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

Disclosures

Am J Epidemiol. 2010;171(8):883-891. 

In This Article

Discussion

In this study of more than 2,500 Danish young men, caffeine intake of ≤800 mg per day and cola consumption of ≤14 0.5-L bottles per week was not associated with reduced semen quality. However, we observed an apparent "threshold" after which especially cola consumption (1 L per day) was associated with a reduction in semen quality. The reduction in semen quality among high-quantity cola drinkers, if causal, must be attributed to constituents in cola other than caffeine because the caffeine content of cola is not high. Alternatively, these associations may be attributed to the less healthy lifestyle and diet of high-quantity consumers.

Coffee has been associated with low levels of estrogen[21] and high levels of testosterone and sex hormone-binding globulin.[22] Previous studies of caffeine intake and semen quality have shown contradictory results[7–13] but had limited control for confounders. One study suggested no associations,[12] whereas others found increased motility.[9–11] Vine et al.[8] found weak evidence for an association between caffeine intake from coffee, tea, and soft drinks and sperm nuclear morphometry, and Parazzini et al.[13] found an increasing risk of poor semen quality with increasing coffee consumption. The only other known study conducted among 343 unselected young Danish men found no adverse effect of caffeine intake on semen quality[7] but increased testosterone levels with increasing caffeine intake,[7] which we did not find in our data. However, that study obtained information only about coffee and tea intake, which may underestimate caffeine intake because many young men drink appreciable amounts of cola. Besides coffee, tea, and cola, we obtained information about chocolate-containing drinks, diet soft drinks, and chocolate bar consumption, providing a more precise estimate of caffeine intake and enabling us to examine the associations with different types of caffeinated products.

Our participation rate was 31%, which is higher than in other population-based semen-quality studies.[15,18,23,24] In addition, because the majority of our young men had no knowledge of their own fertility potential, this factor is unlikely to have affected their motivation to participate. In addition, our goal was to compare semen quality among men with different caffeine and cola intakes, so whether the men were representative of the general population is of secondary importance.

The men in our study reported caffeine consumption the week before they completed the questionnaire because we assumed that to be more accurate to recall than average intake. If this consumption differed from the typical intake, misclassification of exposure may have occurred. We estimated that a cup of coffee contains 150 mL and 117 mg of caffeine, but it will vary depending on cup size, method of preparation, and product brand. In addition, we did not obtain information on type of tea consumed.

The questions about cola and diet soft drink consumption were not very accurate because the men were asked about cola or diet soft drink intake only, not about type of drinks. The caffeine content of diet soft drinks was estimated to be similar to that of cola (70 mg and 0.5 L), although not all soft drinks contain caffeine. The Danish Brewers Association reports that 64% of the sales of soft drinks in Denmark are of cola.[25] In addition, very few men had a high intake of "diet soft drinks," which had no independent effect on semen quality. We also repeated the analyses excluding diet soft drinks from total caffeine and cola intake, which did not change the findings. These potential sources of exposure misclassification are all likely to be random and not related to semen quality, since the men responded to the questionnaire before they knew the result of their semen analysis, and therefore underestimate the associations between caffeine and semen parameters. In addition, the dietary questionnaire was not validated, and the men were just asked approximately how often they consumed different food items.

It is well known that interobserver variability in semen analysis exists and is particularly high for motility assessment, which may help explain the lack of an association of caffeine and cola consumption with motility. However, all analyses were performed blinded, and the same technician assessed all morphology slides. Furthermore, our laboratory participated in an external quality control program. We obtained only one semen sample for each man, and intraindividual variability exists, which may have introduced nondifferential misclassification and thereby underestimated the effects.

Men who consumed no caffeine had better semen quality but also a more healthy lifestyle. High-quantity consumers of cola or caffeine had an unhealthier lifestyle, which has previously been associated with poorer semen quality.[26–30] To the extent possible, we considered these factors in the analyses, and they did not appear to explain the caffeine and cola associations. High-quantity caffeine and cola consumers also had a less healthy diet, and previous studies have found reduced semen quality among men who consumed few fruits and vegetables[31] and had a low intake of antioxidant and trace minerals.[32,33] We repeated the analyses taking into account these factors (data not shown), but they did not explain the negative association we observed with caffeine and cola intake. High caffeine and cola consumption may also be related to in utero exposure to caffeine,[7] working in a sedentary position,[34,35] being less physical active,[12,36,37] or being more stressed,[38] variables that have previously been associated with poorer semen quality. Unfortunately, we did not obtain information about these factors.

Habitual moderate coffee drinking has been associated with a reduced risk of chronic diseases, including cancer, and reduced mortality, whereas high intake has been associated with increased risk.[39] The effect of cola intake on reproduction has not been intensively studied, but it has been associated with increased incidence of osteoporosis.[40] Colas were originally blends of extracts of the coca leaf and the cola nut, mixed with sugar water. The coca leaf is no longer used, but the cola nut remains in the recipes that are public, and it is reportedly also still in the secret Coca-Cola recipe (The Coca-Cola Company, Atlanta, Georgia). In addition, these drinks contain large quantities of sugar. A report from The Danish Institute for Food and Veterinary Research showed that consumption of sugar-sweetened soft drinks increased from 133 mL to 184 mL per day among Danish teenagers and from 110 mL to 121 mL per day among adults from 1995 to 2001.[25] In our study, mean daily cola intake was 290 mL, which was higher than among Danish teenagers in 2001, indicating that intake increased from 2001 to 2005. Therefore, a possible adverse association with semen quality is of public interest, particularly since poor semen quality in young Danish men is unexplained.[14,15,41,42]

In conclusion, we found that moderate caffeine or cola consumption (≤800 mg or 1 L of cola per day) was not associated with a reduction in semen quality. However, among the small fraction of men (3%) who consumed "high" quantities of cola, and possibly caffeine, daily (exceeding 800 mg or >1.0 L, respectively), several semen parameters were reduced. The associations found for high-quantity cola drinkers could not be attributed to the caffeine content in cola, which was not high. We cannot exclude the possibility of a threshold above which cola (and possibly caffeine) negatively affects semen quality. Alternatively, a less healthy lifestyle among these men may explain the findings. Since cola consumption is high and has been increasing among young Danes, our findings, if confirmed, may be of public health concern.

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