David Moreau, Associate Professor of Psychology, University of Auckland, Waipapa Taumata Rau
Ever worried that skipping breakfast might leave you foggy at work? Or that intermittent fasting would make you irritable, distracted and less productive?
Snack food ads warn us that “you’re not you when you’re hungry”, reinforcing a common belief that eating is essential to keep our brains sharp.
This message is deeply woven into our culture. We’re told constant fuelling is the secret to staying alert and efficient.
Yet time-restricted eating and intermittent fasting have become hugely popular wellness practices over the past decade. Millions do it for long-term benefits, from weight management to improved metabolic health.
This raises a pressing question: can we reap the health rewards of fasting without sacrificing our mental edge? To find out, we conducted the most comprehensive review to date of how fasting affects cognitive performance.
Why fast in the first place?
Fasting isn’t just a trendy diet hack. It taps into a biological system honed over millennia to help humans cope with scarcity.
When we eat regularly, the brain runs mostly on glucose, stored in the body as glycogen. But after about 12 hours without food, those glycogen stores dwindle.
At that point, the body performs a clever metabolic switch: it begins breaking down fat into ketone bodies (for example, acetoacetate and beta-hydroxybutyrate), which provide an alternative fuel source.
This metabolic flexibility, once crucial for our ancestors’ survival, is now being linked to a host of health benefits.
Some of the most promising effects of fasting come from the way it reshapes processes inside the body. For instance, fasting activates autophagy, a kind of cellular “cleanup crew” that clears away damaged components and recycles them, a process thought to support healthier ageing.
It also improves insulin sensitivity, allowing the body to manage blood sugar more effectively and lowering the risk of conditions such as type 2 diabetes.
Beyond that, the metabolic shifts triggered by fasting appear to offer broader protection, helping reduce the likelihood of developing chronic diseases often associated with overeating.
What the data showed
These physiological benefits have made fasting attractive. But many hesitate to adopt it out of fear their mental performance will plummet without a steady supply of food.
To address this, we conducted a meta-analysis, a “study of studies”, looking at all the available experimental research that compared people’s cognitive performance when they were fasting versus when they were fed.
Our search identified 63 scientific articles, representing 71 independent studies, with a combined sample of 3,484 participants tested on 222 different measures of cognition. The research spanned nearly seven decades, from 1958 to 2025.
After pooling the data, our conclusion was clear: there was no meaningful difference in cognitive performance between fasted and satiated healthy adults.
People performed just as well on cognitive tests measuring attention, memory and executive function whether they had eaten recently or not.
When fasting does matter
Our analysis did reveal three important factors that can change how fasting affects your mind.
First, age is key. Adults showed no measurable decline in mental performance when fasting. But children and adolescents did worse on tests when they skipped meals.
Their developing brains seem more sensitive to fluctuations in energy supply. This reinforces longstanding advice: kids should go to school with a proper breakfast to support learning.
Timing also seems to make a difference. We found longer fasts were associated with a smaller performance gap between fasted and fed states. This might be due to the metabolic switch to ketones, which can restore a steady supply of energy to the brain as glucose runs out.
Performance in fasted individuals tended to be worse when tests were conducted later in the day, suggesting fasting might amplify the natural dips in our circadian rhythms.
The type of test also mattered. When cognitive tasks involved neutral symbols or shapes, fasting participants performed just as well, or sometimes even slightly better.
But when tasks included food-related cues, fasted participants slipped. Hunger doesn’t create universal brain fog, but it does make us more easily distracted when food is on our minds.
What this means for you
For most healthy adults, the findings offer reassurance: you can explore intermittent fasting or other fasting protocols without worrying that your mental sharpness will vanish.
That said, fasting isn’t a one-size-fits-all practice. Caution is warranted with children and teens, whose brains are still developing and who appear to need regular meals to perform at their best.
Similarly, if your job requires peak alertness late in the day, or if you’re frequently exposed to tempting food cues, fasting might feel harder to sustain.
And of course, for certain groups, such as those with medical conditions or special dietary needs, fasting may not be advisable without professional guidance.
Ultimately, fasting is best seen as a personal tool rather than a universal prescription. And its benefits and challenges will look different from person to person.
David Moreau does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
