Even moderate weekly alcohol intake may be linked to poorer sperm quality in healthy young men, a new study has claimed.
The effects are apparent after more than five units, but impact worsens with increasing weekly tally, researchers said.
They base their findings on 1,221 Danish men between the ages of 18 and 28, all of whom underwent a medical examination to assess their fitness for military service between 2008 and 2012.
As part of their assessment, the military recruits were asked how much alcohol they drank in the week before their medical exam (recent drinking); whether this was typical (habitual); and how often they binge drank, defined as more than 5 units in one sitting, and had been drunk in the preceding month.
They were also invited to provide a semen sample to check on the quality of their sperm, and a blood sample to check on their levels of reproductive hormones.
The average number of units drunk in the preceding week was 11. Almost two thirds (64 per cent) had binge drunk, while around six out of 10 (59 per cent) said they had been drunk more than twice, during the preceding month.
The analysis published in the journal BMJ Open showed that after taking account of various influential factors, there was no strong link between sperm quality and either recent alcohol consumption or binge drinking in the preceding month.
But drinking alcohol in the preceding week was linked to changes in reproductive hormone levels, with the effects increasingly more noticeable the higher the tally of units.
Testosterone levels rose, while sex hormone binding globulin (SHBG) fell; similar associations were also evident for the number of times an individual had been drunk or had binge drunk in the preceding month, researchers said.
Almost half (45 per cent, 553) of the men said that the quantity of alcohol they drank in the preceding week was typical of their weekly consumption.
And in this group the higher the tally of weekly units, the lower was the sperm quality, in terms of total sperm count and the proportion of sperm that were of normal size and shape, after taking account of influential factors.
The effects were evident from five plus units a week upwards, but most apparent among those who drank 25 or more units every week.
And total sperm counts were 33 per cent lower, and the proportion of normal-looking sperm 51 per cent lower, among those knocking back 40 units a week compared with those drinking 1-5.
Habitual drinking was associated with changes in reproductive hormone levels, although not as strongly as recent drinking, while abstinence was also linked to poorer sperm quality.
Researchers point out that the findings could be the result of reverse causation - whereby men with poor quality sperm have an unhealthier lifestyle and behaviours to start with.
