Why does blood sugar increase during a fast?

Ask Dr. Jason Fung

Why is my blood sugar increasing during a fast? Will reversing diabetes prevent arterial plaque production in the future? And can keto and intermittent fasting help reverse thyroid disease (Hashimoto’s)?

It’s time for this week’s Q&A about intermittent fasting and low carb with Dr. Jason Fung:

Increasing blood sugar when fasting

Why does my blood sugar level increase from 86 to 112 mg/dl (4.8 – 6.2 mmol/L) when I do a 20-hour fast? Is that normal? I’m on an LCHF diet.

Thank you doctor,

Answer:This is a normal process. When you fast, insulin levels start to drop and this triggers a surge of counter-regulatory hormones, including noradrenalin and growth hormone. This is normal, and meant to pull some of the stored sugar from the liver into the blood. If your liver is full of sugar, it may release lots of sugar into the blood, causing the blood sugar to rise.

So, yes, blood sugar may rise during fasting. The most important question to consider, though, is where this sugar came from. If you are not eating, the rise in blood sugar may only come from your own body. You are simply moving sugar from the liver to the blood. It means that there is too much sugar stored inside your body and you need to empty it out, either with LCHF diet or intermittent fasting

Dr. Jason Fung

Arterial plaque

Reversing diabetes and therefore inflammation should stop future arterial plaque production, right? Does reversing diabetes cause plaque to recede from arteries already affected? Are there any studies on this?


Answer: Diabetes is a very strong risk factor for heart disease and arterial plaque. Reversing diabetes should theoretically lower the risk of plaque disease, but there are no studies that conclusively demonstrate this. That is probably because most people consider type 2 diabetes to be a chronic and progressive disease, like aging. So, yes, I think it will reduce risk of heart disease, but no, there are no studies to prove this.

Dr. Jason Fung

Hashimoto’s, fasting and keto

I believe there is a way to reverse Hash! I feel so much better on keto and IF than any treatment over the past three years. Have you had any patients reversing Hashi with this program?

Thank you so much for all your work for all of us,

Answer: I don’t see Hashimoto’s very frequently so don’t have much experience. Fasting and low carb diets usually drop insulin so are effective for diseases of excessive insulin such as obesity, type 2 diabetes and PCOS. However, it may also have a smaller effect on inflammation so it may certainly benefit Hashimoto’s as well, but there are no studies to prove this, and I have only limited clinical experience with Hashimoto’s.

Dr. Jason Fung



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Top comment

  1. Carol
    Oh and cut back on the frantic exercise which can be a stressor. Have some recovery days and smell the roses!!!
    Read more →

All comments

    I have just been told I'm prediabetic..this is sending me in to a tailspin..I also have high cholesterol..I'm not overweight 5'8 155lb I workout 6 days a week cardio and weights..I'm female 62 years I don't remember not being physically active I have watched my diet..I have no idea what my next move should be..yet all I keep reading is how it will be affecting my heart this has me terrified...I'm losing sleep..its giving me anxiety..what should I be doing to get a handle on this situation..
    Reply: #15
  2. Marcus
    Stress is really a huge factor for diabetes and heart disease. I know I have had seven heart attacks and one quad bypass. My body hates starches in almost any form. Rice, Pasta, white breads and wheat breads they use brown sugar in some wheat breads. IDK what your blood pressure is, but I meditate and use headphones to listen to music that helps bring down my BP. I use portion control to help.
  3. Carol
    Catherine, assuming (since you are on this site) when you say you "watch your diet" you must be eating LCHF and doing intermittent fasting.....then as the 2 Keto Dudes say...... "keep calm and Keto on". Read everything you can on this site. Listen to the ground breaking presentations of Dave Feldman, Dr Fung and all the amazing experts on this site. Follow their advice and you will avoid diabetes. Get your fasting insulin and HBA1c measured as more reliable indicators than blood sugar.
  4. Carol
    Oh and cut back on the frantic exercise which can be a stressor. Have some recovery days and smell the roses!!!
  5. Mary Ellen
    Keto made my Hashimoto’s worse. But it’s very individual. You just have to experiment.
  6. Lynn Churchill
    Dr Jason Fung. I am a nurse and a nutrition therapist on the Devon/Somerset border. Myself and a colleague (both in The Public Health Collaboration) who is a lead diabetes nurse have been discussing how our Hashimoto thyroids are functioning so much better on low carb/healthy fats. I am lowering my medication gradually and my colleague has come off medication completely. Although neither of us have diabetes I am becoming more and more aware with my clients who do, that the thyroid as well as the liver and pancreas go through a positive healing process. Keep up the good work. Lynn Churchill
  7. Bev
    Hi. I have been following this diet for 2.5 months now. Have lost 29 pounds, have been taken off all diabetes meds and am lowering my thyroid medications as well. My liver is healing from fatty liver disease and I believe that my thyroid will continue to improve with time. I think this diet does help Hashimoto’s disease as it has sure helped mine.
  8. Maria Vilma
    Hi. I am on the insulin pump and I was wondering if I need to release a bolus on my meal times if I'm fasting? I'm thinking I have my basal rate but do I have to do a Bolus? Doing that will plummet my blood sugar if I don't eat. Thank you for your reply.
  9. Sonny

    I am 45 years old and a T2D since 2003. I watched all the videos and started a keto diet, about 45 days ago. I incorporated intermittent fasting with the diet about 30 days ago. I was on Trulicity, Inovokana, and Metformin before the starting the Keto diet. Within a months time, I was completely off my medicines and my glucose levels were better than what I experienced while taking medicine. I am off my medications for 3 weeks now and continuing with strick keto a Intermittent fasting along with HIIT work out. Three weeks into it now I am observing high glucose levels, these levels are the same throughout the day (fasting 130 and 135 all day long) I am checking my sugar levels 3-4 times a day to keep a check. Not sure what is going on and why my levels are rising. Could you please advice on what could be the cause? Many thanks

  10. SueG
    There is a post by Dr. Fung in previous Q&A which explains that if you aren’t taking sugar in, it will start to released into your bloodstream from your liver which isn’t a problem and actually a good thing.
  11. JP Lott
    I having a huge issue with blood-sugar levels. I intermediate fast all day with an eating window of 4pm to 8pm, eating keto diet 70/20/10. (1 meal and snack) What happens is I wake in the Morning to high blood sugars between 11 to 17, after taking meds and insulin, my sugars drop slowly through out the day to end up at 6 or 7 before my eating window. After eating window they climb to about 8 or 9 and then I take meds and about half my insulin I normal take so I don't drop low in the night. I drop to about 5-6 at night, till about 3am, then they slowing climb again till about 10am stopping at 15 to 17, assuming from a dawn effect as I have no other explanation. Its really frustrating me as I get get my AC1 low ensure and the highs are starting to have really negative affects on my body, specially my feet. Any help and suggestions would be very welcome.
    Reply: #12
  12. Kristin Parker Team Diet Doctor
    Please work with your medical provider to see if you should adjust your medicine dose. In the meantime, I would not recommend fasting - instead focus on keeping each meal at ketogenic ratios until you can get in to see your doctor.
  13. Jane D
    Glad to see that this is a common event with IF. Is there any rule about how long these elevated sugars should last (it's a bit disconcerting to see, even knowing that it's normal) or any possible complications that we should watch for?
  14. Keral
    I am noticing same problem, rising blood sugars especially in morning, despite keto diet. Have been keto 2 years, no blood sugar issues until recently. But weight loss stopped cold 6 months in. Yeah, I can see that the liver is releasing sugar...But why are tissues NOT using it? This seems to me to be cellular level problem with mitochondria? If lacking B1,Mn, Mg krebs cycle gets downregulated? Signals cells to shut down insulin receptors? So diet can be perfect. Can fast for days on end, yet sugars stay way too high. I am not highly energized either--it seems to be just hanging out in bloodstream for most part. What the ??
  15. Kathryn
    Have you read the article about higher glucose levels whilst doing lchf and fasting.? It's on this site. You may not be prediabetic at all! Check it out and don't stress.
  16. Roxana

    Do you have experience with fasting with alopecia areata?
    I 'm 37 years old and I have this disease for few years. But This disease starts in childhood..around 10years and after that with short episodes in stress periodes.
    Three months ago I made a gut analysis and I have Zonulin over and other markers.. and seems to be "leaky gut".
    I started a cure with probiotics and enzymes. For 2 months I make IF (16:8)...and now i'm in extended fast ...85hours until now.

    My question is...is any chance to heal my gut and also my autoimmune disease???

    Thank u a lot

    Reply: #17
  17. Kristin Parker Team Diet Doctor
    Unfortunately I do not have any information or research about keto healing alpecia areata.
  18. Susan
    I have been fasting 3 days a week eating between 6:00 pm & 8:00 pm. When I am not fasting I do Keto. The second week of doing this I was getting low blood sugar readings under 70. I don’t want to stop fasting. I am insulin resistant and usually have high blood sugars. I lost 7 lbs in 2 weeks. Any ideas?
  19. Matthew
    I am having the same issue where BG levels are high during the fast mine is from 5.2 to 8.6mmol. My question is when eat doing omad. My BG drops to 4 to 5.6 on average. When I fast BG is up when I eat BG goes down. Any insight?
  20. Carter Califri
    Fasting Question:
    I'm a T2 24 hours into a 48 hour fast. After exercising my sugar went to 150. Should I continue to fast until it reaches 90 or less? Even if it requires me to go beyond the 48 Hours. Im not taking meds and normal diet is Keto 20 Grams <Carb/day??
  21. Jennifer
    Dear Dr. Fung
    As some above I do intermittent fasting three times a week. First time in 3 years, I saw an unusual extraordinary HIGH sugar spike when I tested at 7.45pm in the evening i.e. finishing the 24 hr fast, prior to eating dinner i.e. 8pm. Normally I am in range of 80 or 99mg/dl
    I had a shock it showed 146mg/dl?? I immediately did 2nd test within 3 mins it was 99mg/dl, then as backup to see if I had two normal results, did a 3rd test i.e. result was 89mg/dl. Bascially the results were two normal non diabetic range, v. 1 diabetic abnormal range.

    Should I view this as a one off situation, the temperatures had been soring and was 40 degrees time of taking the test? I had washed my hands as normal, no soap or other, simply warm water and dried thoroughly with clean towel.

    Next day, concerned of course, I did another fasting test the normal out of bed, before breakfast at 8 hours after sleep, result was as I normally experienced 85mg/dl.

    Could it be that the 24 hr fast for my body gives higher rating, it was the first time I had tried this test, I normally do the fasting day, and the next day I do the tests.

    Curious as to how this happened?

    Look forward to any guidance comments, and especially that with a 5 min period the levels dropped from sky high to normality?

    Thank you.

  22. Angela
    Based on the explanation of elevated blood glucose during a fast I shouldn't worry about it because it isn't like I have elevated blood sugar from eating. My question is doesn't this still affect my A1C?
  23. Barbara Throne
    I did a 2 day fast and my blood suger meter said I got down to 72. Weeks later I do a three day fast and on the third day it go to 58 and i ate lightly and it went back to Mid 70s. The next morning it was back to low 50s. I ate fruit and it came up quickly. Is more that a 2 day fast out for me?
  24. kassandra cotto
    I'm fighting with type one diabetes and my blood sugars for a period was all controlled but now they seem to be spiking up more frequently what could this be?
    Reply: #25
  25. Kristin Parker Team Diet Doctor

    I'm fighting with type one diabetes and my blood sugars for a period was all controlled but now they seem to be spiking up more frequently what could this be?

    That could vary based on many factors. Here is our guide for type 1 diabetes that may be helpful.

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