Caffeine Intake and Semen Quality in a Population of 2,554 Young Danish Men

Tina Kold Jensen; Shanna H. Swan; Niels E. Skakkebæk; Sanne Rasmussen; Niels Jørgensen


Am J Epidemiol. 2010;171(8):883-891. 

In This Article

Materials and Methods


Because of the military draft in Denmark, all men 18 years of age, except those with severe chronic disease, are required to undergo a compulsory physical examination to determine their fitness for military service. Some men postpone their examination to continue their education and are therefore called up to serve when they have completed their education.

Trained staff from the University Department of Growth and Reproduction approached these young men when they appeared for this compulsory physical examination in Copenhagen and during 2 time periods in Aalborg, Denmark, and invited them to participate in a study of reproductive function. Men recruited from September 2001 to December 2006 were included in the present study (because the questionnaire they completed included detailed information about lifestyle factors). Participants, who were compensated for their time (500 kr = ~US $100), completed a questionnaire, delivered a semen sample, had a blood sample drawn, and underwent a physical examination. Participants did not differ from nonparticipants with regard to age, but they were better educated than nonparticipants (data not shown). Ethical approval was obtained from the local ethical committee. For a detailed description of the study, refer to Andersen et al.[14] and Jørgensen et al..[15]

Semen Analysis

All men provided a semen sample by masturbation in a room close to the semen laboratory. The period of ejaculation abstinence was recorded, and the semen sample was analyzed according to the World Health Organization's 1999 guidelines,[16] modified in accordance with Jørgensen et al..[17] Since 1996, our laboratory has led a quality control program for assessment of sperm concentration, and the laboratory has kept the interlaboratory difference unchanged in comparison with 2 other laboratories that have also participated since the program started.[15,17,18]

The same experienced technician assessed sperm morphology according to strict criteria[16] within 8 consecutive working weeks.[19] The current analysis includes morphology results for only a subset of men because not all samples had been counted yet. Spermatozoa morphology was assessed in 284 for the 299 men consuming more than 1 bottle (0.5 L) of cola per day, as well as for 97 randomly selected men consuming no cola and 98 randomly selected men consuming less than 1 bottle (0.5 L) of cola per day because we initially found a negative association between cola intake and semen quality.

Physical Examination

Four physicians performed all physical examinations. The Tanner stage of pubic hair and genital development, testicular volumes, the possible presence of a varicocele (stage 1 to 3) or hydrocele, the location of the testes in the scrotum, and the consistency of the testis and epididymis were recorded. Weight and height were measured and body mass index calculated as weight in kilograms divided by squared height in meters. Diseases and conditions found at the physical examination that may affect semen quality (varicocele (stage 2 to 3) or abnormal position of the testes) were summarized in a single variable: "conditions found at the physical examination"; 38 men had more than one condition.


Prior to the examination, all participants completed a questionnaire containing information on previous and/or current diseases and genital diseases such as inguinal hernia, varicocele, epididymitis, gonorrhea, chlamydia, and surgery for testicular torsion. The men were asked whether they were born with both testicles in the scrotum. In addition, they reported whether they had had a fever of >38°C (100.4°F) during the previous 3 months. Self-reported diseases in the reproductive organs affecting semen quality (operation of varicocele, torsion of testes, epididymitis, or sexually transmitted diseases) were transformed into one variable: "self-reported genital conditions"; only one man had more than one of these conditions.

The young men responded to a standard questionnaire about parents' social class conceptualized as parents' education and occupation coded according to the standards of the Danish National Institute of Social Research,[20] which is almost identical to the United Kingdom Registrar General's categorization into 5 social classes from I (high) to V (low), with an additional category: housewife. The social class of the highest ranking parent was used.

Participants were asked, How much did you consume of the following beverages during the last week? Possible responses were as follows: glasses of wine (units), bottles of beer (0.33 L), number of strong alcoholic drinks (12 cL), bottles of cola (0.5 L), bottles of diet soft drinks (0.5 L), and number of chocolate bars (50 g). In addition, they were asked how many cups of coffee, tea, and chocolate-containing beverages they consumed daily during the last week. Alcohol intake was considered the sum of strong alcoholic drinks (approximately 12 g of alcohol in each), glasses of wine, and bottles of beer per week. Each man's daily caffeine intake was estimated by assuming a cup to contain 150 mL and the caffeine content to be 117 mg in one cup of coffee, 70 mg in one cup of tea, 5 mg in one cup of chocolate beverages, 70 mg in 0.5 L of cola and diet soft drinks, and 7 mg in a 50-g chocolate bar.[1] In the analyses, "weekly intake of cola" was calculated as the sum of reported drinks of cola and diet soft drinks (assuming diet soft drinks to be cola).

The men were asked about their dietary habits with the following question: How often do you consume cheese, butter, vegetables, fruits, chicken, lamb or beef, burgers, fish, etc.? Answer categories were never, 1–3 times per month, once per month, 2–3 times per week, once per day, and more than once per day.


Outcome variables were semen volume, sperm concentration, total sperm count, and percentages of motile and morphologically normal spermatozoa. Exposure variables were average daily caffeine intake included as a continuous variable (ln transformed) and categorized as daily intake of 0–100 mg, 101–200 mg, 201–800 mg (1–800 mg), and >800 mg, which corresponds to approximately 1, 2–7, and >7 cups of coffee per day. All analyses were initially performed with total daily caffeine consumption and then for men who reported caffeine intake from coffee, tea, chocolate beverages or bars, diet soft drinks, or cola separately to determine the independent associations with each. Cola consumption was reported as weekly intake of number of 0.5-L bottles; therefore, many men reported intake of 7 and 14 bottles per week, corresponding to daily intake of 0.5 L (70 mg of caffeine) and 1 L (140 of mg caffeine). Cola intake was entered as a continuous variable (transformed by natural logarithm) or categorized as no cola, 1–7, 8–14, and >14 bottles per week.

First, we compared semen quality in men in relation to caffeine and cola intake by the Kruskal-Wallis test. Then, we compared the distributions of the variables from the questionnaires and physical examinations among these groups of men by the chi-square test to identify potential confounders.

Finally, data were examined by using univariate analyses of variance. Normally distributed outcome variables were entered directly as continuous variables in the model, whereas sperm concentration and total sperm count were transformed by use of the natural logarithm to obtain normality and were back-transformed to obtain the percentage change in these semen parameters. Covariates initially included factors possibly associated with semen parameters or caffeine consumption and were then excluded stepwise if they did not change the estimate by more than 10%. Period of abstinence was entered to adjust to 96 hours. The same set of confounders was used for all semen parameters: fever >38°C within the last 3 months, period of abstinence, body mass index, in utero exposure to smoking, conditions found at the physical examinations, self-reported genital conditions and cryptorchidism, and sperm motility time from ejaculation until analysis of the sample. In addition, we estimated the adjusted median of the semen parameters by performing the analyses without including an intercept. Analyses for different types of caffeine intake were performed by including the sources in separate models as well as by simultaneously including all caffeine-containing sources in the same model. In this paper, results are presented as regression coefficients with 95% confidence intervals. We evaluated fit of the regression models by testing the residuals for normality and by inspecting the residual plots.