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Intermittent Fasting: What You Should Know

by John Berardi
Date Released : 25 Feb 2013

If you work in fitness, I know you’ve been hearing a lot about intermittent fasting (IF) lately. It’s no surprise as to why. With feature articles in Time, The NY Times, and The LA Times, as well a TV specials, including one from the BBC, you should be taking notice.

IF is all the rage and, like it or not, your clients are being exposed to it. That’s why it’s important to dig in and learn the basics - so you can answer their questions; so you can stay current. With this particular trend there’s another reason for learning all you can. There is some intriguing new data showing that IF just might offer unique health and body composition benefits.

So, here’s my advice: Don’t dismiss this trend as a fad. By doing so you’ll be missing out on a genuine opportunity to challenge your own beliefs, to learn, and to grow as a professional.

So what’s Intermittent Fasting All About?

The practice of intermittent fasting involves either skipping specific meals (like breakfast) or skipping entire days of eating. While, technically, there are hundreds of ways to “not eat,” the following describes the 5 protocols that seem to have the most scientific and user-driven support:

Alternate Day Fasting
(36-hour fast / 12-hour feed)

With this plan you simply eat every other day. For example, on Monday, you’d eat within a 12-hour window, say, 8 AM to 8 PM. Then you’d fast overnight on Monday, and all day/overnight on Tuesday. You’d eat again from 8 AM to 8 PM on Wednesday. And so on. Alternate day fasters are encouraged to make good eating choices, but they’re allowed to eat what they want on the non-fasting days.

Eat Stop Eat (24-hour fast, 1 or 2 times per week)

On this plan, you fast for a full 24 hours once or twice per week, eating sensibly (higher protein, minimizing processed foods, etc.) on the other days of the week. It’s flexible: You can choose whichever 24 hours you want to fast. Want to fast from breakfast to breakfast? That’s fine. Just eat breakfast on Monday, and don’t eat again until breakfast on Tuesday. Want to fast dinner to dinner? That’s fine too. Eat dinner on Wednesday, and don’t eat again until dinner on Thursday.

LeanGains (16-hour fast / 8-hour feed)

This brand of fasting is based on an 8-hour feeding period followed by a 16-hour fast. However, it also layers a few other food rules on top: The diet should be high in protein, should cycle carbohydrates, should include fasted training, and should use nutrient timing (eating the bulk of your calories during the post-exercise period).

On this plan, you fast from, say, 9 PM on Monday night until 1 PM on Tuesday afternoon. If you’re going to exercise, you’d do so just before 1 PM on Tuesday, with 10 g BCAAs (branched chain amino acids) during training. After training, you eat 2-3 meals before 9 PM, with your biggest meal coming right after exercise. The fast begins again on Tuesday evening until Wednesday at 1 PM, and repeats every day.

The Warrior Diet (20-hour fast / 4-hour feed)

On this plan, you would either fast, or eat very small amounts of specifically recommended foods, for the first 20 hours of each day, working out during this period of under eating. Then, you would eat the majority of your daily intake within a 4-hour over feeding window. After that 4-hour over feeding period, you would repeat the under eating/fasting for the next 20 hours.

Generally, most people place their 4-hour over feeding window at the end of the day, as it’s more convenient for family dinners and after-work training sessions. However, modifications can be made based on individual and scheduling needs.

Meal Skipping (Random)

Some IF proponents believe we should behave like our evolutionary ancestors did. As humans evolved to get their food and exercise randomly, so should we. This brand of IF includes eating unprocessed, “evolutionary friendly” food (think Paleo-diet type). Randomly cycling daily calorie intake and randomly skipping a breakfast or dinner meal once or twice a week. The rules are very flexible. (It is random, after all.)

What About Frequent Eating?

I know, I know…the idea of fasting for a few extra hours every day seems to fly in the face of conventional nutrition wisdom. Many of you probably think that going entire days without eating is sheer lunacy. I get it. As a long-time proponent of grazing (eating smaller meals more frequently), I was a little skeptical of the concept too. Would I get moody? Experience blood sugar drop? Have muscle proteins dissolve and burned for energy? It’s enough to make any fitness buff run screaming from the room.

But here’s the funny thing: If you do intermittent fasting right, none of that actually happens. How do I know? Well, first, there’s the research. In fasting-related studies, muscle isn’t lost like you’d expect, unless there’s a huge energy deficit and there’s no weight training involved. However, I’ll share a dirty little secret: I don’t always believe the research. Even though I spent eleven years in higher education and earned a research-based PhD, I know how poorly research can be conducted. Furthermore, how many actual bodybuilders end up in research studies? Very few.

My Experiments with Intermittent Fasting

That’s why I spent eight months experimenting with intermittent fasting. Indeed, I turned myself into a human guinea pig and tested dozens of different fasting-related protocols. Throughout the process, I meticulously recorded everything from body composition to blood values to lifestyle factors, all in an attempt to figure out whether intermittent fasting is a new and potentially valuable paradigm shift in the nutrition world or just another fad diet.

(For those who are interested, I published my findings in a free E-book called “Experiments with Intermittent Fasting”)

In the end, some of the experiments were a huge success, leading to improvements in my body composition, health, and performance. Others were disastrous, causing me to drop muscle mass and develop food obsessions. Yet, at the end of the day, I was able to accomplish most of my goals. I lost about 20 pounds of body fat while preserving most of my lean mass, strength, and power. According to my Intelametrix device (a validated ultrasound-based form of body composition testing), I went from a fairly lean 10% body fat to a very lean 4%.

Here are some of my progress photos:



Yes, I know the lighting was different in the after photos. I was so happy with my results I decided to schedule a professional photo shoot and that’s why the lighting difference. However, even if you’re a photo skeptic, you can’t deny the obvious changes in body composition. Again, I’ve already reported body weight and body fat measures so you know something was working.

What About Muscle Gain?

Of course, not everyone is interested in getting leaner. So what about muscle gain?

Well, my colleague Nate Green experimented with intermittent fasting too. During his experiments he gained 20 pounds of lean muscle. He also improved his aerobic and anaerobic exercise performance at the same time. (For those who are interested, he also published his findings in a free E-book called “Bigger, Smaller, Bigger”)

His protocols were obviously different than mine, but his results also show that intermittent fasting can assist with either fat loss or muscle gain as long the protocols are a good fit are and are followed correctly.

But I’d Never Be Able to Fast!

For those of you who are deathly afraid of missing a meal, let alone going entire days without eating; for those of you who swear that you’d never, ever fast – I have news for you.You already do intermittent fasting.

That’s right; every night, from the time you eat your dinner to the time you eat your breakfast, you’re fasting. Whether it’s 8 hours, 10 hours, or 12 hours, you’re fasting. Believe it or not, that fasting brings some unique benefits. So before you freak out and summarily dismiss the concept, understand that you’re naturally already doing some form of IF.

The Benefits of Intermittent Fasting

Many of you are probably wondering why I did my fasting experiments in the first place.

Well, a lot of it is curiosity…I’m what you’d call a “professional dieter.” In other words, I’ve tried nearly every diet or nutritional protocol that’s around in order to test its efficacy. In addition, I’ve been pursuing a new goal: track and field. When you’re running competitively, every pound has got to earn its rent, so I wanted to test drive this new way to drop fat and get extremely lean while staying strong and powerful.

Finally, the proposed benefits of IF are quite interesting and extensive. They include:


  • blood lipids (including decreased triglycerides and LDL cholesterol)
  • blood pressure (perhaps through changes in sympathetic/parasympathetic activity)
  • markers of inflammation (including CRP, IL-6, TNF, BDNF, and more)
  • oxidative stress (using markers of protein, lipid, and DNA damage)
  • risk of cancer (through a host of proposed mechanisms; we’ll save them for another review)


  • cellular turnover and repair (called autophagocytosis)
  • fat burning (increase in fatty acid oxidation later in the fast)
  • growth hormone release later in the fast (hormonally mediated)
  • metabolic rate later in the fast (stimulated by epinephrine and norepinephrine release)


  • appetite control (perhaps through changes in PPY and ghrelin)
  • blood sugar control (by lowering blood glucose and increasing insulin sensitivity)
  • cardiovascular function (by offering protection against ischemic injury to the heart)
  • effectiveness of chemotherapy (by allowing for higher doses more frequently)
  • neurogenesis and neuronal plasticity (by offering protection against neurotoxins)

To be frank, most of the research to date has been done in animal models with pretty limited data collection in humans. While the human studies that have been done show some promise, we’re probably a good 5-7 years away from knowing exactly what IF does in humans and why, and 10-12 years from knowing which IF protocols are “best.” That’s another reason I’ve been putting IF to the test, with great results.

Is Fasting a Must?

Of course not! People have been getting in shape for a very long time without using the intermittent fasting ideas I outline above. In fact, the dominant nutrition paradigm suggests that we should be eating smaller meals every few hours…so doesn’t intermittent fasting just fly in the face of everything we’ve been told to do?
Not really. The rules of good nutrition haven’t changed. You still need to eat good foods. Calorie balance still applies. Peri-workout nutrition is still important. The only real difference between more traditional bodybuilding-style eating and intermittent-fasting style eating is how you distribute your calories between days or meals.
This means that for most people, as long as we eat the right foods in the right amounts, meal frequency is a matter of personal preference. You can eat lots of small meals (every few hours) or you can eat a few big meals (with bigger time gaps between them). You can even go an entire day without eating, once in a while.

But what about speeding up the metabolism, controlling appetite, and controlling blood sugar? New data has been published showing that eating more frequently doesn’t necessarily speed up the metabolism. (Check out this study for a full review of the literature on the subject of feeding frequency and metabolism). Although grazing is supposed to enable better appetite and blood sugar control, that effect isn’t reliable. For some people, eating more frequently does help to control both. For other people, the opposite is true; eating less frequently gives them an appetite and blood sugar advantage. This means that your decision to eat small meals more frequently or larger meals less frequently should be based on what works best for your schedule, your mood, your appetite, and how you prefer to spend your time. That flexibility is pretty cool.

In the end, we shouldn’t totally abandon the grazing concept. Instead, we should recognize that we don’t have to graze. It’s not a must; rather, in most cases, it’s a choice.


A growing number of experts claim that short fasts can accelerate fat loss and make you healthier. As a result, I spent eight months testing the most popular Intermittent Fasting (IF) protocols for myself. During this time, I dropped twenty pounds of weight (from 190 pounds to 170 pounds) and reduced my body fat from 10% to 4% while maintaining most of my lean muscle mass.

I also helped others lose fat and gain muscle using a host of different intermittent fasting strategies. Of course, the full details of my experiments are beyond the scope of this article. However, if you’d like to learn more, you can check out my free e-book called “Experiments with Intermittent Fasting.” In the book I cover everything I did, including details of my training programs and my exact eating plans for all of the IF protocols I tried. There’s also measurement data (including blood work) and a host of other features you won’t want to miss. The best part? It’s 100% hosted online so anyone interested in more can pop over to the site and read the entire thing right now, for free, without having to enter an email address or anything.

As a result of my experiments, I learned that IF is a helpful tool and one I’ll continue to use periodically, but it’s not the end-all, be-all of nutrition or fitness.
People have been getting in awesome shape (and staying in awesome shape) for decades without the use of intermittent fasting. Simply put, when people control their calories, eat good quality food and train regularly, they get in shape.

The rest is a matter of personal preference, lifestyle, and individual difference.

Dr. John Berardi is the co-founder of Precision Nutrition and the creator of the Precision Nutrition Certification Program. For more from Dr. Berardi, check out this free 5-day course exclusively for fitness professionals “The Essentials of Exercise and Fitness Nutrition.

Tarrant Oneill, Darrell | 12 Feb 2014, 01:38 AM
Great article John!

I have been doing some research into IF as well as Carb cycling in order to get leaner etc. Would you approve of the 'Lean Gain' Fast on a Mon/Wed/Fri training schedule with those days being the high carb day. In turn, leaving the other days (Tue/Thu/Sat/Sun) to be low carb days?

Would that be a good idea or not?

Thanks a heap!
Webber, Lee | 17 Jul 2013, 14:56 PM
Have you seen success with endurance athletes on this type of nutrition periodization?
Bigelow, Chris | 06 Jun 2013, 21:33 PM
Thanks for posting this up, John! I had recently heard about Intermittent fasting and had been looking for a good 'overview' on the subject. I'm looking forward to what we find out about this type of nutritional strategy in the coming years as more research is done.
Berardi, John | 17 Apr 2013, 17:16 PM
@Steven: thanks for the feedback.
Berardi, John | 17 Apr 2013, 17:15 PM
@Michael: Check out this link... covers everything you need to know about setting up your own fasting schedule.
Ballance, steven | 10 Apr 2013, 19:38 PM
I liked reading the article. Nice pics and I am happy some one has done reserach for themselsves and write about it. I fast my self. it is very good practice. Also, Fasting has been around for thousands of years. In many religions around the world. It is done for spiritual reasons and help creating self discipline and mental clarity. I think With the mental benefits will come first then the physical attributes will follow. Just now in the past few decades has reserach been around.
Brigo, Michael | 11 Mar 2013, 15:21 PM
hi John I've done IF before and its yielded good results but i want to try the eat stop eat and my question is what will my calorie and macro nutrient intake be for the days im not fasting? is it kept the same or do I higher my carbs, protein and fats?
Berardi, John | 27 Feb 2013, 22:35 PM
No problem on the article, Teri.

As far as the photos, I think my wife likes me just fine. But remember, it's not about the "look". I was actually losing weight for an athletic purpose (track and field). And the level of body fat (and body weight) was exactly what I needed to perform at my best.

Besides, with the dark shadows in the picture above, you might not get the best perspective on how I looked post-IF in the daylight.

Here's a better representation:
Chadwick, Teri | 27 Feb 2013, 20:45 PM
From my (female) perspective, I have to say that I prefer your "before" photos. Just sayin. What does your wife think? : )

I heard your interview about IF, and other topics, on Fitcast. For the PTON member who wanted to know about IF results for women, John goes into some detail in that podcast. Google Fitcast, and I think you should be able to find it. If I remember correctly, it was episode 250.

Thanks, as always, for sharing your personal (experiential) knowledge and expertise.
Berardi, John | 26 Feb 2013, 18:04 PM
Thanks for the question (and feedback) Suzanne. I actually dig much deeper into this subject (including IF for women) in this particular video series: Part 5 contains some additional information for women. I even link to the same article you mention here. Hope that helps.

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