Original title: Nut Consumption and Semen Quality: A Narrative Review of the Evidence
Background: With struggles related to infertility on the rise, and semen quality on the decline, research in the area of male factor infertility is becoming increasingly important. Due to the limited amount of data on the subject, there are currently no clear clinical guidelines for male patients seeking fertility treatment. This narrative review looks to investigate the recently proposed connection between nut consumption and semen quality, in hopes of sparking interest in further scientific study in the area.
Methods: Databases searched include Cochrane Library, EBSCOhost Databases, PubMed, ProQuest Nursing & Allied Health Database, and Turning Research into Practice. Topics searched for included nut consumption, omega-3 PUFAs, semen quality, and fertility.
Results: Omega-3 polyunsaturated fatty acids (PUFAs) are an integral element in sperm cell membranes and are powerful anti-inflammatory and antioxidant agents. Nuts are an excellent source of PUFAs, and are being studied for their potential impact on semen quality.
Conclusions: Further research is needed to establish the pathomechanism behind the global reduction in sperm counts and quality in recent years. That being said, there is promising research in the area of nut consumption and improvements in semen quality. This is a potential low-cost low-risk intervention that deserves further investigation.
Roughly one in six couples struggle with fertility, failing to get pregnant within one year. Research has primarily focused on females, despite the fact that male factors are identified in roughly 50% of couples seeking medical attention to address infertility. Human semen quality has declined worldwide in the last 40 years. The decrease is more pronounced in industrialized countries; Recent meta-analyses of men without known fertility problems found a 50-60% reduction in sperm counts in industrialized countries between 1973 to 2011. This leads some to believe that factors such as diet, lifestyle, and environmental exposures are to blame. Common semen quality issues include low sperm concentration in semen (oligospermia), an absolute lack of motility or a decreased motility of spermatozoa (asthenozoospermia), or an insufficient number of spermatozoa of normal structure (teratozoospermia). The rapid decrease in semen quality in recent years creates serious cause for concern in terms of its implications for human fertility. The pathogenesis of this phenomenon is a highly debated subject, but research is investigating factors such as obesity and resulting hypothalamic-pituitary-gonadal axis dysfunction, genetic abnormalities, DNA mutation, varicocele, environmental exposures and pollution, and damaging lifestyle factors such as smoking or poor-quality Westernized diet. Due to this lack of understanding, there are currently no clear clinical guidelines for male patients seeking fertility treatment. Research in the area of nutrition is proving to be promising. Some positive associations between nut consumption and semen quality are being investigated, and omega-3 PUFAs may be the agent that facilitates change. If further studies prove causation, nut consumption could offer men with a low-cost intervention with virtually no negative side effects. This narrative review will culminate the current research on this topic for assessment, and aims to inspire further research. Studies outlined below show promising potential, but large prospective cohort studies and randomized controlled trials are needed to establish causation.
Databases searched include Cochrane Library, EBSCOhost Databases, PubMed, ProQuest Nursing & Allied Health Database, and Turning Research into Practice. The time period searched was from 2012 through January 2021. Search terms used included omega-3 fatty acid, nuts, semen quality, and fertility. Due to the highly specific focus of this paper, there was already a limited number of articles on the topic. Nonetheless, studies were eliminated from consideration that were not relevant to the topic, such as general dietary information or studies specific to other forms of supplementation (ex: COQ10, soy, dairy, beef, etc). Studies were included that covered nut consumption and omega-3 supplementation, the role of omega-3 PUFAs in semen health and development, as well as some general information about the assessment of male fertility.
Many essential components for proper development, maturation, and functioning of spermatozoa are sourced from food. Therefore, nutritional insufficiencies may be crucial with regard to spermatogenesis, sperm quality, and male fertility. Dietary fats are of great importance for the proper development of the sperm cell membrane. Sertoli cells in the testis prefer the conversion of omega-3 polyunsaturated fatty acids (PUFAs) to create a healthy barrier. As sperm mature, there is a corresponding increase in the concentration of docosahexaenoic acid (DHA). In their study, Nassan et al. said, “The proper functioning of this specialized machinery in the testis relies on an adequate supply of metabolic substrates obtained from diet.” Eating foods rich in fatty acids modifies fatty acid composition of the sperm, and effects semen quality. Skoracka et al. found that fatty acids have the ability to modify the composition of the cell membrane by building into it, thus supporting its functioning. They also noted that EPA and/or DHA supplementation significantly increased DHA concentration in semen, which resulted in increased sperm motility, and improved concentration, number and morphology of sperm. In their 2018 study, Nassan et al. also found sperm membrane DHA content to be associated with higher sperm motility, normal morphology, and concentration. In their 2017 study, Salas-Huetos et al. found that intake of omega-3 PUFAs was positively related to normal sperm morphology, and that a lack-thereof was associated with reduced sperm motility. Though we have discussed several studies that describe the impact of omega-3 PUFAs on semen quality, there has been only one RCT conducted in infertile men with idiopathic oligoasthenoteratozoospermia who had lower levels of EPA and DHA in spermatozoa. Salas-Huetos et al. pointed out that, again, it was found that omega-3 PUFA supplementation had beneficial effects on semen quality parameters in these individuals.
In addition to providing structural integrity to the sperm cells, it seems that essential fatty acids play important roles as anti-inflammatory and antioxidant agents. In their 2020 study, Skoracka et al. stated that oxidative stress is reported to play a role in 30%–80% of male infertility cases, and further explained, “Cell membrane lipids, proteins, and sperm DNA are damaged once reactive oxygen species (ROS) overcomes the sperm antioxidant barrier. As a consequence, the higher the intensity of oxidative stress, the lower the motility, live sperm count, and sperm concentration in the semen.” Omega-3 fatty acids are precursors to eicosanoids, which are known to have anti-inflammatory and antioxidant properties that act to prevent cell or DNA damage. Salas-Huetos et al. emphasized seminal plasma’s status as one of the most powerful antioxidant fluids, and that defects in seminal plasma can be associated with oxidative stress. And an increase in ROS may result in sperm DNA fragmentation (SDF), which contributes to male infertility.
In terms of the literature available, scientists seem to be specifically interested in nuts as a source of the coveted omega-3 PUFAs. The Clinical Advisor reports that nuts are a rich source of dietary PUFAs, which are important in sperm maturation and membrane function. Several studies have been performed investigating the connection between nut consumption and semen quality. For example, in Salas-Huetos et al.’s 2018 study, they demonstrated that adding 60 g of mixed nuts/d to a Western-style diet for 14 weeks improved the total sperm count (P= 0.002), and the vitality (P= 0.003), motility (P= 0.006), and morphology of sperm (P= 0.008) in a group of healthy reproductive-aged participants when compared with an age-matched control group. In the nut-group, participants also saw decreases in SDF (P= 0.001) and expression of genes related to a DNA damage response (P= 0.036). Semen and blood samples were collected before and at the conclusion of the trial to establish this data.
In a randomized controlled trial (RCT) by Robbins et al., 117 healthy men age 21-35 years old who routinely consumed a Western-style diet were asked to supplement with walnuts. The RCT was a parallel two-group dietary intervention trial with single-blind masking of outcome assessors. After 12 weeks, the group asked to eat 75g of whole-shelled walnuts per day saw an improvement in conventional semen parameters. When results were compared to baseline, the walnut group saw statistically significant improvement for sperm vitality (P = 0.003), motility (P = 0.009), and morphology (P = 0.04). No change was seen in the individuals that kept their Western diet, and were instructed to avoid tree nuts. Improvements in blood serum and sperm fatty acid profiles were also noted within the walnut group, with increases in omega-3 (P = 0.0007) and the plant source of omega-3, alpha-linolenic acid (P = 0.0001). No similar changes were noted in the control group.
Further literature speaks to the antioxidant nature of nuts, citing that antioxidants contained in nuts might decrease the levels of ROS, and consequently oxidative stress, potentially affecting sperm count, motility, and vitality. In an animal study performed by Kara et al., a hazelnut supplemented diet significantly improved testicular antioxidant function and semen quality in male rats. As compared to young male rats, old male rats were found to have histopathological damage and decreased sperm quality. Young and old control groups were given traditional rat feed, whereas the young and old hazelnut groups were supplemented with 3g hazelnuts per kilogram of body weight. Hazelnut supplementation improved histopathological variables, sperm quality, seminal plasma and plasma oxidative stress in both groups.
Finally, Salas-Huetos et al. discussed an RCT aimed at exploring sperm DNA methylation patterns in subjects after 14 weeks of consuming 60 g/d of mixed nuts (nut group), when compared to a normal Western-style diet avoiding consumption of nuts (control group). The study found 36 genomic regions that were significantly differentially methylated by the end of the trial. Overall, 97.2% of the regions displayed an increase in methylation, which demonstrated the potential for diet to impact sperm quality. No such change was noted in the control group after the 14-week period. The study concluded that some sperm epigenome regions may respond to diet.
This paper seeks to highlight the connection between consumption of PUFAs and sperm structural integrity, resilience in the face of oxidative stress, and resistance to genetic changes. This has been illustrated by viewing the problem through the lens of dietary therapy, utilizing the current research connecting nut consumption to semen quality. This narrative review is limited due to the relatively limited number of RCTs related to nut consumption and semen quality. The preliminary results are promising, but further studies with larger sample sizes are needed before causation can be established, and the results can be generalized and applied to the greater public. Another great limitation of the review is the lack of understanding behind the great decreases in sperm count and quality issues in recent decades. A strength of this review is in its mission to find low-cost, low-risk interventions in a realm where men are generally left without help.
This review has led to the understanding that sperm development, sperm quality, and fertility as a whole may be fueled by dietary factors. Without the proper nutritional building blocks, healthy, resilient sperm cannot develop or thrive. One of these important nutrients is omega-3 PUFAs; they are responsible for sperm membrane structural integrity, and associated with healthy concentration, number, and morphology of sperm. Omega-3 PUFAs are also powerful antioxidant and anti-inflammatory agents that act to protect the cells from free radical damage and destructive genetic variances. In recent years, attention has been given to nut consumption and the impact on semen health. This review acts to integrate the powerful information that is understood about omega-3 PUFAs with the existing studies on nuts; the omega-3 PUFA content of nuts may be the driving nutritional factor that influences their impact on the sperm. In my opinion, there is evidence enough from these preliminary studies to warrant further investigation.
Amidst the epidemic of decreasing semen counts and quality worldwide, there is a great need for exploration in the field of male-factor infertility. Research must be done to determine the cause of these declining numbers in hopes to find solutions. Dietary therapy in the form of omega-3 PUFAs may be a low-cost, low-risk intervention for men struggling with semen quality issues. Further studies aiming to understand the connection between oligospermia, asthenozoospermia, or teratozoospermia and blood serum and sperm fatty acid profiles will solidify this connection. If causation is determined by prospective cohort studies and well-designed randomized controlled trials, this may be among the first treatment modalities offered to men struggling with infertility, and could act to re-shape clinical dietary and nutrition advice in the world of fertility.
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