Intermittent Fasting and Strength Training: The Ultimate Guide

Intermittent fasting is a popular pattern of eating. Millions of people use it to lose weight, improve their health, simplify everyday life, and even live longer.

What is intermittent fasting? Does it work, is it safe, and how do you do it? And can you build muscle while doing it? 

If you’re looking for answers to those questions, you’ll find answers to them and many more in this article. 

Let’s get into it!

Key points:

  • Intermittent fasting is a pattern of eating involving alternating periods of eating with periods of fasting. It’s not so much about what you eat but when you eat.
  • There are several popular ways to do intermittent fasting, with none being inherently superior.
  • Intermittent fasting is a proven and effective way to lose weight.
  • Fasting regularly offers many health benefits, although most of these are attributed to weight loss.
  • You can maintain and build muscle while doing intermittent fasting, although the jury is out on whether it’s as effective as a traditional meal pattern.
  • Intermittent fasting is generally safe, although it might be best to consult your doctor if you’re pregnant or diabetic.

What Is Intermittent Fasting?

Intermittent fasting, or time-restricted feeding, is an eating plan where you alternate between fasting and eating on a set schedule of your choice. It’s more about when you eat rather than what you eat.

With a traditional eating pattern, you spread your meals out over the day. That’s not the case with intermittent fasting. Instead, you only eat during certain hours and fast during the others. The most popular way to practice intermittent fasting is to fast a certain number of hours every day. Another way to do it is by fasting completely some days of the week.

You’ve probably practiced a form of intermittent fasting all your life just by sleeping. Think of intermittent fasting as extending that fast. It’s not more advanced than that.

If you’ve eaten three square meals and a couple of snacks per day your entire life, you might balk at the idea of going without food for extended periods. However, eating every few hours is a pretty modern concept. Living as hunters and gatherers, our forefathers certainly couldn’t sit down to a planned breakfast, lunch, and dinner. Regular fasting is in our genes.

What Are the Different Types of Intermittent Fasting?

You can practice intermittent fasting in several ways. They all involve fasting and eating on a planned schedule, and they offer or at least claim to offer various benefits.

Time-Restricted Feeding

Time-restricted feeding is when you only eat during a pre-determined number of hours each day. Two popular examples are the 16:8 diet and the 20:4 diet. They involve fasting for 16 or 20 continuous hours, respectively, and limiting your “eating window” to 8 or 4 hours. Fasting for 16 hours doesn’t feel as long as it sounds, as you might be sleeping for half of them.

A September 2021 review concludes that time-restricted feeding is “a promising dietary approach to improving body composition and metabolic health while maintaining fitness and muscular function”.1

Alternate-Day Fasting

This method of intermittent fasting is just what it sounds like: eating every other day and fasting every other day. Alternate-day fasting is a strict form of intermittent fasting because you’ll only be eating three out of every seven days. 

There are two forms of alternate-day fasting. One where you don’t eat anything during your fasting days, and one called modified alternate-day fasting. 

Modified alternate-day fasting means you can eat a small meal of up to 25% of your caloric needs on your “fasting” days. That makes the diet much more palatable for most.

Researchers have looked at alternate-day fasting extensively.

A 2016 review found that alternate-day fasting is at least as effective as a low-calorie diet.2 Studies lasting up to 12 weeks show that alternate-day fasting leads to a weight loss of 3–12% of your body weight during that time.3 4 5 

Another study showed that alternate-day fasting does not slow down your metabolism, a common problem with regular weight-loss diets.6

In addition, the 2016 review found that alternate-day fasting helps maintain your muscle mass. A recent study raised some questions over that conclusion. The subjects lost more muscle compared to the control group eating a regular low-calorie diet.7 This study did not involve strength training, however. That’s very important to keep in mind. Lifting weights is the best way to keep your muscles during a diet.

So, the jury’s still out. Alternate-day fasting is effective for weight and fat loss, but how muscle-sparing it is remains unanswered.

Periodic Fasting

Periodic fasting is a more extreme form of intermittent fasting. It involves a longer fast, at least 48 hours, sometimes with water being the only thing you put in your mouth.8

This kind of intermittent fasting is probably something you do for potential health benefits rather than athletic performance and muscle growth.

The 5:2 Diet

The 5:2 diet is a popular method of fasting where you eat your regular diet for five days per week, then fast or consume at most 5–700 calories for two days.

Almost no scientific studies look at the 5:2 method specifically. That doesn’t mean it’s no good. It likely has similar benefits as other types of fasting.

One study using the 5:2 protocol supports that assumption.9 The 5:2 diet was just as effective as a continuous calorie restriction, improving insulin sensitivity and other health markers, and leading to weight loss.

Eat Stop Eat

Eat Stop Eat is another way to do intermittent fasting, and probably the most lenient one. You fast for 24 hours once or twice per week and eat your regular foods on the other days. It is also the least researched of the popular types of intermittent fasting.

There are no reasons why the Eat Stop Eat wouldn’t work through the same mechanisms as other intermittent fasting protocols, but again, no scientific research looks at it specifically.

The Warrior Diet

One of the oldest commercial intermittent fasting methods, The Warrior Diet, has you fasting or eating very sparingly for 20 hours during the day. Upon getting up in the morning, you eat as little as possible until dinnertime, and then it’s anything goes for four hours. You’re encouraged not to overeat on junk food, but to focus primarily on unprocessed foods, though.

Supposedly based on the eating habits of ancient warriors, the Warrior Diet claims to mimic our pre-programmed genetic “survival of the fittest” evolutionary traits.

Science does not necessarily agree with those theories. There is no scientific research on the Warrior Diet specifically, although it’s close enough to other types of intermittent fasting that the principles probably overlap.

The Warrior Diet is stricter than most variants of intermittent fasting and might not be for everyone. Also, nothing suggests that it offers any advantages over other types of time-restricted eating.

A Simplified Lifestyle?

Everyone wants more time to do stuff they want, and cooking and cleaning don’t always top that list.

With intermittent fasting, you don’t have to cook, prepare, and clean up after many meals all day. You also get more uninterrupted time to do other things than eat. It could also make it easier to stay away from tempting snacks. You have a set eating schedule, and you eat when your clock tells you to.

Many people appreciate the freedom and extra time they get from intermittent fasting.

However, that freedom can also be something of a restraint.

Intermittent fasting makes it harder to have impromptu social interactions involving food. Everyone else partakes of the goodies, but not you. “No thanks, it’s two hours until my eating window” can make for awkward situations if you care about things like that. 

Of course, you can make an exception. But one exception leads to another, and then you’re in a downward trend. 

Intermittent fasting can be a fantastic way to simplify your everyday life, giving you more time for things not related to food and eating.

The strict nature of alternating between set periods of fasting and eating can also feel restrictive for some.

It all depends on your personality, schedule, and lifestyle. If intermittent fasting improves on those, it’s a great way to make your life easier.

What Happens in Your Body When You Fast?

When you don’t eat anything, your body quickly responds by adjusting hormone levels and turning on various metabolic switches to adapt to starvation. “Starvation” sounds like something terrible, but fasting is a form of starvation, even if it’s intended and beneficial. Your body doesn’t know why you’re not giving it any food.

In the Short Term

  • Increased ketones: ketone bodies are fuel for your body and brain. They also regulate molecules and proteins that involve health and aging.10 When you fast, your levels of ketones increase, and you switch from using glucose to ketone bodies and fat as fuel.
  • Increased autophagy: autophagy is your body’s natural way of cleaning out damaged cells, making room for fresh, new ones. Autophagy protects you against infections and neurodegenerative diseases like Alzheimer’s.11 Fasting helps your body repair itself.
  • Decreased insulin: intermittent fasting can lower your insulin levels and improve your insulin sensitivity.12 13 Having a high insulin sensitivity means your pancreas doesn’t have to release as much insulin for your cells to absorb blood sugar when you eat something. Low insulin sensitivity can lead to insulin resistance, which is a significant risk factor for diabetes.
  • Increased growth hormone: when you fast, your levels of growth hormone rise a lot.14 15 Human growth hormone, contrary to what you might think given its name, doesn’t have much to do with muscle growth in adults.16 However, it stimulates the breakdown of fat in your fat cells, especially during fasting and exercise.17

In the Long Term

  • Improved cardiovascular health: intermittent fasting improves many markers of cardiovascular health, including blood pressure, cholesterol, blood lipids, blood sugar, and insulin levels.18
  • Healthy gut microbiota: intermittent fasting might benefit your gut microbiota, improving composition and diversity.19 Unfortunately, human research is still in its infancy stage, with most studies having rodent participants. The available research is promising, but there is more work to be done.
  • Decreased inflammation: several studies show that intermittent fasting improves inflammation and oxidative stress markers associated with atherosclerosis, the cause of stroke and myocardial infarction.20 21 22

Most of these long-term benefits come from the weight loss many people experience when doing intermittent fasting. You’d get them any diet that helps you lose weight. Most, but not all. Recent research shows that intermittent fasting improves health even without weight loss.23

Intermittent Fasting for Weight Loss

Is intermittent fasting an effective way to lose weight?

Absolutely.

Is it more effective than other regular calorie-restriction?

Probably not. 

Fasting might offer a few unique health benefits, but most people use intermittent fasting to lose weight. In a 1999 survey, some 14% of Americans reported using fasting to lose weight.24 Since then, fasting and intermittent fasting have become increasingly popular. The number is likely higher today.

To lose weight and body fat, you must burn more calories than you eat.25 Every single diet that results in weight loss does so by creating a negative calorie balance: you eat fewer calories than you expend.

Many weight loss methods do so by changing your behavior.26 You lose weight simply by eating less, but keeping it off requires a lifestyle change. If you go back to your old habits, you’ll eventually regain the weight. 

Intermittent fasting is such a method.

When you limit your eating to only a few hours a day, or a couple of days a week, you make it harder for yourself to overeat. 

Most people trying to lose weight do not keep exact track of their calorie intake.

Let’s face it, weighing everything you eat and counting calories is a hassle. Not to mention trying to track how many calories you burn.

Intermittent fasting can help you lose weight automatically without that hassle. You’d have to cram the food down during your eating window to compensate for the long periods of fasting. Some might do so, but most don’t. Intermittent fasting works without calorie-counting because most people don’t compensate for the calorie deficit during fasting.27

The weight loss is still the result of a calorie deficit, of course, but intermittent fasting makes it happen without any effort from you.

Intermittent fasting offers a couple of hormonal advantages compared to regular dieting.

As we discussed earlier, fasting increases your growth hormones levels and decreases your insulin levels.

In addition to this, you also release a hormone called norepinephrine when you don’t eat.

Norepinephrine is a stress hormone, but it’s also a fat-burning hormone. It releases fatty acids from your fat cells into your blood. You can then use them as fuel, which might even help you lose body fat.28

Also, those higher norepinephrine levels increase your resting metabolic rate.29 30 In other words, you burn more calories per hour, even without exercise, with intermittent fasting.

Which Method is Best – Intermittent Fasting or Regular Calorie Restriction?

The method you like the best!

It’s as simple as that. If you prefer eating your meals spread out over the day, that’s your way to go. If intermittent fasting fits your schedule and allows you to stick to your diet better, go for it.

Regular calorie restriction and intermittent fasting are equally effective for weight and fat loss. Overall, the hormonal benefits you get from fasting don’t seem to sway the outcome over time.31 32 33

Intermittent fasting might help you keep more of your hard-earned muscle mass while losing weight. That’s a huge benefit. The keyword here is might, though. We don’t have enough evidence yet to say for sure. In any case, nothing suggests that regular fasting makes you lose more muscle than traditional dieting. If anything, it’s the opposite.

One significant benefit of intermittent fasting is that you feel less hungry than when you’re on a regular diet.34 Or at least you might be able to handle it better. Instead of feeling hungry all the time, you’re able to fill your stomach during your eating window and still lose weight.

In summary, intermittent fasting is an effective way to lose weight. While it might not be more effective than simply eating less, it helps you do so without counting calories. 

If you don’t enjoy stuffing yourself for a few hours or if you just feel better spreading your meals out over the day, you’re likely better off sticking to a traditional weight-loss method.

However, if you enjoy feeling full regularly while still losing weight, and if it’s the better option for your schedule, intermittent fasting is a good choice for you.

Read more: How to Cut: Lose Fat and Keep Your Muscle Mass

Intermittent Fasting and Lifting Weights

Intermittent fasting offers some exciting health benefits and helps you lose weight.

But what about building muscle? Can you get bigger and stronger while practicing intermittent fasting?

After all, going without food for extended periods can’t be good for your muscles. Or can it?

Contrary to popular belief, intermittent fasting does not eat away at your muscles, at least not if you lift weights. Research shows that intermittent fasting, when combined with strength training, maintains your muscle mass.35 36 

However, only one study reports meaningful gains in muscle mass from strength training while doing intermittent fasting.37 Interestingly, the participants in that study ate more than usual while eating in a time-restricted manner, which made them gain weight.

Another study found that while young men doing intermittent fasting maintained their muscle mass while lifting weights, the control group who didn’t fast gained a lot of muscle from the same training program.38

Intermittent fasting often makes you eat less. Intermittent fasting helps you lose weight without counting calories, but that might be a curse if you’re trying to gain weight and muscle mass. If you want to build muscle, you probably need to keep track of your calorie intake and make sure you eat enough to do so.

Unfortunately, the research to date can’t answer if intermittent fasting is harmful to your gains. The studies are too short, often lasting no more than 4–8 weeks, and they don’t make sure the subjects eat enough.

In a real-world setting, though, we see plenty of athletes with very impressive physiques and large amounts of muscle who practice intermittent fasting.

In theory, regularly fasting for extended periods is not optimal for building muscle.

Low insulin levels while fasting means increased muscle breakdown. Insulin decreases muscle protein breakdown.39 You can’t compensate for that during your eating window either. You only require very modest increases in your insulin levels for a maximal decrease in muscle breakdown. Twenty to thirty grams of carbohydrates will do the trick.

Because intermittent fasting means a more extended fasting period than the time you spend eating, your 24-hour muscle protein breakdown is likely higher compared to eating 3–5 meals during the day.

Muscle protein synthesis over 24 hours is also likely lower with intermittent fasting.

Why? Because you can only use so much protein per meal for muscle-building purposes. You absorb all the protein you eat, but muscle protein synthesis maxes out at around 30–40 grams from a single meal. It then stays elevated for up to six hours, and you can’t stimulate it again by eating even more protein for at least three of those.40

Eating a moderate-sized protein-rich meal every 3–4 hours likely leads to an overall greater muscle protein synthesis than loading up on vast amounts of protein during a few hours.

In summary, you can at the very least maintain your muscle mass while doing intermittent fasting. That’s probably not good enough for most serious lifters, though.

Unfortunately, science does not offer any good answers this time. Intermittent fasting might not be the best if your goal is to build muscle and get stronger. A recent scientific review suggests that we need more research before we can fully recommend time-restricted feeding for athletes involved in sports that demand strength and power.41

However, before you despair, keep in mind that most studies so far are kind of trash or based on the Ramadan fasting protocol, which might not be applicable to everyone’s training and eating schedule.

Most importantly, you need to ensure you eat enough calories and protein to support the gains you want. That could mean cramming a lot of food into your eating window and keeping track to make sure it’s enough.

While we’re talking about eating, let’s look at it more closely.

Eating for Muscle Growth While Doing Intermittent Fasting

Eating to build muscle is both easy and tricky when you’re doing intermittent fasting.

Your diet should look just like when you’re trying to build muscle on any other kind of diet. That’s the easy part.

You need to eat your entire day’s worth of food in only a few hours. That’s the potentially tricky part.

On a 20:4 diet, you only have four hours to eat everything you’re going to eat that day. That could be an unpleasant challenge if you need, say, 4,000 calories per day.

  • While building muscle during a calorie deficit is possible, aiming for at least calorie balance makes it much more manageable. A slight surplus gives you an additional boost. It’s easy to undereat while doing intermittent fasting, so make sure you eat enough.
  • Eat at least 1.6–1.7 grams of protein per kilogram of body weight per day. Aim for 2–2.2 grams per kilogram to be 100% sure you’re getting enough protein.
  • Distribute your carbs and fats according to your personal preferences. Research shows that you can gain muscle mass with almost any diet, from high-carbohydrate ones to ketogenic diets without any carbs at all.
  • Eat a balanced diet with enough vitamins and minerals. You don’t have to eat 100% “clean,” but don’t overindulge in refined junk food either. Make unrefined and healthy foods the staple of your diet, but don’t forget to treat yourself occasionally.

Read more: Eating for Muscle Growth

There you have it! No distinct differences compared to any other diet plan for building muscle. As I said at the beginning of the article, intermittent fasting is more about when you eat rather than what you eat. That goes for gaining muscle as well.

When Is the Best Time To Work Out When You Do Intermittent Fasting?

You might be wondering when’s the best time to work out when you’re practicing intermittent fasting.

The answer is anytime during your eating window or right before it.

Why? Because your muscles rely on protein to grow.

It all comes down to your muscle protein balance. In the fasted state, you’re constantly losing a little muscle. Your body tries to prevent this from happening by reducing breakdown, but it can’t quite make it. 

In the fasted state, your muscle protein breakdown is greater than your muscle protein synthesis.

Enter strength training. 

Lifting weights increases your muscle protein breakdown. That’s a good thing, not something you should try to avoid.42 43 44 45

Strength training also stimulates your muscle protein synthesis, more so than it does breakdown. 

However, without amino acids, i.e., protein, your muscle protein synthesis can’t become greater than your muscle breakdown. Once you eat or drink some form of protein, your muscle protein synthesis skyrockets, and you start building muscle.

protein after training

In other words, after a fasted workout, you lose less muscle than you do fasting and resting. But you don’t start gaining muscle until you eat.46 47

Working out during the hours after a protein-rich meal is also fine, even if you don’t eat anything afterward.48 49 Your muscles can use amino acids from that meal after the workout to repair themselves.

In practical terms, train when your muscles have access to protein after the workout. Without it, they won’t grow. That means sometime during your feeding window or when you’re about to end your fasting period. Training fasted is no problem (as long as you feel ok and can perform adequately) as long as you eat afterward.

The worst time to hit the weight is in the middle of your fasting period. Your muscles can no longer use the protein from the last meal you ate, and you’re not providing them with any building materials after the workout.

Fasting both before and after training means that you’ll lose a little muscle until you finally break your fast.

You probably don’t have to worry too much about that, though. Your muscles are more sensitive to protein for up to 24 hours after a workout.50 That means that every time you eat protein during the 24 hours following a workout, you build more muscle compared to a day without training. 

There is no immediate hurry to chug a protein shake immediately after a workout and no known benefits to time your protein intake to the hour before or after training.51

However, the available studies don’t take intermittent fasting into account. If you don’t eat for hours before working out and wait several hours more afterward before eating, you might be losing out on gains.

A rule of thumb is not to separate your pre-workout and post-workout meals by more than 3–4 hours. If your meals are large, which they likely are if you’re into intermittent fasting, separating your last meal before your workout and your post-workout meal by 5–6 hours is ok.

Working out in the middle of a 16–20 hour fast is probably stretching it. You’re only giving your muscles the nutrients they need to grow for a few hours, and your body can probably only compensate for so much.

In summary: working out right before or during your eating window is the best time if you want to give your muscles the best chances to grow. If you train sometime during your feeding hours, let your schedule and how you feel guide you.

And, if you’re doing alternate-day fasting, it’s probably a good idea to schedule your heavy, muscle-building workouts for your eating days. That way, you ensure you’re not starving your muscles of nutrients when they need them the most. Keep active on your fasting days, but maybe leave the hard-core workouts for the days you eat if building muscle is a priority.

In any case, you don’t have to worry about muscle loss while doing intermittent fasting, as long as you hit the weights regularly.

Safety

Humans have fasted since forever. There is nothing dangerous about going without food for a while if you’re healthy and well-nourished.

That being said, there are side effects to intermittent fasting even if you are otherwise healthy-

Fortunately, these are not dangerous, although they can be unpleasant both for you and the people around you.

The most common adverse side effects of intermittent fasting are hunger, irritability, and impaired cognition.52 In most cases, these aren’t specific to intermittent fasting in most case. Any weight-loss diet makes you hungry. 

Intermittent fasting could be detrimental or even harmful for some people:

  • Pregnant or lactating women. You need the food for energy and milk production.
  • Children and adolescents. During natural periods of growth, fasting is probably not a good idea.
  • Older individuals. Appetite often decreases during aging, and many older adults don’t get enough nutrients. Fasting makes it even harder to get enough energy and vital nutrients.
  • Underweight people. If you’re already underweight, going on a diet that promotes weight loss sounds like a bad idea.
  • Persons with or susceptible to eating disorders. Intermittent fasting is associated with eating disorder symptoms.53 If you have an eating disorder or a history of one, it might be hard to draw the line between healthy intermittent fasting and an unhealthy eating disorder.

Diabetes

If you have diabetes type 2, intermittent fasting can be beneficial. Research shows that intermittent fasting is an effective treatment for diabetes type 2.54 However, you should consult with your doctor before doing intermittent fasting, especially if you take insulin. 

Intermittent fasting with diabetes type 1 can be safe but requires more careful monitoring.55 Consult with your physician before doing intermittent fasting if you have diabetes type 1.

In summary, healthy, normal-weight adults can safely practice intermittent fasting. It’s not for everyone, though, if you’re in one of the categories mentioned above. If you have a medical condition involving blood sugar control, it’s always best to consult your doctor first.

Frequently Asked Questions

Do BCAAs or EAAs Break Intermittent Fasting?

Yes, they do! Everything with calories breaks your fast. If you look at your container of BCAA and see 0 calories, it’s a lie. Supplement manufacturers get away with it because of an old loophole in FDA regulations which states “protein shall not be declared on labels of products that… contain only individual amino acids”.56 In the real world, BCAAs give you 4.65 kcals per gram.57

Read more: BCAA vs. EAA: Which Is Better For Your Gains?

Is It Ok to Lift Weights on an Empty Stomach?

Yes! As long as you eat after the workout, you’re good to go. You don’t build muscle during your training session. Muscle protein synthesis only kicks in 60-90 minutes afterward.58 If you eat some protein after your fasted workout, your muscles have everything they need when protein synthesis takes off. 

Some people feel like crap or can’t perform their best when exercising intensely on an empty stomach. If that’s you, don’t force yourself. But there is nothing inherently wrong with working out on an empty stomach.

Can I Do Cardio While Fasting?

Absolutely! 

Not only can you, but your performance might thank you for it in the long run. 

A concept called “train low, compete high” suggests you might improve your exercise performance by training without many carbs in the system. You teach your body to use fat as fuel more efficiently. You should see a performance improvement when you load up on carbs for an important race or cardio session. In theory, at least.59 60 Current research has no definitive answers, and some scientists advise against high-intensity cardio on an empty stomach.61

If you enjoy fasted cardio, do it. It won’t harm you. However, you might want to stay away from long, high-intensity workouts if you haven’t eaten anything if your goal is to build muscle. It might nibble away at your muscles if you overdo it.

Does Exercising While Fasting Burn More Fat?

Yes, it does.62 You burn more fat if you exercise on an empty stomach. 

Don’t get too excited, though. Unfortunately, no evidence shows that fasted exercise makes you lose more body fat.63 64

Your body compensates for the increase in fat oxidation by storing more fat or burning less fat during the day. The result? Probably no difference compared to exercising during your eating window or soon after.

Some speculate that fasted cardio makes a difference if you’re already in great shape and trying to lose the last bodyfat to get shredded. No studies confirm this, though.

Don’t force yourself to do fasted cardio to lose body fat if you don’t like it. Of course, it could also be an essential time-saver if you only have a specific time slot where you can squeeze some cardio in.

Does Creatine Break My Fast?

Most likely not. No studies look at that question specifically, but creatine is calorie-free and does not trigger an insulin response.65 However, considering that your creatine uptake is better if you take it with some protein or carbs, you might want to save it for your eating window anyway.66

Read more: Creatine: Effects, Benefits and Safety

What Should You Eat After Fasting?

After a prolonged fast, it’s probably best to gradually reintroduce food over several days instead of overloading your stomach with a hefty meal right away.

However, if you’re doing intermittent fasting where you don’t go without food for longer than a day, you don’t have to overthink it. Eating a regular meal is fine. Just make sure you include protein when you break your fast. You want to give your muscles what they want as soon as possible.

Does Skipping A Day of Intermittent Fasting Ruin Your Results?

No. No matter what kind of results we’re talking about. That day is just a non-fasting day, nothing more. It doesn’t have any lingering effects.

That said, consistency is the key to many things. If you use intermittent fasting for weight loss, skipping a day might lead to more missed days, and so on. But if it’s just a day now and then, don’t worry about it.

What Can You Drink While Fasting?

By definition, fasting means you fast. You don’t eat or drink anything with calories. Anything that causes an increase in blood sugar or insulin kicks you out of your fast. The physiological relevance of minor insulin release is debatable, but technically, you’re not fasting anymore.

Let’s talk about what you can drink during your fast without breaking it.

Water. Water is calorie-free and does not break your fast. Plain or carbonated, drink as much water as you want. Flavoring it with a slice of lemon or cucumber is likely also fine.

Diet soda. Technically, diet soda contains no calories. Artificial sweeteners like aspartame and acesulfame-k do contain calories. Still, you need so little of them to sweeten a beverage that the calories become non-significant, very close to zero. Also, they don’t trigger an insulin response.67 68 69 In other words, drinking diet soda does not break your fast.

Read more: Artificial and Non-sugar Sweeteners: the Good, the Bad, and the Science

Coffee and tea. Coffee contains very few calories (it’s not entirely calorie-free, but we’re only talking a couple of calories per cup) and does not kick you out of a fast. As long as you take it black, of course. Adding sugar, milk, or cream is a no-no during a fast. Animal studies suggest coffee can boost the benefits of fasting.70 As for tea, stick with unsweetened black, white, or green tea, and you’re good to add. Adding calories in the form of sugar or milk breaks your fast, just like with coffee.

Read more: Caffeine: Effects, Benefits, and Safety

Beverages like milk, milk substitutes, any alcoholic beverage, fruit juice, sugar-sweetened sodas will break your fast. They are all more or less high-calorie drinks. Save them for your eating window.

Does Intermittent Fasting Slow Down Your Metabolism?

Contrary to common myths, probably not. Meal frequency does not impact how many calories you burn.71 A calorie deficit, eating fewer calories than you expend over an extended time, slows down your metabolic rate.72 However, intermittent fasting might not, at least throughout a couple of months.73 A short-term fast, up to three days, boosts your metabolic rate quite a bit.74

Unfortunately, no real long-term studies compare intermittent fasting with continuous calorie restriction. Weight loss always lowers your metabolic rate a bit, but nothing suggests that intermittent fasting is worse than regular dieting. If anything, research indicates the opposite.

Summary

Often portrayed as a “fad diet” in media and even scientific journals, intermittent fasting has, over the last decades, become more and more popular.75

Recent research shows that intermittent fasting is effective for weight loss and that it might even offer unique health benefits and help you live longer. While long-term human studies are sparse, it’s a promising field of research.

In real-world scenarios, intermittent fasting is as effective as regular dieting to lose weight and body fat. You can even maintain and build muscle when doing intermittent fasting.

Intermittent fasting is more than a diet. It’s a valuable and usable pattern of eating that can simplify your life. But only if it suits you. Some people don’t feel good on a fasting regimen. Some can’t take the idea of fasting.

If intermittent fasting is not for you, you can get all the potential benefits from other healthy lifestyle choices. But if you find it sustainable and unproblematic, you have a flexible way of eating you can tailor to your every need.

The only way to know if intermittent fasting is for you is to try it.

Further Reading

If you liked this article, check out our other articles about protein and eating for performance and health. Almost everything in those apply to intermittent fasting as well.

References

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Andreas Abelsson

Andreas has over 30 years of training experience and is a highly appreciated writer and educator on exercise, fitness, and nutrition. Few people stay more up to date and have a better grasp of the field of exercise science than Andreas.