【Flip the Fat-Burning Switch】Does 16-Hour Fasting Produce Ketones? How It Works, Tips to Boost Them, and Precautions

“I’m dieting, but the fat just won’t come off…”

Is that something you’re struggling with?

The truth is, simply cutting calories won’t make your body burn fat.

When ketones are produced during a 16-hour fast, your body starts using fat as fuel in earnest.

In this article, I’ll walk you through how that works and how to put it into practice, in plain language.

What Happens in Your Body During a 16-Hour Fast? How the Fat-Burning Switch Turns On

Normally, our bodies use the carbohydrates (glucose) we get from foods like rice and bread as their main source of energy.

But as you keep fasting and your blood sugar drops, your body flips a certain switch.

That switch is the shift into “fat-burning mode.”

Glycogen Runs Low After 10 to 12 Hours

About 10 to 12 hours after you stop eating, the glycogen (the stored form of carbohydrates) in your liver and muscles starts to run out.

Glycogen is like your body’s energy “savings account.”

Once those savings are used up, your body tries to break down body fat and use it as its next fuel.

Ketone Production Ramps Up at 16 Hours

Once you pass the 16-hour mark of a fast, your liver begins breaking down fatty acids and producing an energy substance called “ketones.”

These ketones are the sign that your body is using fat as fuel.

In other words, fasting naturally brings about the same state as a low-carb diet (the ketogenic diet).

Your Savings Are Being Spent Even While You Sleep

Even while you sleep, your heart, your brain, and the muscles you use to breathe keep working the whole time.

That’s why your glycogen savings are being withdrawn little by little even between dinner and the next morning.

Because your savings have already dropped overnight, skipping breakfast or eating dinner earlier tends to speed up the switch into fat-burning mode.

Waking up hungry in the morning can be seen as one sign that your savings are running low.

The Timing of the Switch Varies from Person to Person

When ketones start to increase differs from person to person, depending on what you ate the day before, how active you were, and your constitution.

Keeping a log of your weight and hunger should make it easier to get a sense of your own switching rhythm.

On the day after a carb-heavy meal, or on days when you didn’t move much, the switch may come a little later.

What matters is not getting too hung up on the exact hour, and continuing with 16 hours as a general guideline.

What Are Ketones? The Process of Turning Fat into Fuel

Many of you may have heard the word “ketones” but honestly aren’t sure what they are.

Here, I’ll explain them as simply as I can.

What Ketones Really Are

Ketones (or ketone bodies) is the umbrella term for the energy substances produced when the liver burns fatty acids (the components that result from breaking down fat).

Among these, BHB can supply energy directly to the brain, which is said to be what’s behind that clear-headed feeling during a fast.

How Ketones Compare with Glucose

Glucose burns quickly and tends to cause blood sugar swings, while ketones are considered a cleaner, more stable source of energy.

Some research has even reported that ketones produce more ATP, the body’s energy currency, than the same amount of glucose.

In other words, the hallmark of ketones is that they “make a lot of energy from a small amount of raw material.”

“A Second Fuel” the Brain Can Use, Too

The brain normally relies mainly on glucose for energy.

Fat itself has a hard time getting through the brain’s checkpoint, so the brain can’t use fat directly.

Ketones, however, can pass through that checkpoint and be used as energy by the brain.

Think of it as a kind of “reserve fuel tank” built into the body so the brain can keep working even when glucose runs low.

Organs such as your muscles and heart can also use ketones for energy.

Ketones Are Produced More Easily When Insulin Drops

A hormone called insulin plays a big role in whether ketones are produced.

Insulin is a hormone released by the pancreas when blood sugar rises, and it works to deliver sugar to your cells and store fat.

While a lot of insulin is being released, fat breakdown is held back and ketones are less likely to be made.

As the time without food continues and insulin comes down, the brakes come off fat breakdown, making it easier for ketones to be produced.

A “Hybrid” Body That Can Switch Between Sugar and Fat

Your body’s ability to use both sugar and fat as fuel depending on the situation is sometimes called “metabolic flexibility.”

Picture a hybrid car that switches between gasoline and electricity as it drives.

If you’re eating all the time, your body only ever uses sugar, and the chance to use fat rarely comes around.

Regularly building in periods without food is thought to be practice for keeping this switching ability in shape.

Ketones Aren’t “Bad for You”

Some of you may have a vague impression that ketones sound bad for your health.

But when ketones rise in a healthy person who is hungry or has just exercised, that’s a natural mechanism built into the body.

It’s thought that humans have long relied on this mechanism to get through times when there was no food.

That said, there are cases where caution is needed if you have a condition such as diabetes, so I’ll go over that in detail in a later section.

Practical Tips for Maximizing Ketones with 16-Hour Fasting

To produce ketones efficiently, it’s important not just to go 16 hours without eating, but to keep a few key points in mind.

Tip 1: Be Mindful of Carbs During Your Eating Window

If you eat a lot of refined carbs like white rice, bread, and noodles during your 8-hour eating window, glycogen tends to linger into the next fast, slowing down ketone production.

You don’t need to cut carbs out completely, but I’ve found that just going a little lighter on carbs during your eating window makes ketone production during the fast go more smoothly.

Tip 2: Add Light Exercise During Your Fast

A light walk or some gentle stretching exercises during a fast help use up your remaining glycogen sooner.

Intense exercise can sometimes backfire, but a walk of about 30 minutes can be expected to help boost fat burning.

Tip 3: Make Use of Black Coffee

Black coffee during a fast is said to promote lipolysis (the process of mobilizing fat to use as energy).

I drink one or two cups of black coffee every morning, and I can really feel my hunger easing and my focus sharpening.

It’s OK to Start with 12 Hours

If you rush to make it too long, sticking with it at all may become a struggle.

Stay Well Hydrated During the Fasting Hours

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

Since you’re getting less water from food, it’s easy to become dehydrated if you don’t drink consciously.

Avoid drinks with sugar or milk, as they raise insulin.

Break Your Fast Slowly and Gently

If you suddenly take in a lot of carbs after 16 hours without food, your blood sugar is likely to spike.

I recommend starting your first bites with things like miso soup, salad, or a boiled egg, and eating slowly while chewing well.

Be careful, because if what you eat during your eating window gets out of hand, it becomes harder to feel the benefits of fasting.

The Surprising Effects Ketones Have on Your Body and Brain

Ketones aren’t just a “byproduct of fat burning.”

In fact, it’s becoming clear that they have very positive effects on both the body and the brain.

An Excellent Energy Source for the Brain

Glucose is the brain’s main source of energy, but ketones can also be used as fuel for the brain.

What’s more, because ketones don’t cause blood sugar swings, they can keep supplying the brain with steady energy.

This is one of the reasons many people feel “clear-headed and more focused” during a fast.

May Help Calm Inflammation

There are research reports that beta-hydroxybutyrate (BHB) works to suppress chronic inflammation in the body.

Chronic inflammation is considered a root cause of many lifestyle-related diseases, so this point is attracting a great deal of attention.

Weight and Body Fat Loss

When ketones are being actively produced, your body is literally “running on fat as fuel.”

After continuing 16-hour fasting for three months myself, I successfully lost 15 kg (33 lb), going from 70 kg (154 lb) to 55 kg (121 lb).

I’m convinced that fasting with ketone production in mind played a major role in this change.

Hunger May Become Milder

As they get used to fasting, some people find they don’t get as hungry as they expected.

There are also research reports suggesting that ketones may influence hormones involved in appetite.

That said, how people feel varies greatly, so there’s no need to force yourself to endure hunger.

Get Enough Protein to Protect Your Muscles

Some of you may worry that longer periods without food will cause you to lose muscle.

With a fast of around 16 hours, it’s thought that getting plenty of protein during your eating window makes it easier to limit the impact on your muscles.

Make a point of including meat, fish, eggs, soy products, and dairy at every meal.

Keeping up some light strength training as well should make it easier to maintain your muscle.

Links to Autophagy and Cell Health

It’s also thought that during periods without food, a mechanism called “autophagy,” which breaks down and recycles old parts inside your cells, becomes more active.

Think of autophagy as a kind of “cleanup crew” that gathers up the garbage inside your cells and recycles it.

However, much of this is still at the stage of animal studies or short-term research, and the effects in humans haven’t been clearly confirmed.

Rather than thinking “if ketones show up, my illness will go away,” see them as one mechanism that supports your overall health.

Why Some People Produce Ketones More Easily Than Others on a 16-Hour Fast

Even when doing the same 16-hour fast, some people produce ketones easily and some don’t.

Let’s look at the main factors behind that difference.

Lots of Carbs Before Your Fast Make It Harder

If you eat a lot of carbs in the meal before you start fasting, your liver and muscles end up fully stocked with glycogen.

The more savings you have left, the longer it takes before your body dips into its other source of funds: fat.

So if your last meal was mostly carbs, ketones may not rise much even after 16 hours.

On the other hand, making your last meal centered on vegetables and protein and going light on carbs is thought to help speed up the fuel switch.

People Who Stay Active Switch Over Faster

When you exercise, the glycogen stored in your muscles gets used.

Moving your body lightly during your fasting hours, such as with a morning walk, reduces glycogen and makes it easier for ketones to be produced.

As mentioned in Tip 2, even if it isn’t intense exercise, a walk of about 30 minutes can give your fat burning a boost.

However, be careful: intense exercise on an empty stomach can cause dizziness or lightheadedness.

Whether You’re Used to Fasting

If you’re normally eating all the time, your body is completely accustomed to running on sugar.

Even if you suddenly try to make it run on fat, the switch may not flip right away.

It’s thought that as you continue 16-hour fasting, your body gradually gets used to using fat as fuel.

This may be why many people find that, even if they feel hungry or sluggish for the first few days, it gradually gets easier.

Snacks and Sweet Drinks Reset the Clock

If you have sweetened coffee, juice, or even a small sweet during your fasting hours, your blood sugar and insulin go up.

That puts the brakes on the fat breakdown that was just getting started, and the switch goes back to square one.

During your fasting hours, it’s important to choose calorie-free drinks such as water, tea, or black coffee.

Sleep and Stress Play a Role, Too

When lack of sleep or stress continues, the hormones that raise blood sugar tend to increase.

As a result, your blood sugar may have a harder time coming down even during your fasting hours, making it harder for the fuel switch to progress.

If you’re having trouble feeling the effects of fasting, take a look at your sleep and daily rhythm as well.

Precautions When Fasting to Boost Ketones, and How It Differs from Ketoacidosis

16-hour fasting is an easy method to get started with, but there are some points you should know to keep doing it safely.

Ketosis and Ketoacidosis Are Two Different Things

When you look up ketones, you may come across the word “ketoacidosis.”

Ketoacidosis is a dangerous, potentially life-threatening condition in which ketones build up abnormally and the blood becomes acidic.

It’s mainly known to occur in people such as those with diabetes whose bodies produce almost no insulin.

What a healthy person experiences on a 16-hour fast is physiological “ketosis,” in which the body naturally regulates its ketone levels, and it has long been considered distinct from ketoacidosis.

To use a bathtub analogy, picture ketosis as a tub filled with just the right amount of water, and ketoacidosis as a broken faucet with water overflowing everywhere.

People Who Should Talk to a Doctor Before Fasting

If any of the following applies to you, don’t fast based on your own judgment; be sure to consult a doctor beforehand.

Extra caution is needed especially for people taking blood sugar-lowering medication, since fasting could cause low blood sugar.

Don’t Push Yourself When You Feel Unwell

If you feel dizziness, severe fatigue, headache, nausea, or heart palpitations during a fast, don’t push yourself; have something to eat and drink.

On days when you’re not feeling well or haven’t slept enough, choosing to take a break from fasting is also important.

If symptoms persist, see a medical professional promptly.

Be Careful About Combining It with Extreme Carb Restriction

Starting fasting and extreme carb restriction at the same time just because you want to boost ketones can put a heavy strain on your body.

It’s safer to focus first on getting used to 16-hour fasting, and to adjust what you eat little by little.

If you’re curious about how it compares with the ketogenic diet or a low-fat diet, check out the related articles as well.

FAQ: 16-Hour Fasting and Ketones

Concrete Habits to Speed Up Fat Burning and Keep It Going

Now that you understand how ketones work, let’s think about how to keep up daily fasting without strain.

Keep Your Fasting Routine Simple

If you decide to “only eat during the 8 hours from noon to 8 p.m. every day,” you no longer have to think about it, which makes it easier to stick with.

Fixing your eating window also helps regulate your body clock, so the timing of your hunger becomes more stable.

Keep a Fasting Log to Stay Motivated

Simply jotting down your weight, body fat percentage, mood, and focus makes your reasons for continuing visible, which helps you stay motivated.

Even if you only keep it up for the first week, you’ll find it easier to notice changes in your body.

Drop the Perfectionism

It’s fine if there are days like “I couldn’t fast today because I had a dinner party.”

Many people say that if you can keep it up 5 to 6 days a week, you can expect plenty of benefit, and it’s more important to keep at it loosely over the long term than to burn out trying to be perfect.

For a full overview of intermittent fasting, also take a look at The Complete Guide to Intermittent Fasting.

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