Fasting and Blood Glucose | Jason Fung

insulin resistance, diabetes, blood sugar, weight loss

What happens to blood glucose during fasting? Most of the time blood glucose goes down, but not to levels that are dangerous. However, sometimes the blood glucose goes up, in a variation of the Dawn Phenomenon. Check out my blog at

More Information:

The Obesity Code – Reviewing underlying physiology of weight loss and how low carb diets and fasting can help.

The Diabetes Code – Reviewing how type 2 diabetes is a reversible disease and dietary strategies.

The Cancer Code – Scientific exploration of how cancer develops –

Canada –

**Fasting Aids**
Pique Fasting Tea (recommended)

Fasting Community and Coaching:

YouTube Medical Lectures (for specialist physicians):

1. The Roots of the Obesity Epidemic –
2. Therapeutic Fasting – The Two Compartment Problem: ​​
3. Does Calorie Counting work? ​​
4. Two Big Lies of Type 2 Diabetes –
5. Reversing Type 2 Diabetes Naturally –
6. Insulin Toxicity – ​​
7. Saturated Fat – Friend or Foe?
8. Diet and Disease –
9. Dangers of Fructose –
10. Insulin Resistance –
11. Role of Hormones in Weight Loss –
12. The Obesity Code Lecture 1 –

#jasonfung #drjasonfung #fasting #fastingbenefits #weightlosstips #intermittentfasting #drfung #fung #calories #weightlosstips #weightloss #nutrition #diet #health #bloodsugar

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Cameron Long

  • Dennis Moore says:

    I’m off my diabetes meds (Doctor approval) because of your direction. And lost 68 pounds in a year. Thank you!

    • primoxxl71 says:

      very happy for you. any tips?

    • Jayy F says:

      congrats!! how long are your fasts?

    • Dennis Moore says:

      I started keto one year ago, Sept 2020, after reading Doctor Fung’s book. A1c went from 8 to 5.3 in three months. Also been doing IF for many months. Mostly 16/8 to 18/6 with occasional 24 to 30 hours.
      My endocrinologist calls me Diet Controlled diabetic, weaned me off the meds.
      I’m 63, was diagnosed 10 years ago. Now no meds. Freaking crazy good!

    • Jayy F says:

      @Dennis Moore yeah I just read his book a month ago. I was a 6.0 a1c. I go back in November I have been doing 3 36 hour fasts per week for the last month. hoping to have atleast A 5.0 a1c. im down about 20.5 pounds in a month.

    • Jayy F says:

      im 28 and was diagnosed a year and a half ago

  • Joseph Onuche says:

    My fasting blood sugar test went from 484 to 79 in 3 months. Thanks Doc you are an angel. I used no meds. Just the manual method

  • Ms.A says:

    This morning I completed a 36 hour fast. I have been prediabetic for 7 years. 36 hours ago my glucose levels were 109, 24 hours ago it was 101. My final reading was 121. Thank you for this video and your awesome books. If I listened to loved one’s and well meaning friends, I would assume that something went wrong.God bless you doctor!!!!! It is my prayer and goal to be more consistent with a fasting lifestyle so that I can live a healthier life.

    • Miss Kay says:

      Congrats! I’m also on hour 36 right now for the first time ever. I hope you’ve continued to have a good journey this past month.

    • Bua Schmidy says:

      @Miss Kay Wow 36 hours!! That’s pretty awesome to me. 🙂

    • 55karenina says:

      After 20 hours fasting my blood pressure is elevated,so I go and eat ,is any one knows why?
      After ai eat ,blood pressure going down.I am not diabetic and not on blood pressure pills,but I have hashimoto,wonder if is connection?

    • skliros says:

      @55karenina are you drinking enough water while fasting?

    • Michelle Parker says:

      Hello I’m about to break my 48 hrs fast I feel good

  • Overwatch says:

    Finding this man was the best thing to ever happened to me in my life. After my diagnosis of type 2 diabetes I spent a year on medication, wondering around the grocery stores trying to find something to eat and kidding myself. When I went for my 1st year check up and was blind sided by my doctor who has absolutely no bedside manner and left his office like a kid that was just scolded. A friend turned me on to Dr. fung. I was 280 plus. I changed eating habits, changed my lifestyle. Started fasting. I eat once a day. From 4pm to no later than 8pm. The weight fell off. I went from 280lbs+ to 215lbs and eventually found my perfect weight at 220 to 225. The metformin went in the trash 2 years ago. I check my blood sugar about once a week.
    Thank you, Dr. Fung.

    • Vicky Lapore says:

      What foods do u eat during your fasting? Hope you share it to us.thanks in advance reply.may God bless u.

    • Overwatch says:

      @Vicky Lapore 
      I eat vegetables, fruit, nuts, and meat. I do not eat bread of any kind. No dairy, except for butter. No crackers. I do not eat processed food. Salad dressings and BBQ sauces, I pay very close attention to the carb and sugar content. 2 grams of sugar is my max. I feed my cravings to an extent. Bananas are a treat. Baked potatoes are a craving. I use real butter. I eat a lot of raw veggies. I pay very close attention to anything with a nutritional label. Generally if food comes in a can, jar, box, package, or bottle, I don’t even pick it up. Anything else that has a label is inspected for carbs and sugar.
      I hope this helps

    • Gorgeous says:

      @Overwatch Helped a lot. Thanks.

    • pugilist102 says:

      You should keep taking metformin. Apparently it has a lot of other health benefits, such as longevity. You can look up the other benefits.

    • Aayiah aka LadyLuck says:

      Changed your diet, IF and avoid certain foods will help greatly. I am also T2D. The metformin made my foot go numb so yes I stopped taking them. My dr cannot treat the numbness bc there was no pain. Its quite frustrating when my own dr cannot help me but prefer to keep us medicated for life.

  • Darhawk Scott says:

    In March 2021, I lost 13 pounds after two weeks of being sick. I thought what a great start to losing even more weight to get healthy. While searching for fasting ideas I came across your channel and I watched a few videos and became inspired to try it out. So my start date was November 7, 2021 and I started to see the pounds melt away literally. I was losing about 3lbs a week on average. I must admit it was super hard to get through first 2-3 days. I was eating fresh food and no sugar or carbs (Ok maybe a couple carbs got devoured ). But I focused my attention on it and now see the results of this type of daily diet. It’s now February 17, 2022 (4 months approximately ) and I now weigh 193lbs. That’s a 35lbs loss ( before 228lbs-after 193lbs and I’m 5’10) Wow!! I had to buy new clothes and now wear smaller sizes. Now I still need to lose about 15lbs until I reach my long term goal. It’s been amazing and now my wife is interested in this based on my results. Hopefully I can give an update in another 3 months. [ Side note: Yes sometimes I still eat some minimal junk while doing this. Like chips and salas and potatoes and I don’t see it as failure but these food are good and more a treat. I also had to redefine what a treat is. It’s not every night after dinner, every day or week for myself. My sugar cravings have lowered so much that I don’t have a high sugar desire and makes these food overly sweet and they don’t taste as good anymore. So keep up the great work on this style of health. It’s working for me. Dr. Fung, Thanks for your advice! I wish I could thank you in person.

    • Zevi Katoyo says:

      Iam ugandan God bless you for what you’re doing. I have diabetes.

    • Bonny Toy says:


    • Darhawk Scott says:

      @Bonny Toy sure I can write an update. I am as of July 20th 177lbs and my lowest weight was 174. I’m actually on a two day fast as I write this. I just got back from vacation and that was challenging. It was hard to stay focused. I fasted 24hours and lost the vacation weight. So overall I have lost 55lbs total. I still like chips even though they are bad for you ( I’m looking for a great substitute). I also feel more in touch with my body and it’s reactions from eating certain foods. I eat more red meat and fresh vegetables. My waistline was a size 38” but now I am a 32” waist. I went out and bought all new pants and shorts (slowly not all at once). My elbows don’t hurt anymore and I believe it has help heal my leg from Lichen Planus( which is a whole other story). I have also told many friends and family about this channel due to the fact that it’s shocking to see me now. I personally know of 8 people that now fast daily based on my results. Remember I said my wife started fasting too, well she has lost now 30lbs since February or 2022. It’s been great! It’s been a fun journey to travel. Is it hard. Sometimes. Do I cheat and eat bad foods still? Yes but not daily or weekly now. I still love pizza. But not 1-2 a week. Low carbs and tiny amounts of sugar is my daily walk. I’m not perfect but I still am doing my fasting daily until I hit 170lbs. Then I will maintain. I do feel like it’s harder to maintain once I get to 170lbs. As I want to hover around 175-179. So I am still learning and changing daily. If you all have questions for me please ask and I hope I covered everything you might have been wondering.

    • dana102083 says:

      Chips arent good as theyre often slathered in more high sugar items..corn starch..maltodextrin and really poor seed oils. It can also cause more addictive tendancies to the a binge eater and even 4 yrs into keto lifestyle, staying away is best. There are keto french fries and chips by heavenly fan id suggest you try and see what you think! 🙂

    • HMM says:

      @Darhawk…you worked hard for your success and I hope you have continued to do well. I admire and appreciate your story. I sincerely hope you are still enjoying some of your favorite junk foods including chips. Some foods I limit but will never deny or substitute. That is the beauty of fasting and getting back your health can have your health and enjoy a large slice of pepperoni pizza every now and then …I love it…life is a beautiful experience 🙏.

  • Rebeca37 says:

    I really appreciate this video because I was completely clueless as to why my blood glucose was high this morning during fasting, and hearing you say that is normal gives me a great relief. Because I am not diabetic but I was worried. Thanks doctor, keep up the excellent work 👍👍

    • Hussein Najee says:

      Me too

    • jyothi krishnan says:

      I ve lost 10 kgs with intermittent fasting since June 2022 . Was feeling great till I did a routine blood works and my fasting was 120 , pp 154 and Hba1c 6.5 . I expected lower numbers considering I lost weight .
      My total cholesterol and LDL also high .
      I m in shock !!!

    • 琳枫玉树-钱姐 says:

      I was very upset when I found my blood sugar was high after fasting. Thanks for Dr. Fung. This days doctors are so busy, they have no time to speak to you when you have questions. I am glad I have found Dr. Fung.

    • arathi desai says:

      @jyothi krishnan are you doing low carb as well. Curious because I’m planning to start IF as my hba1c is 6.1

    • Dee Dee Israel says:

      @jyothi krishnan oh my goodness, it’s the same with me. I’ve lost 25 pounds and was expecting my numbers to be better but they were worse. I’ve been checking my glucose 2-3 times a day. I’ve been freaking out. I’m so glad I found this video. It calmed me down.

  • Scott Keeney says:

    This man has changed my life! Have lost 50lbs in three months (with no exercise) and my fatty live is healthy again. Thank you DR Fung!

  • Jason Fung says:

    Learn more about “8 Fasting Variants that Get Results” in this video –

  • James Clary says:

    Doing my first 24 hour fast. Checked my sugar this morning after 18 hours and not eating and it was 184! I freaked. I’ve had my blood sugar down to around 100 after six weeks of hard work. Glad I found your video

  • Kimberly Berry says:

    I was just diagnosed with type 2 and I immediately started watching your videos. Went low carb and fasting for 20 hrs per day, within my 4 hr eating I have two regular meals and so far, I feel fantastic!

    • Brian Jenkins says:

      How’s your blood sugar?

    • Secret Wood Project says:

      @Brian Jenkins I saw your question and since I just started the 20 hour fasting per day with a single meal, I am in my third week and my glucose went from average of 146 when I woke up in the morning down to just 97 when waking up in the morning and no higher than 132 after my one meal. My meals consist of 8oz of fatty steak, 6oz ofchicken, or a 6oz salmon filet and a whole plate of leafy spring mix salad with cheese, tomatoes, pickles, jalapenos, onions, and apple cider vinegar & olive oil for my dressing. I hope this helps.

    • Brian Jenkins says:

      @Secret Wood Project That’s pretty much what I eat also.

    • Lisa Defries says:

      Glad you got results that you happy with. But isn’t such extreme /fasting a risk to your gallstones/gallbladder problems. Although there are recognised benefits such as lower blood pressure, reduction in excess weight etc but my guess is all things in moderation and balance. Many people who are pre or diabetic can reduce their levels effectively via exercise/diet/sleep/relaxing/yoga etc. take care 😊

  • Ligaya Annawi says:

    Thank you very much Dr. Fung. I was lucky to have bumped into your channel last year. I did IF last year for more than 2 month and lost weight but because of depression I went back to overeating and gained back the weight I lost and more. I am again ready to do IF and I am more determined this time. I started last week and went on 20/4 for a week. I just started OMAD today. I am going to see to it that I will keep it this time.

  • Nancyann says:

    Thank you Dr. Fung, for explaining why glucose can go UP when not eating carbs. I thought I was doing something wrong because I recently started to get glucose levels in the low 90s while ketones are 1-2. I understand that I just need to keep going and eventually I’ll get to the next plateau of normalcy for my system. I do have another 15lbs to lose. Lost 40 so far. Not diabetic, no medications, age 61.

  • RF H says:

    Dr. Fung, I spread the word about YOU everyday. You are my hero. I began watching you years ago as you simply explained blood sugar in simple understandable terms. I loved the video and a year later, watched as many as I could. I unplugged the microwave, oven/ stove and ate nothing but live foods. I challenged myself to walk for hours. Found out later that dancing worked better than walking lowering blood sugar quicker. Lost 50 Lbs. You are my hero. My doctor took his glasses off, looks at me and asked “ How did you do this?” I told him Dr. Fungs intermittent Fasting. Love eating one meal around 2 pm and go to bed early.

  • Kim Marshall says:

    This is exactly what’s happening to me these days. My blood sugar has been rising despite fasting (6.5 mmol/L). I fasted another 3 hours and it was 4.6. After watching your video, it confirmed on the right track to fast longer until the blood sugar goes back down. Thank you ❤

    • Nan's Global Kitchen says:

      But when you eat after this long period of fasting the blood sugar goes really crazy high at least for me and I have not loss much. I was told it could be hypothyroid for me! At the health food store they put a drop of iodine on my wrist at it was supposed to disappear in one hour’s time but mine disappeared within 5 minutes so that’s the reason they said it’s my thyroids! I am so confused and fed up with all this.

    • Linn Booher says:

      2:53 I’m

  • Gayle Horton says:

    Thank you for explaining this. My husband was recently diagnosed as Type 2. For the past two weeks, he’s had a very clean diet. His after meal readings are around 92. So, I was surprised that his morning reading (after a 12 hour fast) ranged around 130. This is his body unloading the passengers from his train car. Your metaphors really help me understand what’s going on.

  • Kimmie F says:

    My A1C was 13 about 10 months ago. It is now 6.3. My fasting blood sugar rises when I fast. Driving me crazy beacause I couldnt figure out how I could go high when I’m not eating. Thank you doctor for explaining this process. You not only answered my question, but gave me more determination, knowing I’m doing the right thing! 🙏

    • kyoki86 says:

      Same and doctors should know this if he does and explain this to us but instead they’re quick to try to put you on meds that you don’t need.

    • B W says:

      So, do you still fast? How often and how many meals do you eat a day?

    • Kimmie F says:

      @BW-kv9wj  I still fast every day for 16 hours. Only drinking water, plain tea, and an occasional pinch of salt under my tongue. But if you eat enough protein and fat in your 8 hour eating window, I’ve gone as long as 22 hours without 1 hunger pang. Generally, I eat 2 meals (1 after I break my fast and 1 before I start a fast) with a protein focused snack in the middle. If an amount of food is important to your psyche, fill up on cruciferous vegetables or green vegetables. They are low-carb and great fiber. But for me, eggs are my super food. I eat them every day. 😊 The danger comes from what you add to them. If you’re just starting this journey, eat simple whole foods that are 1-3 ingredient recipes that you put together yourself. As you progress in your journey and get to know what affects your blood sugar (a glucose meter is a must. It will never lie to you and tell you it’s ok to eat something if it’s not, lol) You can eventually venture with food choices as you become more confident. But 1 more piece of advice, do not put anything into your body if you can’t say its name or know why it’s in your food, and take a great multivitamin 😊 It’s a lifestyle change that is so worth it. I feel better than I have in 20 years! Now that’s sayin something!

    • Bobbi Plunkett says:

      I’m still having issues with my blood glucose being high when fasting. Mornings are 145 to 180 and by the time I eat lunch (16 to 18 hr fasted) readings are still no lower than 125. After eating it drops to 100 to 120 in half an hr & slowly drops until dinner as low as 85. I’m a 64 yr old t2d female 5’3 183 pounds & no carbs for 3 months. Also no meds. Can someone give me some information on this please.

    • Kimmie F says:

      @bobbiplunkett3267  Watch the video carefully… the Doctor explains it. It will take a while for it to gradually lower. Keep at what you are doing, losing weight and eating low carb will help! It just takes time to rid yourself of the visceral fat. You got this!

  • Frances Koonce says:

    So glad to hear this information again from another well informed Dr.
    Had recent bloodwork and my Dr wants to put me on Metformin because my glucose was above what I prefer it at 130-140`s. I thanked them for their concern and explained that I always get an increase of insulin about 5:00 am and it reduces as the day goes on. I will never go on Metformin. I saw what it did to my mom.

  • Kimi Fia Minor says:

    Ive found some of your really old videos. What surprises me is how many doctors are so set in their ways that they don’t explore your scientific supported content. Thank you for putting so much of your time for producing these video all these years. That is astounding. The docs I’ve had treat me are not even willing to explain all my labs. 🎉

  • Jacqui Samsung says:

    Thank you so much, I honestly learnt so much from you and your kind explanations – you do not make people with diabetes feel inferior or doomed to failure- you encourage us to work on helping us improve. Bless you Dr

  • johfc says:

    I’ve had mixed results from fasting with the blood glucose level going up as mentioned. Dr Fung’s explanation is excellent, but quite simple and so logical when you think about it – your body is releasing stored glucose which you probably have a lot of. I decided to get more aggressive with fasting and probably fasted 36 hours which I have rarely done. I was surprised how effective this was in reducing a typical 12-15mM reading (after fasting) down to 6-8mM. I think the answer is to be persistent with the right extent of fasting, be strict about your diet when you don’t fast and don’t give up.

    • Terrieann Diehl says:

      ~AND~ DON’T GIVE UP!

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