【The Latest Intermittent Fasting Research】Does 16-Hour Fasting Lower Blood Sugar? Why It Can Rise and How to Prevent Blood Sugar Spikes

“Does fasting really do anything for your body?”

“I’ve been doing it on and off, but is there any scientific evidence behind it?”

I imagine quite a few of you are keeping up with fasting while wondering exactly that.

In fact, over the past few years, research on 16-hour fasting (intermittent fasting, or IF) has been advancing rapidly around the world.

And the results are very clear.

In particular, the benefits for improving blood pressure and blood sugar have been confirmed in multiple highly reliable studies.

In this article, I’ll use the latest scientific data to explain, in plain language, how fasting works on your body.

1. How 16-Hour Fasting Lowers Blood Pressure

When people hear “blood pressure,” they tend to think “salt in the diet is the problem,” but in fact, excess insulin secretion is also one of the major causes of rising blood pressure.

When you eat frequently, your blood sugar rises again and again, and insulin keeps being released.

Picture a delivery driver called insulin being summoned over and over every time your blood sugar goes up.

Insulin promotes the reabsorption of sodium in the kidneys, and this is what pushes blood pressure up.

When your kidneys hold on to salt instead of letting it go, your blood volume increases and the pressure on your blood vessels rises.

When you eat less often through fasting, insulin secretion is held down and sodium excretion is promoted.

As a result, your blood pressure naturally comes down.

Research Data: Blood Pressure Improvement in Numbers

A paper published in the New England Journal of Medicine (NEJM) in 2019 reported that
systolic blood pressure (the top number) dropped by an average of 6 to 8 mmHg in subjects who kept up intermittent fasting.

That figure is on par with the initial dose of blood pressure medication.

Look Beyond Salt to “How Often You Eat”

When it comes to managing high blood pressure, people tend to focus only on cutting salt.

Of course, reducing salt is important, but “how many times you eat” is thought to be just as relevant to blood pressure.

A lifestyle where snacks and late-night eating keep insulin flowing is like constantly ordering your kidneys to “hold on to the salt.”

Eating less often and creating periods without food is an approach to balancing blood pressure from a different angle than cutting salt.

Recording your morning and evening readings with a home blood pressure monitor will make it easier to notice changes.

However, if you take blood pressure medication, fasting may lower your blood pressure too much, so be sure to talk to your doctor before you start.

2. Remarkable Effects on Blood Sugar and Insulin Sensitivity

One of the most noteworthy effects of 16-hour fasting is improved insulin sensitivity.

Insulin sensitivity refers to how responsive your cells are to insulin.

When it declines (insulin resistance), your blood sugar has a harder time coming down, and your risk of type 2 diabetes and obesity goes up.

During a fast, your blood sugar and insulin levels stay low for an extended period.

You could say it’s a state where the delivery driver called insulin is getting a proper break.

This restores your cells’ insulin sensitivity, allowing them to process blood sugar efficiently even with less insulin.

Key Research Data

An HbA1c improvement of 0.3 to 0.5% is a very large figure when viewed as a marker for managing diabetes.

The fact that this effect can be achieved simply by changing meal timing, rather than with medication, is drawing a great deal of medical attention.

Blood Sugar Doesn’t Drop to Zero Even When You’re Not Eating

You might worry that going 16 hours without eating anything will make your blood sugar keep falling and falling.

But a healthy person’s body has a built-in system for keeping blood sugar within a certain range.

As time passes after a meal, your liver gradually draws on its store of sugar called “glycogen” and sends sugar out into your blood.

Think of glycogen as “emergency rations” kept in the warehouse that is your liver.

And as the time without food gets longer, your body starts breaking down fat to use as energy.

Your Body Clock and Meal Timing

The human body has a “body clock” that keeps a rhythm of about 24 hours.

How well insulin works also follows a daily rhythm: it’s generally said to work well from morning through the daytime and less well as night approaches.

That’s why, even with the same food, eating it late at night tends to raise blood sugar more.

Cutting down on late-night eating through 16-hour fasting also leads to a way of eating that’s in sync with your body clock.

A Study That Moved the Eating Window Earlier

In a study published in the medical journal Cell Metabolism in 2018,
men with prediabetes tried confining their meals to a 6-hour window from morning to early afternoon.

The results reported improvements in insulin sensitivity and blood pressure, even though they ate the same amount of food and their weight barely changed.

Because changes were seen without weight loss, attention turned to the possibility that not just how much you eat, but “when you eat,” affects the body.

However, this study had only eight participants and a short duration, so we can’t say for certain that it applies to everyone.

A Report That an 8-Hour Eating Window Improved Blood Sugar Control

At the European Association for the Study of Diabetes, a study was also presented showing that when adults at risk of type 2 diabetes limited their daily eating to 8 hours, their blood sugar control improved.

16-hour fasting is exactly this kind of “8-hour daily eating window.”

Based on research like this, keeping your eating to a shorter window is thought to be one possible way to help manage blood sugar.

3. Effects on Inflammatory Markers: Fasting That Calms Chronic Inflammation

On top of improving blood pressure and blood sugar, fasting also has a major effect on suppressing chronic inflammation.

Chronic inflammation is sometimes called a “silent killer” and is thought to be a root cause of many lifestyle-related diseases, including arteriosclerosis, cancer, and dementia.

It’s like a small fire that keeps smoldering inside your body without you noticing.

Autophagy (the cell’s self-cleaning process), which is activated during fasting, works to suppress the production of inflammatory cytokines and calm inflammation in the body at its root.

Changes in CRP (an Inflammation Marker)

A study in the Journal of Translational Medicine (2020) reported that
participants who followed an 8-week IF program saw their CRP (C-reactive protein) drop by an average of 28%.

CRP is widely used as a risk marker for heart disease and arteriosclerosis, and an improvement in this number means a significant reduction in cardiovascular risk.

CRP is sometimes measured in the blood tests at health checkups, so if you’re concerned, talk to your doctor.

Autophagy Is Your Cells’ “Cleanup Crew”

Autophagy is the process by which old parts and waste inside your cells are broken down and reused as new materials.

Think of it as a cleanup crew that clears away the trash piled up in a room and recycles whatever can still be used.

It’s said that this cleanup crew works more readily during periods without food, when nothing new is coming in.

Cells left full of waste can easily become a spark for inflammation, which is why it’s important to make regular time for cleaning.

Pair It with an Inflammation-Conscious Diet

To boost the effects of fasting, what you choose to eat during your eating window also matters.

By getting both your fasting and what you eat in order together, you should be able to shrink that small fire inside your body.

Can 16-Hour Fasting Raise Blood Sugar? Possible Causes

Some people feel that after starting 16-hour fasting, “my blood sugar actually seems to have gone up”.

Here, I’ll go over possible causes of blood sugar rising during or after a fast.

1. The “Dawn Phenomenon,” Which Raises Morning Blood Sugar

From the early hours of the morning, your body releases hormones that raise blood sugar to prepare for waking up.

Because of this, your morning blood sugar may read a little high even if you haven’t eaten anything, and this is called the “dawn phenomenon.”

If you skip breakfast while fasting, you may simply be more likely to notice this natural rise in blood sugar.

2. Blood Sugar Spikes from Overeating When You Break Your Fast

If you wolf down a rice bowl, noodles, or sweets all at once after 16 hours without food, your blood sugar is likely to shoot up.

This rapid rise and fall in blood sugar is called a “blood sugar spike,” and it’s thought to put a strain on your blood vessels.

Sugar can feel like it’s absorbed faster after a period of hunger, so what you eat at your first meal becomes very important.

3. Stress and Lack of Sleep

When you’re under heavy stress or not getting enough sleep, the hormones that raise blood sugar tend to increase.

If you experience fasting as “enduring” and stress builds up, that alone can affect your blood sugar.

Rather than forcing yourself to continue, it’s important to choose a timing and length that you can keep up comfortably.

4. Overeating During Your Eating Window

If you eat more than usual during your 8-hour eating window, it’s less likely to lead to better blood sugar.

Snacking or having sweet drinks over and over because “it’s my eating time” can backfire.

The key is not only to shorten your eating window, but also to review what you eat during it.

5. Too Much Caffeine During Your Fasting Hours

Drinking cup after cup of black coffee during your fasting hours can, for some people, make it easier for the caffeine to trigger hormones that raise blood sugar.

It’s a good idea to limit coffee to a few cups a day and combine it with water or caffeine-free tea.

6. Changes That Happen While Your Body Is Adjusting

When you’ve just started fasting, your body isn’t used to the new eating rhythm yet, so your blood sugar may seem to fluctuate.

If the numbers concern you, don’t judge based on a single measurement; keep an eye on the trend while consulting your doctor.

4. Three Keys to Maximizing the Benefits of Fasting

To maximize the fasting benefits revealed by scientific research, there are some practical points worth keeping in mind.

Key 1: Get Your Diet in Order at the Same Time

The benefits of fasting increase even more when combined with what you eat.

In particular, if you focus on meals centered on fiber, protein, and healthy fats during your eating window (the hours when you’re allowed to eat), the effects on stabilizing blood sugar and suppressing inflammation become even greater.

Key 2: How Long You Keep It Up Matters

Looking at the research data, the effects generally begin to appear after 4 to 8 weeks of consistency.

Quitting after two weeks because you feel “nothing is changing” is too early, even from a scientific standpoint.

That’s because the numbers in your body change slowly over time.

It’s important to keep at it for at least a month.

Key 3: Build Your Sleep into Your Fast

By spending 7 to 8 hours of your 16-hour fast asleep, the time you actually have to endure hunger shrinks to 8 to 9 hours.

This approach is also recommended in research as the easiest way to keep fasting going.

If Possible, Make Your Eating Window Earlier in the Day

As in the study introduced earlier, concentrating your meals in the daytime, when insulin works best, may make it harder for your blood sugar to rise.

If work or family circumstances make that difficult, prioritize what you can sustain and start by avoiding late-night meals.

I cover the basics of 16-hour fasting in Two Super-Simple Habits for Better Health: Intermittent Fasting Improves Metabolism and Helps with Weight Loss.

Start Your First Bites After a Fast with Vegetables and Soup

For your first meal after a period without food, start with vegetables, seaweed, mushrooms, or miso soup.

Next, have protein such as meat, fish, eggs, or soy products, and I recommend eating staple carbs like rice or bread last.

Eating fiber and protein first is said to slow the absorption of sugar and make it easier to keep blood sugar from spiking.

Chew Well and Eat Slowly

After going hungry, it’s easy to eat too fast, so make a point of chewing each bite thoroughly and taking at least 20 minutes to eat.

Eating slowly makes it easier for the fullness signal to reach your brain and also helps prevent overeating.

Move Your Body Lightly After Eating

It’s said that walking for 10 to 15 minutes after a meal helps keep blood sugar from rising, because your muscles use up the glucose.

Even small movements, like clearing the dishes or taking a short walk nearby, are fine.

Stay Well Hydrated During Your Fasting Hours

During your fasting hours, hydrate frequently with calorie-free drinks such as water, tea, or black coffee.

Sugary drinks and juice raise blood sugar, so avoid them during your fasting hours.

Drawbacks of 16-Hour Fasting and Blood Sugar Precautions

16-hour fasting is an easy method to get started with, but there are some points to be especially careful about when it comes to blood sugar.

If You Take Diabetes Medication, Always Talk to Your Doctor

If you use insulin injections or oral medication to lower blood sugar and start fasting on your own judgment, you risk developing low blood sugar.

Low blood sugar is a dangerous condition with symptoms such as cold sweats, trembling hands, heart palpitations, intense hunger, and a foggy mind.

If you’re being treated for diabetes, be sure to talk to your doctor before starting a fast and get instructions on the dose and timing of your medication.

Who Should Avoid Fasting

If any of these apply to you, don’t start fasting on your own judgment; consult a doctor first.

Pay Attention to Changes in How You Feel

If you feel dizzy, lightheaded, very fatigued, or get a headache during a fast, don’t push yourself; have something to eat and drink.

Especially while you’re still getting used to fasting, it’s safer to avoid driving or intense exercise during your fasting hours.

On days when you’re not feeling well, taking a break from fasting is also an important choice.

Hunger and Irritability May Show Up

When you first start fasting, you may experience strong hunger, irritability, or trouble concentrating.

In most cases this settles down as your body gets used to the new rhythm, but there’s no need to push yourself when it’s hard.

Instead of aiming for 16 hours right away, starting with 12 hours and gradually extending the time should make it easier to reduce the burden on your body.

Watch Out for Nutritional Gaps During Your Eating Window

When your eating window gets shorter, you may become more likely to fall short on protein, vitamins, minerals, and fiber.

If you don’t get enough protein in particular, you may lose muscle more easily, and your muscles’ ability to use sugar may decline too.

During your eating window, aim for balanced meals with a staple, a main dish, and side dishes, and make sure you get the nutrients you need.

Check Your Numbers Regularly

If you’re concerned about your blood sugar, use health checkups and doctor visits as opportunities to track changes in your fasting blood sugar and HbA1c.

Moving forward while reviewing changes in your numbers with your doctor will help you continue with peace of mind.

FAQ: 16-Hour Fasting and Blood Sugar

16-Hour Fasting: A Lifestyle Improvement Method Proven by Science

16-hour fasting isn’t just a trendy diet.

It’s a science-backed lifestyle improvement method whose benefits have been demonstrated in medical journals around the world.

Significant improvements in blood pressure, blood sugar, and chronic inflammation have all been confirmed with fasting alone, without medication.

Of course, if you have a chronic condition or are taking medication, be sure to consult your doctor before trying it.

From “something I just do out of habit” to “a habit backed by science.”

Shifting your mindset a little starting today should make fasting even more enjoyable.

If you’d like to make hunger during your fast a little easier, also check out the fasting support supplement I actually use.

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