Intermittent Fasting: What method is best for maintaining lean muscle mass?

Intermittent Fasting

Intermittent fasting involves cycling between periods of eating and fasting. Well known for its weight-loss benefits, the trend has become extremely popular over the past few years. In 2019 intermittent fasting was Google’s most-searched term relating to weight loss and was also prominently featured in a review article in The New England Journal of Medicine (NEJM).

But does the evidence support the hype? Is intermittent fasting healthy? More so, is intermittent fasting safe? We’d make a strong case that, yes, intermittent fasting is healthy, safe, and promotes weight loss alongside many other health benefits. This article aims to provide you with everything you need to know about intermittent fasting in order to get started and which method of intermittent fasting is optimal for building/maintaining lean muscle mass.

What is fasting?

Firstly, it is important to note that fasting and starvation are NOT the same. Fasting has one major difference: control. Starvation is involuntary absence of food for an extended period of time, which leads to suffering.

Any time you are not eating you are fasting. For example, the 12-14 hour period between dinner and breakfast; in this sense, intermittent fasting can be considered part of everyday life.

Consider the term “breakfast.” This term refers to the meal you eat that breaks your fast; which is done daily after sleep. The English language acknowledges that fasting should be performed daily, even if only for a few hours.

What is the difference between fasting and intermittent fasting?

The difference between fasting and intermittent fasting is based on the amount of time that has passed since your last meal. Intermittent fasting is done by skipping meals; this means that a conscious effort is made to eat only at certain periods of the day. Abstaining from food for longer periods (like days) would be considered a “fast.” Which one is best for you depends on your goals; keep reading to find out which method is best suited for you!

How does it work?

Intermittent fasting to lose weight allows the body to use its stored energy, by burning off excess body fat. Body fat is merely food energy that has been stored away, and if you don’t eat, your body will simply “eat” its own fat for energy. The proper term for this is autophagy and literally translates to “self-eating.”

The Stages of Intermittent Fasting

Each stage of intermittent fasting triggers a cascade of events. Each of these events has a unique and profound effect on health.

6-24 hours

During the post-absorptive stage, or fasting-state, food has been metabolized and glucose levels fall. As a result, insulin levels drop and glucagon levels rise. (Glucagon is a hormone secreted by your pancreas when the sugar level becomes too low. It is the inverse effect of insulin.) These hormones then signal the body to metabolize energy from stored glucose (glycogen) in muscle and liver tissue.

Low insulin levels and high glucagon levels stimulate the kidneys to excrete water and minerals. This process facilitates the metabolism of glycogen.

>24 hours

After approximately 24 hours of fasting, your glycogen levels will be fully drained and your body can no longer support a normal blood glucose level. When this happens, the body will synthesize glucose from non-carbohydrate precursors, such as amino acids and glycerol. This process is referred to as gluconeogenesis, which translates to “making new glucose”.

A common concern for many individuals is that gluconeogenesis (and thus fasting) will cause muscle loss. This is not true. Because the body has an abundance of stored energy at its disposal, it would be illogical for the body to use muscle tissue to replenish energy stores.

Amino acids are sourced from a process called autophagy, which simultaneously begins 24 hours into fasting. Autophagy is a natural regeneration process for old cells. During this process, the body degrades damaged cells and reuses the cellular components to generate new cells or glucose.

Autophagy is the foundation of intermittent fasting and the primary reason it has gained a lot of attention for anti-aging, and preventing diseases such as Parkinson’s, Alzheimer’s, and cancer.

>48 hours

Gluconeogenesis alone cannot product enough energy to support our bodies and brains; thus the body transitions to ketosis. Commonly referred to as ‘fat adaption’, ketosis is when the body uses fat, instead of glucose, as its main energy source. Stored fat is metabolized into glycerol and fatty acids.

Glycerol, as we mentioned above, gets used during the process of gluconeogenesis to keep our blood glucose levels stable. Fatty acids are used to power most tissues – except for the brain. The brain can only use glucose and ketone bodies.

FUN FACT: Your brain uses up some 60% of your glucose when your body is in the resting state. When fasting, ketones (ketone bodies) generated by your liver partly replace glucose as fuel for your brain; among other organs. This ketone usage by your brain is one of the reasons that fasting is often claimed to promote mental clarity and positive mood – ketones produce less inflammatory products as they are being metabolized than glucose does.

>72 hours

At the 72 hour mark, insulin levels will be at their lowest, and the body is fully fat adapted – fatty acids and ketones are the main energy source.

By this point, autophagy is in well under way and begins to rejuvenate the immune system — old, defective cells are degraded, which triggers the synthesis of new immune system cells.

Talk about the gift that keeps on giving! Understanding intermittent fasting on the physiological level will help you overcome barriers and reach your fasting goals; whether those goals are spiritual, religious, or simply intermittent fasting to lose weight. Don’t worry if a 72 hour fast isn’t manageable for you at first. The more you practice, the easier it will be too fast for longer durations. Practice makes perfect!

Types of Intermittent Fasting

There are many different diets for intermittent fasting. We suggest starting off with a shorter fast if you are not accustomed to fasting, and progressing to a longer fast gradually.

For the purpose of this article we will be discussing 3 popular types of intermittent fasting methods and explain which method would be best for building and/or maintain lean muscle mass.

Diets for intermittent fasting include:

16:8 (Recommended for maintaining/gaining lean muscle mass)

There are many different approaches one can take when beginning a fast. Probably the most popular fasting regimen is intermittent fasting 16/8. Intermittent fasting 16/8 involves fasting daily for 16 hours and eating all your meals within an 8 hour window. For example, if you had your first meal of the day at 12 PM, you would eat your last meal before 8 PM and fast until lunch the next day. Hence where this “intermittent fasting 16/8” routine get its name. Generally most individuals skip breakfast, but the beauty of intermittent fasting is that it is flexible and completely up to you. If you would rather skip dinner instead, that is perfectly fine.

If your goal is to gain/keep lean muscle while losing fat, this is most likely going to be the best method for you. Jim Stoppani, PhD, is one of the world's most recognized and popular authorities on training, nutrition and dietary supplements. In an interview conducted October 7, 2018 Jim stated he chose the 16:8 method of intermittent fasting because this method best helped him lose fat while maintaining, and even gaining, lean muscle mass.

Jim states that he would train in the “fed-state”, which makes perfect sense because it is in this state you will have the most energy. He also noted (for muscle gaining purposes) the meal with which you break your fast is extremely important.

Uncoupled proteins essentially poke “holes” in the mitochondria; which is where fat and carbs get converted into ATP to produce energy for the body. During the fasting period your body produces more uncoupled proteins, thereby releasing out more heat than energy, meaning you burn more fat and carbs. Studies saw that when a fast was broken with a high-protein meal, the production of these uncoupled proteins saw a huge bump, whereas when a fast was followed by a high carb meal, there was still a bump in uncoupled proteins, but it would significantly deplete after a while whereas a high protein meal would maintain.

Continue your usual training routine while implementing the 16:8 method and you are sure to see a drop in body fat while also building and maintain lean muscle mass.


Similarly, 20:4 method involves fasting for 20 hours and keeping all your meals within a 4 hour window. For example, you might eat two small meals; one at 3 PM and another at 7 PM, and then proceed to fast the following 20 hours.


This method of fasting, sometimes referred to as the Fast Diet, was popularized by Dr. Michael Mosley. The 5:2 fast involves eating regularly for 5 days out of the week and only eating 500 calories the other two days. (Recommended 500 calories for women, 600 calories for men) For those two days, you may eat two small meals (250 calories each) or one big 500 calorie meal.

For fasts that extend past the 36 hour mark it is always important to check with your health care provider to ensure you are not at risk for fasting complications.

FUN FACT: The world record for fasting is 382 days! (12) (Although we don’t recommend this!)

How to Get Started

Firstly, decide what type of intermittent fasting you’re going to try. Keep in mind that if you have never fasted before, it would be wise to start with a shorter fast, such at the 16:8 method, and gradually work up to longer durations. Once you begin your fast, continue with your daily activities and life as usual.

If your goal is to maintain/build lean muscle mass, the shorter 16:8 method will be a good choice. If your goal is fat loss, longer fasts will be even more beneficial, but starting with the 16:8 for this goal will be effective as well.

IMPORTANT: We can’t stress this enough. FOR ANY GOAL, keep in mind that the quality of the food you’re consuming during your fed-state is still extremely important. It’s not possible to binge on junk foods during the eating periods and expect to lose weight/maintain lean muscle and boost your health.

Can I exercise while intermittent fasting?

Absolutely! All types of exercises from cardio to resistance training (weights) are encouraged! The liver will supply energy via gluconeogenesis, and during longer periods of intermittent fasting your muscles will be able to use fatty acids directly for energy.

You should continue your training as normally while fasting; this will exponentially help you maintain and build lean muscle mass, while dropping fat.

How do I break a fast?

Gently. This is important, especially for longer fasts because eating too large a meal right after a fast is likely to lead to a stomach ache; eat as normally as possible. As we suggested above, especially if your goal is to maintain and build lean muscle, following your fast with a high protein meal is optimal.

Top Tips for Intermittent Fasting

1) HYDRATE: Staying hydrated during your fast is extremely important. Aim to drink 2 liters of water daily. Artificial flavors and sweeteners should not be used, however, adding a squeeze of lemon or lime, or even infusing your water with orange slices or cucumber for flavor is completely fine.

Not only is it extremely important to stay hydrated for physiological reasons, but drinking water can also help diminish hunger.

2) DRINK TEA: Tip two, or as I like to call it, Tip Tea! Tea is a great beverage choice anytime, but especially during a fast. Enjoy them hot or cold, spiced, or blended, teas are very versatile and have a variety of options for you to choose from. (Green, oolong, herbal, etc.)

3) DRINK COFFEE: Coffee, caffeinated or decaffeinated, is another great beverage choice. And if you’re a coffee addict, congratulations, you don’t have to forfeit your favorite drink! Cinnamon is a great spice choice to add to your coffee, without adding calories or breaking your fast.

4) STAY BUSY: Experienced clinicians and dieticians recommend staying busy during a fast as a way to ignore or curb hunger.

5) RIDE OUT HUNGER WAVES: According to most individuals that have practiced intermittent fasting, hunger usually comes in waves, meaning it lessens or complete disappears after a couple hours. Other individuals who have practiced intermittent fasting in the long term (multiple days) have reported that hunger cravings actually cease completely after the second or third day into the fast. This is because your body becomes accustomed to fasting and starts to burn its stores of fat, thereby suppressing your hunger.

Who should not fast?

Is intermittent fasting safe? For the average individual, yes, intermittent fasting is safe. However, individuals who are underweight, have an eating disorder(s), pregnant, breastfeeding, or under the age of 18 should not fast.


All in all, is intermittent fasting healthy? Yes! You can see intermittent fasting to lose weight is extremely effective and is what has made it so popular; however the benefits extend way beyond sheer weight loss. Diets for intermittent fasting are flexible, making them a great fit for any lifestyle.

Good luck!

Disclaimer: While intermittent fasting has many proven benefits, it’s still controversial. A potential danger regards medications, especially for diabetes, where doses often need to be adapted. Discuss any changes in medication and relevant lifestyle changes with your doctor. This guide is written for adults with health issues, including obesity, whom could benefit from intermittent fasting.

People who should NOT fast include those who are underweight or have eating disorders like anorexia, women who are pregnant or breastfeeding, and people under the age of 18.

Curious about the science?

Check out these links for more information: