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expert reaction to study looking at fish oil supplements and testicular function in healthy young men

Research, published in JAMA Network Open, reports that fish oil supplements may have a positive impact on young men’s testicular function.


Prof Allan Pacey, Professor of Andrology, University of Sheffield, said:

“This is another interesting study about a possible relationship between diet and the quality of sperm in young men.  In this instance, the study also looked at the key hormones involved in making sperm which gives it an additional dimension.  Moreover, the fact that they looked at healthy men from the general population (military recruits) 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 (unlike many men from a fertility clinic).

“Briefly, the research shows that men who took fish oil supplements during the three months before they produced a semen sample, had slightly larger testicles, ejaculated more sperm and had a healthier looking profile of two key hormones, which regulate sperm production, in their blood (LH and FSH).  Therefore, on the face of it, this would suggest that fish oil supplements might be good for fertility.

“However, 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).”


Dr Rod Mitchell, Research Group Leader at the MRC Centre for Reproductive Health at the University of Edinburgh, and Member of the Society for Reproduction and Fertility, said:

“Current options to improve sperm quality in men with fertility problems are limited, primarily involving lifestyle modification.  This well conducted study, involving a large cohort (1679 men) of young army conscripts, investigated whether intake of supplements containing fish oils (which contain omega-3 fatty acids) was associated with differences in sperm quality or reproductive function in healthy men.

“Previous studies, including randomized 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.

“The research group show an association between the use of fish oil supplements and reproductive function in men.  Sperm count and testicle size were higher in men who had consumed fish oils compared to those who had taken multivitamins or no supplements.

“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.

“Overall, this study supports the theory that fish oils and omega-3 fatty acids may have positive effects on sperm quality in some males, although a large randomized controlled trial would be required to prove that it can increase sperm count in a healthy male population.

“Interestingly, the men who received fish oil supplements were less likely to be regular smokers and although the study did take account of smoking status in their analysis, modification of lifestyle factors such as obesity and smoking remain important for male fertility.”


Prof Sheena Lewis, Emeritus Professor of Reproductive Medicine, Queen’s University Belfast, said:

“This is a large well-designed study and the association between fish oil intake and improved semen quality is compelling.  However, the study focuses on healthy young men; mostly with sperm counts already in the fertile range.  There is no evidence from this study that infertile men with low sperm counts benefit from fish oil.”


Dr Channa Jayasena, Reader in Reproductive Endocrinology at Imperial College London, and Member of the Society of Endocrinology, said:

“This is one of a small number of studies linking fish oil supplement consumption to better sperm function.  The researchers have studied a lot of men, but the results are observational.  Men who choose to take fish oil supplements may have very different lifestyle behaviours when compared with other men.

“So, 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.”


Dr Kevin McEleny, Consultant Andrologist and British Fertility Society Committee member, said:

“This is a large and well-conducted study.  However, it is looking for a correlation between reproductive health and fish oil consumption; it is not a clinical trial where supplements were tested as an intervention for poor sperm quality.

“There is increasing evidence for a link between diet and reproductive health.  Fish oil products have been common as dietary supplements for a long time and have been shown to be safe.  This study suggests that further investigation of fish oil for male reproductive health is warranted.

“In the study, there were very few men who took only fish oil and no other supplement.  They also tended to have healthier lifestyles and in particular tended not to smoke.  Being fit and healthy is important for male and female reproductive health.  This doesn’t help us to say whether fish oil specifically is helpful.

“Ultimately, whatever the reason, young, fit and healthy men who took fish oil tended to have slightly higher semen volume, higher sperm count, larger testicles, and healthier hormone levels.”


Prof Richard Sharpe, MRC Centre for Reproductive Health, University of Edinburgh, and Member of the Society of Endocrinology, said:

“This is a well-conducted, population-based study from a group that leads the world in understanding the relationships between diet, lifestyle, exposures and semen quality/male fertility.  It provides further evidence that the types of fatty acids that we consume can have ‘effects’ on our health and potential reproductive function, although it must be remembered that this particular study is an association study and therefore cannot prove cause and effect.

“There is growing evidence from experimental dietary intervention studies in man and animals (rodents, rabbits, pigs) that higher consumption of so-called PUFAs (ω-3 polyunsaturated fatty acids), that are present in oily fish and in certain nuts, is associated with improvements in sperm count, motility and/or morphology.  The present study adds to this evidence base by demonstrating that adolescent men who take fish oil supplements (but not vitamin supplements) have significantly larger testes, semen volume and higher sperm counts than comparable men who did not take such supplements.  Consistent with this, they found that the men using fish oil supplements had significantly healthier reproductive hormone profiles – a profile that previous studies have shown is associated with better testis function and sperm count.  Mechanistically, we know that PUFAs are incorporated into the sperm plasma membrane which plays a critical role in sperm motility and fertilisation.

“As the authors state, these findings need to be followed up using a randomized double-blind clinical trial before we’d be able to know whether taking fish oil supplements can improve reproductive function in men from the normal population.  Such a trial would be able to exclude the possibility that remains in this study, which is that men taking fish oil supplements may simply be more health conscious and in a better reproductive state because of this, rather than the supplement itself having any effect.  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.”


Dr Frankie Phillips, Dietitian and spokesperson for the British Dietetic Association, said:

“This interesting study shows that omega-3 fatty acids might have a significant role to play in testicular function.  We already know that several aspects of diet have an impact on semen quality and key nutrients have been identified as having some part to play in sperm numbers and quality.  Antioxidants, including vitamin C, selenium and vitamin A, as well as zinc and omega-3 fats all have a role in the production of healthy sperm.

“This study is an observational study suggesting that those who took omega-3 supplements had slightly better quality sperm, but it is not clear what non-supplemental omega 3 was consumed; if these men were already eating oily fish regularly, this could also have an impact, but the study did not include any details on dietary intake.

“There is much focus on the diets of women who are trying to conceive, ensuring that they are in the best possible position to achieve a healthy pregnancy, but diet might also be a factor involved in men’s reproductive health.  Omega-3 is present in a both animal and plant derived foods, but oily fish stands out as an excellent source of long chain omega-3, and the UK population currently consume way below the recommended ‘at least one portion of oily fish per week’.  So including omega-3 is already part of current dietary recommendations – this study on its own can’t prove that upping omega-3 will itself improve testicular function.”


‘Associations of fish oil supplement use with testicular function in young men’ by Tina Kold Jensen et al. was published in JAMA Network Open at 16:00 UK time on Friday 17 January 2020.

DOI: 10.1001/jamanetworkopen.2019.19462


Declared interests

Prof Allan Pacey: “Chairman of the advisory committee of the UK National External Quality Assurance Schemes in Andrology, Editor in Chief of Human Fertility and Trustee of the Progress Educational Trust (all unpaid).  Also, recent work for the World Health Organisation, British Broadcasting Corporation, Purple Orchid Pharma (paid consultancy with all monies going to University of Sheffield).  Co-applicant on a research grant from the Medical Research Council (ref: MR/M010473/1).”

Dr Rod Mitchell: “None.”

Prof Sheena Lewis: “Sheena is CEO of SpermComet Ltd, a university spin-out company marketing a test for male infertility.”

Dr Channa Jayasena: “No conflicts of interest.”

Dr Kevin McEleny: “None.”

Prof Richard Sharpe: “No conflicts of interest.”

None others received.

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