Debunking Intermittent Fasting Myths With Jason Fung, MD

Here are some of the most common fasting myths I hear.

These myths include:

1. Fasting puts you in ‘starvation’ mode
2. Fasting causes muscle loss
3. It’s just another type of calorie restriction diet
4. Fasting will slow your metabolism and you’ll regain all the weight you’ve lost and more
5. You don’t get enough nutrients
6. Fasting will overwhelm you with hunger and you’ll be side-tracked by food cravings all day
7. When you do eat, you’ll overeat
8. During fasting, you’ll feel awful, tired, cranky and “hangry”
9. Fasting causes low blood sugar (hypoglycemia)
10. The brain needs glucose to function
11. Fasting leads to or exacerbates disordered eating
12. Fasting is extreme and maybe more than a little bit crazy

If any of these myths were actually true, we wouldn’t be alive today to fear them.

Let’s take a closer look at  few of the most common myths and let the fasting expert, Jason Fung, MD, debunk them for us.

It’s an unfounded fear that fasting burns muscle.

Once again, we have to look at how our prehistoric ancestors were able to stay alive and perpetuate the human species.

Fung explains:that during long winters, there were many days where no food was available. If the absence of food caused us to burn muscle for energy, we would be severely weakened after the first episode.

After repeated episodes of no or limited access to food, we would be too weak to hunt or gather food.  If we had burned our muscle for enough energy to stay alive, our species would be as extinct today as the dinosaur.

Fung poses this better question:  Why would the human body store excess energy as fat if it needed to burn muscle/protein instead? The answer is, according to Fung, that the body does not burn muscle when body fat is available.

According to Fung,

Fat is able to provide the necessary small amount of glucose for the brain, red blood cells and renal medulla. You do not need to break down muscle for this.

So, fasting does not really ‘starve’ the muscles of fuel. Instead, scarcely available glucose is replaced with widely available fats as the fuel of choice. That’s good, because the body is able to store virtually unlimited amounts of fat, but only a small amount of glucose. Mother Nature, again, has proven herself far, far smarter than us.

Fung further explains:

Breakdown of muscle tissue happens at extremely low levels of body fat – approximately 4%. This is not something most people need to worry about.

At this point, there is no further body fat to be mobilized for energy and lean tissue is consumed.

There is another persistent myth that brain cells require glucose for proper functioning.

Fung tackles this one too.

Human brains, unique amongst animals, can use ketones as a major fuel source during prolonged starvation, allowing the conservation of protein such as skeletal muscle…Fat is simply the body’s way of storing food energy for the long term, and glucose/ glycogen is the short-term solution. When short-term stores are depleted, the body turns to its long-term stores without problems.  [My emphasis.]

Fat is not burned when plenty of glucose is available. Over decades of abundant glucose, fat stores proliferate. What would happen if glucose were suddenly unavailable? Would everything shut down in ‘starvation mode’? Nothing could be further from the truth. Energy, so carefully stored as fat, would be released.

Here’s how Fung torpedoes the myth that fasting drives us into starvation mode.

Starvation mode, as it is popularly known, is the mysterious boogieman always raised to scare us away from missing even a single meal…The human body has evolved to survive episodic periods of starvation. Fat is stored energy and muscle is functional tissue. Fat is burned first.

This is akin to storing a huge amount of firewood but deciding to burn your sofa instead. It’s stupid. Why would we assume the human body is so stupid? The body preserves muscle mass until the body fat becomes so low that it has no choice.

There are, in fact, no species of animals, humans included, that evolved to require three meals a day, every day.

Fung explains why it’s dead wrong that fasting causes a decrease in your metabolic rate.

The daily caloric restriction weight loss model, a hallmark of standard diet advice, leads to decreased metabolism.  Rebound weight gain is the unwanted consequence of calorie restriction diets as metabolism slows even though calorie consumption remains stable.

This scares people about IF.  They fear that eating fewer meals will slow their metabolism and weight gain will be the inevitable result.

According to Fung, nothing could be further from the truth:

If you rely on food for energy, then decreasing food will lead to decreased energy intake, which will be matched by decreased energy expenditure. However, as food intake goes to zero, the body switches energy inputs from [available] food [glucose] to stored food (fat). This significantly increases the availability of ‘food’ and this is matched by an increase in energy expenditure. [Again, my emphasis.]

Everything changes when food intake goes to zero (fasting).

The body obviously cannot take Total Daily Energy Expenditure (TEE) down to zero.  Instead, the body now switches to burning the fat stored on our bodies.  After all, that is exactly why we accumulated it in the first place.

Our body fat is used for fuel when no food is available.

Fung describes how Resting Energy Expenditure (REE) goes UP, not down, during fasting in this post.

Best news ever!  Your metabolism actually revs up during IF.

Adequate fat burning for energy is thwarted by eating frequently.

Fung explains how quickly the tiniest bit of glucose stops the benefits of fasting.

Even a sip of a soft drink is enough to stop your body from burning fat for energy.

Almost immediately after consuming glucose, the body is designed to stop burning fat. It switches quickly to burning the sugar that you are eating, the most easily accessible source of fuel.

So now can you see how IF keeps you in fat burning mode for longer periods of time?

I like to think of it as “dining in” — on my own fat stores.  Your body fat is a perfect fuel for your entire body, brain included.

Repeated concerns are raised that fasting may provoke overeating.

Fung addresses this concern too.  Studies of caloric intake do show a slight increase at the next meal after a one day fast.  Average caloric intake increases from 2436 to 2914.

But over the entire 2-day period, there is still a net deficit of 1958 calories. The increased calories did not nearly make up for the lack of calories on the fasting day.

Experience in Fung’s practice shows that appetite tends to decrease with increased duration of fasting.

In fact, may fasting experts describe a condition called “appetite correction” in which your appetite noticeably decreases when your body becomes fat adapted.

Does fasting deprive the body of nutrients?

Fung tells us that most people have more than enough quantities of nutrients. The whole point is to get rid of some of these nutrients – also known as fat.

If you are worried about micronutrients and minerals – you can always take a general multi-vitamin, he advises.

Personally, I eat whole foods, preferably organic vegetables and fruits, grass fed and pasture raised meat and poultry, and wild caught fish and have never feared nor have had any medical test results that indicate I have any nutrient deficiencies.