People have been fasting for weight loss since before losing weight was even a thing. Up until the invention of agriculture and supermarkets, all human beings went through periods of food scarcity that forced their bodies to use stored fat for energy – it was just normal life. So early, people didn’t care about the weight loss per sé. But they depended on burning fat through fasting so they could thrive even when food was in short supply.

Only in very recent history has ‘round-the-clock eating been possible, let alone viewed as the standard for healthy eating.

But today, after decades of misinformation on calorie restriction and six-meal-a-day fad diets, there’s finally enough research to prove that fasting is the most effective method for weight loss. And we’re waking up to the truth that we don’t need to spend thousands of dollars on pills and diets in order to maintain a healthy weight.

Let’s break down the science of fasting weight loss, and explore fasting weight loss methods that can works for you!

Why people fast for weight loss

Most everyone who’s lost weight from fasting has begun their fasting journey out of frustration. We get frustrated at always feeling hungry; we get frustrated at reading a thousand weight-loss books that all have contradicting information; we get frustrated by all the different diets that have failed us. And after a certain point, that frustration leads us to do something we consider extreme:

To start skipping a meal or not eat for a period of time.

Never mind the fact that fasting is so much a part of human history and culture that it’s embedded in our very DNA. To all of us who’ve been conditioned to think of missing a meal as a metabolic sin – not eating will destroy your metabolism!!! – fasting seems pretty radical, indeed.

But then when the first hunger wave passes, you realize that hunger isn’t a permanent thing. As the pounds continue to melt, you notice that fasting doesn’t feel so bad after all. And eventually, you get to a point where your greatest happiness, productivity, and clarity comes during times of fasting – much like the ancient philosophers and warriors who used fasting as a performance enhancer.

When you find that fasting not only helps you lose weight but also helps you become your best self, that’s when you realize that all those years of frustration were worth it. It made you try something you thought was impossible.

The science behind fasting and weight loss

People have a knee-jerk reaction when it comes to fasting. When they hear about fasting for weight loss, they automatically think Starvation! Muscle loss! Destroyed metabolism! But there is absolutely no scientific evidence to back this up.

In reality, it’s calorie restriction – not fasting – that decreases metabolism.

During a talk on fasting for weight loss, Dr. Jason Fung, author of The Complete Guide to Fasting, referenced the ‘Biggest Loser’ rebound phenomenon. He said that most winners on the popular reality show end up gaining the weight back because of a metabolic slowdown.

In a study done on 14 contestants after a season of The Biggest Loser, levels of the satiety hormone ‘leptin’ plummeted after calorie restriction. This causes insatiable hunger and overeating. Also, the majority of participants ended up with rebound weight gains that brought them back to their starting weight or higher.

This doesn’t happen with fasting!

How fasting helps you burn fat without metabolic setbacks

When fasting, you have plenty of fat to burn as fuel. The average person can safely fast from one week to several months without deleterious health effects. And because the body doesn’t sense scarcity (plenty of fat  plenty of energy), your metabolism stays high.

But during periods of calorie restriction, the constant intake of food prevents your body from fully entering fat-burning mode, so it interprets this long-term food shortage as a crisis and drops its metabolic rate to compensate. This leads not only to sub-optimal performance for your entire body – think how a car would perform if its battery were at 60% – but it also leads to crazy-intense food cravings and rebound weight gain.

If your baseline energy expenditure is 2000 calories, and it drops down to 1200 calories through weeks and months of calorie restriction, that means that you’ll immediately gain weight even if you eat a reduced-calorie diet of 1,500 or 1,700 calories! And that weight gain will stop only once your metabolic rate has returned to normal after enough regular eating.

This issue of reduced metabolism does not happen when you’re fasting for weight loss. Because the body has plenty of fat and diseased/inefficient tissue to burn while fasting, there is no scarcity. Baseline energy expenditure does not decrease during a fast. In fact, it increases.

Scientists believe this fasting phenomenon is the work of evolution.

Since our ancestors didn’t have refrigerators and grocery stores, they frequently went without food. Sometimes they would go weeks and weeks in between big kills, subsisting off of nothing, or an extremely reduced-calorie diet at most. (Studies show that extremely reduced-calorie diets do not negatively impact metabolism over the short term, and can ‘mimic’ the effects of fasting.)

Now, if our metabolism decreased during periods of not having food, that would mean our mental and athletic abilities would have decreased so much that getting the next big meal would have been impossible. (Scientists argue we would have gone, ummm, extinct if this were the case!) But the fact is, our ancestors did manage to find the next meal. We’re living proof.

This all suggests that we’re able to burn fat efficiently during a fast without all the metabolic setbacks and rebound weight gain associated with calorie restriction.

To be fair, if you eat an extreme amount of calories and junk food after a fast, and if you don’t get plenty of movement, you’ll be likelier to regain the weight you lost. So adopting a fasting habit is not a substitute for eating healthy foods and exercising. But when combined with an overall healthy lifestyle, fasting is the surest way to lose weight and maintain your ideal body composition.

Fasting and muscle mass

The common belief is that fasting will consume your hard-earned muscle. But not only is that wrong; it’s not even close to being right. Fasting is known to spare lean muscle mass.

From an evolutionary perspective, muscles (and muscle growth) were part of our survival strategy: when we put on muscle, our ability to get the next meal increased. So if fasting did, in fact consume the muscle that aided our survival, then going days and weeks without food would have made it much less likely for us to have gotten a meal when we needed it most – or to have protected ourselves.

The science shows that this isn’t the case. Studies have concluded that non-fat mass does not decrease at all during prolonged fasting. And even if you happen to lose some, it will only be the damaged/dysfunctional muscle tissue that steals energy from healthy muscle fibers. (This occurs through the almost miraculous process of autophagy.)

For more evidence of fasting not ruining your chiseled biceps, look no further than the muscled-up predators of the wild (tigers, wolves, etc.) who often go weeks in between meals.

How to fast for weight loss

Now that you’re ready to lose weight – and keep it off! – with fasting, it’s time to learn how to fast the right way.

Many people take a haphazard approach to fasting and experience harsh side effects from doing it incorrectly – excess fatigue, headaches, etc.) This negative first impression can keep them from ever trying to fast again and from ever losing the weight.

So along with consulting your doctor prior to the start of a fast, you’ll need to know a few things:

• which fasting method will work best for you
• how to prepare for a fast
• what to do during a fast
• and how to end a fast

 Which fasting method will work best for you

The number of fasts grows almost every day. (Thanks, human ingenuity!) But to make things simple, we’re only going to focus on intermittent fasts and prolonged fasts.

Intermittent fasting is defined as scheduled periods of eating and fasting. These eating periods, or windows, occur on a daily or weekly basis. Many types of intermittent fasts, such as 16/8, 14:10, 12: 12, and OMAD, involve fasting from 12-20 hours a day. (These are usually recommended for beginners since longer fasts can be more challenging.) Other types of intermittent fasts restrict eating to a couple of times per week, such as the 5:2 diet and alternate-day fasting.

(Read this article for more information on intermittent fasting types.)

Prolonged fasts, by definition, are fasts that last 48 hours and longer. Most involve total calorie restriction for the fasting period but allow water, coffee, tea, and other no-calorie beverages. There’s also a special prolonged fast called the Fasting Mimicking Diet (FMD) that restricts calories to 25% of your regular daily intake, and it’s been proven to induce a similar amount of fat-burning as normal prolonged fasts. (You can expect to lose 1-2 pounds per day on a prolonged fast.)

Now to choose the fast for you, you have to determine your fasting ability.

If you haven’t fasted for a day yet and you rely mostly on carbs, you’ll want to start with intermittent fasting*. For five days out of the week, you’ll eat your meals within an 8-12 hour window – from 8:00 am to 4:00 pm, or from 6:00 am to 6:00 pm, for example.

*The FMD is also recommended for beginners.

Prolonged fasts are a good idea if

• you’ve fasted for a day or more before
• you’re currently on a ketogenic diet and have already practiced intermittent fasting
• or if you’re including your medical practitioner for a doctor-supervised fast

(This also applies to longer intermittent fasting schedules such as OMAD, 5:2, and alternate-day fasting.)

How to prepare for a fast

Now that you’ve selected the type of fast you’ll go on, you have to prepare for it properly.

Eating too many greasy/fatty foods in the days before your fast can burden your GI tract and negate the healing benefits. Instead, you should focus on eating light foods in the days before your fast. These include

• rice-based dishes
• fruits and vegetables
• soups
• bone broths
• fish
• juices
• crackers
• nut butters
• grass-fed butter
• kitchari
• plant-based meals
• yogurt
• nutritional shakes
• and sparkling water.

You’ll want to avoid all processed foods while preparing for your fast, as they will negatively impact your insulin, hormone, and hunger levels. (These include soda, chips, candy, store-bought goodies, and most packaged meals.) Heavy proteins such as chicken, pork, and beef should also be avoided on the day before your fast.

Sleep is an essential part of preparing for a fast. Studies show that the hunger hormone ‘ghrelin’ increases when you lose even a couple of hours of sleep and that various pleasure-seeking parts of the brain are more active, too. So make sure to get 7-9 hours of sleep in the days leading up to your fast.

Finally, cut out alcohol.

Yes, we know alcohol is amazing in moderation – and that there are many health benefits associated with ‘cheersing’ 1-2 glasses per day. But when preparing for your weight-loss fast, any amount of alcohol will stress the liver with an extremely toxic compound called acetaldehyde. And since the liver is responsible for metabolizing fat, you’ll want to keep it happy in the days leading up to a fast!

(In case you needed another reminder: yes, you should always consult your doctor before embarking on a fast. They’ll conduct blood tests and determine if you fall in a high-risk category that shouldn’t be fasting.)

What to do during a fast

This step entirely depends on which fast you’ve selected. For instance, many fasting methods (such as alternate-day fasting and FMD) involve small quantities of food during the fast; others call for total abstinence from food. So make sure that you understand precisely what your chosen fasting method calls for.

But in general, you’ll want to stick to these fasting guidelines:

• drink plenty of water (because up to 30% of your regular water intake is through food)
• add a pinch of sea salt to your water – this prevents headaches and electrolyte imbalance
• coffee, herbal teas, and sparkling water are ok – just don’t add sugar!
• continue light exercise such as walking, gardening, lightweight training or bodyweight activities, or bicycling
• avoid heavy activities including weightlifting, sprinting, prolonged running, and intense sports.
• give yourself plenty of rest – listen to your body!
• and sleep as long as you can

The most important thing you can do when you’re fasting is stopping the fast if the fast becomes dangerous. Make sure to break your fast if you notice these symptoms:

• you stop losing weight in a prolonged fast
• when you experience any symptom more severe than fatigue or headache. (Weakness and headaches are typical; feeling like you’re about to pass out is not.)

How to end a fast

When fasting for weight loss, the absolute worst thing you can do is pig out on junk food to break a fast – especially when your digestive system is sensitized through fasting. This can be harmful to your intestines, and it can reverse the calorie deficit you created on the fast.

So, when you’re ready, break your fast with a small meal that’s very easy to digest: bone broth, soups, juices, rice cakes, etc.

Doctors recommend that you follow a refeeding period that’s half as long as your fast. The foods you eat during this refeeding period are very similar to the foods you eat when preparing for a fast, so consult this list for post-fast meals and foods to avoid.

Remember: The meal you break your fast with isn’t the end of your fast. To avoid refeeding syndrome, which is a potentially fatal electrolyte imbalance, you’ll want to choose small, light foods for a period of half as long as your fast.

Conclusion

Now that you’re prepared don’t put your fast off! Some people never experience lasting weight loss because they keep telling themselves, “Eh, I’ll do it next week.” Once you’ve talked to your doctor and are clear to fast, spend a few days preparing for the fast, then do it! You can expend to lose up to two pounds per day, and you can repeat these fasts daily or weekly, if intermittent, and monthly if prolonged.

And if you really want to ramp up your fat-burning potential while fasting, consider trying out a ketogenic diet. Fasting weight loss comes through the process of ketosis. So if you’re metabolically primed to burn fat through a ketogenic diet, your fasting period will not only be more enjoyable – you’ll have more energy and fewer fasting symptoms – but you’ll also lose more weight!