Intermittent fasting is a popular pattern of eating. Millions of people use it to lose weight, improve their overall 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.
• An intermittent fasting diet 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 schedule of your choice. It’s more about when you eat rather than what you eat. Some fast for religious reasons, others for weight loss, for health reasons, or for the convenience of a lower meal frequency.

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 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 20:4 diet and the 16:8 diet. They involve fasting for 20 or 16 continuous hours and limiting your “eating window” to a four- or eight-hour window. Sixteen hours of fasting doesn’t feel as long as it sounds, as you might be sleeping for half of them, making for a relatively short fast during daylight hours.

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 intermittent fasting protocol 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. 

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

In addition, the 2016 review found that alternate-day fasting helps maintain lean muscle mass. A recent study raised some questions about that conclusion. The subjects lost more muscle compared to the control group eating a regular low-calorie diet. 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 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.

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. 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, have 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. It’s one of the main reasons many practice intermittent fasting. 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 in 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 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. 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. Fasting helps your body repair itself.
• Decreased insulin: intermittent fasting can lower your insulin levels and improve your insulin sensitivity. 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. Human growth hormone, contrary to what you might think given its name, doesn’t have much to do with muscle growth in adults. However, it stimulates the breakdown of fat in your fat cells, especially during fasting and exercise.

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.
• Healthy gut microbiota: intermittent fasting might benefit your gut microbiota, improving composition and diversity. 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.

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

Intermittent Fasting for Weight Loss

Is intermittent fasting an effective way to lose weight?

Absolutely.

Is it more effective than other regular calorie restrictions?

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. 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. 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. 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 caloric 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 the fasting periods.

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 hormone 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.

Also, those higher norepinephrine levels increase your resting metabolic rate. 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. For example, a recent one-year-long study found that participants who ate their meals at a certain time lost similar amounts of weight and body mass as those who ate at any time.

For pure weight loss, calories trump everything.

Intermittent fasting might help you keep more of your hard-earned lean 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. 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.

Lifting Weights While Fasting

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 resistance training, maintains your muscle mass.

However, only one study reports meaningful gains in muscle mass from strength training while doing intermittent fasting. 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.

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 muscle gain.

Low insulin levels while fasting means increased muscle breakdown. Insulin decreases muscle protein breakdown. 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.

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 might not be good enough for most serious lifters and bodybuilders, though.

Unfortunately, science does not offer any good answers this time. Intermittent fasting is likely a great option during a cutting diet, but perhaps not the ideal one when the primary goal is adding as much muscle mass as possible. 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.

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 a high number of calories every day. With 16:8 and other more lenient fasting protocols, it shouldn’t be that big of a deal.

• While building muscle during a calorie deficit is possible, aiming for at least a 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.

Source: https://www.strengthlog.com/blog