Scientists found that zebrafish produced fewer eggs and less sperm if they went without food.

Eleanor Haywood

The Times

Gerry Gajadharsingh writes:

“Time Restricted Fasting (or Time Restricted Eating, it depends on how you look at it) is quite a trendy regime used by people trying to lose weight, (commonly people eat within a certain time window such as 12 pm to 8 pm), However, because the Metabolic Balance individual nutritional program works so well, we have never found it necessary to adopt such time restricted fasting in the majority of patients. Eating three meals a day and not snacking in between meals is one of the main principles to balance glucose and therefore insulin. There have been a number of recent research articles, flagging up the negative effect on metabolism, using time restricted fasting and the research below focuses on the effect on fertility. Although the research was focused on zebrafish, which are used in medical research as they have a similar genetic structure to humans, I suspect that when they do the research, there will be numerous adverse effects on metabolism by restricting food intake in such a time restricted fashion.”

Trendy intermittent fasting diets could cause long-term fertility problems, scientists have warned.

Research at the University of East Anglia suggests that restricting food intake reduces the energy available for the body to produce healthy eggs and sperm.

Their study looked at the impact of fasting on reproduction in zebrafish, which are used in medical research as they have a similar genetic structure to humans.

After the fish were forced to go without food, the quality of eggs and sperm they produced decreased — even after the fish returned to their normal diets.

The scientists said that their findings highlight the importance of examining the impact of intermittent fasting on human fertility, and potential risks of the diet should be taken into account alongside any weight-loss benefits.

Intermittent fasting diets have boomed in popularity in recent years. They involve setting an “eating window” each day, such as from 12 noon to 8pm.

Professor Alexei Maklakov, the lead author of the study, said: “Time-restricted fasting is an eating pattern where people limit their food consumption to certain hours of the day.

“It’s a popular health and fitness trend and people are doing it to lose weight and improve their health.

“But 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.

“We wanted to find out more about how these sorts of diets can affect fertility in a popular model organism.”

The research team studied zebrafish to find out what happens when they are exposed to food during and after a period of fasting.

During periods of calorie restriction, the animals reduced the amount of energy they put into their reproductive organs, with detrimental effects on the quality of egg and sperm produced.

The study, which was published in the journal Proceedings of the Royal Society B, adds to previous research suggesting fasting diets may damage fertility by interfering with female sex hormones.

Dr Edward Ivimey-Cook said: “What we found is that time-restricted fasting affects reproduction differently in males and females.

“Once the fish returned to their normal feeding schedule, females increased the number of offspring they produced at the cost of egg quality, resulting in reduced quality of offspring. The quality of male sperm also decreased.

“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.

“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.

“More research is needed to understand how long it takes for sperm and egg quality to return back to normal after the period of fasting.”