Amodel for the relation of job control and job demand with health outcomes was developed by Karasek and Theorell in the 1980s.1 In the field of human reproduction, this model has been used in studies of various end points such as menstrual cycle pattern, spontaneous abortion, birth weight, and gestational age.2–6 In analyses of female job strain, we found no support for a substantial detrimental effect on fecundability, although secondary analyses suggested a possible effect in couples with no corresponding causes of reduced fertility.7 In the same population, the effect of the man’s daily life psychologic stress on his semen quality has been found to be small or nonexistent. The results indicated an effect of stress only on fecundability, and this only among men with low sperm concentration.8 No published studies have dealt with job control and demand in relation to male fertility. The aim of this study was to estimate whether male job strain influences characteristics of semen quality and couple fecundability.
From 1992–1994, 430 Danish couples were recruited through a nationwide mailing to 52,255 trade union members (metal workers, office and commercial workers, nurses, and day-care workers) who were childless, 20–35 years old, and living with a partner. Only couples without previous pregnancies and without previous periods with unprotected intercourse were eligible. A detailed description of the cohort and design is given elsewhere.9 The couples were enrolled when they discontinued use of birth control methods; they were then followed for 6 months or until a pregnancy was recognized, whichever came first. The woman recorded vaginal bleeding and sexual intercourse in a structured diary. Twelve men did not deliver a semen sample and 19 had incomplete answers in the basic questionnaire. Analyses of semen quality and fecundability are based on the remaining 399 couples.
The couples were enrolled at 2 centers: the Department of Occupational Medicine in Aarhus (West Center) and the Department of Growth and Reproduction in Copenhagen (East Center). At enrollment, both partners filled out a questionnaire on job strain and demographic, medical, reproductive, occupational, and lifestyle factors. On the 21st day of each menstrual cycle during follow up, the couples completed questionnaires on changes in job strain, occupational exposures, and lifestyle factors. Psychologic job strain was assessed using questionnaires developed by Karasek and colleagues.1,10 The assessment of job strain includes at least 2 dimensions: job demand and job control (24 items). Information on support from supervisor and coworkers was also obtained (12 items). Based on information collected at the start of follow up, the men were categorized by tertiles in each of the 2 dimensions. Men were also categorized according to the combined dimensions in which job demand and job control were dichotomized at the median, resulting in 4 types of jobs: low-strain jobs (low demand and high control score), active jobs (high demand and high control), passive jobs (low demand and low control score), and high-strain jobs (high demand and low control score). Information on changes in job strain during follow up was collected from men at the 21st day of their partners’ menstrual cycles. Men were asked to state whether their job demand and job control had each increased, stayed the same, or decreased.
Each man provided a fresh semen sample at enrollment. Seminal volume, sperm concentration, and sperm morphology were classified according to the 1992 WHO criteria.11 Computer-assisted analysis was carried out using the CRISMAS system (Image House, Copenhagen, Denmark).12 Serum was stored for later determination of testosterone, estradiol, luteinizing hormone, and follicle-stimulating hormone. Further details are given elsewhere.9,13
Associations of job strain variables with semen characteristics and sexual hormones were analyzed in linear regression models with transformation to obtain constant variance and a symmetric distribution of residuals, when necessary. The odds for pregnancy during a menstrual cycle (fecundability) were analyzed by logistic regression while controlling for cycle number and potential confounders.14 No pregnancies occurred during menstrual cycles in which no sexual intercourse occurred between days 11 to 20, and so these cycles were excluded. Further details are given elsewhere.7,9 The following variables were selected a priori as potential confounding factors in analyses of semen parameters and sexual hormones: age, reproductive disorders, alcohol consumption, and center. Semen parameters analyses also included time of day of collection, season, and logarithm of days of sexual abstinence. Confounding factors included in analyses of fecundability are listed subsequently.
The median score on the job-demand and job-control axes were 29 (range, 12–48) and 72 (range, 28–96), respectively. Characteristics of the 399 men are shown in Table 1. Both high job demand and high job control were most prevalent in men with higher age and education, and high job demand was also more prevalent among smokers.
Median values of semen characteristics and sexual hormones for the various levels of job-strain variables are shown in Table 2. In crude analyses, a reduced straight line velocity was found in men with high strain jobs compared with men in low strain jobs (for transformation of VSL1.5: β = −60,P[r] = 0.005). In the adjusted analyses, there was very little difference by job for any of the outcomes listed in Table 2 (for VSL1.5: β = −33,P[r] = 0.1). Center was the variable with the largest influence on the adjusted estimates, but results from analyses stratified by center gave similar results. Exclusion of 43 men with known reproductive disorders did not change these results.
Within the follow-up period of 6 menstrual cycles, 240 women became pregnant (60.2%). The odds for pregnancy per cycle were not associated with any of the job strain variables (Table 3). Exclusion of couples with reproductive disorders as well as cycles with less than 1 intercourse per week did not change the results. Results were similar after exclusion of 753 cycles in which men reported changes from baseline values in job demands or job control. All results were also unchanged when stratified by sperm count 20 million spermatozoa per milliliter. Stratification by social support had no modifying effect.
Male job demand and job control was not associated with fecundability in this study. A crude association between job strain and one aspect of sperm motility (“straight line velocity”) disappeared when controlling for potential confounders. A large number of comparisons were made, and it seems likely that the crude finding was by chance. The related cancer-associated semen analysis (CASA) variable “curved line velocity” showed no evidence of association with job strain in either the crude or adjusted analyses.
The advantage of this study was the prospective data collection. However, the size of the study limited the possibilities of subanalyses, including analyses of men with extreme job strain. Participation in the study was rather demanding, and some men might have declined the invitation because of high demands at work. The negative results can therefore not be extrapolated to evaluate effects of more severe job strain.
We thank secretary Jane Boilesen, research nurse Charlotte West, and technicians from the laboratories for skillful assistance. We also thank several trade union officials for support: Ernst Bliesmann, Peter Olesen, Rigmor Laulund, and Niels Nedergaard in particular.
The Danish First Pregnancy Planner Study is a collaborative follow-up study on environmental and biological determinants of fertility. The project is coordinated by the Steno Institute of Public Health, University of Aarhus, and is undertaken in collaboration with the Department of Growth and Reproduction, National University Hospital in Copenhagen. The team includes Jens Peter E. Bonde, Niels Henrik I. Hjollund, Tina Kold Jensen, Tine Brink Henriksen, Henrik A. Kolstad, Erik Ernst, Anna-Maria Andersson, Aleksander Giwercman, Niels Erik Skakkebæk, and Jørn Olsen.
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