【Your Brain Is at Risk】How Are Sugar and Dementia Linked? The Danger of Blood Sugar Spikes and How to Eat to Help Prevent Alzheimer’s

Dementia is now said to affect about 7 million people in Japan.

According to one survey, 1 in 5 people aged 65 and over will develop dementia, so if you’re thinking, “Won’t I just get a little more forgetful someday?”, you’re very much mistaken.

With dementia, you don’t just lose memories. You may stop recognizing your own family’s faces, or even become unable to handle everyday life.

What if a disease that serious were actually closely tied to your everyday “diet”?

There’s even research data showing that eating too many carbs doubles the risk of dementia.

After turning 40, my worries about my health slowly started to grow. Drawing on my own experience of feeling changes in my brain and body through fasting and rethinking my diet,
I’ll share eating habits for dementia prevention that you can put into practice starting today.

Eating Too Many Carbs Damages Your Brain! How Blood Sugar Spikes and Dementia Are Linked

Eating something sweet makes you feel “so happy,” right?

But if those sweet meals continue every day, your brain may be slowly getting damaged.

What Is a Blood Sugar Spike?

A “blood sugar spike” is when your blood sugar shoots up after a meal and then crashes.

It tends to happen when you eat white rice, bread, udon (thick wheat noodles), or sweet snacks, and it’s one of the reasons you get sleepy after eating.

It’s easier to understand if you picture your blood sugar going up and down like a roller coaster.

When these spikes happen again and again, the inner walls of your blood vessels get damaged, making chronic inflammation throughout the body more likely.

Picture chronic inflammation as a small fire inside your body that just keeps smoldering.

And the tiny blood vessels in your brain take damage too.

Insulin Resistance Throws Your Brain Off

When high blood sugar continues, “insulin resistance,” a state in which insulin works less effectively, develops.

Insulin doesn’t just bring blood sugar into your cells. It also plays an important role in the brain.

If insulin is “the key that opens the cell’s door,” insulin resistance is like the keyhole rusting so the door is hard to open.

When the brain can no longer use insulin, the “hippocampus,” which is in charge of memory, is more likely to shrink, and this is said to trigger Alzheimer’s-type dementia.

I explain how insulin resistance works in detail in my article on insulin resistance.

The Danger of AGEs (Advanced Glycation End Products)

Another troublesome substance is “AGEs,” which form when carbs react with proteins in the body.

Picture a reaction like the way pancakes turn brown when you cook them, happening slowly inside your body.

AGEs age the body, and when they build up in the brain, they become entangled with “amyloid beta,” which is considered a cause of Alzheimer’s, and lower cognitive function.

AGEs are also abundant in foods cooked at high temperatures (grilled or fried), so people who eat a lot of fried food need to be especially careful.

I used to eat convenience-store desserts on my way home from work almost every day.

Suddenly getting sleepy after eating, that “post-lunch slump,” was an everyday thing.

Looking back now, I realize it was a textbook sign of blood sugar spikes.

Is Alzheimer’s “Type 3 Diabetes”!? A Startling Medical View

In fact, some researchers in the medical world call Alzheimer’s “the third diabetes (type 3 diabetes).”

That’s because they point out that, separate from type 1 and type 2 diabetes, there’s a special condition in which insulin stops working properly in the brain alone.

Picture it as a state where blood sugar in the body is normal, but “the insulin key won’t turn” only inside the brain.

This concept was proposed in 2005 by Dr. Suzanne de la Monte and colleagues at Rhode Island Hospital and published in a medical journal.

There’s also plenty of data showing that people with diabetes have about 1.5 to 2 times the risk of Alzheimer’s.

Keep in mind, too, that “type 3 diabetes” isn’t an official diagnosis, but one idea used among researchers.

The Surprising Relationship Between Amyloid Beta and Insulin

Amyloid beta is a protein that builds up in the brain like “trash,” and it’s considered a hallmark of Alzheimer’s.

This amyloid beta becomes harder to break down because of an enzyme it competes for with insulin.

Think of it as one garbage truck that has to collect both insulin and amyloid beta.

In other words, in a state of excess insulin (the result of a carb-heavy diet over time), amyloid beta builds up more and more easily in the brain.

Comparing Dementia Risk in People With Diabetes

Multiple large-scale studies consistently show this data, which makes it clear that “limiting carbs” means more than just dieting.

Keeping your blood sugar stable is directly tied to keeping your brain young.

In Japan too, the “Hisayama Study,” which has followed residents of the town of Hisayama in Fukuoka Prefecture for many years, has reported that people with diabetes are more likely to develop dementia.

This study led to the view that abnormal glucose metabolism, including diabetes, is a risk factor shared by both vascular dementia and Alzheimer’s-type dementia.

Glucose metabolism is the system by which your body turns the carbs you eat into energy or stores them.

In other words, it’s important to be mindful of protecting your brain from the “blood sugar is slightly high” stage, before you’re ever diagnosed with diabetes.

Effective for Dementia Prevention! 5 Foods That Protect Your Brain

“So what should I eat?” That’s the question, right?

Don’t worry.

There are plenty of foods that protect your brain.

No complicated dietary restrictions required.

You can start with a positive habit: “eat more of this.”

DHA and EPA are fats that serve as building material for the membranes surrounding the brain’s nerve cells, and it’s believed that when these membranes are flexible, information flows more smoothly.

Polyphenols and vitamin E likely act as a “shield” that protects the brain from reactive oxygen species, which cause the body to “rust.”

The Mediterranean Diet Is the Most Effective for Preventing Dementia

The “Mediterranean diet,” which combines the foods above, has been shown in many studies to help prevent dementia.

Its style, centered on olive oil, fish, vegetables, beans, and nuts while limiting red meat and sweets, can be called a model diet for slowing brain aging.

If you translate it into Japanese food, a traditional menu of grilled fish, natto (fermented soybeans), ohitashi (lightly boiled greens in a savory broth), and miso soup (soup made with fermented soybean paste) is a close match.

Since I changed my diet, I feel like the “fog” in my head has lifted.

I used to always have a foggy stretch after lunch, but since I made a point of eating more oily fish and vegetables, my focus in the afternoon has clearly improved.

Combined with fasting, I feel the effects even more.

An easy way to add olive oil is to avoid overheating it and drizzle it over salads or ohitashi instead.

Prevent Blood Sugar Spikes! Eating Tips You Can Use Starting Today

It’s not just about eating brain-protecting foods. “How you eat” is also extremely important.

Even with the same foods, the order and timing in which you eat them can make a big difference in how your blood sugar rises.

Tip ①: Go “Veggies First” to Keep Blood Sugar Gentle

Eating in the order of vegetables → protein → carbs helps curb sharp rises in blood sugar.

That’s because fiber reaching your gut first slows down how fast sugar is absorbed.

Picture fiber acting as a “cushion” in your gut so sugar is absorbed slowly.

Even on a busy lunch break, just eating your salad first makes a big difference.

Tip ②: Swap Out Refined Carbs

Gradually swap out refined carbs: white rice → brown rice, white bread → whole-grain bread, sugary drinks → water or green tea, and so on.

Refined carbs are carbs that have been processed to make them easier to eat by removing fiber and other parts.

Even if it feels like “a hassle” at first, once you get used to it, it won’t bother you.

If you don’t like brown rice, you could start by mixing mixed grains or rolled barley into your white rice.

Tip ③: Switch Your Snacks to Nuts or Eggs

Store-bought snacks and sweets contain large amounts of refined carbs and AGEs.

If you snack, switching to foods centered on protein and fat, such as nuts, boiled eggs, or cheese, keeps your blood sugar stable.

Simply switching sweet drinks to unsweetened tea or sparkling water can also cut your daily carb intake considerably.

Tip ④: Take a Short Walk After Eating

Research has shown that even a light 10–15 minute walk after eating can greatly curb blood sugar spikes.

Try working movement into your day, like taking the stairs instead of the elevator or parking a little farther away.

Muscles use up sugar in the blood quickly, so even moving a little after a meal flattens your blood sugar peak.

Tip ⑤: Chew Well and Eat Slowly

Eating fast not only makes your blood sugar shoot up, but also causes you to overeat before the “I’m full” signal reaches your brain.

Simple tricks like putting your chopsticks or fork down between bites and chewing thoroughly will naturally help bring down how much you eat.

Just changing the order in which you eat greatly reduces blood sugar swings.

Start by trying “veggies first.”

How Fasting Can Dramatically Lower Your Risk of Dementia

Not just “what” you eat, but building in “time without food” is also extremely effective for your brain.

Ketones Become the Brain’s Best Fuel

When you fast, your body turns fat into energy instead of glucose and produces “ketones.”

Ketones are one of the few energy sources that can pass through the blood-brain barrier, and they reach the brain even when insulin resistance is present.

Think of the blood-brain barrier as a “checkpoint” that keeps harmful things from getting into the brain.

In other words, even a brain on the verge of Alzheimer’s (one that can no longer use insulin) can still use ketones.

This is probably one of the reasons people say “fasting may be the brain’s savior.”

Autophagy Cleans Up the Brain’s Trash

What switches on during fasting is “autophagy,” your cells’ self-cleaning system.

It works to break down and clear out waste that has built up in the brain (such as amyloid beta), and expectations are rising among researchers for its role in dementia prevention.

It’s easiest to think of autophagy as the “cleaning and recycling crew” working inside your cells.

Keeping up a 16-hour fast (intermittent fasting) makes it easier for this autophagy to activate.

Better Insulin Sensitivity Is Fundamental to Prevention

When fasting improves your insulin sensitivity, your blood sugar stabilizes and your brain uses insulin more efficiently.

Remember that insulin sensitivity is a term for how readily your cells respond to insulin.

This leads to lowering the risk of Alzheimer’s as “type 3 diabetes” at its root.

A few months after I started 16-hour fasting, the clarity in my head changed dramatically.

My head used to often feel foggy from the morning on, but now I’m sharp right from the moment I wake up and can focus on work.

Note that if you’re taking diabetes medication, fasting can cause hypoglycemia (low blood sugar), so be sure to talk to your doctor before starting.

I go over the basics of how to fast in detail in my complete guide to intermittent fasting.

Don’t Cut Carbs Too Much Either! A Brain-Friendly Way to Handle Carbs

After reading this far, you might be thinking, “Wouldn’t it be better to quit carbs entirely?”

However, the brain normally uses glucose as its main energy source, so cutting out carbs drastically isn’t recommended.

What matters isn’t “cutting carbs to zero,” but getting the “amount, quality, and timing” right.

Amount: Aim for “a Little Less” of Your Staple Foods

Just eating a little less than usual, like skipping a second helping of rice or ordering a regular portion instead of a large, changes how your blood sugar rises.

If you cut back all at once, irritability and hunger can get stronger, and you may end up bingeing on sweets as a rebound.

Quality: Choose “Brown Carbs” Over Refined Ones

Even among carbs, those high in fiber, such as brown rice, mixed grains, whole-grain bread, beans, and potatoes, raise your blood sugar gently.

An easy way to remember it is: choose brown over white.

Timing: Cut Back on Late-Night Carbs and “Nibbling”

Sweets late at night and the habit of nibbling on something all day leave your insulin no time to rest.

Leaving a proper gap between meals secures time for your brain and body to reset.

Know Which Foods Are High in Carbs

Carbs are abundant not only in rice, bread, and noodles, but also in potatoes, squash, fruit, juice, and sports drinks.

If you drink fruit juice or smoothies every day because they “seem healthy,” you may be eating too many carbs without realizing it.

A sweet bun with a sweet café au lait for breakfast is a classic example of a combination where carbs pile up.

Thinking in terms of “adding,” such as adding a salad or a boiled egg if you choose an onigiri (rice ball), is another approach.

If you get in the habit of checking the “carbohydrate” or “sugar” amounts on food nutrition labels, you’ll naturally develop an eye for choosing well.

Habits Beyond Diet Also Protect Your Brain

It’s said that preventing dementia depends not only on diet, but also on sleep, exercise, and social interaction.

Sleeping soundly, walking every day, and talking with family and friends all provide good stimulation for the brain.

Exercise in particular likely helps stabilize blood sugar too, since your muscles use up sugar.

If you have a chronic condition or are concerned about forgetfulness, talk to a doctor before making big changes to your diet on your own judgment.

For the link between all-day grazing and the brain, please also see my article on Alzheimer’s and eating habits.

Frequently Asked Questions About Carbs and Preventing Dementia and Alzheimer’s

Rethink How You Eat Carbs and Protect Your Brain’s Youth

Finally, let’s review the key points of this article.

Dementia doesn’t start suddenly one day. It’s thought to be shaped by decades of accumulated daily habits.

That’s exactly why changing today’s meal just a little should help protect your brain 10 or 20 years from now.

Start with just one thing you can do, like swapping one sweet drink for tea or eating a salad before your rice.

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