According to research, intermittent fasting – one of the most popular diet techniques for celebrities – could trigger fertility issues.
Hollywood stars like Jennifer Aniston, Nicole Kidman, Cameron Diaz and Mark Wahlberg have hopped on the trend since it rose to prominence in the early 2010s.
But, despite numerous studies suggesting it works, experts have remained divided on its effectiveness and potential long-term health impacts.
Now, an animal study suggests that while fasting may be good for your waistline, it could harm your chances of conception.
Experts from the University of East Anglia in Norwich have studied how time-restricted fasting affects zebrafish reproduction.
Hollywood stars like Jennifer Aniston (pictured above), Nicole Kidman, Cameron Diaz and Mark Wahlberg have all jumped on the trend since it rose 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 is Mark Wahlberg, known for his intermittent fasting
The study examined the impact of fasting on the fertility of zebrafish, an animal commonly used in research exploring implications for human health.
Critically, the researchers found that egg and sperm quality were negatively affected even after the fish returned to their normal level of food consumption.
Writing in the journal Proceedings of the Royal Society B: Biological Sciences, the researchers said the findings “call for careful evaluation of the effects of intermittent fasting on fertilization.”
The scientists measured both sperm and egg production and the quality of the resulting offspring.
Q+A: Everything you need to know about intermittent fasting
How it works?
Intermittent fasting involves switching between fasting days and normal eating days.
It generally falls into two categories: reducing meal times to 6-8 hours a 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 diet fast for 16 hours a day and eat whatever they want in the remaining eight hours, usually between 10 a.m. and 6 p.m.
This may be more tolerable than the well-known 5:2 diet — where followers limit their calories to 500 to 600 a day for two days a week, then eat normally for the remaining five days.
What are the benefits of time-limited meals?
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 8 p.m. because that means they only need to fast at night and skip breakfast, but they can still have lunch and dinner, as well as some snacks.
When eating, it’s best to opt for healthy options like fruits, vegetables, and whole grains, and to drink water and unsweetened beverages.
What are the disadvantages ?
The downsides of the fasting schedule can be that people abuse the hours they can eat, which leads to weight gain.
It can also lead to long-term digestive issues, as well as hunger, fatigue, and weakness.
They took 12 male and 12 female fish and put them on a complete diet.
While another group of 18 men and 18 women were placed in a fasting group.
After 15 days, the fasting fish were able to eat normally again.
To test reproductive performance, the 30 male and 30 female fish were randomly paired on days seven, 15, 21, 28, and 35 of the experiment with fish of the opposite sex from the “general population”.
The fish then had a maximum of five hours to spawn.
If unsuccessful, they were given another partner and the process was repeated the next day.
Semen from each of the 30 males was also collected on days seven, 15, 21 and 35, while the eggs were then assessed after two and 24 hours.
Scientists found that female fish on a fasted diet had, on average, “significantly poorer” reproduction on days seven and 15.
“This resulted in a reduction in the total number of offspring for females,” they added, with 163 offspring in those who were fully fed compared to 75 who were fasted.
The reduction in offspring even continued once the females were fed normally, with the fish recording increasing fin growth at the same time.
The researchers suggested this could be a sign that women’s bodies were prioritizing their own post-fasting health over their ability to reproduce.
The fasted male fish also showed a faster decrease in their sperm velocity over time and a “significant decrease in sperm quality” compared to the fed group, the scientists said.
The study authors said the results “cannot be directly compared to a two-week period in a mammal,” due to differences in metabolism between us and fish.
However, they hope the results underscore the importance to consider not only the effect of fasting on weight and health, but also on fertility.
Dr Edward Ivimey-Cook, biological scientist and study author, said: “These findings underscore the importance of considering not only the effect of fasting on body maintenance, but also on the production of eggs and sperm.
“It is important to note that some of the negative effects on egg and sperm quality can be seen after animals return to their normal level of food consumption after a time-restricted fast.
“Further research is needed to understand how long it takes for sperm and egg quality to return to normal after the fasting period.”
Professor Alexei Maklakov, an expert in evolutionary biology and author of the study, said: “The way organisms respond to food shortages can affect egg and sperm quality, and these effects could potentially continue after the end of the fasting period.
Writing in the journal Proceedings of the Royal Society B: Biological Sciences, the researchers said the findings “call for careful evaluation of the effects of intermittent fasting on fertilization.” Pictured above is Nicole Kidman, known for her intermittent fasting
To test reproductive performance, the 30 male and 30 female fish were randomly paired on days seven, 15, 21, 28, and 35 of the experiment with fish of the opposite sex from the “general population”. Pictured above is Cameron Diaz, known for his intermittent fasting
Zebrafish is a common animal used in such studies because its genetic structure is broadly similar to that of humans and its reproductive rate is high.
Although it’s a popular celebrity diet trend, experts are divided on the merits of intermittent fasting.
Some say fasters usually end up consuming a relatively large amount of food all at once, which means they don’t cut calories – a known way to combat bulging.
And in recent years, a small body of research has emerged linking the trend to shorter lifespans and higher risk of heart disease.
Experts suggest this could be because intermittent fasters eat more at one time, which they believe could damage cells.
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