“Just five alcoholic drinks a week could reduce sperm quality,” The Guardian reports. A study involving Danish military recruits found that even moderate drinking, if done regularly, was associated with a drop in quality.
The study involved 1,200 young Danish military recruits (with an average age of 19), and assessed their semen quality, as well as questioning their alcohol intake in the week preceding the sample, or binge drinking in the past 30 days.
Overall, there was no clear association between semen quality and alcohol intake. However, in analyses restricted to the 45% of men who said this was a typical week for them, there was a dose-response relationship, with higher alcohol intakes associated with lower sperm quality.
Men who did not drink any alcohol at all also had impaired sperm quality. They could have had health issues that impacted on their sperm quality and also meant they needed to avoid drinking, although this is pure speculation.
As always, there are limitations. Importantly, as the study assessed alcohol intake and sperm quality at the same time, it cannot prove cause and effect. Various other factors could also influence the relationship.
There is also the possibility of inaccurate recall of alcohol units consumed, although we would suspect that young men tend to underestimate rather than overestimate how much they drink.
We also don’t know whether any of the measures of reduced sperm quality observed would actually have any effect on fertility.
Nevertheless, the detrimental effects of high alcohol intake in various areas of health are well known, so laying off the drink for a few days a week certainly wouldn’t hurt.
Where did the story come from?
The study was carried out by researchers from University of Southern Denmark, University of Copenhagen and Icahn School of Medicine at Mount Sinai, New York, and was funded by The Danish Council for Strategic Research, Rigshospitalet, European Union, DEER, The Danish Ministry of Health and The Danish Environmental Protection Agency, and Kirsten and Freddy Johansens Foundation.
The UK media's reporting of the study is accurate and included some useful observations from independent fertility experts. However, the reporting doesn’t make clear that, overall, there was no clear association between semen quality and alcohol intake. An association was only seen in men who reported habitually drinking five units or more.
What kind of research was this?
This was a cross sectional study which aimed to look at the association between alcohol consumption, semen quality and reproductive hormones.
As the researchers say, several research studies have associated excessive alcohol consumption and binge drinking (defined in the paper as five units or more in a single day; roughly the same as two standard cans of UK premium 5% abv lager) with adverse health outcomes. Some studies have reported an association between alcohol consumption and semen quality, while others have not.
However, few studies have specifically examined the effect of binge drinking.
The main limitation with this type of study is that being cross sectional, it cannot show that alcohol consumption causes poor semen quality. It cannot show that the men previously had higher-quality semen and that they then developed these alcohol consumption patterns and had this effect. There could be other factors (confounders) that explain the association seen.
For example, the results of this study could also be used to suggest that men with poor semen quality are more likely to drink.
A more suitable study design would be a cohort study, where men are followed over many years, but these are both expensive and time-consuming to carry out.
What did the research involve?
This Danish study used a specific population of 1,221 men (average age of 19) recruited to compulsory military service between January 2008 and April 2012. At recruitment, they undergo a compulsory physical examination and were invited to have an assessment of semen quality. The semen samples were analysed for volume, sperm concentration, total sperm count and percentage mobile and morphologically normal. Blood samples were also tested for levels of sex hormones such as testosterone.
All the men completed a questionnaire which, as well as collecting medical information, also included assessment of alcohol intake. They completed a diary reporting their daily intake of red and white wine, beer, strong alcoholic drinks, alcopops and others during the week prior to the semen and blood samples. They were asked to give their intake in units, being told that one standard beer, one glass of wine or 40ml of spirits contained 1 unit of alcohol (≈12g of ethanol), one strong beer or one alcopop contained 1.5 units of alcohol, and one bottle of wine contained 6 units.
Alcohol intake was calculated as the sum of daily reported unit intakes within that week. They were asked whether the intake in that week was typical for them (habitual intake). They were also asked how many times during the past 30 days they had been drunk or had consumed more than five units of alcohol on one occasion, which was defined as binging.
In their analyses they considered alcohol intake in five unit intervals, with the intake of one to five units as the reference category to which all others were compared. They also categorised the number of binge episodes and the number of times a person was drunk in the past week.
What were the basic results?
The median (average) alcohol intake in the preceding week was 11 units, and beer was the most common drink (making up an average 5 units). In the past month 64% of the men had binge drunk and 59% had been drunk more than twice. Almost half of the men (45%) said the preceding week had been a typical intake week for them.
Semen quality generally decreased with increasing alcohol intake and binge drinking. Men with an intake of 30 units, or who frequently binged, tended to have higher caffeine intake, were more likely to be smokers, were more likely to report having had sexually transmitted infections and were younger. Overall, after adjusting for confounders, such as time since last ejaculation, smoking and body mass index (BMI), there was no clear association between semen quality and alcohol intake or binge drinking.
There was a dose-response association between increasing units per week (or increasing binge episodes or being drunk) and higher blood testosterone levels, and lower sex hormone-binding globulin (SHBG), indicating more testosterone is freely available to the body tissues. This dose-response association remained after controlling for confounders.
In analyses restricted to the 45% of men who said that this was a typical week for them, there was a dose-response association: as alcohol intake went up, sperm concentration, total sperm count and percentage morphologically normal sperm went down, even after adjustment. The trend was more pronounced among men with a typical weekly alcohol intake above 25 units.
No alcohol intake at all was also associated with reduced semen quality. It is unclear why this is the case.
How did the researchers interpret the results?
The researchers conclude that, “Our study suggests that even modest habitual alcohol consumption of more than 5 units per week had adverse effects on semen quality, although most pronounced associations were seen in men who consumed more than 25 units per week. Alcohol consumption was also linked to changes in testosterone and SHBG levels. Young men should be advised to avoid habitual alcohol intake.”
This study of more than 1,200 young Danish military recruits finds some associations between alcohol intake and measures of semen quality and sex hormones.
Overall, after adjustment for confounders, there was no clear association between alcohol intake in the past week or binge drinking in the past 30 days and semen quality. However, in analyses restricted to the 45% of men who said this was a typical week for them, there was a dose-response relationship, with higher alcohol intakes being associated with lower sperm concentration, total sperm count and percentage morphologically (structurally) normal sperm.
Increasing alcohol intake was also associated with increased levels of free testosterone in the body.
However, there are various points to consider when interpreting this study:
- The main limitation with this study is that, being cross sectional, it cannot prove cause and effect. We do not know that the alcohol consumption has directly influenced the measures of sperm quality. Various other health and lifestyle factors could also be having an influence on the relationship (adjustment was only made for confounders of time since last ejaculation, smoking and BMI). For example, men who drink more may have poorer overall diet and activity and lifestyle habits, and these things may all be associated.
- There is the possibility of inaccurate recall or inaccurate calculation of units of alcohol consumed in the previous week, or numbers of past binge drinking episodes.
- Also, though the researchers asked whether this was a “typical week”, it cannot be known how representative it was of longer-term patterns. This is especially the case as this was a week in which they were due to be called up to military service which, depending on individual personality, may cause them to drink either more or less than usual.
- Though this is a large sample of men, they were all young adult Danish men recruited to the military. Therefore, they may not be representative of all populations.
- We don’t know that any of the measures of reduced sperm quality observed would actually have any effect on fertility.
Overall, this study is a valuable contribution to the body of literature assessing the relationship between alcohol intake and effects on semen quality, but it doesn’t provide conclusive answers on its own.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Guardian, 3 October 2014
The Daily Telegraph, 3 October 2014
Mail Online, 2 October 2014
Links to the science
BMJ Open. Published online October 2 2014