Sperm quality is more heavily and more rapidly influenced by diet than previously thought, researchers at Linköping University have found.
The Swedish study, published in PLOS Biology, revealed rapid effects on sperm mobility when healthy, young male participants consumed a high-sugar diet.
"We see that diet influences the motility of the sperm... our study has revealed rapid effects that are noticeable after one to two weeks," head of the study and senior lecturer in the university's Department of Clinical and Experimental Medicine, Anita Öst, said in a press release.
The study was initiated to investigate whether consuming high amounts of sugar affects RNA fragments in human sperm.
Scientists have speculated that RNA fragments may be involved in epigenetic phenomena - the changing of gene expression, even when the DNA sequence is not altered. In some cases, these changes can lead to properties being transferred from a parent to their child via the sperm or egg. It is too early to say whether RNA fragments are involved in epigenetic phenomena in humans.
The researchers examined 15 non-smoking young men, who each followed a specific diet for two weeks. Their food, provided by the scientists, was based on the Nordic Nutrition Recommendations for healthy eating - with one exception.
During the second week, the researchers added high amounts of sugar - the equivalent of roughly 3.5 litres of fizzy drink or 450 grams of sweets per day.
The participants' health and sperm quality were investigated at the beginning of the study, at the end of the first week after following a healthy diet, and at the end of the second week in which they consumed high quantities of sugar.
At the start of the study, one third of the participants had low sperm motility - one of several factors that influence sperm quality. However, the sperm motility of all the men became normal throughout 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," Öst said.
"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."
Small RNA fragments, which are linked to the mobility of sperm, also changed.
The researchers are planning to continue investigating whether a link exists between RNA fragments in sperm and male fertility.
In a previous study, the scientists found that male fruit flies who consumed excess sugar shortly before mating tended to produce offspring which became overweight.
Similar studies on mice have suggested that RNA fragments play a role in these epigenetic phenomena that appear in the next generation. Large amounts of these fragments are found in the sperm of species including humans, fruit flies and mice.
So even if you tend to eat healthily during the year, feasting on a high-sugar diet over the festive period may be enough to alter your sperm quality.