Intermittent fasting is a popular diet trend that many celebrities including Jennifer Aniston, Nicole Kidman, Cameron Diaz and Mark Wahlberg practice.

But research published in the journal Proceedings of the Royal Society B: Biological Sciences suggests that the dieting technique can cause fertility issues.

A new study suggests that intermittent fasting could possibly be beneficial for weight loss — but it could negatively impact reproduction.

Intermittent fasting is a diet that involves only eating during a specific time frame, switching between fasting and eating on a regular schedule, according to Johns Hopkins Medicine.

Researchers at the University of East Anglia in Norwich, England, tested how the fasting trend affected zebrafish reproduction and measured both production of sperm and eggs as well as the quality of offspring.

Zebrafish are commonly used in studies like this one since they have all the main organs involved in metabolism, according to Oxford University Press.

A group of 12 male fish and 12 female fish were put on a fully-fed diet, while a different group of 18 male fish and 18 female fish were on a fasting diet. The fish in the fasting group were allowed to eat normally after 15 days.

The combined 30 male and 30 female fish were randomly paired at random on days 7, 15, 21, 28 and 35 of the study with fish of the opposite sex from the “general population” in order to test reproductive performance.

Fish were given no more than five hours to reproduce, and if they failed, they were given another partner to repeat the process with the following day.

On days 7, 15, 21 and 35 of the study, sperm from each of the 30 male fish was collected, and the eggs were assessed after two and 24 hours.

Findings showed that female fish in the fasting diet group had “significantly lower” reproduction on days 7 and 15, on average — resulting in a lower number of offspring for females in total.

There were 163 offspring from the group who were fully fed compared to the 75 from the fasting group.

Even after the females in the fasting group were fed normally once again, there was still a reduction of offspring, and the first had an increased growth in their fins.

Scientists discovered that the quality of both egg and sperm was negatively impacted after returning to normal levels of eating.

“The way organisms respond to food shortages can affect the quality of eggs and sperm, and such effects could potentially continue after the end of the fasting period,” professor Alexei Maklakov, an expert in evolutionary biology and author of the study, said.

They suggested that these findings could be a result of female bodies prioritizing their own health and recovering from fasting at the expense of reproduction.

Male fish that fasted also had an increased decline in the speed of their sperm over time and a “significant decline in sperm quality.”

However, the study authors noted that the results “cannot be directly compared with a two-week period in a mammal” because of metabolism differences.

Experts hope the research will encourage people to understand the importance of the effect of fasting on fertility and not just on weight loss.

“These findings underscore the importance of considering not just the effect of fasting on body maintenance but also on the production of eggs and sperm,” study author Dr. Edward Ivimey-Cook, an expert in biological sciences, said. “Importantly, some of the negative effects on eggs and sperm quality can be seen after the animals returned to their normal levels of food consumption following time-restricted fasting.”

They noted that more research would need to be done in order to properly understand how long it takes the quality of sperm and eggs to get back to normal after fasting.

Meanwhile, a study earlier this year found that intermittent fasting does not help dieters lose weight, with calorie counting still the most effective way to drop any unwanted pounds.

Researchers also previously found that intermittent fasting did not actually lead to any more weight loss for obese people than daily calorie caps.

Another study from November 2022 determined that intermittent fasting and skipping breakfast are associated with a higher risk of dying from cardiovascular disease.