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    If losing body fat is your goal, slowing your metabolism is the last thing you want.

    Yet that very thing tends to happen when we cut calories to lose weight. (We know; sometimes it feels like the human body is quite obnoxious, but this is actually a primal survival response that makes sure we don’t conk out in times when food is scarce.)

    So now you’re wondering: Does intermittent fasting slow metabolism too?

    Ready to rev up your metabolism? Discover the truth about intermittent fasting and metabolism. No more slowing down, only forward progress!

    Good news! All the research points the other way. Intermittent fasting may actually speed up and improve your metabolism. 

    How does it do that? What magical processes are at work here?!?! 

    Let us take you on a SIMPLE journey of discovery. 

    Key takeaways

    • Intermittent fasting may speed up your metabolism.
    • Traditional calorie-counting can slow down your metabolism due to adaptive thermogenesis.
    • Intermittent fasting increases production of metabolism-boosting hormones.
    • Even short fasts have been shown to positively affect metabolic processes and deliver weight loss results.

    What exactly does metabolism mean?

    Metabolism is a complex beast but, very simply put, it’s the internal chemical reactions that convert the food you eat into the energy and substances your body needs to function. 

    Your resting metabolic rate (RMR) — also known as your basal metabolic rate (BMR) — is the amount of energy you need to function at rest. Your body is still pretty active even when you’re not and uses calories to do things like make your heart beat, keep you breathing, power your brain, and so on. 

    Essentially, your RMR is the amount of calories you need to stay alive. 

    If you have a low RMR, you’ll use fewer calories at rest than a similar-sized person with a higher RMR who eats the same amount, and from this, we can see how metabolic rate links to weight management. The lower your RMR, the fewer calories you burn doing anything, and the harder it is to lose weight. 

    The good news is, you can speed up our metabolic rate with both what and how you eat. 

    Cue stage left… intermittent fasting.

    How intermittent fasting affects your body at the metabolic level

    When you fast intermittently, many specific metabolic changes happen.[1] 

    For instance, your body finds alternative ways to create energy. 

    When you eat, your body breaks down the carbohydrates in your food into glucose. When you fast, and carbohydrate isn’t available, your body will make glucose from other, non-carbohydrate sources. 

    This process is called gluconeogenesis: When your body uses glycogen reserves in your liver and water stored within your tissue to create glucose for energy.[2]

    Once your glycogen stores are used up, your body turns to other things for fuel, such as your fat cells to burn ketones. 

    This process is called ketosis, and it’s one of the ways intermittent fasting creates adaptations in your metabolic processes. Regular calorie-counting diets can’t do this. 

    Intermittent fasting may increase your metabolic rate

    Where calorie restriction slows your metabolism (more on this later), short-term fasting with regular refeeds may have the opposite effect.

    Intermittent fasting can maintain your metabolic rate, as shown in a study where fat oxidation increased on fasting days and RMR stayed the same.[3] 

    And, it may increase your metabolic rate — at least in the short term — as in one study where metabolism was boosted by 14% by intermittent fasting.[4]

    Driving this effect was a hormone called norepinephrine. 

    One of norepinephrine’s jobs is to prevent hypoglycemia. To do so, it tells the liver to make more glucose. As we’ve just seen, this is gluconeogenesis, and it’s an energy-hungry process.[4] 

    Because it takes a lot of energy, gluconeogenesis raises RMR. 

    One of the cool things about metabolism is that, just as it responds to doing less by slowing down, it also responds to doing more — i.e., using more energy — by speeding up. (That’s why exercising is also a great way to speed up your metabolism. Just FYI.)

    Does intermittent fasting increase fat-burning hormones?

    Another part of intermittent fasting’s effect on metabolism is how it upregulates (a fancy word for increases) certain hormones. 

    Hormones affect nearly every process in your body, including your physical growth and development, emotions and mood, sleep, stress response, and so on. They are also big players in metabolism and have a significant role in weight management. They influence your desire to eat, what you eat, how you get energy from your food, your ability to build muscle and burn fat, and so on. 

    Intermittent fasting increases production of some key metabolism-boosting hormones, including insulin, human growth hormone (HGH), and norepinephrine. 


    Insulin is a hormone at the forefront of fat metabolism. It tells your body when to store fat and when to stop breaking it down. Studies show intermittent fasting can reduce fasting insulin levels, as well as increase your insulin sensitivity and reduce insulin resistance.[5,6] 

    This enhances your metabolism because when your insulin levels are regulated, your RMR increases, and your ability to burn fat improves.[7]

    Human growth hormone

    Human growth hormone (HGH) is critical for retaining muscle mass and promoting fat loss. 

    Scientists found a 24-hour fast can increase HGH production by 2,000 percent in men and 1,300 percent in women.[8] An increase in HGH can stimulate protein and carbohydrate metabolism, again helping to improve your RMR.[9] 


    Lastly, as we’ve already seen, there’s a link between intermittent fasting and an increase in norepinephrine, a stress hormone that helps your body’s fat cells to release fatty acids.[4] 

    Though more research is needed to confirm this, this could mean that as more norepinephrine circulates in your blood, more fat becomes available for your body to burn, which helps you to lose weight more effectively. 

    So, does intermittent fasting actually slow your metabolism? 

    From everything we’ve seen so far, it looks like the idea that intermittent fasting slows down metabolism is a false one. 

    The study from earlier that demonstrated a 14% uplift in metabolism was done on a 3-day fast. But, you can accomplish the same increase in norepinephrine with a milder fast. 

    Researchers have found that fasting for short periods, such as a 16:8 schedule, may increase your metabolism.[10] In fact, even 12-hour fasts have been shown to have a positive impact on metabolic processes and support weight loss.[11]

    That’s not so scary, huh! Why not give it a go yourself? Try our SIMPLE quiz and we’ll get you all set up. 

    Traditional diets vs. the intermittent fasting effect on metabolism

    Traditional calorie restricted diets have a profound slowing effect on metabolism. 

    When you eat less than you burn for a prolonged period of time, the calorie deficit you create means you lose weight. 

    BUT… the body then says, “you know, if we’re gonna do this low-cal thing long term, I gotta turn the engine down”.

    This is called adaptive thermogenesis, and it means you burn fewer calories at rest than you did before. That makes it harder to lose weight, because you now have to do more to burn the same amount of calories you used to burn doing less

    This drop in metabolic rate can be pretty long-lived, too. One study on The Biggest Loser participants found that their metabolisms were still running slower six years later [12], and another research found calorie restriction can decrease metabolism for good.[13] 


    On the flip side, a small group of men and women practiced intermittent fasting for over a year.[14] Fasting did not produce adaptive thermogenesis, and their metabolic rate at the end of that year was 10% higher than predicted. 

    Intermittent fasting works differently for two reasons. 

    1. Intermittent fasting is not a constant calorie restriction, but rather an alternation between fasting and eating “normally” (whatever normal is for you). 

    On the 5:2 diet, for instance, you have 2 days where you eat 500/600 calories. On the other days there’s no calorie restriction. 

    If you were to eat just 500/600 calories every day, your metabolism would certainly drop. (A good reason to not get carried away and fast more than your intermittent fasting schedule asks you to, even if you’re loving it and feeling great!)

    1. Intermittent fasting preserves more muscle. 

    From the research, it seems we lose less muscle when losing weight with intermittent fasting than with traditional dieting. 

    More muscle means a higher RMR, because the more muscle we have, the more calories we burn for every activity we do. 

    Intermittent fasting and strength training make a great partnership.[15] Maybe time to dust off that barbell / kettlebell / bodyweight circuit / [insert your favorite style of strength workout here]?

    Is it okay to skip meals?

    Yes, it is. So long as that works for you. 

    If you practice 16:8 intermittent fasting, the likelihood is you might skip either breakfast or dinner. There’s no problem there. 

    The key is to eat enough for your needs, so you have enough energy to live your life and your body gets all the nutrients it requires. 

    You can do that with 1 meal a day or 6+ meals a day — lots of meal structures have been shown to work. Whatever fits your lifestyle — and your chosen intermittent fasting schedule — best is just fine and dandy. 

    Why do people gain weight even while fasting?

    There’s a few reasons someone might gain weight while fasting. 

    First, it can be easy to overeat after a fast, especially when you’re new to it. 

    Eating windows can become overeating windows simply due to not being super-skilled at managing your hunger, and/or breaking your fast in a way that’s healthy.

    Both of these things can be improved over time. 

    Second, intermittent fasting isn’t a silver bullet for obliterating dodgy eating habits. We might fast like a champion and yet still have ice-cream in our coffee and deep dish pizza for breakfast on a stressful day. 

    When it’s time to eat, work on making food choices that are nourishing, give you good energy, and support your health. We’ve got some solid advice for what to eat during intermittent fasting — check it out if you could use some pointers.  

    And, if you’re new to intermittent fasting, we can help you to avoid some of these intermittent fasting mistakes (we all make ‘em).

    Is intermittent fasting for you?

    First things first: 

    For some, fasting is not a great choice. Don’t take up intermittent fasting — unless you have your doctor’s express permission and support — if you: 

    • have (or are at risk of having) an eating disorder, or have a history of one
    • take medications (especially those which affect blood glucose or blood pressure levels)
    • have a Body Mass Index (BMI) in the “underweight” category
    • are pregnant, breastfeeding, or trying to conceive
    • are under the age of 18, or 80 years old or more 
    • have a health condition, like diabetes or anemia
    • have nutritional deficiencies
    • have high daily physical activity demands

    OK, now we’ve covered that: If you’re clear to give it a go and you’re keen to pump up your metabolism as we’ve described, all you need to do now is choose your intermittent fasting schedule!

    (Not sure which one to choose? Take our SIMPLE quiz, and we can help you figure out the best plan for your unique body, preferences, and lifestyle.)

    Time-restricted feeding (TRF)

    Time-restricted feeding is fasting for a period of time, and eating within the time that’s left, every day. 

    With 16:8 intermittent fasting, for example, you’d fast for 16 hours and eat within 8 hours. With 14:10, you fast for 14 hours, eat within 10. With 12:12… you get the picture. 

    TRF is a SIMPLE, accessible option, so if you’re just starting out, we’d recommend you start here (and have a read of our intermittent fasting for beginners guide to get you feeling comfortable and confident as you do). 

    The warrior diet 

    The warrior diet is also in the TRF family. It’s a 20 hour fast with a 4 hour eating window. 

    If this feels like it could be right for you, start with a shorter fast and expand that over time rather than just dropping straight into the full 20:4. With a 20 hour fast, the hunger can be heavy and insistent, and it takes practice and skills to be able to squeeze all the nutrients you need into a 4 hour window. 

    Alternate day fasting (ADF) 

    With alternate day fasting, on day 1, you fast. Day 2, you eat. Day 3, you fast. And so on. 

    If ADF is your bag, we recommend you do a modified fast on the fasting days and eat 500/600 calories. That makes the hunger easier to cope with, and helps you get all the nutrition your body needs. 

    The 5:2 diet

    The 5:2 diet is a modified fast of eating only 500/600 calories for 2 days, and eating as you normally do on the other 5 days. Another good option for beginners, especially if you are keen to ultimately try ADF. 

    Simple’s expert opinion and final thoughts 

    If you’ve ever wondered “Will intermittent fasting slow down my metabolism?”, let us set your mind at rest. 

    All the research points to no. 

    If that clears a mental block for you and you’re ready to try intermittent fasting, why not get started today? Try out our quiz and we’ll help you get the ball rolling! 

    Frequently asked questions about intermittent fasting and metabolism

    How do you restart your metabolism after fasting? 

    It never stops, so you don’t need to restart your metabolism after fasting. To keep it stoked and working well while you’re following an intermittent fasting schedule, all you need to do is eat a plentiful, nutritious, healthy diet.

    What happens to metabolism during fasting? 

    What happens to metabolism during fasting is that the body turns to other resources for energy, stimulates greater amounts of various hormones, and our resting metabolic rate can increase.

    Does intermittent fasting interfere with metabolism? 

    From the research, it doesn’t seem that intermittent fasting interferes with metabolism, but rather makes it more efficient. 

    Does the 16:8 fast slow your metabolism? 

    Quite the contrary: Not only does 16:8 fasting not slow your metabolism, it may actually increase it for a short while![10]

    What are signs of slow metabolism? 

    Signs of a slow metabolism can be things like weight gain, low energy levels, feeling cold, finding it hard to lose weight, as well as other things like problems with skin, hair, and nails, having difficulty concentrating, headaches, low mood, and cravings. 

    Does intermittent fasting help with fat loss, or weight loss? 

    Fat loss vs weight loss are two different things. Weight loss is when you reduce your overall body weight, and fat loss is when you reduce your body fat percentage. When you lose fat, you’ll often lose weight at the same time, but not always! Intermittent fasting can help with both.

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