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• Time-restricted eating is an increasingly popular method of weight control that involves consuming all your meals and snacks within a set time period and fasting outside that period.
• Some people find that it helps them to lose weight or maintain a healthy weight, and there is some evidence that it may also lower the risk of metabolic diseases, such as diabetes.
• A new study in zebrafish has found that time-restricted eating adversely affected their fertility.
• More research is needed to determine whether similar effects might be seen in humans.

Time-restricted eating (TRE) is a form of intermittent fasting that focuses on the timing of eating, rather than on calorie intake. It involves eating all meals and snacks within a set time period — usually between 6 and 12 hours each day — and consuming only water and calorie-free drinks outside that time.

A person on a TRE plan will choose the eating window that best suits their lifestyle. Many people find that by restricting the time during which they can eat, they tend to eat less, which may make it a straightforward method of weight control. The practice has become increasingly popular in recent years.

Studies have shown some benefits from TRE. One study showed clinically meaningful weight loss and improvements in fasting blood glucose in people with obesity who followed a 10-hour eating – 14-hour fasting plan for 8 weeks. Another found similar weight loss for women with obesity following an 8:16 fasting plan for 3 months.

However, not all results have been entirely positive. A meta-analysis of 43 studies found that, while intermittent fasting resulted in greater weight loss than a non-intervention diet, phentermine tablets reviews it had less benefit than calorie restriction.

Now, a study has found that, in zebrafish, TRE had negative effects on the quality of both sperm and eggs and that the adverse effects continued after usual feeding resumed.

The study, from the University of East Anglia (UEA), U.K., is published in Proceedings of the Royal Society B.

Little effect on body growth

The researchers used zebrafish (Danio rerio), small tropical fish which share more than 70% of their genome with humans, for the study. Zebrafish are widely used for research as they are small, live happily in large shoals in tanks, and breed rapidly.

All fish were reproductively mature and had been fed an unrestricted diet before the experiment. The researchers then divided them, at random, into two groups. One continued with the unrestricted diet, and the other fasted. After 15 days, the researchers returned all fish to the unrestricted diet.

In the 15-day experimental period and after the return to unrestricted feeding, the researchers assessed both somatic (body) growth (by measurement of the tail fin) and reproductive performance, including the quality of eggs and sperm produced.

The researchers found that during the study, there was no difference in somatic growth between the fasted and fully-fed fish. However, once the fasted fish were returned to their usual diet, the females showed faster fin growth than the non-fasted fish.

Decline in egg and sperm quality

During fasting, the total number of offspring in fasting females was reduced compared with those feeding freely. However, once they began re-feeding, the differences between fasted and fed fish disappeared.

The researchers did see a difference in offspring quality during and after fasting. During fasting, females produced fewer offspring, but they were high quality. Once the fasted females started feeding again, the number of offspring increased, but their survival rate was lower.

Similarly, both during fasting and once feeding restarted, there was a reduction in male sperm quality.

So, for both sexes, gamete quality appeared to be negatively affected by fasting, and the effects continued after typical feeding resumed. The researchers suggest that when food was restricted, the fish were investing more resources in body maintenance and survival and less into reproduction.

Could there be a similar effect on people?

Prof. Alexei Maklakov, the corresponding author of the study, from UEA’s School of Biological Sciences, said:

“Time-restricted fasting is […] 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.”
— Prof. Alexei Maklakov

There have, so far, been few studies into the effects of TRE on fertility and reproduction, and most have been in rodents. The few human studies, most with small sample sizes, have raised as many questions as they have answered.

Human studies

A recent study, in women with obesity, found that a very restrictive time window (4–6 hours) for eating reduced levels of DHEA, a steroid hormone that is important for creating both estrogen and testosterone. However, this was a small study, and experts highlighted the need for similar research in people of a healthy weight.

Another review of studies found that intermittent fasting might reduce androgen markers in both men and women, although the authors noted that there was limited research in this area. This effect may be beneficial to women with polycystic ovary syndrome (PCOS) but could have undesired consequences, such as a reduction in muscle mass, in men.

Dr. Krista Varady, professor of nutrition at the University of Illinois, Chicago, who was not involved in the study, has written widely about TRE. She told Medical News Today:

“Overall, I don’t think these results in zebrafish can be applied to humans. Results from human TRE studies show that fasting has virtually no adverse effects on fertility hormones in men or women.”

More research needed

Although this study was conducted in fish, the researchers say that their findings highlight the importance of considering the effect of fasting on fertility in people.

Dr. Edward Ivimey-Cook, from UEA’s School of Biological Sciences, corresponding author of the study, commented:

“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.”

“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,” he added.

But Dr. Varady is not convinced that the research has implications for human health:

“There are hundreds of TRE papers published each year in humans. I think we should focus on the human findings, instead of worrying about what is happening in other non-mammalian species. Fish and humans are very different creatures with vastly different reproductive systems. This paper would have been much more impactful if it was done in humans.”

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