Analysis Reaffirms Smoking’s Impact on Semen Quality

Cigarette smoking remains a major factor in reducing semen quality in adult men, a new study has verified, despite significant changes to the criteria that laboratories worldwide are supposed to use to evaluate semen samples and assess fertility.

The linkage between smoking and decreased sperm quality had previously been documented, but until the study’s recent publication in European Urology, it was unclear how that connection would be affected by the updated laboratory reference values for semen characteristics that the World Health Organization (WHO) instituted in 2010. The updated semen benchmarks, the first to be derived from a population-based study of fertile men, are markedly lower than those used in previous WHO guidelines.

Researchers led by Ashok Agarwal, PhD, HCLD, Director of Cleveland Clinic Glickman Urological & Kidney Institute’s American Center for Reproductive Medicine, set out to reassess the smoking/semen quality connection in light of the WHO testing changes.

“This is important because WHO manuals are widely used by laboratories worldwide, and smokers with abnormal semen analysis results based on older WHO criteria might be considered normal according to the new criteria,” Dr. Agarwal says. “That would affect patient counseling and case management.”

A study of studies

Dr. Agarwal and his colleagues undertook a meta-analysis of 20 studies that were published after the release of the 2010 WHO semen testing manual and that dealt with smoking’s effect on semen volume, sperm count, motility and/or morphology — the most commonly used male infertility semen parameters.

The 20 studies evaluated in the meta-analysis involved a total of 5,865 male participants age 13 and older and compared infertile and fertile smokers and non-smokers. Only seven of the 20 studies used the latest 2010 WHO semen testing guidelines; the remaining 13 employed older criteria published in 1992 and 1999. Consequently, the researchers conducted a subgroup analysis to determine whether the semen testing guidelines (new or older) would change the magnitude of any observed effect of smoking on semen quality.

Smoking’s effects on semen

The meta-analysis confirmed cigarette smoking’s overall negative impact on conventional semen parameters. Study participants’ smoking was associated with reduced sperm count (mean difference [MD] -8.92 x 106 mL; 95 percent confidence interval [CI] -12.40 to -5.44; p < 0.001), motility (MD -3.48 percent; 95 percent CI -5.53 to -1.44; p < 0.001) and morphology (MD -1.37 percent; 95 percent CI -2.63 to -0.11; p < 0.001).

The 2010 WHO laboratory manual for the examination of human semen had an insignificant effect on the observed pooled effect size of all semen parameters except sperm morphology.

Subgroup analysis found more negative consequences on semen quality in infertile men compared with the general population. In addition, moderate/heavy smoking (from 10 to more than 20 cigarettes per day) caused more substantial sperm damage than mild smoking (1 to 10 cigarettes per day).

Future research directions

Since the majority of studies in the meta-analysis employed previous WHO semen analysis criteria, more research is needed to fully understand how modified methods affect the smoking/semen quality connection.

Additional research also is necessary to illuminate the mechanisms by which smoking degrades semen quality, the impact of smoking duration and cessation on semen characteristics, and the clinical effects of smoking on fecundity rates.

Nonetheless, the meta-analysis’s findings have implications for smokers, clinicians and policymakers. Since smoking is a modifiable behavioral factor, Dr. Agarwal says clinicians should promote smoking cessation programs to men seeking infertility treatment not only as a way to reduce health risks but as a way to improve semen quality.

“To our knowledge, this meta-analysis is the first to summarize the evidence currently available on the association between cigarette smoking and sperm quality after the publication of the latest WHO laboratory manual for the examination of human semen,” Dr. Agarwal says.