How Fasting Affects Your Brain, Cognition and Mental Health

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.

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). So, increasing your BDNF levels by fasting is one of the ways it can improve your brain health.^[2]

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, which in turn has been linked to better brain health and mental health (as well as many other diseases).^[3]

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 and 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 and 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. Other studies have shown that regular intermittent fasting is associated with better psychological health, mood and lower rates of depression and anxiety. However, the results aren’t consistent across all people in all situations.^[7][8]

Fasting may not be appropriate for people who are already under high physical or mental stress (e.g., competitive athletes, shift-workers, or those in demanding, dangerous or risky jobs). In fact, fasting may actually worsen their mental health and mood.

Fasting and 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). However, in our opinion, the evidence isn’t strong enough yet to recommend fasting to treat people with serious brain diseases or disorders.^[1][8]

Putting it into practice