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Taking fish oil supplements was associated with better testicular function in healthy young men, according to results of an observational study.

Among more than 1,600 young Danish men, those who reported taking fish oil supplements had larger testes (β=1.3 mL, 95% CI 0.5-2.1 mL), higher semen volume (β=0.49 mL, 95% CI 0.18-0.80 mL), and higher total sperm count (β=0.39 million, 95% CI 0.03-0.76 million) compared with men who did not use supplements, reported Tina Jensen, PhD, of the University of Southern Denmark in Odense, and colleagues.

Men who used fish oil supplements also had 20% lower levels of follicle stimulating hormone (95% CI 9%-31%). Their luteinizing hormone (LH) levels were 16% lower (95% CI 8%-24%), and their free testosterone to LH ratio was 8% higher (95% CI 0%-17%), they stated in .

However, there were no differences in inhibin B or testosterone levels, the authors reported.

They also found a dose-response relationship, with testicular function stronger in men reporting supplement use on 60 or more of the past 90 days compared with those who said they took supplements on fewer than 60 days. For example, men with intake on 60 or more days had 0.6 mL larger semen volume, 1.5 mL bigger testis, and a 15% higher free testosterone to LH ratio, a difference that could be clinically relevant, they said.

"To our knowledge, no studies on the association of intake of ω-3 fatty acids supplements with semen quality among healthy men from the general population have been performed," the authors wrote. "These findings suggest that intake of fish oil supplements was associated with better testicular function, which is less likely to be due to confounding by indication as no associations of intake of other supplements with testicular function were found."

Previous research has shown the rich fatty acid content of the sperm membrane is critical for sperm function, Jensen's group said. However, acids such as polyunsaturated fatty acids (PUFAs) cannot be synthesized by the body and must be obtained from dietary sources, they added.

The cross-sectional study included 1,679 Danish men (average age 18.9) who had undergone an examination to determine fitness for military service. The men were approached after the exam and invited to participate in the study, regardless of their fitness for service. Participants completed a questionnaire on health, lifestyle, and diet. They also underwent another physical exam and provided blood and semen samples for analysis.

Human semen quality has been declining worldwide, especially in developed and industrialized countries, Albert Salas-Huetos, PhD, of the University of Utah School of Medicine in Salt Lake City, said in an .

Studies suggest the reasons for the decline may be environmental and lifestyle factors such as pollution, smoking, lack of physical activity, stress, and unhealthy diets, he said.

"The evidence supporting an association of diet with sperm quality is growing very rapidly. In fact, dozens of observational studies have concluded that diets rich in foods such as fish, shellfish and seafood, poultry, cereals, vegetables and fruits, low-fat dairy, and skim milk were positively associated with several sperm quality parameters," Salas-Huetos wrote.

"This is the first well-designed study from a general population and including healthy individuals published to date, making the findings more interesting," he added. "Another strength of the study is the number of individuals analyzed ... Furthermore, the study ... included an adequate phenotypic evaluation of the men, which is uncommon in similar articles. It is important that future studies consider phenotypic and lifestyle characteristics, including but not limited to diet, obesity, medications, smoking, and physical activity, as Jensen et al did."

However, the dose-response relationship the researchers claimed to find is less clear, Salas-Huetos said. This is because only two cutoff points were used (less than 60 or more than 60 days of supplement use). Jensen's group acknowledged this as a study limitation, along with the fact that there was lack of information on the concentrations of ω-3 fatty acids in the fish oil supplements the men took.

"While these limitations are significant, the study is meaningful and insightful," Salas-Huetos said. "Jensen et al are to be commended for their creative study. Moreover, the article contains very interesting observational data that could affect assisted reproductive clinics if the findings are reproduced in the future by well-designed randomized clinical trials."

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