【What Research Shows】How Does Fasting Affect the Brain? How BDNF and Ketones Work, Memory, Dementia Prevention, and Precautions

“Lately, I feel like I’ve been forgetting things more often…”

Have you been telling yourself, “It must be my age”?

Actually, one of the causes may be related to your eating habits.

Recent research has been revealing, one finding after another, that fasting (intermittent fasting) isn’t just about weight management: it’s also deeply connected to brain health and rejuvenation.

In this article, based on scientific data, I’ll clearly explain how fasting affects the brain, from the mechanisms to how to put it into practice.

Along with its relationship to memory, focus, and dementia prevention, I’ll also cover how to handle worries like “Isn’t fasting bad for the brain?”, such as headaches and stroke.

What Happens in Your Brain When You Fast? 3 Mechanisms

When you fast, three major changes are thought to take place in the brain.

They are “an increase in BDNF,” “the use of ketone bodies,” and “the activation of autophagy.”

These are all unfamiliar terms, so let’s go through them one at a time with everyday comparisons.

BDNF Increases and Activates Brain Nerves

During fasting, the brain increases its secretion of a protein called “BDNF (brain-derived neurotrophic factor).”

BDNF is also called “the brain’s nutrient,” and it works to promote the growth, maintenance, and repair of nerve cells.

If you compare the brain to a field, BDNF is like the “fertilizer” that helps the crops, your nerve cells, grow.

According to research by Professor Mark Mattson of Johns Hopkins University, fasting increases the amount of BDNF secreted, which may improve memory and learning ability.

Conversely, a drop in BDNF is also said to raise the risk of depression and dementia, so increasing BDNF through fasting is extremely important for protecting brain health.

Ketone Bodies Become the Brain’s Most Powerful Fuel

When fasting causes your body to run low on carbohydrates (glucose), the liver begins producing a substance called “ketone bodies”.

Ketone bodies are something like a “reserve fuel” made by breaking down fat.

Ketone bodies are one of the few energy sources that can pass through the blood-brain barrier, and they’re said to be a more efficient source of energy for the brain than glucose.

The blood-brain barrier is like a “checkpoint” that decides which substances get into the brain.

In fact, in research on Alzheimer’s disease, attention has focused on the fact that while the brain loses its ability to use glucose properly (more on this in a later section),
ketone bodies can work as a bypass.

Picture a car that’s running low on gas switching over to another tank and continuing to drive.

Producing ketone bodies through fasting leads to delivering fresh, high-quality energy to the brain.

Autophagy Cleans Up the Garbage in Your Brain

During fasting, a cellular self-cleaning process called “autophagy” also becomes more active.

Autophagy is the process by which cells break down and recycle their own worn-out parts.

Autophagy breaking down and removing waste proteins (such as amyloid beta) that build up in the brain may help lower the risk of dementia.

Picture it like regularly clearing the trash out of a room: your brain is being “rejuvenated” at the cellular level.

Proven by Research! 4 Scientific Benefits Fasting Brings to the Brain

So how do these changes connect to how the brain actually works?

Here, let’s look at the effects on memory, focus, and dementia risk that have been reported in research.

Memory and Learning Ability Improve

As BDNF increases, the nerve cells in the hippocampus (the part of the brain responsible for memory) become more active.

The hippocampus is like “the brain’s notepad,” a place that temporarily stores new information.

In experiments with mice, the group that fasted showed significantly better maze memory scores than the group that didn’t.

In humans as well, people with higher BDNF are said to learn new things faster and retain memories better.

For those who feel “I haven’t been remembering things as well lately,” fasting may turn out to be an unexpected solution.

Focus and Cognitive Function Improve

When ketone bodies are used as the brain’s energy during fasting, focus tends to be more stable than when the brain depends on glucose.

Because post-meal blood sugar spikes (sharp rises and crashes) disappear, “post-meal sluggishness” and “afternoon sleepiness” decrease, and your focus is less likely to break from morning to evening.

For me, since starting 16-hour fasting, the so-called “dreaded sleepy hour” around 2 to 3 p.m. has almost completely disappeared.

Once I felt the quality of my work improve, my motivation to keep fasting went up even more.

When your blood sugar shoots up and then crashes, your brain falls into something like a temporary energy shortage, which can make you foggy or crave something sweet.

It’s thought that eating fewer times and having gentler blood sugar waves makes it easier to keep a clear head.

Research on Ketone Bodies and BDNF

Results showing that fasting raises BDNF and improves cognitive function have been consistently reported in animal studies.

Also, a small study of a ketogenic diet, which cuts carbohydrates way down, reported that BDNF was 47% higher and blood flow to the brain increased by about 22%.

Only 11 people completed this study, and it was a study of a way of eating rather than fasting itself, but it can be seen as one piece of data showing the relationship between ketone bodies and the brain.

A Lower Risk of Alzheimer’s and Dementia Can Be Expected

The anti-inflammatory effects of fasting and the activation of autophagy are thought to suppress the buildup of proteins in the brain that cause dementia.

A review published in the New England Journal of Medicine in 2019 mentioned that fasting may lower the risk of neurodegenerative diseases.

Research on the long-term effects in humans is still ongoing, but even now, many researchers are paying attention to it as a promising possibility.

I’ll explain the mechanisms in detail in the next section.

Fasting and Dementia Prevention: The Connection to Alzheimer’s

Of all the connections between fasting and the brain, the one attracting particular attention is its relationship with dementia prevention.

Here, let’s sort out how Alzheimer’s disease and fasting are connected.

Alzheimer’s Is Also Called “Type 3 Diabetes”

Research on Alzheimer’s disease has been revealing that the brain loses its ability to use glucose properly.

Because of this, Alzheimer’s disease is sometimes called “type 3 diabetes.”

When the brain runs short on energy, nerve cells weaken, and memory and judgment can easily decline.

On the other hand, attention has focused on the fact that ketone bodies can work as a bypass (detour) in place of glucose.

Picture ketone bodies delivering energy to the brain the way a delivery uses another road when the main one is jammed.

The Potential to Curb Inflammation and Garbage Buildup

Alzheimer’s-type dementia is also thought to involve ongoing inflammation in the brain and the buildup of garbage such as amyloid beta.

Inflammation is like a small fire burning inside the body, and if it goes on for a long time, brain cells become more easily damaged, too.

Using fasting to shrink the fire of inflammation and move the garbage cleanup along may help protect the brain.

Mentioned in a Medical Journal Review as Well

The 2019 New England Journal of Medicine review introduced in the previous section also mentioned the relationship with the risk of neurodegenerative diseases.

Neurodegenerative diseases are diseases in which nerve cells are gradually lost, such as Alzheimer’s disease and Parkinson’s disease.

In terms of preventing Alzheimer’s disease too, fasting seems to be attracting the attention of many researchers.

Please think of fasting not as a way to “cure” dementia, but as one option for protecting your brain as part of your lifestyle.

Think About Dementia Prevention in Terms of Your Whole Lifestyle

Does Fasting Have Negative Effects on the Brain? Worries About Headaches, Stroke, and Shrinkage

Some of you may have heard that “fasting is bad for the brain” and feel uneasy about it.

Here, I’ll go through common concerns about fasting and the brain, and how to deal with them.

Headaches and Fogginess Can Occur

During the first few days before you’re used to fasting, headaches and fogginess can occur while your body is switching energy sources, or when you’re short on water and salt.

People taking diabetes medication, for example, are at risk of hypoglycemia, where fasting lowers blood sugar too much, so if you experience lightheadedness, cold sweats, or strong fogginess, don’t push yourself: eat something.

If You’re Worried About Stroke, Watch Out for “Dehydration”

Some people worry about the relationship between fasting and stroke (cerebral infarction), but what you need to watch out for is dehydration rather than not eating itself.

You already take in less water from food, and if you also drink less, your body ends up in a state close to dehydration.

When you’re dehydrated, your blood gets thicker and closer to a sludgy state, which can put a strain on people with blood vessel conditions.

Hydration is extremely important while fasting.

Dehydration significantly lowers focus and judgment, so make a conscious effort to drink 1.5 to 2 liters of water a day.

In particular, people with high blood pressure, heart disease, or a history of stroke must consult a doctor before starting to fast.

The Worry That “Your Brain Will Shrink”

You’ll also see people worried that “fasting makes your brain shrink,” but problems are most likely when extreme undernutrition continues for a long time.

Fasts where you don’t eat for days, or extreme calorie restriction where you barely eat even during your eating hours, can put a strain on your whole body, including your brain.

Also, overly restricting calories during fasting or eating an unbalanced diet within your eating window can backfire, so it’s important to pay attention to what you eat as well.

People Who Should Hold Off on Fasting

If any of the following applies to you, don’t start fasting on your own judgment; talk to a doctor.

The more concerned you are about your health, the more important it is to first consult a doctor and then try it for short periods.

The Right Way to Practice Fasting to Protect Your Brain

If you’re hoping for positive effects on your brain, the best thing is to choose a method you can keep up without strain.

Here, I’ll introduce a way of fasting that’s easy for beginners to start.

A Sample 16-Hour Fasting Schedule

If you’re hoping for brain benefits, I recommend starting with “16:8 fasting.”

It’s a simple method where you fast for 16 hours of the day and set an 8-hour eating window.

For the first week or two you’re likely to feel hungry, but after about three weeks your body gets used to it, and you’ll notice that the fasting hours actually clear your head.

Starting from 12 Hours Is OK

If jumping straight to 16 hours is hard, it’s fine to start with a shorter time.

For example, just finishing dinner at 8 p.m. and having breakfast at 8 a.m. the next morning gives you a 12-hour fast.

Once you get used to it, gradually extend your fasting time to 14 hours, then 16 hours.

Stress is a burden on the brain too, so “not trying too hard” is also a key to protecting your brain.

Eat Brain-Friendly Food During Your Eating Hours

During your eating hours, it’s important to get plenty of the nutrients your brain is built from.

Combine protein from oily fish, eggs, and soy products with deeply colored vegetables, nuts, and so on, in a balanced way.

On the other hand, if you eat nothing but sweets and sweet pastries, your blood sugar will swing up and down, weakening the benefits of all your fasting.

Be Careful with Your First Meal After Fasting

After a long fasting period, your body absorbs sugar easily, so if you suddenly eat a rice bowl or ramen, your blood sugar is likely to spike.

When your blood sugar spikes, it then drops sharply, which can lead to strong sleepiness and fogginess after eating.

For your first meal, start with dishes like vegetables, miso soup, and meat or fish, and save rice or bread for last.

Chewing well and eating slowly is also one brain-friendly way to eat.

Brain-Friendly Habits That Boost the Effects of Fasting

During fasting, you can expect a synergistic effect by not just abstaining from food but also making it a time to actively use your brain.

Here, I’ll introduce some ways to make your fasting hours more meaningful.

Combine Fasting with Light Aerobic Exercise

Light aerobic exercise like walking is also known as a habit that increases BDNF.

The combination of fasting and aerobic exercise in particular is said to maximize BDNF secretion.

Once you try it, I think you’ll feel a world of difference in how sharp your mind is.

Even just taking a 20- to 30-minute walk on an empty stomach in the morning can help you feel refreshed.

However, intense exercise on an empty stomach can cause lightheadedness, so keep the intensity light.

Do Activities That Use Your Brain

Activities that use your brain, like reading and studying, are said to help maintain the brain’s plasticity.

Plasticity is the brain’s ability to change by forming new connections.

It’s also a good idea to use the morning, when hunger tends to leave your head clear, for studying or work that requires thinking.

Get Enough Sleep

With 16-hour fasting, your sleeping hours count toward your fasting time, so make a point of finishing dinner early in the evening and sleeping well.

Eating right before bed keeps your digestive system working, which tends to lower the quality of your sleep.

Thinking of fasting, exercise, learning, and sleep as a set is a shortcut to keeping your brain healthy.

Keep Caffeine in Moderation

Drinking black coffee during your fasting hours is fine, but you need to be careful not to overdo it.

Too much caffeine on an empty stomach can upset your stomach or make you feel restless.

Also, drinking coffee in the late afternoon or evening can make it harder to fall asleep, which may shorten the time your brain spends cleaning up during sleep.

I recommend keeping coffee to about one or two cups in the morning, and switching to water, mugicha (roasted barley tea), or herbal tea in the afternoon.

Don’t Let Stress Build Up

When intense stress continues, your brain stays in a constant state of tension, and fatigue builds up more easily.

Continuing to tough it out even when hunger is hard can also become a source of stress.

Reset your mood with deep breathing or light stretching, and keep the flexibility to take a break from fasting on tough days.

Frequently Asked Questions About Fasting and the Brain

Get a Brain Transformed by Fasting, and Better Everyday Productivity

To sum up once more why fasting is good for the brain,
there are three main mechanisms: nerve activation through increased BDNF, a steady energy supply from ketone bodies, and the removal of garbage in the brain through autophagy.

At the same time, you need to watch out for dehydration, low blood sugar, and extreme undernutrition.

Concerns like “my head feels vaguely foggy” or “I can’t stay focused” may be improved just by changing your eating pattern.

There’s nothing difficult about it: just have dinner a little earlier and push breakfast back a little.

If that alone can create an environment your brain loves, I think it’s well worth trying.

Combine that with light exercise and enough sleep, and it becomes an even more brain-friendly habit.

If you think of fasting not as “time spent forcing yourself not to eat,” but as time for maintaining your brain and body, it becomes much easier to keep going.

If you have health concerns or a chronic condition, don’t forget to consult a doctor before you start.

Start with 12 hours, and try it within a range that’s comfortable for you.

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