I Keep Running into Fung-damentalists

I am a huge fan of Dr. Jason Fung. You can see throughout this BLOG how Dr Fung’s video on Diabetes and Intermittent Fasting turned my life around. But I also keep running into people who misunderstand Dr Fung’s words in his book and lectures.

An Extended Fast is not Short Fast

One of the ways that people frequently misunderstand Dr Fung is to take what Fung said about Short Term Fasts (3-4 days) and make it apply to longer (Extended) fasts (5+ days).

The study Dr Fung quotes on this point was (Zauner, Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine).

With any study it is important to know at least a few things:

  • Who were the participants? Are they like me?
  • How long was the study for?
  • What type of study was it?
  • What special conditions were there for the study?

In the case of this study the participants were:

Eleven healthy, lean volunteers (7 women and 4 men) participated in the study.

The study only lasted four days.

The first measurement was made after an overnight fast [started at 2100 the previous day (day 1)]. Further measurements were undertaken 36 h (day 2), 60 h (day 3), and 84 h (day 4) after the beginning of starvation. All volunteers entered the metabolic unit at 0700.
So, quoting the study to say that it is applicable to an Extended Fast it not justified based on the study. And that is not what Dr Fung has said. And if someone specifically asked Fung if the study warrants drawing conclusions about metabolism on longer fasts I seriously doubt he would say it does.
The study was done on an outpatient basis to mimic real life except the the study did limit sporting activity. Also, if you were told to limit yourself to only “necessary activities” would you do less? I’d skip doing the laundry for four days, etc.
The study was performed on an outpatient basis to keep subjects under normal living conditions. However, the subjects were instructed to perform only necessary physical activities (ie, to avoid sports).

What Did Fung Actually Write?

On several pages of Dr Fung’s book, he writes that Fasting does not lower metabolism. For instance:

In fact, metabolism revs up, not down, during fasting (The Complete Guide to Fasting, p 73).

And here:

And, studies show that after a four day fast, resting Energy Expenditure increased by 12-percent. Rather than slowing the metabolism, fasting revs it up. (TCGTF, p 49).

 As I have showed above the context of what Fung is writing is about a fast which lasted only four days. Further, Fung includes the data from the study in the chart (TCGTF, p 74).


Dr. Fung is talking about a short (four day fast). It is a mistake to extrapolate from a four day fast to a much longer fast.

The chart actually shows a decline in metabolism (the red line in the figure above) from day three to day 4 (45.3 to 44.3).

The reason that the body gets that energy burst at the start of any fast is explained in the study:

Our results indicate that an increase in serum norepinephrine concentration rather than a decrease in serum insulin concentration initiated by the decline in blood glucose concentration may be the primary initial signal of metabolic changes during early starvation.
This is a short term effect until ketone levels rise a few days into the fast and are able to provide energy at that point.
And to be fair to those who misunderstand him, Dr. Fung has not made this any easier due to the expansive language he uses. When he says “metabolism ramps up during fasting” in this context he means intermittent and short fasts. To take what Dr Fung says specifically about short term fasts and extrapolate it to Extended Fasts is not warranted either from the evidence from the study nor from Dr Fung’s own words.

Author: Doug

I'm an Engineer who is also a science geek. I was pre-diabetic in 1996 and became a diabetic in 2003. I decided to figure out how to hack my diabetes and in 2016 found the ketogetic diet which reversed my diabetes.

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