Researchers from the University of Southern Denmark carried out the study in an aim to find a link between omega-3 and semen quality as well as reproductive hormone levels.
Previous studies, including randomised controlled trials, have shown that supplementation of omega-3 fatty acids improve sperm parameters in men presenting with infertility, whereas this study involved healthy men.
Men who took the omega-3 supplements regularly (at least 60 out of the past 90 days) were found to have testicles 1.5ml larger and to ejaculate 0.64ml more sperm, on average.
Larger testicles and more sperm creation is linked to higher testosterone levels and better fertility, although the study did not test how fertile the men were.
Whilst experts praise the study for being well administered, they note several drawbacks due to the use of an observational study.
Prof Allan Pacey, Professor of Andrology at University of Sheffield, says the fact this study looks at the key hormones involved in making sperm gives it an additional dimension to other studies looking into nutritional impacts on sperm health.
He adds that the use of healthy men from the general population makes the study interesting because it rules out many other health confounders that could influence the result and the men would not have been actively looking to improve their sperm, as would be the case with many men from a fertility clinic.
But, as with many other expert responses to the study, he points out that causality is not proven in this study.
“Before men or their partners reach for the cod liver oil, it is important to note that this study is not a randomised controlled trial. As such, we cannot conclude absolutely that it’s the fish oil which is causing this improvement.
"Having said that, the study is large, was carried out by an established group with a very good reputation in this area and attempts to control for many variables, such as alcohol intake and smoking. But to be absolutely sure that it’s the fish oil that had sperm boosting qualities, someone would have to run a side by side comparison of men who were taking it compared to those taking a dummy pill (placebo).”
Prof Sheena Lewis, Emeritus Professor of Reproductive Medicine at Queen’s University Belfast, says that the study's focus on healthy young men, mostly with sperm counts already in the fertile range, means there is no evidence from this study that infertile men with low sperm counts benefit from fish oil.
Dr Rod Mitchell, Research Group Leader at the MRC Centre for Reproductive Health at the University of Edinburgh, agrees and points out that the researchers could not be sure how much omega-3 the participants had actually consumed.
“The study was based on questionnaire reporting of fish oil use without direct measurements of intake. Therefore, it is not possible to determine how much fish oil (and omega-3 fatty acids), these men had consumed.
“Detailed information on the dietary history was not available for these men. Given that these fatty acids are present in foods such as oily fish, some men who did not take fish oil supplements may already have a high intake of omega-3 fatty acids through their diet.
“Whilst sperm counts and testicular size were higher in men who had consumed fish oil supplements, the difference in sperm count was modest and the clinical significance in terms of reproductive function and fertility is unclear.
“The study was conducted in a population of healthy young men, the majority of whom will have a normal sperm count and testicle size, regardless of whether they use fish oil supplements."
Another issue with this observational study is that it could be that men who choose to take fish oil supplements may have very different lifestyle behaviours when compared with other men.
Dr Channa Jayasena, Reader in Reproductive Endocrinology at Imperial College London, and Member of the Society of Endocrinology, explains: “This report does not prove that fish oil itself has any direct influence on male fertility. Keeping your weight down, and not smoking or drinking excessively are likely to be much more effective New Year’s resolutions.”
As the authors state, these findings need to be followed up using a randomised double-blind clinical trial in order to exclude the possibility that men taking fish oil supplements may simply be more health conscious and in a better reproductive state because of this.
However, the report states: "Nevertheless, because modern Western diets contain drastically reduced proportions of ω-3 PUFAs, compared with the diet humans were exposed to during their evolution, the present study would add to the substantial evidence suggesting that a return to such diets can have numerous health-beneficial effects, and those may include reproductive effects.”
Source: JAMA Network ]
Jensen. T. K., et al.
‘Associations of fish oil supplement use with testicular function in young men’
DOI: 10.1001/jamanetworkopen.2019.19462