Is fasting good for you?

Written by Trey Reck

Medically reviewed by Josie Porter, RD

Published on January 31, 2023 | Updated on June 11, 2024

Is intermittent fasting good for you?

That’s what we’re here to find out. 

Of course, what we suspect you really want to know is whether fasting will help you achieve your goals. Improve your health. Lose weight. Transform your body. Feel better about yourself. Feel better within yourself. 

Making progress on these priorities matters. And people say intermittent fasting can make a difference. 

We’ll cut through the hype and look at what the science says about exactly why intermittent fasting is thought to be so good for you. 

Let’s see what stands up to our Simple scrutiny. 

What is intermittent fasting?

Intermittent fasting is a dietary approach that alternates periods of eating and not eating. 

Those periods of not eating are called fasts. 

You could fast every day using a time-restricted eating schedule (TRE). TRE fasts are between 12 and 20 hours and typically include zero calories.

Or, you could fast a few times a week, using a method like 5:2 or alternate-day fasting (ADF). Fasts with these methods are usually 24 hours and include 500/600 calories. 

At its heart, intermittent fasting is simple. There aren’t any complex rules about what you should and shouldn’t eat — nothing is off-limits during your eating window. (That said, you’ll get the best results from eating a healthy, balanced diet; check out what to eat while intermittent fasting to learn more.)

During your fasting window, you’d stick to drinks with zero calories or whatever foods you like to make up your 500/600 calories. 

While choosing a fasting schedule comes down to what works for you — your body, goals, needs, lifestyle, and preferences — we don’t recommend more restrictive fasting schedules where you eat either nothing or a limited amount of calories for 18+ hours (especially if you plan to fast daily). These fasting schedules involve more potential risks and safety concerns, and there isn’t really evidence to suggest they’re more effective results-wise.

Regardless of what fasting schedule you’re considering, you should also always check in with your healthcare provider before making any major changes to your eating habits or lifestyle. 

If they’re on board and you’re raring to go, check out our Simple quiz, and we’ll help you get started! 

What happens to your body when you are fasting?

So, why bother fasting? What does it actually do for / to your body?

Well, when you eat in a regular way throughout the day, the food you eat provides energy for whatever you’re doing. 

When you fast, your body doesn’t get that ready energy supply coming in. So it has to draw on the energy it has stored. 

First, it’ll turn to glucose and burn through your glycogen stores. Then, it’ll switch to using your fat stores. This happens anywhere upwards of eight hours into your fast and is called ketosis. 

This is the metabolic switch that draws many people to try intermittent fasting.[1] Through a simple, regular back-and-forth between eating and fasting, we can prompt this fat-burning state to occur, which can have several benefits for our health. 

Is fasting an effective way to lose weight?

One of those benefits is weight loss.[2]

Plenty of studies have shown intermittent fasting is just as good as regular calorie-counting diets for weight loss — including the 2022 study, where both intermittent fasting and calorie-counting dieters lost weight (17 lbs and 14 lbs, respectively).[3]

Where intermittent fasting has the edge, you might find, is in the lack of calorie counting you have to do!

Another study with 101 participants looked at the effects of two specific intermittent fasting methods: 16:8 intermittent fasting and alternate-day fasting (ADF).[4]

At a three-month follow-up, those doing 16:8 had lost an average of 9.7 lbs, and the ADF-ers lost 32.6 lbs on average.

Clearly, the effect of any dietary change is an individual thing. What works for one person may not work for another. With intermittent fasting, the science suggests that it supports you by helping control your appetite, improve your hormonal profile in weight-loss-supportive ways, and naturally reduce your calorie intake by reducing the amount of time you have available for eating.[5] 

Likewise, even if certain studies suggest that one method of fasting may be more effective than another, it’s really about finding a method that works well for you. Everyone responds to fasting differently, and there’s no universal gold star approach. (Plus, remember: we don’t recommend more restrictive fasts like ADF since they tend to carry more potential risk.)

Ultimately, what matters most of all is this:

Is it a safe method for you, and can you stick to it?

If it is and you can, then it could be an effective way to lose weight. 

Benefits of intermittent fasting

One benefit of intermittent fasting is that it doesn’t slow down your metabolism.

But why else is fasting good for you? What other neat things can it do for your body, mind, and life? 

Let’s check out intermittent fasting’s top hive headline benefits.

1. Your risk of diabetes drops.

Intermittent fasting has been shown to help improve our body’s ability to manage blood sugar,[6] which means a lower risk of pre-diabetes and type 2 diabetes. 

Alternate-day fasting also has a positive impact on improving insulin sensitivity and blood sugar control.[7]

If you’re concerned about your HbA1c, intermittent fasting could help get it into a good range.[8] 

2. Your body fat goes down; your muscle sticks around.

Intermittent fasting can also help reduce body fat. 

In one study, following ADF for 3 to 12 weeks led to a body fat loss between 6.6 and 12 lbs.[9] (With intermittent fasting, this fat is often lost from the belly area, too.[10] Bonus!) 

Fat loss with intermittent fasting often goes hand in hand with preserving your muscle mass,[11,12] which boosts your overall metabolic health.[13] 

There are some studies that show muscle can be lost,[14] but these tend to involve multiple zero-calorie, 24-hour fasts [15] — the kind we don’t recommend at Simple. 

For maximum muscle-preserving benefits, pairing intermittent fasting with strength exercises and a balanced, protein-rich diet is a strong move.[11] 

3. Intermittent fasting can make you brainier.

It’s not just your body: intermittent fasting is good for your mind, too. 

Studies suggest that cognition (a fancy word for thinking) can be enhanced.[16]

Your brain’s neuroplasticity and stress resilience get a boost, and your risk of neurodegenerative and psychiatric disorders is reduced.[17] 

That’s kinda cool, right? You might be able to improve the condition of the pink blob of matter in your head simply by how you approach your eating.  

4. Your heart gets healthier.

Intermittent fasting is a heart helper for sure. 

Many studies provide evidence that intermittent fasting improves the body in all sorts of heart-healthy ways, some of which we’ve already talked about, like improved insulin sensitivity and lower body fat.[18]

But we also get reduced blood pressure, lower levels of inflammation, and better cholesterol levels in the mix with intermittent fasting.[19,20,21]  

Every one of these nudges your risk of heart disease down.[7] 

Not bad, intermittent fasting, not bad at all! 

5. You’ll have a happier gut .

The gut and the role it plays in your health is currently a hot topic, and there’s growing evidence to suggest that intermittent fasting could be a friend to your gut microbiome by:

• increasing levels of beneficial bacteria [22]
• increasing the diversity of your microbiome [23]

What does all that mean in real-life terms? 

Better metabolic health,[23] reduced risk of disease, a healthier heart,[22] reduced inflammation,[24] and all that kind of good stuff. 

When gut health benefits and intermittent fasting benefits combine — talk about a win-win! 

(We’ve laid out six more intermittent fasting benefits here if you’re keen to keep learning.)

Risks of intermittent fasting

We’ve covered some of the reasons why it’s good to fast. Now, let’s look at the dark side — intermittent fasting does have some risks. 

Hunger and friends

Understandably, when you first start a fasting schedule, one of the things you’ll experience is hunger. 

You’ll also be on a first-name basis with tiredness and grouchiness. Maybe a headache or two will drop by. You might not sleep as soundly for a little while. Your energy might tank. 

Nobody panic. All of this is temporary. It. Will. Pass. 

You could become dehydrated

When you’re fasting, you gotta get organized with your water intake. 

Without the water that comes from the food you eat (about 20% to 30% of your overall fluid intake), and as your body burns through its glycogen stores (which causes the loss of stored water, too), you run the risk of not getting enough.

So, make sure you’re drinking lots of water to avoid this. 

Your food choices might get a little … iffy

Sometimes fasting will kick off your hunger and cravings hard, you’ll hit toddler mode, and when that happens … yeesh. It gets a lot tougher to make good food choices. 

To get the most from intermittent fasting, you need to make good food choices more often than not, and fierce hunger and cravings can get in the way. You can beat it, though! It just takes practice. 

That said, some extra insights into how to fast well and build great habits around food could really help. Give our Simple quiz a go and hook yourself up with our app. We have lots of great, practical ideas on how to navigate challenges like this. 

Nutrient deficiencies (and constipation) can occur

When you restrict your eating to specific time windows or calorie counts, it can be easy to undereat. 

Not enough food equals not enough nutrients, which means you won’t feel or perform at your best. 

A specific side note here: not enough fiber equals not enough … umm, how to say this politely … Number Two time. 

There are lots of reasons to make sure you eat enough! 

Some unwanted habits might pop up

Intermittent fasting has an inevitable element of restriction. 

For some, food restrictions can provoke some challenging feelings, thoughts, and behaviors.[25]

Look after yourself as you fast and pay attention to these signals. If intermittent fasting doesn’t feel like a supportive approach for you, stop. 

Who should avoid intermittent fasting?

There are some people who need to avoid intermittent fasting. 

If you see yourself on the list below, don’t try intermittent fasting without your doctor’s permission and support:

• are pregnant, breastfeeding, or trying to conceive
• are prescribed medication (especially those that affect blood glucose or blood pressure levels or need to be taken with food)
• have a history of or are currently diagnosed with disordered eating
• are very active and/or have high daily energy demands
• are under 18 or 80 years old or older
• have a health condition, like diabetes or anemia
• have a body mass index (BMI) of less than 18.5

Is intermittent fasting right for you?

We’ve addressed the question of whether intermittent fasting is good for you. 

Here’s the next big one: is intermittent fasting right for you? 

It doesn’t matter how many benefits intermittent fasting has if it’s not something you can practically and sustainably do because none of the benefits persist if you don’t persist — meaning that once you stop intermittent fasting, your body fat, blood pressure, cholesterol, etc., will likely return to how they were before. 

Whichever intermittent fasting schedule you choose has to be something you can do long term. So, how will you know if intermittent fasting is right for you?

First, ask yourself: am I safe to try it? 

If yes, then how does it feel as you think about it? Does fasting every day / week feel doable? Could it fit into your lifestyle?

If yes — even if you’re not quite sure — try it out. Pick a schedule and get started. Our intermittent fasting for beginners guide will have you up and running in no time. 

If you’re not quite ready to jump in just yet, that’s OK. Let’s address the next juncture …

Which intermittent fasting schedule should you choose?! Let’s take a look at the options. 

Types of intermittent fasting

There are a number of possibilities, but once you whittle them down to what’s effective and safe, it’s not so overwhelming. 

We’ll guide you through it so you can see which fasting schedule might suit your lifestyle best. 

Time-restricted eating (TRE)

First up, we have …

[drum roll]

16:8 intermittent fasting!

This gives you a daily eating window of eight hours.  

The fasting period is 16 hours, and a lot of this happens overnight while you’re asleep. (That really helps with the hunger and irritability, honestly.)

If 16 hours sounds like a lot, you can get your intermittent fasting feet wet by starting with a shorter fast, like 14:10 intermittent fasting or even 12-hour intermittent fasting.

These fasting approaches are our most recommended schedules: They still give you plenty of time to fill up on nutrients, they’re easier to manage on a day-to-day and regular basis, and they typically carry less potential health risk when compared to a longer fast. 

The next five fasting schedules involve 18+ hours of either no eating or limited calorie intake. These schedules can be tricky to manage safely and sustainably, so we usually recommend less restrictive approaches. While we always think it’s a good idea to involve your healthcare team in any changes to your eating habits, if you’re going to give any of the following fasting methods a try, they should definitely be involved in the planning process.  

The Warrior Diet

On the Warrior Diet, the fasting window is longer: 20 hours.

This means your eating window is just four hours long! 

That could be a toughie for making sure you nourish your body properly (remember the nutrient deficiency risk from earlier?), so that’s why we only recommend it if your doctor has agreed it’s a good choice for you.

The 5:2 diet

The 5:2 diet is the one you might have heard of at the gym or around the office. It’s quite a popular way to practice intermittent fasting. 

With 5:2, you fast two days a week. On your fast days, you’ve got 500/600 calories to play with, so it’s not a strict, 0-kcal, 24-hour fast. 

The rest of the week, you eat as you wish. 

It may sound more manageable than daily fasting, but 500/600 calories still isn’t a lot. That’s why it’s essential to involve your healthcare team if you’re going to try it — they’ll help you develop a solid nutrition plan.    

Alternate-day fasting (ADF)

Alternate-day fasting (ADF) also has fast days that include 500/600 calories. 

However, with ADF, these fast days happen every other day. So on Monday, you eat as you like; on Tuesday, you fast. On Wednesday, you eat; on Thursday, you fast. And so on, like a relay race. 

Basically, it’s the 5:2 diet bonus package. That means even more concerns over nutrient deficiencies and potential health and well-being side effects — and, subsequently, more reasons why coordinating with your healthcare team is important for your safety.

Eat Stop Eat

Eat Stop Eat lets you eat one full meal on your fast day, and that’s it. You’re done on the food front till the same time the next day. There are two days like this each week. 

On the other five days, how much you eat and when is entirely your choice. 

We don’t like this approach because the fasting period is 24 hours and zero calories. (Remember: we don’t recommend fasting longer than 18 hours.) The risks of dehydration, nutrient deficiencies, hormonal disruption, and so on are far higher with a fast like this. 

Water fasting

Water fasting makes our “please don’t try this at home” list because it involves going 24–72 hours with no food and drinking only water. 

If you’re doing it with medical supervision, fine. Otherwise, it’s not a smart path to your goals. Any weight loss or health benefits you get from water fasting will be short-lived, and, again, the risks are pretty high. 

Written by Trey Reck

Medically reviewed by Josie Porter, RD

Published on | Updated on June 11, 2024