We Were Wrong About Fasting—Massive Review Finds
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. To find out whether we can reap the health rewards of fasting without sacrificing our mental edge, we conducted the most comprehensive review to date of how fasting affects cognitive performance. 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. 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.
The Metabolic Switch: How Fasting Rewires Brain Fuel
Fasting isn't just about skipping meals; it triggers a deep metabolic shift. 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 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. 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.
The Big Picture: What the Data Show About Fasting and Cognition
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. This section also includes the broader context and caveats about how age, timing, and test type can influence outcomes. David Moreau, Associate Professor of Psychology, University of Auckland, Waipapa Taumata Rau. Read the original article.
