- Time-restricted eating is an increasingly popular method of weight control that involves eating all of your meals and snacks within a set time frame and fasting outside of that time frame.
- Some people find it helps them lose weight or maintain a healthy weight, and there’s some evidence it may also reduce the risk of metabolic diseases, such as diabetes.
- A new study on zebrafish has found that time-restricted feeding negatively affects its fertility.
- Further research is needed to determine if similar effects could be seen in humans.
Time-restricted eating (TRE) is a form of intermittent fasting that focuses on when you eat rather than calorie intake. This involves eating all meals and snacks within a set time frame – usually between 6 and 12 hours a day – and consuming only water and calorie-free beverages outside of this time frame.
A person on a TRE plan will choose the feeding window that best suits their lifestyle. Many people find that by limiting the time they can eat, they tend to eat less, which can make it a simple method of weight control. The practice has become
Studies have shown some benefits of TRE.
However, not all results have been entirely positive.
Now, a study has found that, in zebrafish, TRE had negative effects on sperm and egg quality, and the adverse effects persisted after resuming the usual diet.
The study, from the University of East Anglia (UEA), UK, is published in Proceedings of the Royal Society B.
The researchers used zebrafish (Danio rerio), small tropical fish that share more than 70% of their genome with humans, for study. Zebrafish are widely used for research because they are small, live happily in large schools in tanks, and reproduce quickly.
All fish were reproductively mature and had been fed an unrestricted diet prior to the experiment. The researchers then randomly divided them into two groups. One continued the diet without restriction and the other fasted. After 15 days, the researchers put all the fish back on the unrestricted diet.
During the 15-day experimental period and after returning to an unrestricted diet, the researchers assessed both somatic (body) growth (by caudal fin measurement) and reproductive performance, including quality of eggs and sperm produced.
The researchers found that during the study, there was no difference in somatic growth between fasted and fully fed fish. However, once the fasted fish returned to their usual diet, the females showed faster fin growth than the non-fasted fish.
During fasting, the total number of offspring in fasted females was reduced compared to free-feeding ones. However, once they started refeeding, the differences between fasted and fed fish disappeared.
Researchers found a difference in offspring quality during and after fasting. During the fast, the females produced fewer offspring, but they were of high quality. Once the fasted females resumed feeding, the number of offspring increased, but their survival rate was lower.
Similarly, both during the fast and once feeding restarted, there was a reduction in male sperm quality.
Thus, for both sexes, gamete quality appeared to be negatively affected by fasting, and the effects continued after resuming the typical diet. The researchers suggest that when food was restricted, fish invested more resources in body maintenance and survival and less in reproduction.
Professor Alexei Maklakov, the study’s corresponding author, 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 how organisms respond to food shortages can affect egg and sperm quality, and these effects could potentially continue after the fasting period ends.”
— Professor Alexei Maklakov
There have been, to date, few studies on the effects of TREs on fertility and reproduction, and most have been conducted in rodents. The few human studies, mostly with small sample sizes, have raised as many questions as they have answered.
A recent study, conducted in obese women, found that a very restrictive time window (4-6 hours) for eating reduced levels of DHEA, a steroid hormone important for the creation of estrogen and testosterone. However, this was a small study and experts have pointed to the need for similar research in people at a healthy weight.
Another review of studies found that intermittent fasting may reduce
Dr. Krista Varady, professor of nutrition at the University of Illinois at Chicago, who was not involved in the study, has written extensively about TRE. She said 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 effect on fertility hormones in either men or women.
Although this study was conducted in fish, the researchers say their findings underscore the importance of considering the effect of fasting on fertility in people.
Dr Edward Ivimey-Cook from the UEA School of Biological Sciences, corresponding author of the study, commented:
“These findings underscore the importance of considering not only the effect of fasting on body maintenance, but also on egg and sperm production.”
“Further research is needed to understand how long it takes for sperm and egg quality to return to normal after the fasting period,” he added.
But Dr. Varady isn’t convinced the research has implications for human health:
“There is hundreds TRE papers published annually in humans. I think we should focus on human discoveries, instead of worrying about what’s happening in other non-mammalian species. Fish and humans are very different creatures with very different reproductive systems. This article would have had much more impact if it had been done in humans.
