A large observational study out of Denmark has linked fish oil supplements with improved semen quality and testicular function in healthy men. The research cannot prove causation, however, it does echo prior research reaching similar conclusions in men with fertility issues.

Over the past few years evidence supporting the health benefits of taking fish oil supplements containing omega-3 fatty acids has been mixed at best. Doubts have been raised over how beneficial the popular supplement is in helping prevent heart disease, stroke, and cancer. On the other hand, evidence has shown possible benefits in lowering childhood asthma risk, especially if mothers consume supplements during pregnancy.

A new study is adding to the discordant volume of study on fish oil, finding supplements may improve testicular function in healthy men. The new research recruited nearly 1,700 young Danish men, all who completed a lifestyle and diet survey that included information of vitamin and fish oil supplements consumed across the prior three months. Semen and blood samples were taken alongside a complete physical examination.

The results revealed a significant correlation between fish oil supplement consumption and higher sperm count. The fish oil was also linked to larger testicular size and a healthier profile of two hormones related to sperm production. The study notes the correlation was not seen with any other vitamins or supplements noted by the subjects.

A number of experts not associated with this particular study suggest that, while the research is rigorous and compelling, it is still only generating observational correlations, and any causation could only be established through a randomized clinical trial. Richard Sharpe, from the University of Edinburgh, calls the new study “well-conducted,” but affirms the need for a clinical trial to confirm causation.

“… 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,” says Sharpe, who didn't work on this new study. “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.”

Kevin McEleny, from the British Fertility Society, also says that, although there is increasing evidence of numerous links between diet and reproductive health, it is unclear in this study whether the subjects taking fish oil supplements simply led healthier lives.

“In the study, there were very few men who took only fish oil and no other supplement,” says McEleny. “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.”

Despite these inherent limitations, this is not the first published study to investigate links between fish oil and male fertility. In fact, a large systematic review published last year examined 16 studies on the topic, comprising over 1,000 individual subjects with potential fertility issues. The conclusion was by no means certain but it did conclude, “omega-3 supplements and dietary intake of omega-3 might improve semen quality parameters in infertile men and men from couples seeking fertility treatment.”

The primary novelty of this new study is its examination of semen quality in healthy men. As with many observational studies, this research is certainly not the final say on the matter. Plus, there is no suggestion that these particular reproductive health metrics directly improve fertility or increase the chances of couples becoming pregnant. However, there certainly is no evidence to suggest fish oil supplements are harmful to a man’s reproductive health in any way.

The new research was published in the journal JAMA Network Open.