【It Progresses Without Symptoms】What Is Insulin Resistance? Causes, How to Check, and the Diet and Fasting That Improve It, Explained Simply

“My checkup said my blood sugar was a little high, but I don’t have any symptoms, so I’m probably fine.” Is that what you’re thinking?

The truth is, insulin resistance progresses quietly with almost no noticeable symptoms,
and by the time people notice, they’ve often moved past prediabetes into full-blown type 2 diabetes. This happens very often.

In my late 30s, I was also told at a checkup that my “fasting blood sugar was slightly high.”

At the time I brushed it off with “Well, it’s not diabetes yet,” but afterward my weight climbed to 70 kg (154 lb), and I kept feeling tired easily and struggling to concentrate.

I still think that if I hadn’t ignored those signs, I could have done something about it much sooner.

That’s exactly why I don’t want anyone reading this article to make the same mistake.

In this article, I’ll explain in plain terms what insulin resistance is, why it progresses without you noticing, and how you can tackle it with fasting and better eating habits.

I’ll also cover the main causes of insulin resistance and the tests you can get at a clinic.

What Is Insulin Resistance? Why Does It Progress “Without You Noticing”?

Insulin is a hormone released by the pancreas, and it acts as a “key” that lets blood sugar (glucose in the blood) into your cells.

When a meal raises your blood sugar, picture insulin being inserted into a cell’s “keyhole,” the door opening, and glucose flowing into the cell.

Once inside the cell, glucose is used as energy to power your body.

Insulin resistance is a state in which this “key” no longer works well. In other words, your cells become less responsive to insulin.

You can picture it as the cell’s “keyhole” being broken.

When this happens, the pancreas decides, “I need to release more insulin!” and keeps pumping out excess insulin.

If the lock is hard to open, it tries to force it open by adding more and more keys.

Because blood sugar stays in the normal range for a while, the biggest problem is that it progresses for years without symptoms until test results finally show something abnormal.

HbA1c (hemoglobin A1c) is a number that shows your average blood sugar over roughly the past 1–2 months.

Standard health checkups often test fasting blood sugar and HbA1c, but it’s said that the early-stage “too much insulin” state is hard to catch with these two alone.

In other words, even if your checkup says “no abnormalities,” your pancreas may already be overworking inside your body.

When the pancreas works so hard that it becomes exhausted, it can no longer release enough insulin, and blood sugar starts rising sharply.

Only at this stage do the numbers finally show something abnormal, which is probably why so many people feel they “suddenly got diabetes.”

In particular, if you’ve ever been told at a checkup that your “blood sugar is slightly high,” “triglycerides are high,” or “waist is on the large side,” it may be a sign that insulin resistance has already begun.

5 Serious Risks Caused by Insulin Resistance

Don’t brush it off as “just slightly high blood sugar.”

If insulin resistance is left unchecked, serious damage builds up throughout your body.

① Progression to Type 2 Diabetes

It’s said that about 30–40% of people with prediabetes (fasting blood sugar of 100–125 mg/dL) progress to type 2 diabetes within 5–10 years.

Once it develops, it’s hard to cure completely and requires lifelong management.

Prediabetes refers to a “borderline” state in which blood sugar is higher than normal but not high enough to be diagnosed as diabetes.

As diabetes progresses, there’s also a risk of complications in the eyes, kidneys, nerves, and elsewhere.

② Higher Risk of Heart Disease and Stroke

Excess insulin damages blood vessels and promotes atherosclerosis.

Atherosclerosis is a condition in which blood vessels become stiff and narrow, so the passageway for blood gets smaller.

It’s been reported that people with insulin resistance have 2–4 times the risk of heart disease compared with healthy people, and their stroke risk is likely significantly higher as well.

Heart disease and stroke can be life-threatening or leave lasting effects, so taking action early is essential.

③ The Vicious Cycle of Visceral Fat Buildup and Obesity

A high-insulin state is also a signal to “store fat.”

As visceral fat increases, insulin resistance gets even worse, trapping you in a vicious cycle where it’s hard to lose weight even if you eat less.

Many people whose diets just don’t work are caught in this vicious cycle.

④ Higher Risk of Dementia and Alzheimer’s Disease

Insulin resistance affects the brain too.

Recent research has strongly shown a link with Alzheimer’s disease, which is sometimes called “type 3 diabetes”, and impaired insulin signaling in the brain raises the risk of memory decline and dementia.

⑤ Complications Such as Cancer, PCOS, and Fatty Liver

Because hyperinsulinemia promotes cell growth, it’s also linked to a higher risk of some cancers (such as breast and colorectal cancer).

It’s also one of the main causes of polycystic ovary syndrome (PCOS) in women and of non-alcoholic fatty liver disease (NAFLD) in everyone.

Think of PCOS as a condition in which eggs don’t develop properly in the ovaries, leading to irregular periods and other issues.

If you’re concerned about fatty liver, please also read my article on non-alcoholic fatty liver disease.

The Main Causes of Insulin Resistance

Insulin resistance doesn’t happen suddenly one day.

Everyday lifestyle habits pile up, and your cells gradually become less sensitive to insulin.

Here are five main causes.

Overeating and Snacking Keep “Insulin Flowing Nonstop”

The biggest cause of insulin resistance is “insulin staying high all the time.”

With the modern habit of three meals a day plus snacks, insulin keeps being released all day long, and your cells become desensitized to it.

Just as you’d gradually get used to a blaring alarm clock every morning until it no longer wakes you, your cells likely get used to insulin and respond less and less.

I explain the link between how often you eat and insulin in detail in my article on why three meals a day make you gain weight.

Visceral Fat Increases

The visceral fat that builds up around your belly isn’t just a savings account of energy.

Visceral fat gives off substances that cause inflammation, and this makes insulin work less effectively.

Even if you don’t look very overweight, you should be careful if only your belly sticks out.

On the other hand, visceral fat also has the characteristic of being relatively easy to lose once you rethink your diet and exercise.

Lack of Exercise Leads to Muscle Loss

Muscles are the “destination for sugar” that takes in a lot of the glucose in your blood.

When you lose muscle from lack of exercise, glucose has nowhere to go, and your blood sugar has a harder time coming down.

The large muscles of the thighs and buttocks in particular use a lot of sugar, so just adding more everyday movement, like walking or taking the stairs, will likely make a difference.

Lack of Sleep and Stress

When lack of sleep or stress continues, a hormone called cortisol increases and works to raise blood sugar.

As a result, even more insulin is needed, putting more strain on the pancreas.

The way staying up late on your phone and running short on sleep makes you crave sweets the next day is arguably part of this vicious cycle too.

Aging and Genetics

As we get older, we tend to lose muscle mass, and insulin tends to work less effectively.

Also, if you have family members with diabetes, you may be genetically more prone to insulin resistance.

You can’t change your genes, but you can start rethinking your lifestyle today.

How Many Apply to You? An Insulin Resistance Checklist and Clinical Tests

Even without noticeable symptoms, signs of insulin resistance show up all over your body.

Check which of the items in the list below apply to you.

If 3 or more apply, insulin resistance may be progressing.

It’s important to start improving your lifestyle right away.

If 5 or more apply, I recommend getting tested at a medical clinic.

Looking at this checklist, I realized that 5 or more items used to apply to me, too.

Who knew “post-meal sleepiness” and “cravings for sweets” were such important signs!

I wish I’d known sooner…

It’s also a good idea to look back at your checkup results and check your fasting blood sugar, HbA1c, triglycerides, and HDL cholesterol.

Clinics Can Test for It With HOMA-IR

At a clinic, insulin resistance can be estimated using “HOMA-IR (Homeostatic Model Assessment of Insulin Resistance),” which is calculated from two values: fasting insulin and fasting blood sugar.

HOMA-IR is known as an index devised by Matthews and colleagues in 1985, and it’s been used around the world.

According to clinical explanations in Japan, a HOMA-IR of 2.5 or higher means insulin resistance is almost certainly present.

However, a low number doesn’t immediately mean you don’t have insulin resistance.

Also, when fasting blood sugar is above 140 mg/dL, HOMA-IR is not considered appropriate for making a judgment.

If you’re concerned, talk to your regular doctor and ask for a blood test.

How Fasting Improves Insulin Resistance

As we saw earlier, at the root of insulin resistance is the problem of “insulin never getting a break.”

One of the biggest benefits of fasting (intermittent fasting) is that setting aside time without food keeps insulin low and restores your cells’ insulin sensitivity.

Think of insulin sensitivity as a term for how readily your cells respond to insulin.

Even ears that have gotten used to a blaring alarm clock will likely start picking up quiet sounds again after spending some time in a quiet place.

Why Fasting Works on Insulin Resistance

The especially big point is that as visceral fat is used as fuel, the sources of inflammation that were making insulin work poorly decrease.

A study published in Cell Metabolism reported that when men with prediabetes limited their eating to earlier in the day (early time-restricted eating),
their insulin sensitivity, pancreatic beta-cell responsiveness, and blood pressure improved even without weight loss.

Pancreatic beta cells are the cells in the pancreas that make insulin.

In other words, simply changing when they ate produced all these changes without medication.

I go over the types of fasting and the basics of how to do it in detail in my complete guide to intermittent fasting.

Note that if you’re already taking diabetes medication, fasting carries a risk of hypoglycemia (low blood sugar), so be sure to talk to your doctor before trying it.

Precautions When Fasting

While fasting, drink water or tea regularly to prevent dehydration.

Also, if you break your fast by wolfing down sweets or white rice just because you’re hungry, your blood sugar will spike.

There’s no point giving your insulin a rest only to undo it with a binge.

It’s important for people who are pregnant or breastfeeding, growing children, and people with chronic conditions not to fast on their own judgment, but to talk to a doctor first.

If you feel lightheaded or very sluggish, don’t push it. Eat something right away.

Start Right Now! Practical Steps to Improve Insulin Resistance

You can start improving insulin resistance today.

Not with medication, but just by changing how you eat every day, your body will definitely respond.

Step 1: Start With a 12-Hour Overnight Fast

Make a habit of not eating for 12 hours from after dinner until the next morning.

If you finish dinner at 8 p.m., don’t eat or drink anything until 8 a.m. the next morning (water and tea are OK).

This alone secures time for your insulin to come down.

Your sleeping hours count too, so simply cutting out late-night snacks makes it easy to achieve.

Step 2: Cut Back on Foods That Cause Blood Sugar Spikes

White rice, white bread, sweet drinks, and sweets make blood sugar shoot up.

You don’t need to ban them all. Just “eating less of them” or “switching to brown rice or whole grains” has a big effect.

Changing your “eating order” so you eat fiber, protein, and fat first is also effective.

For example, just having a vegetable side dish or some miso soup (soup made with fermented soybean paste) before your rice makes your blood sugar rise more gently.

Step 3: Stop Snacking and Give Your Insulin a Rest

If you have sweets, juice, or fruit between meals, insulin keeps being released all day long.

Limit yourself to 2–3 meals a day, and make a habit of eating nothing between meals (for at least 4–5 hours).

Step 4: Add a 15–30 Minute Walk After Meals

A light walk after meals (15–30 minutes) activates “GLUT4 transporters,” which let muscles take in blood sugar without insulin.

This is said to curb post-meal blood sugar spikes by 30–40%.

Think of GLUT4 as a kind of “entrance for sugar” on your muscle cells.

When you move your muscles, these entrances open, so sugar can get into your cells more easily even without insulin’s help.

Step 5: Get Plenty of Sleep

Lack of sleep increases stress hormones, which makes insulin work less effectively.

Create an environment where you can sleep soundly, for example by going to bed and getting up at the same time every day and avoiding your phone before bed.

Step 6: Focus on Protein and Fiber in Your Meals

It’s not just when you eat, but also what you eat that matters.

Protein from fish, soy products, eggs, and meat is the building material for muscle, and it protects your muscles, the place where sugar gets taken in.

Fiber, which is abundant in vegetables, mushrooms, and seaweed, slows down sugar absorption and likely also helps keep your gut environment healthy.

I explain how to choose your first meal after a fast in detail in my article on the first meal after a 16-hour fast.

Frequently Asked Questions About Insulin Resistance

Stop Silent Insulin Resistance Now and Protect Your Health

Let’s review what we covered.

The feeling of “I’m still fine” is your biggest enemy.

Starting while you still have no symptoms is the best choice you can make to prevent serious illness in the future.

For a fasting program focused on insulin resistance and blood sugar management, check out this science-based program for balancing blood sugar and insulin.

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