Fasting Does Not Necessarily Harm Cognitive Performance, Major Review Finds

Fasting can be an effective way to lose weight, yet many people assume that withholding food from the body must impair brain function.

Does fasting necessarily come with a decline in cognitive performance?

A large review published recently indicates that this is not invariably true.

Drawing on 63 scientific articles covering 71 independent studies and 3,484 participants in total, the review found no meaningful difference in cognitive performance between people fasting and those eating regular meals.

The findings provide a substantial challenge to the belief that moderate, short-term limits on food intake drain healthy people's mental resources. That belief appears in everything from snack advertisements ("you're not you when you're hungry") to the familiar claim that breakfast is the day’s most important meal.

Psychologist Christoph Bamberg of Paris Lodron University in Austria and cognitive neuroscientist David Moreau of the University of Auckland in New Zealand conducted the analysis. They do not want concerns about cloudy thinking to discourage people who might benefit from fasting.

"For most healthy adults, the findings offer reassurance," Moreau explained in a commentary for The Conversation.

"You can explore intermittent fasting or other fasting protocols without worrying that your mental sharpness will vanish."

Fasting and cognitive performance

To examine the extensive body of evidence, the researchers applied a Bayesian statistical method. Rather than delivering a simple yes-or-no result, this approach produces a probability distribution.

Here, it was used to assess whether fasting influences cognitive performance, and is especially valuable when evaluating numerous different statistical sources.

The studies measured cognitive abilities including memory recall, decision-making, and the speed and accuracy of responses. Considered together, these tests showed that short-term fasting, with a median length of 12 hours, did not significantly alter scores.

When fasting may affect mental performance

There were, however, some important qualifications.

Cognitive performance fell modestly during fasting periods longer than 12 hours, while children and teenagers experienced "noticeable declines". Children represented only a small share of all participants.

This may mean that young, developing brains are more vulnerable to lengthy periods without food, and that eating three regular meals each day is particularly important for children and teenagers.

Notably, effects were most apparent in food-related cognitive tasks. Certain highly specific brain circuits may begin to signal during fasting, although more research will be needed to establish this.

"Performance deficits were often evident only in tasks involving food-related stimuli, such as looking at pictures of food or processing food-related words," Moreau said.

"In contrast, performance on tasks using neutral content was largely unaffected."

"Hunger might selectively divert cognitive resources or cause distraction only in food-relevant contexts, but general cognitive functioning remains largely stable."

People who were fasting also generally performed less well on cognitive tests conducted later in the day. This may suggest that not eating intensifies the natural concentration dips linked to the body’s internal circadian rhythms.

Potential health effects of fasting

In addition to helping some people manage their weight, scientific research has linked fasting with other possible health benefits, including better cardiovascular health and lower inflammation levels.

Scientists believe fasting brings substantial changes in how the body functions. Among these is a shift away from using energy stores made from glycogen towards using body fat more broadly, particularly through molecules known as ketone bodies.

With these findings considered, Moreau concludes that "fasting is best seen as a personal tool rather than a universal prescription".

There is no universal approach, and fasting is best undertaken with advice from a medical professional.

"The primary takeaway is a message of reassurance: Cognitive performance remains stable during short-term fasting, suggesting that most healthy adults need not worry about temporary fasting affecting their mental sharpness or ability to perform daily tasks," Moreau said.

The research was published in Psychological Bulletin.