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The A-Z of Intermittent Fasting: Everything You Need to Know

Intermittent fasting (IF) is one of the simplest, healthiest lifestyle changes available to most people.

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It costs nothing, it actually saves time rather than occupying your day, and it offers a wide variety of benefits for health, fitness, and body composition.

However, there are many different ways to fast, and not all of them are appropriate for everyone (especially not if you’re a beginner).

Additionally, the science of how fasting actually works can be overwhelming for lay people. While you don’t have to understand every nuance of IF science, a basic grasp can be helpful for optimizing your fasting results.

In this article, you’ll learn:

We’ll also cover more in-depth areas like how to combine ketosis and IF, some of the most effective forms of intermittent fasting, a closer look at IF science and benefits, and more.

The Science of Intermittent Fasting

Intermittent fasting (IF) is an eating schedule that alternates between fasting and feeding. You set a time window for eating, eat only within that window, and fast the rest of the time.

For example, you might eat only between the hours of 10:00 AM and 6:00 PM (a pattern called 16/8 fasting).

IF is not a diet in the conventional sense, but rather, an ancient pattern of eating. 

Historically, during hunter-gatherer days and even earlier, our ancestors were often in a natural fasting state while seeking food[*].

Modern industrial agriculture may have eliminated hunger for most people, but the widespread availability of packaged, processed food has also led to many new diseases stemming from obesity and high blood sugar[*].

Essentially, it turns out that having food available all the time is unnatural, and modern approaches to fasting work by reintroducing periods of scarcity for health purposes.

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For most people today, fasting is a voluntary act, whether undertaken for health or religious reasons.

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The health benefits of fasting relate to cellular pathways that remain inactive or minimally active when you’re fully fed, such as[*]:

  • Autophagy
  • AMPK
  • Fat oxidation
  • Growth hormone release
  • Lipolysis

In a nutshell, avoiding food intake while fasting enables your body to naturally cleanse, repair and regenerate cells and tissues for optimal function[*].

Fasting also works incredibly well for fat loss because it allows you to eat fewer overall calories and burn more fat with less hunger[*].

Added bonus benefits include better cognition, brighter mood, and greater energy levels[*].

Not coincidentally, many of the benefits of fasting overlap with those of ketosis, which we’ll cover more later in this article.

But first, keep reading to learn whether fasting is a good idea for you or not.

Is Intermittent Fasting Right for You?

Fasting can work for most people, and its plethora of benefits offer something for nearly everyone.

However, if you struggle with disordered eating, or have in the past, intermittent fasting may not be advisable for you. Fasting and skipping meals may make your condition worse, or trigger it to return.

To work correctly, fasting needs to be part of a healthy, balanced, and well-rounded approach to eating. If you’re not sure whether it would be a good idea, speak to a doctor, therapist, or counselor who is qualified to help you decide.

Most types of fasting may also be risky during pregnancy or breastfeeding. You can still ask your doctor, but there’s not much data at this point, so it’s probably better not to take the risk.

Likewise, here are some conditions that can make fasting risky:

  • Hypoglycemia (low blood sugar)
  • Type 1 diabetes
  • Medication-controlled type 2 diabetes
  • Very low body weight or body mass index (BMI)

While it may still be possible to fast, the safest approach is to ask your doctor first if you have a medical condition.

Also, some medications can make fasting riskier, especially blood pressure medications that may result in electrolyte imbalances. Talk to your doctor before you try fasting if you take prescription medication.

Beyond any medical risks, some people simply do better with fasting than others.

For example, if you struggle with hunger or get “hangry,” you may need to ease into intermittent fasting or fix your metabolism first.

You also need to have a clear understanding of your goals to decide which type of fasting is right for you, which we’ll cover below.

For Beginners: How to Begin Intermittent Fasting Today

Step 1: Define Your Goals

The vast number of different fasting methods can seem overwhelming, but there’s a solution: start by thinking about your goals first.

When you begin with a clear picture of your goals in mind, getting started with IF is much simpler:

  • Weight loss is one of the best-known benefits of fasting, but not everyone wants to lose weight. Your approach to fasting will vary depending on whether or not you want to get leaner or shed fat. (Fasting less frequently, rather than daily, might be the best choice if you prefer not to lose weight.)
  • If you’re like most people, plenty of energy and mental clarity would be a welcome benefit from fasting. Shorter fasts tend to boost energy and focus immediately, while longer fasts may have the opposite result.
  • Immune function and cellular health are benefits nearly everyone could use, but the extent to which you want to focus on them is a personal decision. Longer fasts could have additional health benefits compared to shorter ones.
  • Plenty of people could also benefit from decreasing inflammation, which is another potential outcome of fasting. Again, longer fasts may work better for this health benefit.
  • If you’re an athlete or train hard, add faster recovery and optimal performance to the list. Fasting might benefit recovery by decreasing inflammation, but longer fasts could slow recovery due to a lack of protein and other nutrients.
  • Last but not least, longevity is one of the most coveted benefits of fasting. While the long-term nature of measuring this outcome in human beings makes it difficult to predict at this time, animal lifespan studies are promising[*]. 

Now that you’ve got a clear idea of what you want from fasting, it’s time to think about what goes into choosing a fasting method.

Step 2: Choose the Right Fasting Method for Your Body and Goals

When you’re deciding how to begin fasting, your unique psychology, metabolism, and physiology each play a role:

  • If you’re the type of person who prefers to jump in with both feet, you may find the challenge of extended or more difficult fasting patterns rewarding.
  • If you’re feeling a bit uncertain about fasting, or aren’t sure how your body will respond to fasting, ease in instead.
  • People who struggle with hunger or get “hangry” should start with a minimal fasting approach (modified fat fasting, skipping one meal daily, or skipping a meal once per several days).
  • Anecdotally, men may have an easier time with prolonged fasts, while women may need to adjust their fasting strategy at different times of their cycles.

Before you choose a fasting strategy, be sure to think about the ways your mindset and your body may affect your fasting experience.

Most people should probably start with 16/8 (which we’ll discuss more below) or another less-challenging approach like non-daily fasting, but there’s no one-size-fits-all recommendation for fasting.

Broadly speaking, here’s how your goal choice may affect your intermittent fasting approach and duration:

  • For weight loss, the simplest, easiest form of fasting for your body is often the best. Skipping a meal daily, 16/8 style, or using fast-mimicking techniques (like fat fasting, covered below) is probably easiest and likely works as well as any other method for fat loss. Extended fasts are more difficult and, for most people, don’t add much in the way of weight loss benefits.
  • When it comes to immune function, cell health, and decreasing inflammation, any fasting method is beneficial. However, some research indicates that occasional longer fast periods of several days may offer added benefits in this regard[*]. You can mix and match long and short fast periods for the best of both worlds.
  • Athletes and others who value physical performance can also benefit from fasting, but too much fasting is likely to reduce recovery and performance[*][*]. If you’re an athlete, you’ll need to balance your other fasting goals against these limitations and be sure to measure its effects on your progress in the training room and on the field.
  • Due to a lack of human longevity trials, the best fasting method for living longer is currently unknown. But some animal evidence suggests that prolonged fasts boost longevity benefits, so you may want to incorporate extended fasting if your goal is to live into the triple digits[*].

Step 3: Intermittent Fasting Checklist

Before you try out intermittent fasting, use this checklist to make sure you’re ready.

  1. Are you eating a healthy, well-rounded diet (whether keto, low-carb, or other)? If not, fix your diet before you attempt to incorporate fasting.
  2. Are you 100% prepared to stick with your chosen fasting schedule? Forming new habits isn’t instantaneous, so consider setting daily alarms or other reminders to keep track of fasting windows and eating periods.
  3. Do you know which foods and beverages break a fast, and the best ways to end a fast for your goals? Make sure to educate yourself and have a gameplan for how to ensure you stay fasted as long as you intend and break each fast in a healthy way.
  4. Have you stocked up on fasting supplements? Most people should take electrolytes to stay hydrated during extended fasts, and MCT oil or BHB ketones are good choices for fat fasting.
  5. Studies show support for a healthy lifestyle and diet can increase your odds of success, so you may want to enlist friends or family members to lean on during your fasting journey–especially if they’re also interested to begin fasting[*].

In the next section, you’ll learn more details about the most popular and effective intermittent fasting methods.

The Types of Intermittent Fasting

In total, there are at least seven types of intermittent fasting you can follow. The best way to incorporate fasting is to experiment with a few types, as needed, to find out which works for you.

For simplicity, here are the five most popular types to get you started:

#1: 16/8 (16 Hours Fasting, 8 Hours Eating) Fasting

This is the most common and arguably simplest and easiest method of IF, where you eat for a period of eight hours and fast for 16 hours. You either eat later in the day, skipping breakfast, or eat an early dinner and not eat again until breakfast the next day.

IF veterans may reduce the eating window to less than 8 hours. When you read about 23/1 or 20/4 splits, this terminology refers to only eating within a window of 1 or 4 hours during the day.

Advantages of daily IF approaches like 16/8 include:

  • Simplicity
  • Ease of use
  • Flexibility for individual preferences (you can skip breakfast or dinner, and easily adjust your fasting window as desired)

Terminology note: fasting for 20 hours or more each day is also known as The Warrior Diet.

#2: Alternate Day Fasting or ADF (24-Hour or 36-Hour Fasting)

Alternate-day fasting, also known as Eat-Stop-Eat and the Up-Day, Down-Day diet, involves fasting on alternating days of the week and eating unrestricted the other days.

As an example, if Mon-Wed-Fri-Sun are your eating days, then Tue-Thurs-Sat would each be fasting days. (This schedule could stay the same each week, or it could “flip” week to week for a true alternate day schedule.)

Staying with the above example, your last meal on Monday would be at dinner time, and then you wouldn’t eat again until Tuesday dinner or Wednesday breakfast.

Choosing a true 24 hour fast would generally involve fasting from dinner one day to dinner 24 hours later, the next day. 

On the other hand, some people prefer to use an even more aggressive 36 hour fasting window, prolonging the fast from dinner one night to breakfast time, the day after next.

Although the prolonged fast periods of ADF may be harder for some people compared to daily IF fast windows, ADF offers the following unique advantages:

  • The longer fast periods allow people to implement fewer restrictions on food intake during eating periods
  • Longer fasting windows may offer increased health benefits
  • For athletes or people who train intensively, a full day of eating may also help fuel performance and recovery

For many people, ADF isn’t the ideal starting point, but that doesn’t mean you can’t give it a try. If you’re drawn to alternate-day fasts but aren’t sure whether they’re a good idea yet, consider using another fasting method with shorter fasting periods first for 4-12 weeks, then switch over to ADF.

#3: 5/2 Fasting (5 Days Eating, 2 Days Fasting)

With 5/2 fasting, you choose two days of the week where you reduce your caloric intake to a quarter or less of your usual daily intake. 

For example, someone who typically eats 2000 calories would instead consume 0-500 calories for two days per week. If you prefer not to count calories, it’s okay to estimate–just eat nothing on fast days, or at most a quarter of what you normally eat in a day.

5/2 fasting offers some of the same advantages as alternate-day fasting, such as prolonged fasts and full-day eating windows, but with added flexibility and less difficulty since there are fewer fast periods each week.

#4: Water Fasting

Water fasting is a prolonged form of fasting that involves consuming no food, only water, for a few days or longer.

While there’s probably no added fat loss advantage over basic methods like 16/8, extended water fasts may offer additional benefits for cellular health and longevity.

For someone who doesn’t already follow the keto diet, ketosis occurs by the second or third day of a water fast, when your body turns to stored fats for energy[*].

Most people don’t use prolonged fasts as their main strategy, but rather augment their core fasting approach with periodic extended water fasts. 

However, if you’ve never fasted before, beginning with water fasts lasting longer than a day or so would be unwise.

Potential downsides include excessive hunger, lethargy, lack of focus, and electrolyte depletion.

#5: Fat Fasting (Fast Mimicking)

True to its name, the fast mimicking diet attempts to mimic a fast– particularly the effects of a water fast–but unlike a “true” fast, you eat healthy fats and sometimes other nutrient-dense foods during the fast (hence the name “fat fasting”).

Advocates say that your body doesn’t distinguish dietary fat from stored fat, and therefore remains in something close to a natural fasted state. 

In theory, this approach gives you at least some of the benefits of fasting while still allowing you to consume the macro- and micronutrients your body needs to sustain itself[*].

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Fast mimicking or fat fasts are excellent choices for people who have difficulty with true intermittent fasting, or who can’t fast for whatever reason. However, you should still speak to your doctor first if you have a medical condition or take prescription medication.

8 Scientific Health Benefits of Intermittent Fasting

In case you’re still on the fence about fasting or aren’t sure whether it’s worthwhile, here are the most important benefits associated with fasting:

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Below, you’ll find more details about the current science supporting intermittent fasting.

Intermittent Fasting For Weight Loss

Not unlike ketosis, intermittent fasting helps your body become better adapted to oxidizing (burning) fat for energy.

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Because of the decreased window of time for eating, insulin levels are lower, allowing adipocytes (fat cells) to release fatty acids[*].

The lower levels of glucose and glycogen encourage the body to use these fatty acids to generate energy for the body and brain rather than store fatty acids in fat cells.

In other words, while fasting, you use up fat instead of storing it–and soon burn what you’ve already stored.

A recent study showed that increasing the nightly fasting duration to greater than 14 hours led to a significant decrease in caloric intake and weight with improvements to:

  • Energy levels
  • Sleep satisfaction
  • Satiety at bedtime[*]

Intermittent fasting also reduces nighttime eating, which contributes to poor sleep quality and reduced sleep duration leading to insulin resistance and increased risk of obesity, diabetes, cardiovascular disease and cancer[*][*][*][*].

Adherence is another important aspect for IF as a tool for weight loss. 

Some researchers suggest that compared to conventional dieting that relies on calorie restriction, calorie reduction, or “continuous energy restriction,” intermittent fasting may be easier to stick with long-term, and thus may work better[*].

Intermittent Fasting for Self-Image and Willpower

One recent study found that women who practiced intermittent fasting had positive experiences related to willpower and associated with increased sense of achievement, pride, reward and control[*].

Willpower influences your sense of accomplishment and self-esteem through being able to exhibit self-control, which can help you feel better and reach your goals more effectively[*].

Intermittent Fasting for Disease States

Recent studies support the use of intermittent fasting as a means of lowering blood glucose in people with diabetes and leading to overall improved health outcomes[*].

In particular, one study published in the World Journal of Diabetes found that subjects with type 2 diabetes mellitus implementing short-term daily IF significantly reduced body weight, fasting glucose and improved post meal glucose variability[*].

IF has been shown to:

  • Improve markers of stress resistance
  • Lower inflammation and blood pressure
  • Improve glucose circulation and lipid levels, leading to a lower risk for cardiovascular disease, neurological disorders (such as Alzheimer’s and Parkinson’s) and cancer[*][*][*].

Beyond its application for diabetes management and reducing the risk of other diseases, IF is also proven to be as effective as some approved drugs for reducing some types of seizures and seizure-related brain damage and for addressing symptoms of rheumatoid arthritis[*][*][*].

Additionally, research is beginning to emerge showing positive effects of alternate day fasting on reducing the toxic effects of chemotherapy and decreasing morbidity rates associated with cancer[*].

Intermittent Fasting for Healthy Aging

Fasting is one of the biological stressors that triggers autophagy – a process where your body clears out dead or underperforming cells and regenerates and recycles damaged proteins[*].

Autophagy is extremely important and a natural process that plays a significant role in preventing diseases such as cancer, neurodegeneration, diabetes, cardiomyopathy, autoimmune diseases, liver disease and much more.

Regular fasting also puts an adaptive cellular stress on the body which in turn allows your body to cope with more severe stressors that may occur and thus protect against potential disease progressions[*][*].

This concept is known as hormesis – when an exposure to a mild stress causes cells in your body to become more resilient against other, more severe stressors.

Many of the benefits of fasting are due to these essential physiological processes.

Similar to ketosis, IF’s positive effects on aging are coming to light as more studies show that it has a profound ability to decrease blood pressure, reduce oxidative damage, improve insulin sensitivity and glucose uptake, and decrease fat mass – all factors that contribute to enhancing health and longevity[*].

Intermittent Fasting for Superior Mental Performance

At least in some promising animal studies, intermittent fasting improves cognitive function and helps boost brain power[*].

As mentioned earlier, IF induces autophagy. This includes neuronal autophagy in your brain and central nervous system, which allows your brain cells to recycle and repair themselves for optimal function[*].

Studies suggest that disruption of neuronal autophagy can lead to neurodegeneration, causing your brain to function insufficiently and prevent you from performing at your full potential[*]. 

In contrast, the absence of food during fast periods promotes neuronal autophagy, leading to better cognitive outcomes.

Intermittent fasting also increases a protein in the brain called brain-derived neurotrophic factor (BDNF).

This protein interacts with the parts of your brain that control learning, memory and cognitive function. Animal studies suggest BDNF helps protect brain cells and even stimulates the growth of new ones[*]

IF also triggers ketogenesis or ketosis, where your body turns to fat for energy, metabolizing fat into ketones[*]. Ketones easily cross the blood-brain-barrier, feeding your brain and resulting in better mental acuity, energy, and productivity.

Combined with a well-formulated whole-foods diet, IF avoids the blood sugar spikes caused by a high carb diet, which leads to brain fog and low mood issues like depression.

Intermittent Fasting for Better Physical Fitness

Research into the effects of fasting on fitness is early but promising.

In one study, scientists tested the VO2 max levels of a fasted group (no breakfast) and a fed group (one hour after a cereal breakfast)[*]. (VO2 max is a marker of aerobic capacity and maximal exercise performance, which directly affects athletic performance.)

Both groups had starting VO2 levels of around 3.5 liters per minute (L/min), which is close to standards for regular, untrained individuals. The study participants underwent endurance cycle ergometer training and had the following change in VO2 max:

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In short, the fasted group increased their VO2max significantly more than those who were fed breakfast, which equates to a promising result for athletes and people who need aerobic fitness.

Intermittent Fasting for Male Hormonal Benefits

The benefits of fasting aren’t confined to one gender, but here are the findings on its effects on male hormone levels:

  • In one study, fasting increased luteinizing hormone levels by 67% and testosterone by 180%[*]
  • In a separate study, a 24-hr fast elevated GH (growth hormone) levels by up to 2000%[*]. 
  • GH and testosterone are intertwined, supporting each other and supporting male health in a variety of ways[*][*][*].

As we’ve already discussed, fasting burns body fat. And as an added bonus, the less body fat a man has, the more testosterone he can produce and utilize[*].

However, for that reason, the above effects on GH, LH and T levels were observed in non-obese men[*]. The same way you can’t expect to see sculpted muscle before you lose fat, it’s unrealistic to count on increased testosterone before you lose any extra fat in your body first.

How to Combine Ketosis and Intermittent Fasting

In a nutshell, ketosis (from following the keto diet) and IF are extremely complementary, but if you’re new to both, you should probably start with one or the other.

Good news first: people who follow the keto diet can become fat adapted, which can make fasting much easier. 

However, the potential for problems exists when people who’ve never tried a very low carb diet or fasted before combine the two strategies.

Issues like keto flu or trouble dialing in your keto diet can be made much worse by fasting, and the brain fog and lethargy some people experience early on during fasting can contribute to feeling worse on keto.

We recommend that you go keto first, allow your body to adapt to a healthy low-carb diet and produce ketones for at least a month or two, and incorporate fasting only after you’ve got a handle on the keto diet. 

Done correctly, fasting on keto can help you break through weight loss plateaus along with all the other many health benefits mentioned previously in this article.

On the other hand, if you’ve already been fasting for several weeks or months but aren’t keto yet, you can consider adopting a ketogenic diet if fasting is comfortable for you.

But if you’re not fully comfortable fasting yet, you’d be better served by holding off on going keto until your body adapts to fasting.

Although going keto is beyond the scope of this article about fasting, don’t miss the important insights about keto vs. fasting in the following sections.

Fasting vs. Keto: Similarities and Differences

Intermittent fasting and the ketogenic diet have overlapping benefits because they both lead to ketosis, the metabolic state of producing ketones due to burning fat instead of glucose. 

Your body gets “lazy” at this healthy natural process when you keep it regularly supplied with carbs. 

But when you fast, you force your body to burn stored fat because there’s no readily available glucose from eating carbohydrates.

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Fasting triggers several desirable bodily processes, including:

  • ketosis
  • lipolysis
  • autophagy

You get into ketosis faster through fasting than you would by slowly transitioning to a fat-adapted state.

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Intermittent Fasting and Keto Combined

Fasting helps you get into ketosis, and being in ketosis also helps you fast.

Along with achieving the typical benefits of fasting, a lot of keto diet followers fast regularly because it helps them break through weight loss plateaus, and it’s an easy way to further limit calorie intake if necessary.

And a ketogenic diet also helps with longer periods of fasting. The benefits and easier transition are mutual:

  • Following the ketogenic diet macro portions (high fat, moderate protein, low carb) helps you transition to fasting easier. In a ketogenic, fat-adapted state, your body is already accustomed to tapping into your fat stores for energy instead of relying on carbs for glucose.
  • On the other hand, experience with fasting helps you avoid the keto flu if you begin the keto diet as an experienced faster.
  • A fat-adapted or ketogenic state also prevents similar symptoms that can occur when you first begin fasting.

Protection Against Muscle Loss

A high fat diet like the ketogenic diet also makes sure your body has a source of fat and uses it during your fast, instead of breaking down muscle protein for energy. For this reason, keto diet followers who fast for the day sometimes drink black coffee with either heavy, full-fat cream, butter, ghee or MCT oil or powder.

Exogenous Ketones for Intermittent Fasting

During a prolonged fast, your body runs out of stored glycogen (glucose) as an immediate energy supply, so it turns to stored fat and converts it into fuel. One form of metabolic fuel, created during fat-burning in a process called ketosis, is ketones.

Exogenous ketones are ketones available in supplement form.

Your body produces and utilizes ketones while in ketosis. And when you take exogenous ketones, you add more ketones for your body to use.

Will exogenous ketones break you out of a fast? No

Rather, ketones feed your brain with minimal impact on the fasting state or its benefits.

Final Thoughts

For most people, choosing the correct intermittent fasting plan can assist with fat loss, disease prevention, and possibly even aid longevity and increase lifespan–all with minimal side effects.

But remember: fasting is an eating pattern, not a complete solution to your dietary needs. Regardless of which form of intermittent fasting you choose, you still need healthy foods to stay healthy.

Fortunately, intermittent fasting is complementary to the keto diet. When incorporated correctly, ketosis and fasting each enhance one another’s effects.

But if you’ve never fasted or eaten a keto diet before, choose one or the other and allow your body to adapt before adopting the other.

While fasting is very natural and is safe for most people, it’s probably not suitable for people with eating disorders. Likewise, people with medical conditions or who take prescription medication should ask their doctor before fasting.

Side effects dieting calorie intake muscle mass eating disorders eating pattern hour window live longer intermittent fasting plan form of intermittent fasting healthy foods lifespan.

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