【Must-Read Before You Start】An Honest Comparison of the Pros and Cons of Fasting, Who It Suits, Who It Doesn’t, and How to Do It Safely

“I hear fasting is good for you, but does it actually work?”

“Isn’t fasting just painful and nothing else…?”

I bet a lot of you are thinking something like this.

Fasting is a health practice you hear about a lot these days, but the benefits tend to get all the attention, while the downsides and risks often go unmentioned, don’t they?

In this article, I’ll give you an honest, point-by-point look at everything from how fasting basically works to the effects you can expect, its downsides, the different types, and how to start safely.

I’ll also include a table comparing the pros and cons, along with a guide to who it suits and who it doesn’t.

What Is Fasting? How It Differs from “Danjiki” and Common Misconceptions

Fasting Is a “Regular Day Off” for Your Digestive System

Fasting is a health practice where you rest your digestive organs by not eating for a set time or period.

In Japanese it’s sometimes called “danjiki,” but it’s a little different from fasting done as religious discipline;
fasting as a scientific approach aimed at health, beauty, and weight loss is what’s attracting attention now.

It may be easier to understand if you think of it like giving your stomach and intestines a day off, the same way a shop has a regular closing day.

By creating time when you don’t eat, your body gradually switches to a different way of using energy than usual.

You Can Still Drink Fluids While Fasting

One common misconception is that “you can’t even drink water while fasting.”

But that’s wrong.

Even while fasting, water, unsweetened tea, black coffee, and the like are OK.

In fact, getting plenty of fluids is essential for reducing the strain on your body.

Drinks with sugar or calories are a no-go.

Even a small amount starts digestion the moment you take it in, resetting your fasting state.

I go over how to choose drinks while fasting in detail in my article on what to drink while fasting.

“Not Eating” Doesn’t Mean “an Endurance Contest”

Another misconception is the image that “fasting means enduring several days without food.”

In reality, there are lots of easy methods where you simply create a window of about 16 hours without food, including your sleep time.

Just finishing dinner a little earlier and pushing breakfast back a little is a perfectly good fast.

This method of “dividing your day into eating time and non-eating time” is also called intermittent fasting.

Compared with full-on fasting where you don’t eat at all for several days, its key feature is that it’s easy to fit into everyday life.

What matters is not going a long time without eating, but choosing a method that fits your health and lifestyle.

5 Benefits You Can Expect from Fasting

Fasting is expected to have a variety of effects.

Let’s look at them one by one.

1. Your Digestive Organs Get a Rest, and Your Gut Environment Improves

When you go a stretch of time without eating, your stomach and intestines get a proper rest.

People today use their digestive organs almost nonstop with three meals a day plus snacks, so fasting could be called a “refresh vacation” for your gut.

It’s like a workplace stuck in nonstop overtime finally getting a day off.

A healthier gut environment can lead to better bowel movements and more efficient nutrient absorption.

For people bothered by an upset stomach or a heavy feeling after meals, this may be the benefit they notice most easily.

2. Autophagy Kicks In and Your Cells Are Rejuvenated

When your fasting time continues for 16 hours or more, a self-repair function called “autophagy” becomes active in your body.

Autophagy is the process by which cells break down old proteins and waste themselves and remake them into new energy.

Think of it like taking apart unwanted items around the house and recycling the usable parts.

Research is moving forward on the possibility that it may slow the pace of aging and help prevent lifestyle-related diseases.

3. Blood Sugar and Insulin Balance Become More Stable

Eating fewer times a day makes sharp rises and drops in blood sugar (blood sugar spikes) less likely.

As a result, it’s thought that insulin secretion settles down and your body enters a state where fat is less likely to be stored.

Insulin is a hormone that acts like a “courier,” delivering sugar in the blood to cells and storing the excess as fat.

With the courier called on less often, your body can more easily use its stored fat.

The fact that some people feel less sudden sleepiness and sluggishness after meals is also thought to be related to this more stable blood sugar.

4. Your Head May Feel Clearer and Your Focus May Improve

After a meal, energy goes to digestion, so it’s easy to feel sleepy or foggy-headed, isn’t it?

In a fasted state, on the other hand, blood flow to the brain increases, and many people feel more focused and clear-headed.

Being hungry may feel a bit like the noise in your head quieting down.

By the way, in my case too, I sometimes feel I get more work done in the morning on days I skip breakfast.

It varies from person to person, but I think it’s worth a try.

One approach is to make good use of your fasting hours by timing them to when you have important work or studying to do.

5. Managing Your Weight and Body Fat Becomes Easier

Because the time you can eat is limited, your calorie intake tends to be controlled naturally.

If you want to learn about the changes fasting causes in the body from a research perspective, also see my article on the health benefits of fasting shown in a medical journal.

The Downsides and Risks of Fasting You Should Know

While fasting has effects you can look forward to, if you don’t approach it correctly, it can also put a strain on your body.

Before you start, make sure you understand the downsides too.

You Can Lose Muscle More Easily

When you run short on energy during a fast, your body may try to break down not just fat but muscle (protein) as well to turn it into energy.

Muscle is like the “engine” that burns energy in your body.

When your muscle mass drops, your basal metabolism (the energy you burn while doing nothing) also goes down, which could actually make your body more prone to gaining weight.

To prevent this, it’s important to get plenty of protein during your eating window.

Also, keeping up light strength training or walking makes it easier to protect your muscles.

You May Experience Hunger, Headaches, or Dizziness

Until you get used to it, you may experience discomforts such as hunger, headaches, dizziness, and sluggishness.

This is a sign that your body senses “this is different from my usual eating pattern,” and the symptoms often ease as you get used to it.

A lack of fluids or minerals makes these discomforts more likely, so sip water or tea regularly.

However, if the symptoms are severe, don’t push yourself; stop.

Binge Eating After a Fast Makes Rebound Weight Gain More Likely

Binge eating out of a sense of release, thinking “I can finally eat!”, is the biggest cause of rebound weight gain.

After a fast, your body digests and absorbs food more efficiently, so if you eat a lot all at once, you’ll absorb more calories than usual.

Picture a dry sponge soaking up water all at once.

When you start eating again, the golden rule is to begin with small amounts of foods that are easy to digest.

It Can Be Hard to Match Mealtimes with Others, and Nutrition Can Become Unbalanced

A downside that’s easy to overlook is that your mealtimes may no longer line up with your family’s or friends’.

Some people may feel stressed when they find themselves turning down more lunch invitations or drinking parties.

On days like that, being flexible enough not to force the fast and to adjust the next day instead is the key to keeping it up long term.

Also, when you eat fewer meals, the variety of nutrients you can get from each meal tends to be limited.

In particular, if you don’t get enough vegetables, seaweed, and fruit, you’ll fall short on dietary fiber, vitamins, and minerals.

Try thinking of your limited eating window as a “bento box” that you pack full of nutrients.

It Isn’t Right for Everyone

If any of the above apply to you, be sure to consult a doctor before trying it.

Comparing the Pros and Cons: Who It Suits and Who It Doesn’t

Pros and Cons Comparison Table

Let’s organize everything so far into a table.

Looking at it this way, you can see that most of the downsides can be reduced by “how you do it.”

In other words, whether fasting is good or bad depends on how you approach it more than on the method itself.

It’s the same as how a kitchen knife can be a tool that helps you cook, but if you use it the wrong way, it can cause an injury.

To make the most of the benefits, the basics are to stick to three things: “start with short periods,” “get plenty of nutrition during your eating window,” and “rest when it gets tough.”

Just keeping these three in mind can shrink many of the downsides in the table.

Who Fasting Suits

People like the following are the type who tend to feel the benefits of fasting.

Since all you have to do is decide “when to eat,” it’s a good match especially for people who find simple rules easier to stick with.

Also, people who have a habit of mindlessly eating late at night may find their daily rhythm gets back on track by setting limits on when they eat.

Who Should Think Carefully

On the other hand, if you fall under “It Isn’t Right for Everyone” in the previous chapter, it’s important not to start on your own judgment.

Beyond that, people who are naturally light eaters, or who easily feel unwell if they don’t eat at set times, don’t need to force themselves to take it up.

People who are extremely thin or whose physical strength has declined will also feel safer consulting a doctor first.

Even healthy people should make the call to take a break from fasting on days when they feel unwell or during periods of ongoing sleep deprivation.

Types of Fasting and How to Choose the Right Method for You

Although we call it all “fasting,” there are several types.

The key to sticking with it is choosing one that fits your daily rhythm.

16-Hour Fasting (Recommended for Beginners)

This method makes 16 hours of the day your fasting time and has you eat within the remaining 8 hours.

For example, if you finish dinner at 8 p.m., you’d have your first meal at noon the next day.

Because your sleep counts toward the fast, the time you actually feel hungry is relatively short.

If you’re trying fasting for the first time, this is the safest method to start with.

I’ve put the details of how to do it together in my article on the benefits of 16-hour fasting and how to do it right.

The 5:2 Diet (Fasting 2 Days a Week)

With this method, you eat normally 5 days a week and keep your calories to about 500 to 600 kcal on the remaining 2 days.

Since you don’t fast every day, it has the benefit of having little impact on your work and daily life.

On the other hand, you’re more likely to feel hungry on low-calorie days, so it’s a good idea to pick days when you don’t have much planned.

One-Day Fasting (24-Hour Fasting)

This is a method where you fast for 24 hours once or twice a week.

While it tends to produce results, it also puts more strain on your body, so jumping straight into a long fast is risky.

Think of it as being for people who are already used to fasting.

If 16 hours feels tough, you can also start with 12 hours and gradually extend it.

Rather than aiming for the top level from the start, moving forward one step at a time, like climbing stairs, is the surest way to avoid the downsides.

How to Choose Based on Your Lifestyle

I recommend thinking about which method to choose based on your daily rhythm.

What matters is choosing based on “whether you can keep it up.”

Frequently Asked Questions About the Pros and Cons of Fasting

What to Know to Keep Fasting Safely

If you approach fasting with the right knowledge, it becomes a health habit that’s very gentle on your body.

On the other hand, forcing it for long periods or neglecting your recovery meals can damage your body.

Recovery Meals (When You Start Eating Again) Are Especially Important

The “recovery period,” when you finish fasting and start eating again, is just as important as the fast itself.

Start your first meal with foods that are easy on your digestive system, such as okayu (Japanese rice porridge), soup, or easy-to-digest vegetables.

Suddenly eating ramen or fried foods puts a heavy burden on your body.

Even with a short fast like 16-hour fasting, it’s reassuring to start your first bites with something gentle, such as miso soup (fermented soybean paste soup), salad, or a boiled egg.

Chewing well and eating slowly also helps prevent binge eating and keeps your blood sugar from spiking.

Focus on “Quality” During Your Eating Window

Many of the downsides of fasting can be reduced by how you spend your eating window.

Get protein at every meal to protect your muscles, and make up for dietary fiber and minerals with vegetables, seaweed, and mushrooms.

Try to balance your staple food, main dish, and side dishes, and include a little of foods in different colors.

If you end up eating mostly sweets and fried foods, the benefits of your fast will be weakened.

Just eating in the order “vegetables → protein → carbs” can make it easier to keep your blood sugar from spiking.

You Don’t Need to Do It Every Day, and Don’t Aim for Perfection

With fasting, the mindset that “3 to 4 times a week is good enough” is plenty.

Even if there are days when you can’t keep it up because of meal plans or how you feel, that’s no problem at all.

Keeping it up comfortably over the long term is the shortcut to improving your constitution.

Jotting down simple notes on changes in how you feel and your weight makes it easier to find a pace that suits you.

If you experience severe dizziness, heart palpitations, or shaky hands while fasting, stop immediately and eat something.

Changes such as irregular periods or a run of sleepless nights may also be signs that your body is under strain.

Take your first step into fasting by starting with having dinner a little earlier tonight.

If you want to learn more about the basics of fasting overall, please also read my complete guide to intermittent fasting.