Intermittent fasting – one of the most popular celebrity dieting techniques – could trigger fertility problems, research suggests.
Hollywood A-listers like Jennifer Aniston, Nicole Kidman, Cameron Diaz and Mark Wahlberg have jumped on the trend since it shot to prominence in the early 2010s.
But, despite swathes of studies suggesting it works, experts have remained divided over its effectiveness and the potential long term health impacts.
Now a study on animals suggests that while fasting may be good for your waistline, it could damage the chances of conception.
Experts at the University of East Anglia in Norwich investigated how time-restricted fasting affected reproduction in zebrafish.
Hollywood A-listers like Jennifer Aniston (pictured above), Nicole Kidman, Cameron Diaz and Mark Wahlberg have all jumped on the trend since it shot to prominence in the early 2010s
In this latest study, scientists from the University of East Anglia in Norwich investigated how time-restricted fasting affected reproduction differently in male and female zebrafish. Pictured above, Mark Wahlberg, who is known to intermittent fast
The study looked at the impact of fasting on the fertility of zebrafish an animal commonly used in research exploring implications for human health
Critically, researchers found both egg and sperm quality were negatively affected even after the fish returned to their normal levels of food consumption.
Writing in the journal, Proceedings of the Royal Society B: Biological Sciences, researchers said the findings ‘call for careful evaluation of the effects of intermittent fasting on fertilisation’.
Scientists measured both the production of sperm and eggs, and the quality of the resulting offspring.
Q+A: Everything you need to know about intermittent fasting
How does it work?
Intermittent fasting involves switching between days of fasting and days of eating normally.
It generally falls into two categories – narrowing eating times to 6-8 hours per day, also known as the 16:8 diet, and 5:2 intermittent fasting.
The 16:8 diet is a form of intermittent fasting, also known as Time Restricted Eating.
What is the difference between time restricted eating and 5:2 intermittent fasting?
Followers of the 16:8 eating plan fast for 16 hours a day, and eat whatever they want in the remaining eight hours – typically between 10am and 6pm.
This may be more tolerable than the well-known 5:2 diet – where followers restrict their calories to 500–to-600 a day for two days a week and then eat as normal for the remaining five days.
What are the benefits of Time Restricted Eating?
In addition to weight loss, 16:8 intermittent fasting is believed to improve blood sugar control, boost brain function and help us live longer.
Many prefer to eat between noon and 8pm as this means they only need to fast overnight and skip breakfast, but can still eat lunch and dinner, along with a few snacks.
When you do eat, it is best to opt for healthy options like fruits, vegetables and whole grains and drink water and unsweetened beverages.
What are the drawbacks?
Drawbacks of the fasting plan may be that people overindulge in the hours they can eat, leading to weight gain.
It can also result in digestive problems over the long-term, as well as hunger, fatigue and weakness.
They took 12 male and 12 female fish and put them on a fully-fed diet.
While another group of 18 males and 18 females were put on a fasting group.
After 15 days, the fasted fish were allowed to eat normally once again.
To test reproductive performance, the 30 male and 30 female fish were paired at random on days seven, 15, 21, 28 and 35 of the experiment with fish of the opposite sex from the ‘general population’.
Fish were then given a maximum of five hours to spawn.
If unsuccessful, they were given another partner and the process was repeated the next day.
Sperm from each of the 30 males was also collected on days seven, 15, 21 and 35 while eggs were then assessed after two and 24 hours.
Scientists found female fish on the fasting diet had, on average, ‘significantly lower’ reproduction on days seven and 15.
‘This resulted in a reduced total number of offspring for females,’ they added, with 163 offspring in those who were fully-fed compared to the 75 who were fasted.
The reduction in offspring even carried over once the females were fed normally, with the fish recording increasing growth in their fins at the same time.
Researchers suggested this could a sign that the females bodies were prioritizing their own health post-fasting at the expense of their ability to reproduce.
Fasting male fish also exhibited a faster decline in the velocity of their sperm over time and a ‘significant decline in sperm quality’ compared to the fed group, the scientists said.
The study’s authors said the results ‘cannot be directly compared with a two week period in a mammal’, due to differences in metabolism between us and fish.
However, they hope the findings highlight the importance of considering not just the effect of fasting on weight and health, but also on fertility.
Dr Edward Ivimey-Cook, an expert in biological sciences and a study author said: ‘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.’
Professor Alexei Maklakov, an expert in evolutionary biology and an author of the study said: ‘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.
Writing in the journal, Proceedings of the Royal Society B: Biological Sciences, researchers said the findings ‘call for careful evaluation of the effects of intermittent fasting on fertilisation’. Pictured above, Nicole Kidman, who is known to intermittent fast
To test reproductive performance, the 30 male and 30 female fish were paired at random on days seven, 15, 21, 28 and 35 of the experiment with fish of the opposite sex from the ‘general population’. Pictured above, Cameron Diaz, who is known to intermittent fast
Zebrafish are a common animal to use in such studies because their genetic structure is broadly similar to humans and they have a high reproductive rate.
While a popular celebrity dieting trend, experts have been divided about the merits of intermittent fasting.
Some argue that fasters usually end up consuming a relatively large amount of food in one go, meaning they don’t cut back on their calories — a known way of beating the bulge.
And in recent years a small body of research has emerged linking the trend to a shorter lifespan and higher risk of heart disease.
Experts suggest this may be because intermittent fasters eat more in one go, which they said could damage cells.