Small study shows how quickly diet influences sperm quality

A diet laden in sugar has some strange effects on sperm.

candy, chocolate, sugar

It doesn’t take long for a sugary snack or a fizzy drink to burn through your body, but the effects of that sugar bomb linger. At least one team of scientists has early evidence that, within weeks, a diet laden in sugar has some strange effects on sperm.

A small study of 15 men conducted by a team of scientists in Sweden noted that when men ate healthy diets for one week and then sugary diets the next week, they saw strange changes in sperm motility — which is how well sperm swim, and is used as a measure of sperm viability. Counterintuitively, the team found that diets high in sugar tended to increase sperm motility over that short time period.

The study’s senior author Anita Öst tells Inverse this study does not suggest that sugar is beneficial to sperm quality. It’s possible that the increase in sperm motility could have come from the week of healthy eating, not the added sugar. Moreover, the added sugar came laden with side effects that probably aren’t doing sperm any long-term favors.

After just a week of eating lots of sugar, the number of triglycerides in the men’s blood doubled, which can increase the risk of heart conditions. To add to that, keeping up a sugary diet increases the risk of obesity, has been shown to negatively impact sperm quality:

“If I have to speculate over long-term effects, I think that a high-sugar intake, in the long run, will have negative effects on both sperm quality and the overall health,” Öst tells Inverse

Sperm motility increased after just two weeks following radically different eating patterns, according to this study. 

The study was published last week in PLOS Biology

Population-based research has shown sperm quality declines between 50 and 60 percent in Western nations from North America to Australia, New Zealand, and Europe between 1973 and 2011. Scientists have searched for explanations that range from stress to exposure to pollution.

This study’s first author, Daniel Nätt, tells Inverse that diet may play a large role in the decline in sperm viability. This study is an early attempt to link the two on a nuts-and-bolts level:

“Noteworthy is that the world has recently experienced a global decline in sperm quality and a simultaneous increase in obesity. Maybe they are connected on a molecular plane,” he tells Inverse

Sugar and Sperm

As part of the experiment, 15 men were fed personalized diets by the researchers over two weeks. In the first week, they ate healthy diets of three meals per day. The second week, they added an additional 50 percent of their daily calories in sodas, candies, and confections.

If you’re someone who burns about 2,000 calories per day, that would be about six Mountain Dews, or four traditional Snickers bars on top of three regular meals.

When men ate an additional 50 percent of their daily calories in sweets, there were changes in sugar-sensitive small RNAs that could impact sperm quality differently in the short and long terms. 

Although the team didn’t find any negative impacts on sperm, they did see “very specific” changes to small RNAs, which are non-coding pieces of genetic material that influence the way that genes are translated into proteins. Small RNAs are particularly of interest in sperm research. Changes to small RNAs that can occur because of a father’s lifestyle and may influence the way that offspring develop.

This team identified a “never before described” sugar-sensitive small RNA that “specifically increased when we introduced sugar,” Nätt says.

Though this increase seemed to be related to better sperm quality, the team cautions that, in the long term, the outlook isn’t so good. This small, short-term study can’t speak to long-run effects directly but tried to compensate for that by comparing sperm in obese men to the lean and otherwise healthy men in their experiment.

This is far from a perfect comparison. The data on sperm quality in obese men was taken from another study and reanalyzed by this team. But it does start to hint at the bigger picture behind this study’s results.

They found that obese men had lower expression of that small, sugar sensitive RNA. That suggests that while the sugar-sensitive small RNA was elevated and related to higher sperm motility in the healthy men in the short term, the effects may dissipate over time.

“A high-sugar diet that in the long-term leads to obesity may therefore negatively affect the molecular pathways that control male reproductive health,” Nätt says.

Taken together, the study hints that it doesn’t take very long for sugar-related small changes to start playing out in sperm — long before they show themselves elsewhere in the body.


The global rise in obesity and steady decline in sperm quality are two alarming trends that have emerged during recent decades. In parallel, evidence from model organisms shows that paternal diet can affect offspring metabolic health in a process involving sperm tRNA-derived small RNA (tsRNA). Here, we report that human sperm are acutely sensitive to nutrient flux, both in terms of sperm motility and changes in sperm tsRNA. 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. Small RNA-seq on repeatedly sampled sperm from the same individuals revealed that tsRNAs were up-regulated by eating a high-sugar diet for just 1 week. Unsupervised clustering identified two independent pathways for the biogenesis of these tsRNAs: one involving a novel class of fragments with specific cleavage in the T-loop of mature nuclear tRNAs and the other exclusively involving mitochondrial tsRNAs. Mitochondrial involvement was further supported by a similar up-regulation of mitochondrial rRNA-derived small RNA (rsRNA). Notably, the changes in sugar-sensitive tsRNA were positively associated with simultaneous changes in sperm motility and negatively associated with obesity in an independent clinical cohort. This rapid response to a dietary intervention on tsRNA in human sperm is attuned with the paternal intergenerational metabolic responses found in model organisms. More importantly, our findings suggest shared diet-sensitive mechanisms between sperm motility and the biogenesis of tsRNA, which provide novel insights about the interplay between nutrition and male reproductive health.

Media via Shutterstock