After coming home from a stressful day at work, cracking open a fresh, cold beer to relax can be tempting. But a new study published in BMJ Open suggests that even a moderate weekly alcohol intake of five units is linked to lower sperm quality in men who are otherwise healthy.

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Men who drink as little as five units of alcohol per week, which is a moderate amount, have reduced sperm quality, according to the latest study.

According to the research team, including Prof. Tina Kold Jensen of the University of Southern Denmark, previous studies conducted in animals have suggested that alcohol could have a direct impact on sperm quality.

“This is, to our knowledge, the first study among healthy young men with detailed information on alcohol intake,” say the researchers, “and given the fact that young men in the Western world [drink a lot], this is of public health concern, and could be a contributing factor to the low sperm count reported among [them].”

To conduct their study, they assessed 1,221 Danish men aged 18-28, who underwent a medical exam for military service between 2008-2012.

The assessment included questions about how much alcohol the men drank in the week before their exam, whether this was typical, how often they binge drank (defined as more than five units in one sitting), and whether they had been drunk in the preceding month.

Additionally, the men provided a semen sample and a blood sample, which assessed their reproductive hormone levels.

The researchers found that 64% of the participants had engaged in binge drinking during the preceding month, and around 60% said they had been drunk more than twice during that period. The average number of units the men drank during the preceding week was 11.

After considering certain influential factors, the researchers found that there was no significant link between sperm quality and alcohol consumption or binge drinking in the preceding month.

However, they did find that drinking alcohol in the preceding week was associated with changes in reproductive hormone levels, and these changes increased significantly as the number of weekly units increased. In detail, testosterone levels increased and sex hormone binding globulin (SHBG) decreased.

Nearly half of the men said that the amount of alcohol they drank in the previous week was typical of their usual weekly consumption.

Additionally, within this group of men, the higher the amount of weekly units, the lower the sperm quality. Assessments for sperm quality include total sperm count and the ratio of sperm that are of normal size and shape.

The researchers say these effects were noticeable in men who drank five or more units a week, but they were most apparent in men who drank 25 or more units each week. In men who drank 40 units per week, total sperm counts were 33% lower, and the proportion of normal-looking sperm was 51% lower, compared with men who only drank one to five units.

It seems that drinking between one and five units per week is the magic number for healthy sperm, as – interestingly – abstinence was also linked to poorer sperm quality.

When asked why men who abstained from drinking had poorer sperm quality than those men who drank between one and five units of alcohol per week, Prof. Jensen told Medical News Today that she and her team did not have any hypotheses.

However, she added that other studies have found similar outcomes, suggesting that men who do not drink at all are different in some way, “and maybe some don’t drink because of an underlying disease.”

Despite the strength of the large sample size, the study does have certain limitations. For example, it was observational, so no concrete conclusions can be made regarding cause and effect.

The investigators note that their findings could even be the result of reverse causation, in which men with poorer sperm quality have unhealthier lifestyles and behaviors to begin with.

The team also adds that because the men in their study reported daily alcohol consumption in the week before the visit, this consumption could differ from their actual typical weekly intake, potentially leading to “misclassification of exposure.”

Still, their results prompt them to suggest that young men should steer clear of what they call “habitual drinking,” adding:

It remains to be seen whether semen quality is restored if alcohol intake is reduced, but young men should be advised that high habitual alcohol intake may affect not only their general, but also their reproductive health.”

In June of this year, MNT reported on a study that suggested cell phones may reduce sperm quality.