Ketogenic Diet

#167: Jason Fung, MD- Intermittent Fasting & Ketogenic Diets for Fat Loss

by Deanna Blow




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About Jason Fung, MD

Dr. Fung is a Toronto based nephrologist. He completed medical school and internal medicine at the University of Toronto before finishing his nephrology fellowship at the University of California, Los Angeles at the Cedars-Sinai hospital. He joined Scarborough General Hospital in 2001 where he continues to practice.


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Books and Resources

The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting

The Obesity Code: Unlocking the Secrets of Weight Loss

Keto Clarity: Your Definitive Guide to the Benefits of a Low-Carb, High-Fat Diet

Related Interview w/ Jimmy Moore

Click here to listen on your mobile device in iTunes

Show Notes

02:17 History of Fasting: Fasting has been done since the dawn of humanity. Religions prescribe periods of fasting. Ancient Greeks did it for mental clarity. In the last 50 years there has been a strong move against fasting. Dr. Fung works with diabetes patients, many of whom are overweight. Diabetes must be addressed or the illness progresses to kidney disease. In order to deal with diabetes, they must lose weight.

03:06 Nothing Wrong with Fasting: Dr. Fung investigated fasting and found that fasting had a great many benefits. Periodic fasting allows your body to burn off fat. Dr. Fung uses it with all of his overweight patients. Blood sugar medications need to be monitored closely once implementing fasting, or blood sugar could go too low. When you don’t eat, your blood sugar comes down.

05:25 Hypoglycemia: If you are taking blood sugar meds, you may become hypoglycemic. Most people who fast have a lowering of blood sugar, but it most often stays within normal range. Your body will produce some of the sugar. It will also reduce the need for sugar. Your muscles will begin to burn fat. Your brain, which cannot burn fat, burns ketone bodies produced by your body. You need a small amount of glucose for the brain, but that can come from your body fat. You often hear that the brain needs 130 grams of carbohydrates every day, so you should eat that much each day. This is not true. If you don’t eat, your body breaks down glycogen and provides glucose. People fast all the time for medical testing and interventions.

09:42 Muscle Loss and Fasting: Starvation mode and muscle loss are two of the many myths. Hypoglycemia is not a big concern unless you are taking blood sugar meds. As you fast, you will consume glycogen. When you eat, you fill your glycogen stores. These are chains of glucose. These stores will last from 24 to 36 hours. For a 24 hour fast, you generally have enough glucose in your liver. At some point in the 24 to 36 hours there is a window where you use protein to produce glucose. This is gluconeogenesis. We store energy as sugar and fat, so we burn sugar and fat. A portion uses protein, but protein burning is not increased. Studies show little change in muscle mass with fasting. An obese person will burn a larger percentage of fat than a lean person. Any time you lose weight, your lean mass is reduced. Fasting is 4 times better at preserving lean mass as compared to caloric restriction.

16:53 Hormones of Fasting: Fasting increases growth hormone. It increases noradrenaline. These are counter regulatory hormones, which counter the effect of insulin. Insulin lowers blood glucose. When fasting, we want to use the sugar and need to pull it back out. Noradrenaline gives you more energy and keeps your metabolic rate high. Human growth hormone makes it so when you start eating again, you will lay down lean mass. Your body knows that you will have periods of starvation/fasting, so it has developed a mechanism so you do not lose all of your lean muscle.

18:01 All Protein is Not Good: In Alzheimer’s disease, there are abnormal proteins in the brain. Fasting provides a mechanism for breaking down protein and getting rid of it. This process is autophagy. The 2016 Nobel Prize in medicine went to one of the pioneers in autophagy research. Theoretically you can prevent diseases like cancer. Your body will burn the excess proteins of flabby skin left over from weight loss. Long fasts are not a good fit for someone who is lean and trying to build muscle.

22:04 Three Day a Week Fast: Dr. Fung does 24 hour fasts. He skips breakfast and lunch, going from dinner to dinner. This allows him to have dinner with his family. You can do any duration you choose. Jimmy Moore did a 21 day fast. Fit it into your lifestyle. For autophagy, you may need to go longer than 24 hours.

26:32 Anti-Aging Benefits: Skin is protein, so skin may alter with intermittent fasting. During the process of fasting, you get rid of old cells as part of renovation.

28:44 Autophagy: mTOR is a major regulator of autophagy. Protein turns off autophagy. mTOR is a nutrient sensor, sensing protein. If your aim is autophagy, it is best not to use bone broth because it contains protein. In theory, you could eat pure fat and still achieve autophagy. With a classic water only fast, you may achieve autophagy in about 24 hours. Branch chain amino acids may negate autophagy as well.

33:37 Salt and Fasting: One reason why Dr. Fung’s patients use bone broth during a fast is for salt intake. If you are doing a water only fast, you are getting little salt, causing some people to get dizzy or have headaches.

35:49 Kidney Function: Dr. Fung had hoped that his patient’s kidney function would improve as they lost weight. Once chronic damage of kidney disease is done, it is almost irreversible.

37:07 Hormone Impacts: The main hormone affected by fasting is insulin. There may be minor effects upon the thyroid. Sex hormones are little affected. Polycystic ovarian syndrome is part of the metabolic syndrome and does improve slightly. Pregnant or breastfeeding women should not fast. If you are underweight, fasting is not a good idea.

40:18 Anorexia: Anorexia nervosa is a psychiatric disorder of body image. It is not just that you are not eating. Fasting is a symptom.

42:17 Cortisol and Fasting: Cortisol does rise with fasting. It is part of the counter regulatory hormones. Fasting is a stress to the body. Exercise is also a stress to the body and the body responds by getting stronger.

43:14 Noradrenaline: As this hormone rises, you get more energy and you don’t get the basal metabolic rate decrease that you get with caloric restriction. This is the key to weight loss. It can cause sleep disturbances because you are so full of energy.  You can lose weight on nearly every diet for 6 months. After that, weight rises again. On diets you lose weight, but your basal metabolic rate goes down so you are burning fewer calories. Your weight plateaus and goes back up.

48:09 Calories: Calories have nothing to do with weight loss. Fasting is about the time spent not eating. Fasting is about changing your hormones. It has nothing to do with calories. Cutting calories will not impact the hormonal imbalance.

49:50 1950s: There was little obesity. People ate cookies, white bread and ice cream. You ate within a 10 hour window. You ate your meals with family or coworkers, but you ate no snacks. You did not eat mindlessly in front of the TV or computer. This made a 14 hour daily fast.

54:51 Bullet Proof Coffee/Fat Fasting: Many of the benefits of fasting come from reducing insulin. Pure fat has almost no insulin effect. By consuming all fat, you are receiving the benefits of fasting. It works for some people and not for others.

56:26 Ketogenic Diet: Intermittent fasting and ketogenic diet are along the same spectrum, close cousins. The ultra-low carb ketogenic diet lowers insulin. Low carb diet delivers 71% of the insulin lowering of fasting. Many diseases of hyperinsulinemia, obesity and type 2 diabetes, can be treated effectively with a ketogenic diet. Fasting is easy for those of us on a ketogenic or low carb diet because our bodies become well adapted to fat metabolism after the first 2 weeks. Ketones may suppress the appetite. Dr. Fung’s obese patients do not need to worry about nutrient density, but you might. Context is everything.

01:00:39 Dr. Fung’s Morning Routine: Monday through Friday he doesn’t eat breakfast. He eats breakfast with his family on the weekend. When he is extra busy, he does more fasting to get more time for work.

01:03:45 Dr. Fung’s Favorite Nutrient: Other than a general multivitamin, he does not make recommendations. His patients are on the other end of the spectrum. He uses omega 3s for fighting inflammation. He also uses Epsom salts (magnesium salts). Many of us are deficient in magnesium, more than most other minerals. Magnesium taken orally is often not absorbed. The magnesium is absorbed through your skin when you take an Epsom salts bath. It is also relaxing.

01:07:54 Dr. Fung’s Elevator Pitch: We need to teach people about the therapeutic value of fasting. It has the potential to change the entire health system of the world. Most of what we do is treat diabetes, high blood pressure and all of the downstream issues. All of this can be taken care of for free. We need to spread the knowledge of how to do it.



Join the conversation

  1. Well I found that 6-18 is great for me ann when I dropped it down, I gained back 20 lbs of 130 lbs I lost by eating zero carb and being on 6-18 intermittent fasting. I tried extended fasting a few time, ( 7 days, 7, 6,4,3) and not a single time I was able to keep the weight loss I had theough them. I always gain back all the weight I lose during the extended fasting and even more which is very depressing putting in mind that fastint is a very stressful process. So imagine if there is no results.i would love to hear a reply or an advice.

    • HI there,

      Without knowing more about your exercise program and lifestyle, it’s hard to figure out exactly what will work/won’t work.

      Might be best to stick with a 16/8 schedule and keep activity levels up; also periodizing carbohydrate intake one to two days per week when activity is high can help.



  2. Hi Mike, this interview gave me the courage to try skipping meals…after all I have been Paleo/low carb for Several years and wasn’t hungry that often anyways. 22 hours was about how long until hunger set in, and I slept fine. Not like many years ago when I tried it before.

    Now, I work in SF downtown. I have probably been overdoing protein in order to get the fat via meat, lots of beef and lamb. How do I know I’m getting too much protein? How can I get more fat when I eat out without excessive protein? There is very little healthy fat around like coconut oil and lard. Best I can do is avocado. Thinking of just carrying some fat around with me.

    Learning a lot from the podcast, and ordering books through your website. Galland on allergies was illuminating! Have you interviewed Nora Gedgaudas? Love her book on Beyond Paleo.

    Also, can you refer me (offline) to any functional med docs with MTHFR experience in SF? Mine is a bit far away now but available via phone I guess. Just for maintenance and optimization.

    • Thanks, Jonathan!

      There’s magic number with protein, you need to do self-discovery. (Many in the traditional Paleo community overdo it in my opinion.)

      Vegetables should be a the cornerstone of your diet, balanced out with clean proteins and fats.

      Glad the content has been helpful!

      As for a practitioner, I think Stephanie Daniel, DO ( is a great practitioner.

      Hope this helps,


  3. Hi Mike,
    what about lemon water or apple cider vinegar (shot) in the morning, would that affect the fasting?
    I believe they have the same effect as coffee…?

    ACV shot is even recommended during fasting?


  4. Saying that protein increases insulin is somewhat disingenuous as if that is by itself a villain and others in the IF field have and are doing the same, Why!? Afteral, every thing you put in your mouth increases insulin even some liquids do. All animals and eben most insects live off fats and proteins, directly or indirectly, we being no different. A bowl of rice n beans can spike insulin more than a few ounces of meat can. Otherwise I give you credit for a well written article.

  5. Fasting okay if your cortisol is a bit high already? I need to lose 20 lbs, but the cortisol belly isn’t budging no matter what I have tried.

  6. Hi Mike
    Dr Fung mentioned that fasting doesn’t effect sex hormones, but can you tell me if a strict keto diet can raise SHBG? And how do you lower it to access free testosterone?
    Thank you

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