How does fasting affect brain function?


Intermittent fasting sounds like an attractive dieting scheme. A quick visit to bodybuilding forums reveals no shortage of people who will attest to its effectiveness. Yet as someone well versed in human physiology, I am suspicious of what less noticeable effects it may have on the human body. For one, the brain runs entirely on glucose and absorbs it via facilitated diffusion, so low blood glucose means low fuel for the brain. I decided to investigate further.

Green et al. had similar concerns (1995). They assessed the effects of short-term fasting on cognitive function using dieting women that either skipped 1, 2, or all meals 24 hours prior to testing. Cognitive function was compared using the Bakan vigilance task, a two-finger tapping task, a simple reaction time (SRT) task, an immediate free recall task, and a measure of focused attention. There were no significant differences in cognitive performance between experimental groups in testing except for the two-finger tapping test. Participants who had fasted 24 hours prior to testing tapped significantly slower. Notably, all fasting groups exhibited reduced heart rate than did non-fasted controls in a dose-dependent manner. Results suggest metabolism is altered by food deprivation though most measures of cognitive performance are not.

bakan test

A similar but more recent study by Green et al. arrived at the same conclusion but concerning dieting in general as opposed to just fasting. Women on a 12-week dieting plan were compared to women who maintained usual diet and exercise habits during the same time. Researchers found that there was no significant difference between dieters and non-dieters in terms of the neuro-psychological tests of speed of information processing, executive function, working memory, immediate and delayed recall and recognition, and verbal ability.

dieting women

There is some merit to my concerns of hypoglycemia and cognitive performance. Kanarek et al. tested memory-recall (simple number sequence recall) and reasoning (arithmetic reasoning, reading, attention) abilities of male college students after administering certain snacks. Half the subjects skipped lunch prior to the testing. In both subjects who skipped lunch and those who did not, consuming higher-calorie foods resulted in improved performance. Subjects ate sugar-containing snacks performed significantly more math problems and solved the problems in less time than did students who snacked on sugarless snacks (e.g. diet soda).

My takeaway? If intermittent fasting does impair brain function, it probably doesn’t do so to such a degree that it will be noticeable to other people. Sleep deprivation or uncomfortable clothes are probably comparable in how they take away from cognitive performance.

tl;dr IF is probably fine, but you might want to eat up if you’re planning to take the bar exam or MCAT soon.

Source 1, 2, 3, 4


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