Drinking five units of alcohol every week could reduce the quality of a man's sperm, new research suggests.
A study published in the journal BMJ Open examined 1,200 Danish male military recruits between the ages of 18 and 28, all of whom underwent a medical examination between 2008 and 2012.
They were asked about their drinking habits and invited to provide sperm and blood samples. The average number of units drunk in the preceding week was 11.
Researchers found that drinking alcohol in the previous week was linked to changes in reproductive hormone levels, with the effects increasingly noticeable as more alcohol was consumed.
Testosterone levels rose while sex hormone binding globulin (SHBG) fell.
Almost half said the quantity of alcohol drunk in the preceding week was typical of their normal weekly consumption.
Researchers found that the higher the tally of weekly units, the lower was the sperm quality in terms of sperm count and the proportion of sperm that were of normal size and shape.
The effects were evident in those who drank five or more units a week and most apparent in those who drank 25 units or more.
Those who drank 40 units a week had a 33pc lower sperm count and fewer "normal-looking" sperm than men who drank between one and five.