How Fasting Affects Your Brain, Cognition & Mental Health
By R.Davies, PhD・Fasting
Published May 20, 2026 | 4 min read
Fasting triggers several changes in the way your brain functions, so there is interest in how (or if) it affects your 'cognitive function', your mood, brain health and mental health. In this article, we’ll look at whether or not fasting affects your brain and, if it does, how it works.
Your brain is the most active, energy-demanding organ in your body, using around a fifth of your body’s energy (despite only being 2% of your body weight). So, as you can imagine, it is very sensitive to changes that go on in your body, like what types of energy are available, changes in your hormones, your environment and inflammation — a lot of things that are affected by fasting.
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Effect of Fasting on the Brain
Ketones
Under normal conditions, when you’re eating carbohydrates on a semi-regular basis (in your meals), your brain runs almost entirely on a sugar called 'glucose'. When you start fasting, your glucose levels drop, and your body will start to release its own glucose stores (from your liver). But when these stores start to run dry, your body starts breaking down fat, and your liver starts producing 'ketones', which are used for energy.
These ketones can get into your brain and can be used for energy to keep your brain working. Ketones aren’t just a “backup" energy source when glucose is in short-supply; they appear to have positive health effects on the brain as well [1].
"BDNF"
Fasting increases substances in your brain called “brain-derived neurotrophic factor” (or “BDNF” for short). It plays a really important role in keeping your brain cells healthy. Low levels of BDNF are associated with depression, poor cognitive function, and some brain diseases (like Alzheimer’s and brain cancer) [2]. So, increasing your BDNF levels by fasting is one of the ways it can improve your brain health.
Inflammation
Your brain isn’t an isolated, sealed-off organ; it senses and can react to inflammation in your body (even if it is in a distant part of your body). Inflammation is a natural and essential process that helps the body fight and recover from injury or illness.
However, there are occasions when your body gets inflamed when it doesn't really need to. It then goes on to cause unnecessary damage to the body (including your brain). Fasting is shown to reduce inflammation [3], which in turn has been linked to better brain health and mental health (as well as many other diseases).
Autophagy
Fasting also triggers another natural process called “autophagy” — sometimes referred to as a clean-up job for your cells, which keeps them healthy (most of the time).
Fasting & Cognitive Performance
In young healthy adults, the effects of fasting on the brain’s performance are mixed. Some studies find small improvements in attention or brain processing speed, but other studies found that it actually makes some mental tasks (like memory or endurance tasks) worse [4].
Short-term fasts or repeated fasts don't seem to reliably produce noticeable changes in your brain’s cognitive performance. A normal, healthy brain is well equipped to go without food for the best part of a day, so you’re unlikely to notice any meaningful change to how you think if the fast is less than 16 hours. Blanket statements about the “brain-boosting benefits” of fasting are not proven (for young, healthy brains anyway).
Results for older adults are maybe more positive, particularly those who have experienced cognitive declines already. Researchers reported that intermittent fasting in older adults who already had mild cognitive issues had better chances of reversing their cognitive declines [5]. Another study found that older adults who ate all their food within a 10-hour time window each day had lower chances of having decreases in their cognitive function [6].
The same study also found the same results for those who ate breakfast, which seems to favour “early time-restricted eating” as the preferred intermittent fasting pattern for health. So fasting may benefit brains that are ageing and may be in a slightly more vulnerable (lower functioning) state. However, the evidence that fasting can potentially prevent, delay and reverse cognitive declines is promising.
Fasting & Mental Health
A study of people who did “Buchinger fasts”, which are very long (4 days to 3 weeks), medically supervised fasts where you’re allowed to consume a little bit of food (under 500 calories, usually liquids), reported improved “emotional well-being”.
The people who did the fast reported feeling more vigorous, cheerful, calmer and relaxed during it, along with a load of other health benefits [7]. Other studies have shown that regular intermittent fasting is associated with better psychological health, mood and lower rates of depression and anxiety [8]. However, the results aren’t consistent across all people in all situations.
Fasting may not be appropriate for people who are already under high physical or mental stress (e.g., competitive athletes, shift-workers, or those demanding, dangerous or risky jobs). In fact, fasting may actually worsen their mental health and mood.
Fasting & Brain Health
Fasting activates several biological processes that are, on paper, good for your brain — we’ve mentioned some of them above. Some studies show that fasting can help people who have brain conditions (like Alzheimer’s, multiple sclerosis, epilepsy and people with mild cognitive impairment) [1][8]. However, in our opinion, the evidence isn’t strong enough yet to recommend fasting to treat people with serious brain diseases or disorders.
Putting It Into Practice
If you're young or middle-aged, healthy with no brain issues, you aren’t likely to notice any brain-boosting effects of intermittent fasting. You're more likely to notice some drop-offs in your cognitive performance (particularly if you’re new to fasting).
The 'brain-boosting' effects of fasting are more obvious in older adults, and those who are noticing some cognitive declines already. Preliminary evidence suggests that time-restricted eating windows of 10-12 hours, earlier in the day, may be effective.
For mental health, fasting may offer some benefits (brain and general health) alongside other treatments. But it’s not a cure or a replacement for primary mental healthcare treatments. You’re generally better off selecting a type of fast you can sustain without adding any extra stress than you can manage. There are also negative effects of fasting (generally, the more extreme fasts, like anxiety, poor sleep or mood changes), which can make your brain health and mental health worse.
When you do eat, eat healthy, nutritious, whole foods. The type of food you eat is also strongly associated with brain health and mental health. Mediterranean-style diets, rich in healthy fats, plants, and low in junk foods, have some of the best evidence out there compared to most other diets.
People with (or recovering from) mental health, sleep issues, eating disorders, psychiatric conditions, under psychological stress and/or taking medication are generally not advised to undertake prolonged fasts or significantly alter their eating patterns. They need to consult with a doctor and the healthcare team first. There are reports that fasting (even mild fasts) can worsen health in some instances.
Key Takeaways
Fasting has some proven beneficial effects on brain health and cognitive function, particularly for older adults with or without noticeable cognitive declines. For mental health, there is some evidence that certain fasts can help, but proceed with caution, as they are not for everyone.
The benefits of intermittent fasting on brain health probably aren’t as obvious as they are for other health conditions. Fasting isn’t going to increase your mental acuity overnight; in fact, it may blunt it.
Sources
1. Brocchi A et al. Effects of Intermittent Fasting on Brain Metabolism. Nutrients. 2022 Mar 17;14(6):1275. PMID: 35334932
2. Colucci-D'Amato L et al Neurotrophic Factor BDNF, Physiological Functions and Therapeutic Potential in Depression, Neurodegeneration and Brain Cancer. Int J Mol Sci. 2020 Oct 21;21(20):7777. PMID: 33096634
3. Turner L et al. The effects of time-restricted eating versus habitual diet on inflammatory cytokines and adipokines in the general adult population: a systematic review with meta-analysis. Am J Clin Nutr. 2024 Jan;119(1):206-220. PMID: 37865184
4. Benau EM et al. A systematic review of the effects of experimental fasting on cognition. Appetite. 2014 Jun;77:52-61. PMID: 24583414
5. Ooi TC et al. Intermittent Fasting Enhanced the Cognitive Function in Older Adults with Mild Cognitive Impairment by Inducing Biochemical and Metabolic changes: A 3-Year Progressive Study. Nutrients. 2020 Aug 30;12(9):2644.. PMID: 32872655
6. Currenti W et al. Association between Time Restricted Feeding and Cognitive Status in Older Italian Adults. Nutrients. 2021 Jan 9;13(1):191. PMID: 33435416
7. Wilhelmi de Toledo F, Grundler F, Bergouignan A, Drinda S, Michalsen A. Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects. PLoS One. 2019 Jan 2;14(1):e0209353. PMID: 30601864
8. Gudden J et al. The Effects of Intermittent Fasting on Brain and Cognitive Function. Nutrients. 2021 Sep 10;13(9):3166. PMID: 34579042
Published: May 20, 2026
Lead Author: R.Davies, PhD | Author Bio
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