- Researchers report that intermittent fasting of less than 24 hours does not appear to diminish a person’s mental performance during the periods where they are not eating.
- They say the human body has adapted to fasting intervals by burning fat as an energy source.
- Past research has shown that short-term fasting can help individuals lose weight, improve insulin sensitivity, and reduce inflammation.
Intermittent fasting does not appear to diminish a person’s mental sharpness during periods when they are not eating, according to a new study.
Researchers report in their online-first paper, published in Psychological Bulletin, that short-term fasting of less than 24 hours does not affect the mental performance of adults.
“People often worry that if they fast they won’t be able to concentrate at work or study effectively,” said David Moreau, PhD, a neuroscientist at the University of Auckland in New Zealand and the senior author of the paper, in a statement.
“Our results show that, for most adults, short-term fasting is unlikely to have a major impact on mental sharpness,” he added.
Researchers noted that children and adolescents did show some erosion of mental capabilities while fasting. That reinforced the importance of having a proper breakfast before school.
In addition, the researchers noted that adults who fast tend to perform worse in tests later in the day. They suggested the lack of food may amplify natural dips in circadian rhythm.
“Humans evolved with periods of food scarcity, so it makes sense our cognitive systems can function well without constant refueling,” Moreau said. “That said, children and teenagers appear more vulnerable, which fits with what we know about their high energy demands.”
Experts who weren’t involved in the study say the research re-emphasizes the benefits of intermittent fasting.
“The important message is fasting is not going to cause a problem with your mental performance,” said Mir Ali, MD, a general surgeon, bariatric surgeon, and medical director of MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center in California.
“This may reassure a lot of people,” he added.
However, Debra Safer, MD, a professor of psychiatry and behavioral sciences at Stanford University in California, pointed out that the study only dealt with short-term fasting of less than 24 hours.
“The study included research investigations primarily examining short term effects of fasting (average 12 hours fasts on cognitive performance),” Safer told Healthline. “Assurance for people who want to fast would require having research available under conditions more similar to those who want to fast as a regular practice.”
“This would include testing the effects of fasting for longer durations of time, research that included participants who were not necessarily in a healthy weight range, testing with participants who may also have co-occurring medical and psychological disorders, particularly including those with eating disorders (or at risk for eating disorders),” Safer continued.
The meta-analysis comprised 63 studies from around the world, involving more than 3,400 individuals.
Most participants regularly fasted for between 8 hours and 24 hours, with a median fasting period of 12 hours.
The researchers concluded that fasting “neither enhances nor impairs mental performance.” They did note three factors that can influence mental acuity.
- fasting duration
- context of the cognitive test
- individual characteristics (i.e., age and body composition)
In particular, researchers noted that people tend to score lower on mental tests near the end of their fasting period.
“These findings suggest that although most adults need not worry about temporary fasting affecting their mental sharpness, implementation across specific populations and situations may require more careful consideration,” the researchers wrote.
Researchers said that glycogen, the primary stored form of glucose, serves as the main energy source for individuals who eat regularly throughout the day.
They said glycogen levels may decrease while a person is fasting, but the body finds other energy sources, such as ketone bodies, that are produced from fat tissue.
“This metabolic flexibility likely supported survival during periods of food scarcity, particularly before the advent of effective food preservation methods,” the researchers wrote.
The researchers stated that this prehistoric adaptation in the body continues to serve humans well.
“Comparative biological evidence and evolutionary theories suggest that humans developed enhanced fat storage capacity as an adaptive response to resource fluctuations,” the researchers explained.
“This energy reserve not only supported daily metabolic demands but also enabled the prolonged development and expansion of the human brain, which requires substantial energy input,” they added.
“[The study] does seem to provide reassurance that the human body is very adaptable and people – in general — can maintain cognitive performance for periods of fasting similar to what is usually observed when people have short-term perturbations in their sleep,” Safer said.
“However, some factors are important to watch out for, like how long the fasting interval is, the context one within which one is performing a cognitive task, and individual characteristics such as younger age,” she added.
Intermittent fasting is described as a dietary pattern that switches between periods of eating and periods of fasting.
Most intermittent fasting plans don’t focus on what a person eats. The emphasis is on when they eat.
There are several popular fasting methods. They include:
- 16/8: This involves an 8-hour eating period and a 16-hour fasting period. Some people do this by skipping breakfast, while others reverse the approach by skipping dinner or eating it early.
- 5:2: This method involves eating only 500 to 600 calories on two nonconsecutive days of the week and then eating as usual the other 5 days.
- Eat Stop Eat: This plan involves fasting for 24 hours once or twice a week.
Fasting can work because of how the body reacts to an extended period of time without food. For example:
- Fasting causes an increase in human growth hormones, which in turn induces fat loss and muscle growth.
- Fasting can also improve insulin sensitivity and decrease insulin levels, making stored fat more accessible.
- In addition, fasting can initiate the process of cellular repair. This repair can include autophagy, in which cells digest and remove old and dysfunctional proteins that build up inside them.
“Fasting basically switches the body into the mode of fat burning,” Ali explained to Healthline.
Research has shown that improvements in insulin sensitivity during fasting can help prevent and manage type 2 diabetes.
“Fasting is effective for a lot of people for a lot of reasons,” Ali said.
The most obvious is hunger. There are others, however. They include:
Additionally, intermittent fasting may not be the best choice for a person with a history of eating disorders or someone who is pregnant or nursing.
On the plus side, there is some flexibility with intermittent fasting schedules.
You can drink water, coffee, tea, and other liquids while fasting. You can also adhere to a daily exercise schedule.
It’s important to consume nutrient-dense foods while following an intermittent fasting plan, as well as to engage in regular exercise and ensure you get adequate sleep.
Ali recommends that people start slowly when implementing a fasting regimen. He says initially fasting for 6 to 8 hours is a good place to start. Then, a person can graduate to 8- to 10-hour fasts and eventually to the more optimal 12- to 14-hour fasts.
Ali recommended that the final meal of the day, such as dinner, contain a high amount of protein and vegetables. He suggested a lower amount of carbohydrates because those substances can burn off more quickly and induce hunger pangs.
