Intermittent fasting has long been praised for its benefits to metabolism and weight control.
Now, researchers are exploring its effects on the brain.
A comprehensive new review published in Psychological Bulletin suggests that short-term fasting lasting less than 24 hours does not significantly impair mental performance in healthy adults.
This finding challenges the common belief that skipping meals inevitably dulls concentration or slows reaction time.
According to lead researcher David Moreau, PhD,
From the University of Auckland, the results show that for most adults, short periods without food do not meaningfully impact cognitive function.
The analysis drew from 63 studies and included data from more than 3,400 participants around the world.
Across a wide range of cognitive tasks memory, attention, reasoning, and problem-solving researchers found no consistent evidence that fasting impaired performance.
In some cases, participants even reported feeling slightly more alert, a response thought to be linked to evolutionary adaptations for food-seeking behavior.
This aligns with what scientists know about metabolic flexibility.
When the body’s usual energy stores of glucose begin to drop, it naturally transitions to burning fat and producing ketone bodies, an alternative fuel for both muscles and the brain.
This metabolic shift is one reason many people who practice intermittent fasting describe a sense of clear-headedness after the initial hunger subsides.
A 2021 review in Nature Reviews Neuroscience noted that ketones may protect brain cells and enhance resilience during temporary energy shortages.
The findings, however, are not universal.
The new review highlighted that children and adolescents showed some cognitive decline during fasting periods, underscoring the importance of consistent nutrition for growing bodies and developing brains.
Similarly, adults tended to perform slightly worse on tests later in the day when fasting, likely reflecting natural dips in circadian rhythm combined with extended energy depletion.
Experts say these nuances matter.
Mir Ali, MD, a bariatric surgeon at MemorialCare Surgical Weight Loss Center, emphasized that short-term fasting is generally safe for healthy adults and unlikely to cause problems with concentration.
However, others urge caution before generalizing these findings to everyone.
Debra Safer, MD, a psychiatrist and eating behavior researcher at Stanford University, noted that longer-term fasting, underlying health conditions, or a history of disordered eating could change the picture significantly.
Future studies, she says, should examine how extended fasting, calorie restriction, or medical conditions might influence cognitive outcomes over time.
For now, the evidence supports that the human brain is remarkably adaptable.
Glycogen the body’s short-term energy store may fall during fasting, but ketones derived from fat help sustain brain metabolism.
This flexibility is thought to be an evolutionary advantage, allowing early humans to stay mentally sharp while searching for food.
In modern settings, it may help explain why some adults report sustained focus even during fasting intervals.
Still, fasting is not a one-size-fits-all strategy.
Those who experience fatigue, mood swings, or brain fog during fasting may be better served by shorter intervals, balanced meals, and adequate hydration.
Each body’s response depends on genetics, daily habits, and metabolic health.
The takeaway from this growing body of research is reassuring.
Short-term fasting appears safe for mental performance in most healthy adults, with the brain equipped to adapt quickly to temporary changes in fuel availability.
Yet fasting for longer periods or under specific health conditions requires individualized guidance.
As Dr. Moreau and his colleagues concluded, the key is balance recognizing both the body’s resilience and the boundaries of its adaptability.
Our brains are built to endure short stretches without food, but long-term focus still depends on sustainable nourishment, rest, and overall well-being.
To understand the science behind fasting’s metabolic benefits and potential brain effects, you can explore this discussion from the Huberman Lab Podcast, which delves into how fasting influences energy, focus, and cellular health.