March 27, 2021

Intermittent Fasting: How It Works and Why It's So Powerful

A man waits for his dinner
Intermittent Fasting

Intermittent fasting results in greater physical and cognitive health. Fasting will also help you lose fat while preserving muscle mass. Our complete intermittent fasting guide will inspire you with the benefits of fasting and teach you everything you need to know to get started. Keep reading, or jump ahead to the section you need.

What is Intermittent Fasting?

We’re fasting when we abstain from food. Of course, we need food to fuel our bodies and lives, so we cannot fast all the time. Intermittent fasting is the process of alternating between periods of fasting and periods of eating.

Intermittent fasting, or IF, isn’t a diet but a way of living. In fact, the total calories or type of food you consume might remain the same. The truth is, caloric reduction is not the only way to lose weight. And as a bonus, intermittent fasting benefits go far beyond weight loss.

Does Intermittent Fasting Work?

It’s not much of a stretch to imagine our pre-agricultural ancestors practiced intermittent fasting. Mornings or entire days would be spent hunting and gathering food, which was enjoyed in the evenings or the following day. The human genome largely developed during conditions such as these. The body today thus functions best when under these same conditions.

Even during periods of history marked by abundance, fasting was touted for its healing properties, both physically and spiritually. Ancient Greeks fasted1 before athletic and intellectual events for greater stamina and clarity. They also fasted as a means of healing when they were sick2 (something many of us instinctually do today).

Proven Benefits of Intermittent Fasting

Losing weight via caloric or food restriction isn’t always the healthiest thing to do. While the end result may be beneficial, caloric restriction can cause fatigue and loss of both muscle and fat. Our performance suffers, and we get irritable and moody. To avoid the loss of muscle mass during a fast, consider an essential amino acid supplement that won’t break your fast.

Intermittent fasting is not only one of the simplest methods of weight loss, but it also provides us with far more benefits than traditional ‘diets’ based on caloric restriction.

Simple to Follow Intermittent Fasting Plans

To some extent, you’re already accustomed to intermittent fasting; you do it every evening while you sleep. You’ve likely had experience skipping breakfast or lunch just because you were too busy. Most intermittent fasting plans simply take these natural breaks in the steady rhythm of meals and extend them further.

There’s no calorie counting and no list of foods you can and cannot eat—just an on and off schedule to follow. Of course, if you are fasting while on a specific diet, you will need to keep the necessary restrictions in mind.

For example, Dr. John Jaquish follows a carnivore diet and typically eats just one meal per day (OMAD). So he tries to consume all calories in one large, meat-centric meal while periodically incorporating longer, multi-day fasts.

Intermittent Fasting for Weight Loss

Does intermittent fasting work? Typically, when we lose weight via caloric restriction, our muscles suffer. When the amount of protein we consume is limited, we try to compensate by supplementing or working out harder. But increasing the intensity of our workouts only breaks down muscle further when the body is in a starvation state. Starvation mode isn’t triggered by fasting, which we’ll explain later.

Fasting is a sure-fire way to lose fat while preserving muscle. In his latest book, Weightlifting is a Waste of Time, Dr. John Jaquish refers to 20103 and 20164 studies in which participants who fasted not only lost more fat than the calorie-restricting control group but did it without losing muscle.

Fasting Repairs Cells and Makes You Smarter

Autophagy is the body’s natural process of repairing and removing damaged cells. Autophagy in the brain protects us from cognitive diseases such as Alzheimer’s. It makes sense then, that science is interested in studying what might upregulate this process. Intermittent fasting is one of those things.

A 2010 study5 found short term fasting indeed boosts autophagy in the brain, not just in organs such as the liver, as had previously been known.

Fasting Increases Human Growth Hormone

Human growth hormone is necessary for our cellular health, muscle growth, and fat utilization. HGH production naturally declines as we get older. Some have speculated preventing this decline could be the fountain of youth. One way to do this? You guessed it: intermittent fasting.

One study found a 5-fold increase6 in growth hormone levels among ‘normal’ men who had completed a 2-day fast. While in a fed state, HGH release is unpredictable7. In a fasted state, HGH is regularly released.

Fasting and Insulin Sensitivity

Over 10% of the US population8 has been diagnosed with type 2 diabetes, while 4 in 10 qualify as pre-diabetic. Insulin resistance is an epidemic9. Weight-loss is typically the go-to solution, but intermittent fasting improves10 insulin sensitivity even in those who don’t lose weight11

IF’s positive influence on glucose uptake12 makes sense historically. Our earliest ancestors needed energy for the hunt, even after 1-2 days without a meal.

Fasting Upregulates Testosterone

There’s one more benefit to insulin sensitivity; it naturally increases13 the availability of testosterone. Among the benefits of elevated testosterone? Stronger bones, less fat, increased muscle mass, better memory, improved mood, and strengthened libido.

While these intermittent fasting benefits each lead to greater health, weight loss is still the number one reason most people fast. So let’s learn more about how it works.

Intermittent Fasting and Weight Loss

Using caliper to measure body fat

For years, we’ve been told that weight loss is a simple equation of calories in versus calories out, but it’s simply not true.

Restricting calories has the effect of slowing your metabolism, making it even harder to lose weight. What’s more, this detrimental effect is long-lasting, even after you reach your ‘goal weight’ and let go of the diet.

In 2016, when a group of researchers14 followed up with contestants from the popular television series, The Biggest Loser, not only had all but one of the contestants gained their weight back, but their metabolism was slower than before their appearance on the show, an astonishing six years earlier.

Pre-caloric restriction, they were each burning (at rest) an average of 2,900 calories per day. After severe caloric restriction, this number had dropped to 2,000 per day. Six years later? Contestants were burning just 1,900 calories per day.

There’s a better way to lose weight. Intermittent fasting isn’t about starving for weight loss—it’s about teaching your metabolism to speed up and target fat. So how does it all work? First, you need to know about the metabolic switch.

Metabolism and Fasting

Successful weight loss depends on more than just a simple math equation. Various hormones and chemical messengers all play a role. When we fast, things happen on a molecular and cellular level that just aren’t triggered by ‘dieting’ alone.

In a fed state, insulin levels remain high, glucose remains our primary energy source, and it’s difficult for the body to burn fat. This is true even if we’re eating a minimal amount. But roughly 12 hours after our last meal, something different happens. Scientists refer to this as the metabolic switch15.

In a fasted state, both available and stored glucose become depleted. When this happens, a switch is flipped. The body turns to fat as our primary energy source, converting fatty acids to ketones. Ketones are actually a far more efficient energy source than glucose, for both your muscles and your brain.

If we look back for a moment at our ancestors, this makes perfect sense. Muscle mass and brain function are preserved in a ketogenic state. Even without a recent meal, we can still think, run, jump, and hunt.

Keto and Intermittent Fasting

A similar effect can be achieved by following the popular keto diet, but let’s admit it, intermittent fasting is far easier. With IF, you can reap the benefits16 of ketosis without micromanaging what you eat.

If the keto diet is important to you, then by all means, do it. We’ll talk later about what to eat to enhance your fasting. First, the intermittent fasting guide continues with recommendations on how to fast.

Intermittent Fasting Schedule

The best intermittent fasting plan is the one that’s easiest for you. The method you choose might also change over time. We review the most common intermittent fasting plans below. Find your ideal intermittent fasting schedule to optimize your results.

Intermittent Fasting 16 8

The 16:8 split is the most popular intermittent fasting method primarily because it’s the simplest to follow. If you’re sleeping enough, you already fast at least 8 hours each day. With the 16:8, you simply extend this overnight fast to 16 hours by eliminating late night and early morning meals. Most people using the 16:8 method eat their meals between 11 AM and 7 PM.

Intermittent Fasting 5 2

With the 5:2 plan, you’ll eat as you typically would five days per week and select two non-consecutive days each week in which to fast. For some, choosing two days in which to fast is easier than following a daily protocol. For others, a full 24 hours without food is just too much. For this reason, one variation of this plan allows for up to 600 calories on fasting days. But is that fasting? We’ll talk about what breaks a fast in the next section.

Eat Stop Eat

With the Eat Stop Eat plan, you eat as you typically would on day one and abstain from food entirely until dinner on day 2. Repeat. Because this plan allows for a minimum of one full meal every day, some find it easier to swallow than the 5:2 method. Pun intended. If you’d rather eat breakfast than dinner, just flip this plan upside down.

Alternate Day Fasting

Alternate Day Fasting looks a little like the Eat Stop Eat plan, but it’s more restrictive. On this plan, you’ll eat your typical meals and snacks on day 1, eat nothing on day 2, then repeat. This extends your fast from 24 to 36 hours. As with the 5:2 plan, some people allow themselves up to 600 calories on fasting days.

OMAD and Building Muscle

One meal a day (OMAD) is a great option if you’re trying to pack on muscle. Just be sure you consume enough protein. Supplements like Fortagen will certainly help.

As mentioned, the one meal a day plan is John Jaquish’s preferred method. With this fasting plan, you consume all your calories within one meal, or one hour, while fasting for the remaining 23 hours of each day. It doesn’t matter if your one meal is breakfast, lunch, or dinner. Choose which works best for you, and keep it the same daily.

Extended Periods: 48 vs 72-Hour Fast

One final method of fasting includes a once-monthly 48 or 72-hour fast. Which is better? It depends on your goals. 72-hour fasts are very difficult for many people and we recommend starting with 48 hours. The benefits after the 48-hour period are tremendous.

Extending fasting is of particular benefit to your immune system, which will fully regenerate17 its T-cells after three days without eating.

What To Eat During Intermittent Fasting

IF will benefit you as long as you eat a reasonably healthy diet. That said, your intermittent fasting results will improve if you optimize your diet. We discuss the pros and cons of the three popular diet plans below.

The Ketogenic Diet

You’re on a keto diet when the amount of carbohydrates you’re consuming is low enough to induce ketosis. As we discussed earlier, you can just as easily (or more easily) reach ketosis by fasting than by eliminating carbs. You can also practice both together.

Keto Diet Pros

  • Your energy use is more efficient in a ketogenic state.
  • Ketosis induces fat loss.

Keto Diet Cons

  • Requires a carbohydrate intake of fewer than 50 grams per day.
  • It can be challenging to limit carbs on ‘normal’ eating days.

The Carnivore Diet

Steak to Show Carnivore Diet

On the carnivore diet, meals include nothing more than meat, fish, eggs, and limited dairy products. If building muscle is your goal, protein is important. The carnivore diet is one way to ensure your protein needs are met.

Carnivore Diet Pros

  • If your eating window is limited, meat-only ensures you’ll get enough protein.
  • It’s an easy-to-follow and low-carb diet plan.

Carnivore Diet Cons

  • It can be too restrictive for some.
  • Some people simply crave more variety.

Plant-Based Diets

Some choose to follow vegetarian or vegan diets for ethical reasons, while others believe it’s the healthiest way to eat. When fasting on a plant-based diet, it can be difficult to get the protein you need for muscle synthesis.

Plant-Based Pros

  • You might choose this diet for personal ethical reasons.
  • You’ll need to eat a wide variety of fruits and vegetables.

Plant-Based Cons

  • Every gram of protein is encased in carbohydrates.
  • It’s difficult to meet your essential amino acid requirements.

At least one gram protein per pound (2.2 g/kg) of body weight is recommended. For some, including Dr. Jaquish, trying to meet these protein needs while on the OMAD plan left very little room for additional calories. So the carnivore diet works best for him.

What works best for you may differ, especially if you’ve chosen a longer eating window. Experiment and see. What you eat during the fasting window is another, very important matter.

How Many Calories Will Break a Fast?

Any food or drink which triggers an insulin response will break your fast. Water, tea, and unsweetened coffee are the only items universally accepted as ‘safe.’

If you’re digesting food, you’re technically not fasting. You’ll hear of plans which allow up to 600 calories on ‘fasting days.’ This is psychologically necessary for some but will indeed trigger a broken fast.

If you must have a bite of something, sugars and carbohydrates are an absolute no. They’ll trigger an insulin spike, and you’ll forego all the benefits of fasting. Proteins are less harmful but can still kick you out of the keto zone. Fats might keep you ketogenic, but they’ll negate other benefits like autophagy.

To the purist, if you’re eating anything at all, you’re not fasting.

Intermittent Fasting Myths Debunked

Isn’t skipping breakfast bad?

Contrary to what you might have heard, there’s no scientific evidence that skipping breakfast is detrimental to health. In fact, the opposite18 is true. Foregoing breakfast extends the overnight fasting window and keeps you in a fat-burning state.

Isn’t it better to eat frequent small meals?

No. Eating small meals often keeps insulin high and metabolism low. There’s simply no research19 that supports the theory that greater meal frequency leads to weight loss.

Won’t I be hungry all the time?

Hunger is indeed an issue when you first begin intermittent fasting. But then an interesting thing happens. You actually produce fewer hunger hormones, not more. In fact, some studies20 correlate frequent meals with greater hunger, not less.

Won’t my body go into ‘starvation mode’ and hold on to fat?

Intermittent fasting isn’t about restricting calories; it’s just a shift in when you eat them. So no, your body won’t go into ‘starvation mode.’ In ‘starvation mode,’ resting metabolic rate slows to preserve energy. Intermittent fasting, on the other hand, increases21 resting metabolic rate, even if you are cutting calories.

Isn’t it bad to work out in a fasted state?

No. In fact, it’s recommended. When the body is digesting food, function is compromised. Working out in a fasted state promotes a more efficient hormonal and muscular response.

Many people prefer training with X3 or lifting weights while intermittent fasting. A 2009 study22 found that athletes who trained before breakfast had greater muscle gains than those who fueled up before their workouts.

How to Get Started with Fasting

If you’ve read the evidence, you’re probably eager to reap the benefits of intermittent fasting. Choose the plan that’s easiest to implement according to your schedule and personal tolerance for hunger. If you’re brand new to fasting, it’s a good idea to experiment with a shorter fasting window.

Experts recommend giving it a good eight weeks before deciding whether or not it’s right for you. After eight weeks of success, keep doing what you’re doing, or switch to a plan with a longer fasting window.


  1. Voluntary fasting ↩︎

  2. Fasting adn Purification ↩︎

  3. Improvements in coronary heart disease risk indicators by alternate-day fasting involve adipose tissue modulations ↩︎

  4. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males ↩︎

  5. Short-term fasting induces profound neuronal autophagy ↩︎

  6. Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men ↩︎

  7. Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man ↩︎

  8. National Diabetes Statistics Report ↩︎

  9. Diabetes is Epidemic ↩︎

  10. Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings ↩︎

  11. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes ↩︎

  12. Effect of intermittent fasting and refeeding on insulin action in healthy men ↩︎

  13. Pituitary-testicular axis in obese men during short-term fasting/ ↩︎

  14. 6 Years after The Biggest Loser, Metabolism Is Slower and Weight Is Back Up ↩︎

  15. Flipping the Metabolic Switch: Understanding and Applying Health Benefits of Fasting ↩︎

  16. Impact of intermittent fasting on health and disease processes ↩︎

  17. Prolonged fasting reduces IGF-1/PKA to promote hematopoietic-stem-cell-based regeneration and reverse immunosuppression ↩︎

  18. Effect of breakfast skipping on diurnal variation of energy metabolism and blood glucose ↩︎

  19. Meal frequency and energy balance ↩︎

  20. Effects of Increased Meal Frequency on Fat Oxidation and Perceived Hunger ↩︎

  21. Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine ↩︎

  22. Increased p70s6k phosphorylation during intake of a protein–carbohydrate drink following resistance exercise in the fasted state ↩︎

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