Molecules influencing the motility of sperm cells are affected strongly by a person's diet, according to a study which may lead to new diagnostic methods to measure sperm quality.

The study, published in the journal PLOS Biology, said the type of diet a person consumed may cause rapid noticeable changes, in just a week or two, in specific molecules in sperm cells which affected their motility.

As part of the study, the researchers, including those from Linkoping University in Sweden, investigated whether a high consumption of sugar influenced fragments of the essential life molecule Ribonucleic Acid (RNA) in human sperm.

The scientists assessed if the sugary diet influenced activity levels of certain genes even when the genetic material, the Deoxyribonucleic Acid (DNA) sequence, was not changed.

They said in some cases such alterations in the activity of genes lead to the transfer of traits from a parent to offspring via the sperm or the egg.

In the current study, the researchers examined 15 normal, non-smoking young men, who strictly followed a diet provided by the scientists for two weeks.

According to the study, 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 scientists assessed the sperm quality and other indicators of the participants' health 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.

About a third of the participants had low sperm motility at the beginning of the study, corresponding to a similar fraction in the general population found in earlier studies to have low motility.

However, the researchers said the sperm motility of all participants became normal during the study.

"Over the course of a 2-week diet intervention, in which we first introduced a healthy diet, followed by a diet rich in sugar, sperm motility increased and stabilized at high levels," the researchers wrote in the study.

However, the scientists cautioned that the dietary effects may not be due to the high sugar in the diet.

They added that the findings agreed with their earlier study in fruit flies in which just 2 days of dietary intervention, before mating, transmitted a signal through the sperm to induce metabolic changes in the next generation.

"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," said study co-author Anita Ost from Linkoping University.