Men who smoke more than 20 cigarettes a day have a 14% lower concentration of the protamine P2 than their nonsmoking counterparts, according to a study of 53 smokers and 63 nonsmokers. P2 helps package DNA into chromosomes; poor packaging is associated with reduced chances for fertilization after intercourse or assisted reproductive techniques, a higher incidence of miscarriage, and other fertility problems.

According to the Human Reproduction report by Mohamed Hammadeh, MD, of the University of the Saarland in Saarbücken, Germany, and colleagues, smoking also throws off the ratio of P1 to P2 protamines, signifying some form of infertility.

The study also revealed that levels of oxidative stress were higher in smokers than non-smokers.

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