbalance makes them highly vulnerable to DNA damage.
"The DNA alphabet of these sperm has one or two letters missing. And this cannot be repaired," Hammadeh tells WebMD. "When we inject these damaged sperm into an egg cell, the sperm is not capable of fertilizing the cell. And even if it does, the [miscarriage] rate is very high."
What appears to be happening is that smoking-damaged sperm lose much of their ability to fight off destructive oxygen molecules -- free radicals -- in the seminal fluid. Interestingly, in addition to making sperm cells more sensitive to oxidative stress, smoking itself increases the concentration of free radicals in the seminal fluid.
"Free radicals can cause sperm DNA fragmentation as well as issues with sperm motility and fertilization," reproductive endocrinologist Adam Griffin, MD, of the University of Rochester, N.Y., tells WebMD. Griffin was not involved in the Hammadeh study.
In earlier work, Hammadeh and colleagues showed that men with fertility problems have higher levels of free radicals than fertile men.
Both Hammadeh and Griffin advise men and women with fertility problems to stop smoking.
"We tell couples seeking treatment for fertility to stop smoking at least three months before we attempt in vitro fertilization, because sperm needs at least that much time to be produced from germ cell to full sperm," Hammadeh says.
Infertility: Can Antioxidants Help?
Griffin notes that smoking also affects female fertility. And even if only one member of a couple smokes, secondhand smoke tends to affect the fertility of his or her partner.
"I usually tell people there is no magic bullet as far as supplements are concerned, but taking antioxidants like vitamin C and vitamin E can do no harm and may provide some marginal benefit," Griffin says.
Hammadeh and colleagues report their findings in the journal Human Reproduction, published online on Sept. 7.