How fasting affects cognitive performance

Ever wondered whether missing breakfast could leave you mentally hazy at work? Or whether intermittent fasting might make you grumpy, unfocused and less productive?

Snack adverts tell us that "you're not you when you're hungry", strengthening the widespread idea that we need to eat to keep our minds working at their best.

That belief is firmly embedded in our culture. We are often told that continually topping up our energy is the key to remaining alert and productive.

But time-restricted eating and intermittent fasting have become extremely popular health practices during the past decade. Millions of people follow them for potential long-term gains, including weight management and better metabolic health.

This poses an important question: is it possible to gain the health benefits of fasting without losing mental sharpness? To answer it, we carried out the most extensive review so far into the effects of fasting on cognitive performance.

Why fast in the first place?

Fasting is more than a fashionable dietary shortcut. It draws on a biological mechanism refined over thousands of years to help people survive periods of food scarcity.

When food is consumed regularly, the brain relies chiefly on glucose, which the body stores as glycogen. However, after roughly 12 hours without eating, glycogen reserves start to run low.

The body then makes an ingenious metabolic shift, breaking down fat into ketone bodies - such as acetoacetate and beta-hydroxybutyrate - that can serve as an alternative source of fuel.

This ability to adapt metabolically was once essential to the survival of our ancestors, and it is now associated with a wide range of health benefits.

Several of fasting's most encouraging effects may result from how it alters processes within the body. For example, fasting stimulates autophagy, a cellular "cleanup crew" that removes damaged material and recycles it. This process is thought to promote healthier ageing.

Fasting can also increase insulin sensitivity, helping the body regulate blood sugar more efficiently and reducing the risk of conditions including type 2 diabetes.

In addition, the metabolic changes caused by fasting may provide wider protection by lowering the chances of chronic diseases commonly linked with overeating.

What the fasting cognitive performance data showed

These physiological advantages have helped make fasting appealing. Yet many people are reluctant to try it because they fear their mental performance will collapse without a regular supply of food.

To investigate this concern, we performed a meta-analysis - a "study of studies" - examining all available experimental research that compared cognitive performance in people who were fasting with that of people who had eaten.

Our search found 63 scientific articles covering 71 independent studies. Together, they included 3,484 participants who completed 222 separate measures of cognition. The studies stretched across almost seven decades, from 1958 to 2025.

Once the results were combined, the finding was straightforward: there was no meaningful difference in cognitive performance between healthy adults who were fasted and those who were satiated.

Participants performed equally well on tests of attention, memory and executive function, regardless of whether they had eaten recently.

When fasting does matter

However, our analysis identified three important factors that can influence the effects of fasting on the mind.

Age was the first. Fasting produced no measurable reduction in adults' mental performance. Children and adolescents, however, performed less well in tests when they missed meals.

Their developing brains appear to be more responsive to changes in available energy. This supports long-established advice that children should have a proper breakfast before school to help them learn.

The length of the fast also appeared to matter. Longer fasts were linked with a smaller difference in performance between fasted and fed states. One possible explanation is the metabolic move towards ketones, which may provide the brain with a stable energy supply as glucose becomes depleted.

Fastened participants generally performed less well when testing took place later in the day, indicating that fasting may intensify the natural low points in our circadian rhythms.

The nature of the test was important too. In cognitive tasks using neutral symbols or shapes, fasting participants did equally well, and in some cases marginally better.

Yet when tasks featured food-related signals, fasted participants' performance fell. Hunger does not produce a general mental fog, but it can make us more distractible when food is at the forefront of our thoughts.

What this means for you

For most healthy adults, these findings should be reassuring: intermittent fasting and other fasting approaches can be explored without fearing that mental clarity will disappear.

Nevertheless, fasting is not suitable for everyone in the same way. Children and teenagers require particular care, as their brains are still developing and they seem to need regular meals to perform at their best.

Likewise, fasting may be more difficult to maintain if your work demands maximum alertness later in the day, or if you are regularly confronted with tempting food cues.

Naturally, some people - including those with medical conditions or particular dietary requirements - may need professional advice before fasting, as it may not be appropriate for them.

Fasting is ultimately best regarded as a personal option, not a universal recommendation. Its advantages and difficulties will vary between individuals.

David Moreau, Associate Professor of Psychology, University of Auckland, Waipapa Taumata Rau

This article is republished from The Conversation under a Creative Commons license. Read the original article.