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.