Home Blog Intermittent Fasting: Installment 2.0

Intermittent Fasting: Installment 2.0

Written on October 15, 2008 at 7:30 am, by Eric Cressey

Last week, I published a guest blog on the topic of intermittent fasting. Brad Pilon, Author of Eat Stop Eat, contacted me shortly thereafter with respect to the previous blog in question, and I encouraged him to pull together a submission of his own on the topic. I’m all for hearing all sides of every argument – and you can find Brad’s perspective below.

I am largely known as the fasting guy, but what many people don’t know is that when I went back to school in 2006, I went back to “destroy” fasting.

After seven years working in Research and Development for a sports supplement company, I was ready to go back to school to complete graduate studies in Nutritional Sciences. My plan was simple. I was going to spend a couple years studying all of the rules of nutrition, and then I was going to write my own nutrition book.

After working in sports supplements for years, I thought I had a pretty good handle on exactly what I would find, the tricky part was figuring out where to start. After some thought, it became apparent to me that the obvious place to start my journey was to examine exactly what happens to the body in the absence of food – when we are fasting. Then, from there, I could start to investigate what happens when you eat different types of food.

I was positive that the research would clearly show that after a couple of hours of not eating your metabolism would slow down. This isn’t what I found.

Instead, study after study kept showing convincing evidence that even fasting for as long as 72 hours did not slow down your metabolism. This research was so convincing that I had no choice but to switch my plan and study the metabolic effects of short term fasting as the focus of my graduate work.

So, I can completely understand why someone might be mislead to believe that fasting for a period of 12-72 hours could drastically suppressed their metabolism. After all, I thought this myself for a long period of time. However, once I became educated on the topic I realized that this belief is simply not supported by the available published research.

So let’s take a look at the effect that fasting has on our metabolisms.

When we say metabolism, or “thermogenesis,” we are really talking about the amount of calories we burn, typically in a 24-hour period. Obviously, from a weight loss perspective we want this to be as high as possible, and any evidence that would suggest a diet might lower our metabolisms is definitely not ideal.

It has been a long held belief that our bodies quickly adapt to short periods of low calorie intake by lowering of our metabolism, but what does the research say?

When I looked at the metabolic effects of short term fasting, I was shocked to find that even when a person does not eat for THREE DAYS, measures of metabolic rate either remain the same or actually increase during this short period of fasting. This has been found by a large group of papers, including those by Mansell in 1990, Klein in 1993, Carlson in 1994, Webber in 1994, Zauner in 2000, and most recently Gjedsted in 2007.

In fact, the available body of research on short-term fasting is remarkably consistent in this finding: for both men and women, fasting for a period of 12-72 hours does not decrease metabolic rate.

To examine this even further, we can take a closer look at the paper written by J. Webber and I.A. MacDonald, specifically because it has a large number of subjects, and it included both men and women.

In this trial, all the people were studied on three different occasions after a 12, 36, or 72-hour fast. The studies were conducted in random order, and there was a gap of at least seven days of normal eating between each fast. Metabolic rate was calculated from a continuous recording of oxygen and carbon dioxide consumption and production, using a ventilated canopy (indirect calorimetry), which is a pretty standard measure of metabolic rate in research studies.

The results of this trial showed that not only was there NOT a decrease in metabolic rate, but that there was actually a significant INCREASE in resting metabolic rate between 12 and 36 hours of fasting.

We’re talking about roughly 100 calories, so nothing to get overly excited about, and I’m even willing to ignore this increase and say that while it was statistically significant, it’s probably not “real-world” significant. That being said, even when we ignore the increase in metabolic rate, we have to admit that there was definitely NO DECREASE in metabolic rate.

So, in men and women who fast for as long as 72 hours long, there is NO decrease in metabolic rate.

Based on this evidence, we can say that the practice of intermittent fasting (where a person fasts for 24 hours) will not decrease metabolic rate.

There are other questions that need to be answered about the benefits of fasting, including how it affects fat burning, hormones like growth hormone, and muscle mass.

For more information on the metabolic effects of short term fasting in humans, you can check out my book, Eat Stop Eat.

8 Responses to “Intermittent Fasting: Installment 2.0”

  1. One of the King's Servants Says:

    I’m curious if all other “normal” activities continued during these studies, including, if any, training stresses. I would also be interested in finding out when it would be most effective to implement periodic fasting, if indeed it is a viable option. In other words, should you choose a fasting day during your rest days or during one of your training days?

    Sure. Fasting doesn’t decrease the metabolism; however, what energy stores are being used during this period? Fat? Lean mass? Both? Or even glycogen?

    Just curious…not convinced.

  2. Hi, my name is klue. Says:

    Thanks for your post Brad, and you EC for posting it on your blog.

    What I’m curious about is your opinion on daily 16hr fasts. Your method advocates 1-2 24hr fasts per week, while others suggest fasting for about 16hrs every day and only eating in a smaller time window (Fast-5, Leangains, Warrior Diet).
    Therefore my question is, do you think fasting everyday for 16hrs would effect metabolism? (Assuming you’re eating just as many calories per day).

  3. Scott Kustes Says:

    One Of The King’s Servants,
    The body uses stored glycogen, then moves to recycling amino acids like lactate and alanine. While recycling amino acids, lipolysis is ramping up to start using fat mass. Of course, all three of these mechanisms are going on at all times, but one typically predominates.

    I’ve been Intermittent Fasting in one form or another for over 3 years and have a lower bodyfat percentage at the same weight (meaning more muscle) as I had before I fasted. Check out CrossFit and The Performance Menu to see more success stories from fasting.

    I wrote a 6-part series on some of the effects of fasting on the body that you can read here.

    Scott Kustes
    Modern Forager

  4. cakeman Says:

    I’ve been doing 2 24 hour fasts a week for a month and a half. I’ve dropped about an inch and a half around my waist and have continued to hit PR’s on Wendler’s 5/3/1 program. Quite frankly, I’m shocked at how well things are going, as I assumed I would be weaker without the normal 6 meal a day eating schedule, but that has not been the case.

    The fasting has been much easier then I expected as well, and I think it is mostly because I know I will get to eat normally the other 5 days a week. It doesn’t really feel like a diet. We’ll see how it goes moving forward but so far it’s been an eye opening experience.

  5. oyaji Says:

    Thank you EC for posting the response, it’s interesting to hear two opinions on a single topic.

    I have two comments, one about each article.

    1. For Brian, he seems to have made the mistake that other opponents of IF make, and that is to typify IF as one single method. As was mentioned in the questions for the second article there are many approaches including 24-hour fasts (6pm to 6pm for example), various eating windows (5hours, 8 hours, etc), every other day (36 hour fasts), and so forth. What Brian and other opponents seem to do is build a strawman argument then shoot it down. They described one case of IF and make claims about why it’s bad and cover all approaches to IF with the same argument against one specific case by saying “And because of these specific cases, IF is bad, final blow, case closed”.

    He did this again with the “significant caloric deficit” that was the “final blow” to IF for him. What is the difference between people who IF on too low calories and people who eat frequent meals but too low calories? The significant caloric deficit is the problem, not IF. Their is a difference in using the wrong tool for the job, and using the tool in the wrong way.

    When making reference to what “research has found” it’s helpful to say which studies and especially what the participants ate. Most IF proponents make it very clear, you cannot IFOC – IF On Crap. See leangains, Mark’s Daily Apple, or Mike OD at performance menu.

    At the end of the day, if the results in terms of health and body composition are the same for a low-carb IF or low-carb multi-meal diet with the same food being consumed, then it typically comes down to convenience and individual preferences.

    2. For Brad’s article, it’s nice to see that references were provided. Even though I follow an IF eating plan, I don’t think many conclusions can be drawn about IF in general from Brad’s article due to the short time frame of the studies. However in particular to Brian’s comments it does address the fact that metabolism is not negatively affected by short term fasting.

    But what seems to be missing from the literature now are long term studies of IF being done low-carb and by people who exercise, over a longer period of time. In the network of strength and nutrition, the main proponents of IF practice this low-carb IF, meanwhile the opponents are using studies that did not follow anywhere near the protocols of people who have had success with IF to refute the beneficial claims. That is the gap,and the perhaps unintentional strawman argument that is occurring in the online debates.

    Apple and oranges.

    Thank you again to both Brian and Brad for posting their comments, and to EC for letting us take up space on his nice blog for this discussion.

  6. Intermittent Fasting: New Diet Solution or Passing Fad? | EricCressey.com Says:

    […] Note from EC: You can find a rebuttal to this post HERE. […]

  7. Healthy Girl Says:

    Great blog! Tomorrow I am finishing my first ever IF (2 days with no calories other than coffee, tea, etc), and you know what? I feel great…! In fact, I sense that you reconnect with your body in a way that you can’t when it’s numbed by food (or carbs.. or insulin!!).

    I am pretty healthy in general and active too – I’d normally do 40 mins of intensive cardio 3 times a week, followed by some good 15 mins of muscle building/weight lifting/pulling each time, and some yoga/pilates twice a week. Also, I am not obsessed with the scales (I haven’t weighted myself for ages).

    By letting my body rest from food (and carbs!) I can actually feel the extremely pleasant muscle-repairing sensations in my body. Wonderful stuff! I think that you can fully enjoy the muscle restoration process when you try an IF (or even a few days of “carbs-rehab”).

    You know what’s even more curious?? I always wanted to try this, naturally, but didn’t even know the expression “IF” until tonight! Now that is what I call a biological need! All the things I knew instinctively, have been confirmed by this and many other cool sites on the subject.

    One question: isn’t the Growth Hormone released only during our sleep, or would we also experience some diurnal spurns when practicing an IF?

    Thanks a lot!

  8. Melissa Says:

    Thanks for the posts…I just stumbled onto IF because the only way my body lets go of excess weight is by fasting. I had lost 40 pounds a while back, and my family thought I was becomming anorexic. I was just eating when my bocy said it was hungry. Thanks for the info…and I’ve lost 3# since I started on Monday. I’m stoked!!

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