Sperm are influenced by diet, and the effects arise rapidly. This is the conclusion of a study by researchers at Linköping University, in which healthy young men were fed a diet rich in sugar.
The study, which has been published in PLOS Biology, gives new insight into the function of sperm, and may in the long term contribute to new diagnostic methods to measure sperm quality.
“We see that diet influences the motility of the sperm, and we can link the changes to specific molecules in them.
Our study has revealed rapid effects that are noticeable after one to two weeks,” says Anita Öst, senior lecturer in the Department of Clinical and Experimental Medicine at Linköping University, and head of the study.
Sperm quality can be harmed by several environmental and lifestyle factors, of which obesity and related diseases, such as type 2 diabetes, are well-known risk factors for poor sperm quality.
The research group that carried out the new study is interested in epigenetic phenomena, which involve physical properties or levels of gene expression changing, even when the genetic material, the DNA sequence, is not changed. In certain cases such epigenetic changes can lead to properties being transferred from a parent to offspring via the sperm or the egg.
In a previous study, the scientists showed that male fruit flies which had consumed excess sugar shortly before mating more often produced offspring who became overweight.
Similar studies on mice have suggested that small fragments of RNA known as tsRNA play a role in these epigenetic phenomena that appear in the next generation.
These RNA fragments are present in unusually large amounts in the sperm of many species, including humans, fruit flies and mice. So far, their function has not been examined in detail.
Scientists have speculated that the RNA fragments in sperm may be involved in epigenetic phenomena, but it is too early to say whether this is the case in humans.
The new study was initiated by the researchers to investigate whether a high consumption of sugar affects the RNA fragments in human sperm.
The study examined 15 normal, non-smoking young men, who followed a diet in which they were given all food from the scientists for two weeks.
The diet was based on the Nordic Nutrition Recommendations for healthy eating with one exception: during the second week the researchers added sugar, corresponding to around 3.5 litres of fizzy drinks, or 450 grammes of confectionery, every day.
The sperm quality and other indicators of the participants’ health were investigated at the start of the study, after the first week (during which they ate a healthy diet), and after the second week (when the participants had additionally consumed large amounts of sugar).
At the beginning of the study, one third of the participants had low sperm motility.
Motility is one of several factors that influence sperm quality, and the fraction of people with low sperm motility in the study corresponded to that in the general population.
The researchers were surprised to discover that the sperm motility of all participants became normal during the study.
“The study shows that sperm motility can be changed in a short period, and seems to be closely coupled to diet.
This has important clinical implications. But we can’t say whether it was the sugar that caused the effect, since it may be a component of the basic healthy diet that has a positive effect on the sperm,” says Anita Öst.
The researchers also found that the small RNA fragments, which are linked to sperm motility, also changed.
They are now planning to continue the work and investigate whether there is a link between male fertility and the RNA fragments in sperm.
They will also determine whether the RNA code can be used for new diagnostic methods to measure sperm quality during in vitro fertilisation.
Decreasing trends in sperm counts during the last decades have been reported in multiple studies (1–4).
The reasons for this downward trend have not been fully elucidated, but exposures adversely affecting prenatal testicular development as well as exposures and lifestyle factors during childhood and young adulthood have been suggested(4, 5).
However, relatively little work has been devoted to understanding the modifiable determinants of semen quality, as a marker of men’s reproductive potential,(6) which could lead to the design of clinical and public health interventions(7).
Diet is a modifiable risk factor that can be assessed in clinical practice and inform actions for disease prevention.(8)
Previous epidemiologic studies suggest that diet may be associated with semen parameters(9–14).
Animal food intake, and in particular intake of red meats and fish, has received particular attention since their nutritional profile and their contamination with persistent organic pollutants and hormonal residues may affect testicular function(11, 15–19).
Although literature is scarce, red meats have consistently been associated to worse semen quality, while fish intake appears to have the opposite relation(19–22).
However, these studies have been conducted in different populations and settings, with diverse age and health status, and only one previous work was performed in healthy young men (19).
Therefore, the aim of this study is to investigate the associations of intakes of meats, including red, white, processed and fish meats, with semen quality parameters and reproductive hormones among healthy young men from Spain.
In this cross-sectional study of healthy young men, we found that total meat intake was unrelated to semen quality or reproductive hormone levels.
When subgroups of meat were separately considered, however, shellfish intake was associated with higher progressive motility and organ meat intake was inversely related to this outcome. We also found associations with reproductive hormones that did not explain the associations with semen quality.
The remaining types of meat were unrelated to semen quality parameters or reproductive hormone levels. The relevance of the observed associations of shellfish and organ meat intakes with semen quality and reproductive hormones should be further evaluated given that these specific relations have only been evaluated in a small number of previous studies (19).
Our findings are consistent with those of previous studies that explored the association between fish and seafood intake with semen quality.
Two case-control studies among men in fertility clinics have reported significantly higher shellfish intake among normospermic men than among oligoasthenoteratospermic patients(19) and greater seafood intakes as a protective factor for asthenozoospermia(21).
Greater sperm motility has also been previously related to higher consumption of fish and other seafood(33), while in other study no associations were found with any semen indicators(34).
Afeiche and colleagues reported a positive association between intake of dark meat fish intake and total sperm count, as well as a positive relation between white meat fish intake and morphologically normal sperm among men presenting to a fertility clinic(22), but not among healthy young men(19).
Shellfish is an important source of polyunsaturated fatty acids (PUFA)(35) and previous work has related higher consumption of omega-3 PUFAs to better sperm motility(36).
However, because we did not observe an association between dark meat fish intake and semen quality in this study, and previous work in this cohort has not found associations between intakes of omega-3 and semen quality parameters or reproductive hormones(12, 24), intake of omega-3 PUFAs is unlikely to explain our findings.
Instead, the relation between shellfish intake with sperm motility may be due to intake of micronutrients highly concentrated in shellfish such as zinc(35).
Zinc is involved in male reproductive function including spermatogenesis, sperm maturation and sperm activation(37).
A recent meta-analysis showed that zinc supplementation in infertile men was associated to better semen quality parameters, including sperm motility(37).
To our knowledge, only three previous studies have addressed the association between intake of organ meat and semen parameters.
Organ meat intake has been previously hypothesized to lower semen quality(19). Contrary to our findings, a similar study among young men in the United States found organ consumption to be positively related to sperm motility(19).
Previous studies among infertile men have not found a relationship between organ consumption and sperm motility(19, 22). As literature is scarce and inconsistent, this association may represent a chance finding and more studies are needed to clarify this discrepancy.
Several limitations should be discussed. The cross-sectional design is not strong to distinguish causal from non-causal relationships and the small sample size could result in missing true associations. However, results of the semen analysis were unknown to participants, essentially blinding them to the study outcome, and thus decreasing the likelihood of reverse causation.
Diet was assessed using a FFQ and like all other diet assessment tools, FFQs are subject to measurement error.
However, this questionnaire has been validated previously in Spanish populations(27). In addition, FFQs are known to be better at ranking than at estimating exact intakes. Our analytic strategy of making comparisons between quantiles of intake rather than relying on continuous measures of intake is not only better aligned with this characteristic of FFQs but also protects against the influence of individuals with very high intakes thus resulting in conservative estimates of the associations examined.
Moreover, as the consumption of shellfish and organ meats was very low (3–5%) and the multiple comparisons developed in the statistical analysis, we cannot rule out the possibility of residual confounding or chance findings that could explain the associations reported. However, we adjusted for a large number of potential confounders, including known predictors of semen quality as well as lifestyle factors associated with meat intake including other dietary behaviors as captured by data-derived dietary patterns.
Finally, although the homogeneity of study participants may have increased the internal validity of the study, it may limit the generalizability of our results to men facing difficulties with fertility.
In conclusion, we found no relation of total meat intake nor intake of major types of meat (red meats, white meats, fish) with markers of testicular function. Nevertheless, analyses of subgroups of meat suggests that intake of shellfish may be beneficial for sperm motility whereas the intake of organ meats may have the opposite relation.
Due to the scarcity of literature in this population and in specific types of meats, additional research is needed to confirm or refute these findings.
More information: Daniel Nätt et al, Human sperm displays rapid responses to diet, PLOS Biology (2019). DOI: 10.1371/journal.pbio.3000559