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What does not eating do to your brain?

Not eating, also known as fasting, has become an increasingly popular health trend in recent years. Proponents claim that abstaining from food for set periods of time can provide a variety of health benefits, including improved brain function. But what does the science say? This article will examine how not eating impacts the brain, looking at how it affects cognition, memory, mood, and more. Keep reading to learn about the effects that fasting can have on the most complex organ in the human body.

How Fasting Impacts Brain Function

The brain requires a steady supply of glucose from food to function properly. This begs the question – what happens when that supply is cut off or reduced? Here’s an overview of some of the key ways that abstaining from food affects brain function:

Cognitive Performance

Several studies have suggested that short-term fasting may temporarily improve some aspects of cognition, such as mental sharpness and focus. One reason is that when you fast, more of the brain’s energy can be devoted to cognitive tasks rather than digestion. However, research indicates cognitive gains are generally modest and tend to plateau after 24 hours. Prolonged fasting of 3 days or more can impair function.


Animal studies link fasting with enhanced learning and memory by increasing factors like BDNF (brain-derived neurotrophic factor), which supports neuron health. But human research is mixed – one study found improved verbal memory after a 2-day fast, while another saw decreased performance on memory tests. More research is needed in this area.


Hunger during fasting can negatively impact mood and emotions. Feelings of irritability, anxiety, and depression often increase with fasting duration. However, some research indicates mood may stabilize after 2-4 days, once the body adapts to using fat for fuel through ketogenesis. Those with mood disorders should proceed cautiously.


Emerging research suggests fasting may provide neuroprotective benefits by enhancing the brain’s resistance to stress and disease. For example, it can increase antioxidant defenses, repair damaged neurons, and stimulate growth of new brain cells. More clinical trials are underway, but animal models have shown promising effects.

Brain Neurochemistry

Fasting causes key changes in neurochemicals like norepinephrine, serotonin, and brain-derived neurotrophic factor (BDNF). Norepinephrine increases alertness and attention, while serotonin influences mood. BDNF supports learning, memory and neuron health. These effects may underlie some of fasting’s impacts on cognition and stress resilience.

How Long Can the Brain Go Without Food?

So how long can the brain continue functioning when deprived of food? Here’s a look at how long humans can withstand a complete fast:

12 Hours

Most people will begin to feel hungry around this time as liver glycogen stores become depleted. Blood sugar levels drop, but the brain still has ample glucose reserves with no significant changes in cognitive function.

24 Hours

People may notice increased fatigue, irritability, and mental “fogginess” after a full day without food. But cognition is still relatively unimpaired if hydration is maintained. Ketone bodies begin ramping up as fat stores are increasingly accessed for energy.

48 Hours

Two days without food result in further degraded glucose reserves, forcing the brain to adapt to using ketones from fat breakdown for up to 60% of its energy needs. Mental and physical performance starts to slip as the body and brain work harder to maintain basic function.

3+ Days

Three days without food signals a transition into prolonged starvation as the body’s fat stores become increasingly depleted. People may struggle with concentration, alertness, and short-term memory. Brain volume can start to decrease as neural inflammation rises and neurons become compromised. Death generally occurs around 21-40 days without food.

While the brain can withstand short periods without food, cognitive deficits and mood issues become more pronounced the longer someone goes without eating. Hydration can help prolong mental stamina a bit, but the brain inevitably suffers without sustained nutrition.

How Fasting Impacts Different Areas of the Brain

Now that we’ve looked at the general effects of fasting, let’s dive deeper into how it impacts specific regions and systems within the brain:


The hippocampus plays key roles in memory and learning. Animal research suggests fasting stimulates hippocampal function through increased BDNF and neural stem cell growth. But human studies are mixed – some show improved verbal memory after brief fasts, while others find no effect or even impairment after several days without food.

Prefrontal Cortex

This region governs complex cognition, including focus, planning, and self-control. Fasting may temporarily boost prefrontal cortex function by increasing arousal and motivation, but this effect declines as starvation sets in and blood sugar drops. After several days of fasting, the prefrontal cortex demonstrates impaired activity.


The hypothalamus produces hunger and satiety hormones, regulating appetite and food intake. During fasting, hunger signals increase dramatically while satiety hormones are suppressed. Prolonged starvation can disrupt normal hypothalamic function.

Reward System

Fasting increases impulsive responses in the brain’s reward system, including the striatum and amygdala. This may be linked to higher levels of the neurotransmitter norepinephrine. A hyperactive reward system may make it harder to stick to fasts long-term due to increased cravings.

Default Mode Network

This neural network is active during resting states and mind-wandering. Some research points to increased connectivity in the default mode network during short-term fasting. This could relate to mild cognitive enhancements seen with shorter fasts.

In general, most regions of the brain are negatively impacted by starvation and deprivation after 72 hours or more without food. But there are some promising areas of research on how fasting may enhance brain function and resilience – primarily through promoting neuron growth and regeneration in key regions like the hippocampus. However, more clinical trials in humans are still needed.

How Fasting May Provide Brain Health Benefits

Based on the emerging research, several mechanisms have been identified through which fasting may yield brain health benefits when done on a limited, periodic basis:

Enhances Brain Plasticity

Fasting increases plasticity, or the brain’s ability to reorganize itself and form new connections between neurons. This is believed to enhance cognition and may help counteract age-related mental decline.

Boosts Neurotrophic Factors

Intermittent fasting stimulates the expression of BDNF and other growth factors that support neuron health, development, and activity. This may promote neurogenesis, synaptogenesis, and cognitive function.

Reduces Oxidative Stress

Fasting can increase production of antioxidants and antioxidant enzymes within the brain, enhancing neural resistance to oxidative stress and degeneration. This may support overall brain health.

Stimulates Autophagy

During fasting, autophagy activity ramps up, clearing away damaged cells and protein aggregates that can impair neural processes and contribute to neurodegeneration. This benefits neuron structure and function.

Modulates Inflammation

Some findings indicate fasting can reduce neuroinflammation through actions on the NLRP3 inflammasome and microglia within the brain. This process may help protect neurons and counteract cognitive decline.

However, longer-term starvation has detrimental effects on the brain – it is only *periodic, limited* fasting that seems to offer benefits by spurring adaptive cellular response pathways. More research is still underway to further establish optimal protocols.

Potential Risks and Side Effects of Fasting on the Brain

While the potential brain benefits of fasting are compelling, there are also some possible downsides to consider:

Cognitive Impairment

Prolonged fasting for over 72 hours can result in deficits in mood, concentration, memory, and alertness as the brain becomes starved for glucose. Mild impairment may arise after 24 hours.

Mood Changes

Common mood-related side effects include increased irritability, anxiety, anger, depression, and sleep problems. These seem to peak around days 2-4 of fasting and improve once ketosis is established.

Disordered Eating

Fasting protocols may trigger or reinforce eating disorders or unhealthy views on food and weight in vulnerable individuals. Those with a history of eating disorders should avoid fasting.


As fasting persists, people may experience drops in blood sugar that lead to symptoms like sweating, heart palpitations, tremors, and confusion. Hypoglycemia can impair brain function.


Not drinking enough fluids during fasting can cause dehydration, reducing blood volume to the brain. This exacerbates cognitive deficits and mood instability caused by lack of food.

Electrolyte Imbalances

Fasting can deplete key micronutrients like sodium, potassium, and magnesium essential for neural activity and health. Imbalances in these electrolytes may lead to symptoms like headaches, dizziness, and muscle cramps.

Overall, fasting risks outweigh benefits the longer someone goes without food. Consulting a doctor before fasting and carefully monitoring physical and mental health is advisable. Those with certain medical conditions or vulnerabilities should avoid extended fasting protocols.

Tips to Minimize the Risks of Fasting on the Brain

Here are some tips to help safeguard brain health and minimize adverse side effects if engaging in fasting:

– Start with intermittent fasting protocols of 12-16 hours to gauge effects on cognition and mood before attempting longer fasts.

– Keep fasts under 24-48 hours to avoid pronounced cerebral glucose deprivation.

– Stay hydrated by drinking water, herbal tea, broth, or electrolyte beverages during fasts.

– Get enough rest during fasting and avoid intensive mental workloads.

– Consider exogenous ketones like MCT oil to support ketosis during extended fasts.

– Watch for warning signs like lack of concentration, mood swings, dizziness, and hunger pangs.

– Break fasts if you experience side effects like brain fog, cognitive issues, sleep problems, or feeling unwell.

– Avoid fasting if you have medical conditions like diabetes, history of eating disorders, depression, or anxiety.

– Speak with your doctor before attempting fasting if you take any medications or supplements.

Following basic precautions and closely monitoring both physical and mental health can help minimize risks from temporary food restriction. Fasting safely requires cutting off intake before cognition is impacted.

Who Should Avoid Fasting?

While fasting may have brain benefits for some people when done cautiously, it is not recommended for everyone. Here are some groups who should generally avoid fasting protocols:

Children and Teens

Fasting is not advised for those under 18 as it may impair growth and development.

Pregnant or Breastfeeding Women

Expectant mothers need ample nutrition to support fetal health. Fasting can jeopardize mom and baby.

Those with Diabetes

People with diabetes are at high risk of blood sugar crashes and complications from fasting. Careful medical supervision is imperative.

Eating Disorders

Fasting may precipitate or worsen eating disorders like anorexia, bulimia, and binge eating disorder.

Mental Health Conditions

Those with depression, anxiety disorders, and other mental health issues may experience worsening symptoms from fasting.


People who are underweight or malnourished should work to improve their nutritional status rather than fasting.

Certain Medications

Fasting can interact with certain prescriptions, supplements, and over-the-counter drugs. Medical guidance is key.

While fasting has its merits, it is not healthy or safe for everyone. Children, pregnant women, and people with certain medical conditions or nutritional needs should focus on maintaining regular, balanced meals instead.


In summary, abstaining from food triggers complex physiological and neurological changes as the brain is temporarily deprived of its main fuel source. Shorter fasts of 12-24 hours may confer mild cognition and mood benefits for some people. However, fasting longer than 3 days can lead to impaired mental function, mood instability, and other side effects. Periodic, time-restricted fasting shows promise for brain optimization, but longer fasting protocols should be pursued cautiously under medical guidance. Those with certain pre-existing conditions and vulnerabilities may be better off avoiding fasting altogether in favor of a consistent, nourishing diet. As with any dietary strategy, fasting requires careful consideration of both benefits and risks on overall health.