In this study, our results showed moderate associations between dietary fats and semen quality, some of which reached statistical significance. Total fat intake was negatively related to total sperm count and sperm concentration. These associations appeared to be driven primarily by intake of saturated fat. Conversely, intake of omega-3 PUFAs was positively correlated to normal sperm morphology. Although dietary fats were not strongly associated with sperm or seminal plasma fatty acid levels, fatty acid composition of sperm and seminal plasma had modest correlations with semen quality.
To our knowledge, no previous study has evaluated the relation between habitual dietary fat intake and semen quality parameters. However, our results can be compared to some extent with previous cross-sectional analyses of diet and semen quality. Our data are in agreement with the results of a Spanish study showing intake of processed meat, an important source of saturated fats, to be associated with poorer semen quality (Mendiola et al., 2009
). In contrast, men adhering to a traditional Dutch dietary pattern (high in meat products) had higher sperm concentration when compared with men who consumed a diet with high intake of fruits, vegetables and fish (Vujkovic et al., 2009
). Clearly, further research on the relation between dietary fats and semen quality is warranted.
We also found that higher intake of omega-3 PUFAs was significantly related to a more favorable sperm morphology as well as a suggestion of a lower frequency of head defects. Our data are in agreement with two clinical trials examining the effect of omega-3 fatty acids on semen quality. Conquer et al
. randomized 28 Canadian asthenozoospermic men to placebo, or 400 or 800 mg of DHA for 3 months, and DHA supplementation failed to improve sperm motility or concentration, in agreement with our null findings for these parameters, while results on sperm morphology were not reported (Conquer et al., 2000
). Safarinejad randomized 238 Iranian oligoasthenoteratospermic men to receive a supplement containing 1840 mg EPA and DHA or placebo for 32 weeks (Safarinejad, 2011
) and found that omega-3 fatty acid supplementation increased the total percentage of sperm with normal morphology, in agreement with our findings. In addition, supplementation also improved total sperm count, concentration and motility, and, in agreement with our data, supplementation significantly increased sperm and seminal plasma EPA and DHA. Differences in the patient population, omega-3 fatty acid dose and duration of use, background omega-3 fatty acid intake and involvement of unspecified amounts of omega-6 fatty acid (corn oil) in placebo may account for differences across studies. Furthermore, our results are also supported by animal data. Dietary supplementation of omega-3 fatty acid in the boar improved sperm morphology in specific breeds, while results regarding the total sperm number and motility have been inconsistent (Mitre et al., 2004
; Estienne et al., 2008
; Yeste et al., 2011
). Despite the consistency of our findings with previous human and animal data, there is currently no known biological mechanism to account for the relation between omega-3 fatty acid intake and improved normal sperm morphology or specific morphological defects.
We examined the correlation between dietary fatty acids and levels of each fatty acid in sperm and seminal plasma in a subgroup of 23 men for whom sperm and seminal fatty acid composition was available. Overall, dietary fatty acids were unrelated with the sperm or seminal plasma levels of the same fatty acid. This is not an unexpected finding, especially for saturated fat intake, as saturated fatty acids can be synthesized endogenously from acetyl CoA. Therefore, the presence of saturated fatty acids at a cellular level may serve as a marker for de novo
lipogenesis and enzymatic activity. In contrast, PUFAs are essential fats as they are not synthesized endogenously by mammals and must be obtained from foods, such as cold water fish, flax seeds and nuts, or in supplements. Nonetheless, only modest correlations were observed between dietary intake of omega-3 PUFA and fatty acid levels in sperm or seminal plasma. This may be related to imperfect measures of dietary intake or may reflect the increased local metabolism at the level of the testes, as expression of enzymes involved in PUFA metabolism and those involved in de novo
lipogenesis are elevated in the testes, specifically in Sertoli cells, when compared with other body tissues (Leonard et al., 2000
; Tvrdik et al., 2000
; Zhang et al., 2001
; Saether et al., 2003
; Mandal et al., 2004
). Peripheral (i.e. Sertoli cell) conversion rates of PUFAs in the testis may also be substantially higher. Thus, fatty acid composition of sperm and seminal plasma may reflect both diet and local metabolism of both unsaturated and saturated fats.
Sperm levels, and to a lesser extent seminal plasma levels, of saturated fatty acids were negatively related to sperm concentration and motility. This is in agreement with previous work showing higher saturated fatty acid concentrations in sperm of asthenozoopermic (Aksoy et al., 2006
; Tavilani et al., 2006
) and oligospermic males when compared with normozoospermic subjects (Aksoy et al., 2006
). Similarly, sperm and seminal plasma levels of omega-3 PUFAs, specifically sperm DHA and seminal plasma EPA, were positively related to sperm concentration and motility. These findings are consistent with those presented in the literature. Levels of omega-3 PUFAs in human sperm, specifically DHA, have been positively correlated to sperm concentration, motility and morphology (Zalata et al., 1998
; Conquer et al., 1999
; Gulaya et al., 2001
; Aksoy et al., 2006
; Tavilani et al., 2006
; Safarinejad et al., 2009
; Safarinejad, 2011
It is important to consider the strengths and limitations of our study. To our knowledge, this is the largest study to date examining the influence of specific dietary fats on male fertility. A potential limitation of this study is the use of a FFQ to assess habitual dietary intake. Although the FFQ has been shown to have adequate validity and reproducibility for use in epidemiological studies (Willett and Lenart, 1998
), it is nonetheless prone to measurement error usually leading to attenuation of the associations of interest. In addition, analysis of a single semen sample from each participant should be acknowledged, although it has been suggested that analysis of multiple semen samples per subject may not be superior in research studies (Carlsen et al., 2005
; Stokes-Riner et al., 2007
). Furthermore, use of disposable chambers for analyzing sperm concentration and motility may result in an underestimation of sperm concentration in men with oligospermia when compared with the method using the haemocytometer (Tomlinson et al., 2001
). However, consistent use of the Leja slides throughout our study allows relative comparisons to be made. In addition to the small sample size for fatty acid analysis, another important limitation of this study is the possibility that any of the observed associations is the result of reverse causation, specifically that men with abnormal semen quality changed their diet in response to knowing that they have semen quality abnormalities. We believe, however, that reverse causation is an unlikely explanation for our findings. First, the observed associations with dietary fat intake are not in the direction that would be expected in the case of reverse causation. If men had adjusted their diet in response to a diagnosis of abnormal semen quality, one would expect that men with abnormal semen parameters would improve the quality of their diet, resulting in spurious associations between intake of higher quality fats (e.g. n
-3 fatty acids) and lower semen quality, as well as higher intake of low quality fats (e.g. saturated fat) and higher semen quality. Secondly, we would not expect that men with a low sperm concentration would adjust their diet differently compared with men with low sperm motility or poor morphology. As a result, we would expect that spurious associations created by reverse causation would be observed for all semen quality parameters simultaneously. In contrast, we observed specific associations for total and saturated fat with sperm concentration (but not motility or morphology), and for n
-3 fatty acids with morphology (but not concentration or motility). In addition, if knowledge of one's semen quality led to changes in diet resulting in a spurious association, we would expect that the associations would be limited to men with greater knowledge of their fertility status. However, our data suggest that the observed associations do not differ according to whether or not men had previous infertility exams or whether or not they had a concurrent abnormal analysis. Lastly, it is important to acknowledge that cross-sectional studies are weak in terms of their usefulness in establishing causality. It is therefore important that the relation between fat intake and semen quality be evaluated in other studies, ideally in prospective studies.
In summary, we observed significant relationships between dietary fats and specific measures of semen quality. Diets containing higher amounts of PUFAs and lower amounts of saturated fats were associated with more favorable semen quality parameters. Given the limitations of the current study, in particular the fact that it is a cross-sectional analysis and that it is the first report of a relation between dietary fat and semen quality, it is essential that these findings be reproduced in future work. However, adopting these lifestyle modifications may not only be beneficial for reproductive health but also for global general health.