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Does fasting reduce metabolism? No, notes Jason Fung, MD
Wednesday, May 16, 2018 12:15 pm Email this article
You might think that fasting to lose weight would cause a drop in metabolism—I certainly assumed that it would—but this is not true according to a new, very interesting book called The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting by Jason Fung, MD and Jimmie Moore.
Fasting does not reduce basal metabolism.
In fact, one study suggests that the exact opposite happens.
4 days of continuous fasting increased metabolism by 12%
They noted that one study found that four (4) days of continuous fasting increased basal metabolism by 12 percent.
Dr. Fung explains why this is.
“If short-term fasting dropped our metabolism, humans as a species would not likely have survived,” Dr. Fung writes.
“During long winters back in the Paleolithic era, there were many days where no food was available. After the first episode, you would be severely weakened as your metabolism falls. After several repeated episodes, you would be so weak that you would be unable to hunt or gather food, making you even weaker. This is a vicious cycle that the human species would not have survived. Our bodies do not shut down in response to short-term fasting.”
“In fact, metabolism revs up, not down, during fasting. This makes sense from a survival standpoint. If we do not eat, our bodies use our stored energy as fuel so that we can find more food.”
Low-calorie diets lower metabolism, but fasting does not
As Dr. Fung explains why low-calorie diets reduce basal metabolism, but fasting does not.
The reason for this is that low-calorie diets keep insulin levels up which prevents fat from being released from fat cells to be used for fuel, so the body cannot get at the fat to be used for fuel, so the body reduces metabolism to compensate for this.
However, during fasting, insulin levels fall, which allows fat to be released from fat cells to be burned for fuel so then the body can maintain basal metabolism.
(When insulin levels are high, this tells fat cells “Don’t release any fat right now, I have to burn these carbohydrates first” because the body can only store a small amount of carbohydrates as glycogen, but can store endless amount of fat.)
Interview with Dr. Fung About Fasting
Watch the very interesting interview of Dr. Jason Fung in which he talks about the health benefits and weight loss benefits of fasting.
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