The evidence is mounting that intermittent fasting — setting limits on when you eat, instead of what you eat — can be a successful weight management strategy. But will skipping meals make it harder for you to concentrate?
“I think most people assume that if they feel hungry, they’ll struggle to think clearly,” says David Moreau, PhD, an associate professor of psychology at the University of Auckland in New Zealand. “It’s a very intuitive belief — we associate hunger with low energy, irritability, and distraction, so it feels logical that cognition would suffer, too.”
But a new scientific review led by Dr. Moreau found that short periods of fasting generally do not affect cognitive performance in healthy adults: Across more than 70 experimental studies, people who hadn’t eaten performed just as well in mental tasks and tests as those who had full bellies.
[1]
What Happens to Your Brain Power When You Go 12 Hours Without Food?
To evaluate how short-term fasting or skipping a meal might affect thinking, scientists looked at outcomes from research focusing on cognitive performance in about 3,500 healthy adults who were either fasting or had recently eaten.
Using standardized tests, the studies tested participants’ mental abilities, including memory recall, decision-making, and response speed and accuracy. On average, fasting participants had gone about 12 hours without food.
Taken together, the results of all those studies found that fasting neither impairs nor enhances mental performance.
“We expected at least mild declines in performance when people were hungry, but the results showed that the brain is far more resilient than commonly assumed,” says Moreau.
The analysis did note modest reductions in cognitive performance for people who’d been fasting longer than 12 hours.
While the scientific review relied on rigorous statistical modeling, the findings are limited by the high variability in the designs of the 70-plus studies analyzed: There were several different fasting protocols, testing times, and types of cognitive tasks, according to the authors.
The studies also didn’t control for diet quality, so it’s not possible to conclude how that might affect cognitive performance between participants.
Not Surprising: Thinking of Food While Fasting Can Be Distracting
There was some evidence from the scientific review that thoughts of food specifically could disrupt attention: Of all the tasks and tests, fasting participants did have a harder time with prompts that involved looking at pictures of food or processing food-related words, according to study authors.
“In contrast, performance on tasks using neutral content was largely unaffected,” says Moreau.
“This research contradicts the widespread assumption that any skipped meal will degrade thinking ability,” says Sera Lavelle, PhD, a clinical psychologist and founder of NY Health Hypnosis and Integrative Therapy in New York City.
Still, says Dr. Lavelle, intermittent fasting can lead to mood changes that may have an impact on cognitive function. “Reductions in blood glucose or increased hunger can trigger irritability, mood shifts, or increased thinking about food,” says Lavelle, who specializes in treating emotional eating, eating disorders, and weight management. She was not involved with the new scientific review.
Charlotte Markey, PhD, a professor of psychology at Rutgers University in Camden, New Jersey, adds that more research needs to be done on whether a negative mood can hamper a person’s cognitive ability.
“It’s hard to be happy when hungry — this is where the expression ‘hangry’ comes from,” says Dr. Markey, whose research focuses primarily on eating‐related behaviors, body image, and weight management, and who was not involved in the scientific review.
Fasting Can Put the Body in Fat-Burning Ketosis
The study authors recognize that fasting may have positive or negative effects on different aspects of health, depending on the individual.
For some, fasting may have a positive effect on how the body uses energy. Glycogen, the primary stored form of carbohydrates, serves as the body’s main energy source when you eat regularly throughout the day.
When you’re fasting, however, glycogen eventually gets depleted, leading the body to start burning fat as an alternative energy source. This is called ketosis, because the body is using ketones, a byproduct of fat burning, for fuel.
[2]
“Emerging evidence suggests that relying on ketones may confer broad health benefits, modulate hormonal systems, and activate cellular repair processes linked to longevity,” says Moreau.
Is Intermittent Fasting Right for You? Consider These Questions
Moreau cautions that short-term fasting is not for everyone, and people with medical conditions, or those who are underweight or prone to disordered eating should consult a clinician before making dietary changes.
According to Lavelle, the key question is: Does this eating-fasting pattern fit your body, your mind, your relationship with food, and your day’s demands?
“Short-term fasting appears not to impair core cognitive functions on specific tasks, which is reassuring,” she says. “But that doesn’t mean there’s no cost, such as the psychological and attentional burden of hunger — thinking about food, mood shifts, and distraction — that can reduce functional clarity.”
