Chapter 20 | When to Eat

Nov 05, 2019


Check out a replay of the Facebook live discussion over this chapter here

You Will Learn

  • What is intermittent fasting?
  • The body’s response to fasting. 
  • Hormonal responses to fasting. 
  • Myths about fasting. 
  • Intermittent fasting vs chronic caloric restriction. 


About Dr. Fung, Author of The Obesity Code and The Diabetes Code

Dr. Jason Fung is a medical doctor, nephrologist by trade, who specializes in kidney disease, type 2 diabetes, and obesity. He acknowledged that traditional medicine wastes time and resources attempting to treat symptoms of disease, rather than the cause of disease.  

You can purchase The Obesity Code book HERE


Overview of the Book

Here is the outline of the book. This post covers chapter 20 in Part 6.  

Part 1: “The Epidemic,” explores the timeline of the obesity epidemic and the contribution of the patient’s family history. It highlights the underlying causes of obesity. 

Part 2: “The Calorie Deception,” reviews the current caloric theory in depth and highlights the shortcomings of the current understanding of obesity. 

Part 3: “A New Model of Obesity,” describes how hormones are involved in the development of obesity. These chapters explain the central role of insulin in regulating body weight and describe the vitally important role of insulin resistance. 

Part 4: “The Social Phenomenon of Obesity,” dives into childhood obesity and why obesity is associated with poverty. 

Part 5: “What’s Wrong with Our Diet?,” explores the role of fat, protein, and carbohydrates, the three macronutrients, in weight gain. In addition, it examines one of the main culprits in weight gain - fructose - and the effects of artificial sweeteners. 

Part 6: “The Solution,” provides guidelines for lasting treatment of obesity by addressing the hormonal imbalance of high blood insulin through proper nutrition, sleep, and stress management. 


What is Intermittent Fasting?

Fasting is a normal part of life. We fast every eventing when we are sleeping. Many world religions practice fasting regularly, and fasting has been used in medicine for many years.  

Intermittent fasting is not a new concept, but its popularity as a weight loss intervention is on the rise. Intermittent fasting is essentially just going longer periods of time without eating. There are endless ways to intermittently fast. If you are going to try it for weight loss, the most important thing to consider is to pick a schedule you can stick to for a few weeks to give yourself a fair shot. 

When I was losing the rest of the baby weight, this was as simple as not eating from 7 PM to 7 AM. 12 hours of being fed, 12 hours of fasting. For some people it may mean skipping breakfast. Another popular model is to fast on alternate days, so you eat 3 meals on Monday, then skip breakfast and lunch on Tuesday and just eat dinner. Wednesday is back to 3 meals a day, then Thursday skip breakfast and lunch again. Again, there are numerous ways to fast, Dr. Fung provides two different fasting schedules in Appendix A on page 252.  

I would also encourage you to start with shorter fasts like my 7 PM to 7 AM and build up to longer fasts to see how your body handles it. I view fasting as another tool in the toolbox, but not as important as learning what to eat and ensuring your body is properly nourished when you are not fasting. 

When you are fasting, supplements and non-calorie liquids like coffee, water, eat, etc are allowed. 

Intermittent fasting can be used with any other diet. It doesn’t cost anything, and saves shopping, meal prep, and clean up time. It can be very practical to work into your lifestyle when trying to lose weight. 


The Body’s Response to Fasting: “Hitting the Wall” and “Fat Burning” during the Keto Diet 

The body, especially one with excess body fat, is well adapted to handle periods of fasting. Under normal circumstances, your body uses glucose for energy. Your body usually gets this from the carbohydrate foods you eat. See this article to learn more about how carbohydrates are broken down into glucose. 

Protein and fat can also be converted into glucose if needed, but under normal circumstances your body runs on carbs. Extra glucose is stored in your liver and muscle tissue as glycogen. When glycogen storage room runs out, the rest of the glucose is stored as body fat. 

When you fast, your body does not have glucose readily available from food, so it taps into the glycogen stores in your liver and muscle and breaks this down into glucose. Dr. Fung reports this starts happening six to twenty-four hours after the start of a fast. Most people have 1,500 to 2,000 calories worth of glycogen stored, so if you are not exercising, this is about a day’s worth of fuel, but if you are exercising, especially doing endurance training, you will run out of glycogen much faster. 

The phrase “hitting the wall” occurs when endurance athletes experience sudden fatigue and loss of energy when they run out of this available glycogen in their liver and muscles.

That is why you often see them eating fruit, gels, or other drinks high in carbohydrates during a race to replenish their carbohydrates and get more glucose for their muscles to use to avoid “hitting the wall.”

Dr. Fung describes on page 239 more specifics about how your body adapts after glycogen stores run out but essentially you start burning fat for fuel which is a more inefficient process.

Keto Side Note: People who are on the Keto diet force their body into the “fat burning mode” by severely restricting their carbohydrate intake to 15-30 grams of net carbs per day.

Net carbs are the total carbs minus fiber in a food. By eating mostly fat, protein, and fiber, they force their body into this “fat burning” state. 

Unfortunately, many people are still very undernourished when doing Keto and don’t get enough fiber or micronutrients. Personally, I don’t think Keto is a sustainable lifestyle for most people, especially those who want to follow a more plant-based lifestyle. I believe the Keto diet is another tool in the weight loss toolbox.


Hormonal and Electrolyte Responses to Fasting




Insulin causes salt and water retention in the kidneys so fasting results in an initial fast weight loss, which is mostly water weight. Prolonged lower periods of low insulin will lower the body’s insulin and thus body set weight. 

Growth Hormone


Maintain bone and muscle tissue mass during the fasting period. 


Increases, starting at around 24-hour mark. 

Increases metabolism 



Improved insulin sensitivity, reduced inflammation, reduced plaques in the arteries. (1)



A multivitamin along with salt ingestion from broth can help keep these stable, otherwise the body pulls the needed magnesium, calcium, and phosphorous from bones.

*Dr. Fung didn’t mention adiponectin in his book but I found this from supplemental research. 


Myths About Fasting 


Fasting will make you lose muscle

Your body isn’t stupid and won’t burn your more important tissue before it burns the lesser important, more readily available fat tissue. Dr. Fung says your body won’t start burning muscle until you are at about 4% body fat, which most of us don’t have to worry about. Growth hormone levels increase to preserve your muscle mass. 

The brain needs glucose to work

The brain can use a byproduct of fat metabolism, called ketones, for fuel. 

Fasting lowers your metabolism

Your body adapts to whatever it experiences regularly. Chronic caloric restriction will reduce metabolism (see next section). Intermittent fasting will not. As described above adrenaline and adiponectin increase and thus increase your metabolism. 

Fasting will overwhelm you with hunger

While you will be hungry initially, you will likely overcome that quickly, especially if you are drinking plenty of fluid and staying busy. The hunger comes in waves and tends to get better over time.

Fasting causes overeating when you aren’t fasting

Studies show a slight caloric increase at the next meal following a fast but not enough to make up for the caloric deficit of the fast itself. Dr. Fung reports in his clinical experience appetite tends to decrease with increased fasting duration. 

Fasting deprives the body of nutrients

The fat stores in your body provide ample energy and a multivitamin with some broth can help prevent micronutrient deficiencies. 

Fasting causes low blood sugar

Again, unless you are diabetic, the body will be able to turn the fat in your body into glucose for energy and your blood sugar (glucose) levels will remain in a lower but healthy range. 

It’s just crazy

One-third of the world’s population is supposed to fast according to their religious traditions. We fast daily overnight. What’s crazy is the health epidemic in our country and fasting is another tool to help reduce weight and reduce the risk for debilitating chronic diseases that require a “crazy” amount of medications, surgery, and doctor’s appointments. 


Fasting Vs. Chronic Caloric Restriction 

The metabolic benefits of greater insulin sensitivity, lower insulin levels, increased human growth hormone, and increased adrenalin do not happen with the traditional chronic caloric restriction dieting model.  

Your body adjusts to anything it is exposed to over a long period of time. If you cut your caloric intake by 500 calories everyday, eventually your body will get smart and reduce its energy expenditure by 500 calories a day. You will lose weight initially, but there will be a plateau (which is normal with weight loss), but because you didn’t actually lower your body set weight (insulin levels), the weight will come back on. 

Dr. Fung stated on page 247 that “intermittent diets produced far greater benefits by introducing periods of very low insulin levels that help break up the resistance. Further studies confirm that the combination of intermittent fasting with caloric restriction is effective for weight loss. The more dangerous visceral fat seems to be preferentially removed.”


Bottom Line

  1. In otherwise healthy overweight or obese individuals, intermittent fasting is a safe and effective way to lower your insulin levels and augment your weight loss. You can try any schedule you like that works with your lifestyle. The eat:fast schedules of 12:12 or 8:16 are popular, as is alternate day fasting. See Appendix A in The Obesity Code for more ideas. 
  2. What you eat when you are not fasting is still very important. Reduce your consumption of refined and processed grains and sugars, eat plenty of healthy fat, moderate your protein, increase protective factors like vinegar and fiber, and strive to eat real, whole, unprocessed foods. 
  3. Fasting between meals is another way to lower your insulin levels. Try to structure your meals so they are filling and nutritious enough to not need snacks. If you find yourself wanting to snack, try and figure out why. Are you bored? Hungry? Stressed? Tired? Dehydrated? Deal with the underlying emotion of physical symptom in a way that doesn't involve food. 
  4. While Dr. Fung gave us the framework for elevated insulin levels as the underlying contributing factor for obesity, there is not one single “cause” of obesity. Each person will have different risk factors that are more impactful than others. Some people may be eating too frequently, some may have high cortisol levels from chronic stress or sleep deprivation, some may have too many refined carbohydrates in their diet. It’s important to identify which factors are contributing to the excess weight and address those on an individual basis. 



  1. Achari AE, Jain SK. Adiponectin, a Therapeutic Target for Obesity, Diabetes, and Endothelial Dysfunction. Int J Mol Sci. 2017;18(6):1321. Published 2017 Jun 21. doi:10.3390/ijms18061321.
  2. Chapter 18. (2016). In J. Fung, The Obesity Code: Unlocking the Secrets of Weight Loss. Vancouver: Greystone Books. 

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