When we talk about fasting, the conversation usually focuses on weight loss, insulin, or metabolic health. But you may be wondering, what happens to your brain when you go without food for a period of time?

Your brain needs a steady supply of energy to work properly, and fasting changes how your body provides that energy. Let’s look at what happens to your brain during a fast and what current research tells us. 

How does your brain get energy during a fast?

Under normal conditions, your brain relies mostly on glucose for energy. When you eat, your body breaks down carbohydrates into glucose, which your cells can use for fuel. 

Between meals, your body keeps blood sugar levels steady by using stored energy. As a fast continues, your body uses more stored fat for fuel and starts making ketones.

Your brain can use ketones as another source of energy. This becomes more important as fasting continues and ketone levels rise. 

That doesn’t mean your brain stops using glucose. Your body can adjust to changes in its energy supply, but how quickly this happens depends on your eating habits, activity level, and other factors. 

Fasting and Brain-Derived Neurotrophic Factor (BDNF)

One area of research getting attention is a protein called brain-derived neurotrophic factor, or BDNF. 

Think of BDNF as part of your brain’s support system. 

It helps nerve cells survive and supports the connections between them. These connections are important for learning, memory, and how the brain adapts. 

Animal studies suggest that Intermittent Fasting may affect BDNF and other processes linked to brain health. Researchers want to know whether these changes could help keep the brain healthy as we age. 

However, human studies have found mixed results.

A 2024 review published in Medicina looked at 16 studies on calorie restriction, Intermittent Fasting, BDNF levels, and brain function in humans. Some studies found increases in BDNF, others found decreases, and several found no clear change. 

In other words, we can’t say that fasting reliably increases BDNF in humans. The research is interesting, but more high-quality human studies would be helpful. 

Can fasting improve focus and mental clarity?

Have you ever heard someone say they feel more focused or mentally clear when fasting? Many people report this experience. Others have the opposite reaction. They may feel distracted by hunger, irritable, tired, or unable to concentrate. 

Both experiences are possible, and fasting doesn’t always affect everyone in the same predictable way. 

If you feel mentally sharp during your usual fasting window, that’s worth paying attention to. 

If you struggle with concentration when you haven’t eaten, that doesn’t mean you’re doing anything wrong. Your fasting schedule may simply need adjusting. 

Could fasting support long-term brain health?

Researchers are also studying whether fasting could help protect the brain as we age. 

Some animal studies suggest that fasting may affect inflammation in a positive way and in the way that brain cells handle stress. It may also affect processes that help brain cells stay healthy and maintain their connections.

These findings have led scientists to explore whether fasting could play a role in supporting cognitive health over time. 

However, results from animal studies don’t always apply to humans. A 2025 review in The Journal of Nutrition, Health & Aging, examined Intermittent Fasting and brain-related disorders. The authors found promising preclinical evidence, but emphasised that human research remains limited. 

How to support your brain while fasting

The goal should always be to find an approach that supports your overall health, rather than pushing yourself to see how long you can go without food. Here are a few things to keep in mind:

• Choose a sustainable fasting schedule. Longer fasts aren’t automatically better, and you don’t need to push through significant discomfort to get potential benefits. 

• Eat enough nutritious food during eating windows. When you do eat, make sure your meals provide adequate protein, healthy fats, vitamins, minerals, and other essential nutrients. Think “refuel” instead of “refeed.” 

• Stay on top of your water and electrolytes. Dehydration and electrolyte imbalances can contribute to headaches, fatigue, and difficulty concentrating. 

• Prioritise sleep. Sleep plays an important role in memory, learning, and overall brain function. 

• Pay attention to how you feel. Persistent dizziness, weakness, confusion, or difficulty functioning are reasons to stop fasting and seek appropriate medical advice. 

What we know and what we don’t know

Fasting changes how your body manages energy, including how it fuels your brain. Some people report improved brain function, while others not so much.

There are interesting connections between fasting, ketones, BDNF, and long-term brain health, much of which is currently anecdotal evidence. As far as scientific research, the evidence in humans is still developing. 

What we do know is that eating around the clock keeps insulin levels high and can eventually lead to insulin resistance and metabolic disease, none of which is good for brain health. 

The best approach is to incorporate fasting into your lifestyle in a way that works for you, and focus on sustainable habits, nutritious meals when you eat, adequate hydration, and good sleep. 
 

Citations:

Alkurd R, Mahrous L, Zeb F, Khan MA, Alhaj H, Khraiwesh HM, Faris ME. Effect of Calorie Restriction and Intermittent Fasting Regimens on Brain-Derived Neurotrophic Factor Levels and Cognitive Function in Humans: A Systematic Review. Medicina (Kaunas). 2024 Jan 22;60(1):191. doi: 10.3390/medicina60010191. PMID: 38276070; PMCID: PMC10819730.

Beveridge J, Montgomery A, Grossberg G. Intermittent fasting and neurocognitive disorders: What the evidence shows. J Nutr Health Aging. 2025 Apr;29(4):100480. doi: 10.1016/j.jnha.2025.100480. Epub 2025 Jan 10. PMID: 39798403; PMCID: PMC12180010.