Why is blood glucose elevated when fasting?
, medical review by Dr. Bret Scher, MD
in Diabetes, Intermittent fasting, Q&A 
Why is blood glucose elevated when fasting? Do you need keto macro guidelines when fasting? What is the optimal way of breaking a fast? And, can you talk about the psychological part of fasting?
It’s time for this week’s Q&A about intermittent fasting and low carb with Dr. Jason Fung:
Fasting & elevated blood glucose
Type 2 diabetes for 20+ years. Started keto with intermittent fasting in January 2019. Why would my fasting blood glucose numbers during my 18-24-hour fast be elevated, then come down to normal ranges after eating?
Mindy
Fasting can definitely raise blood glucose. This is due to the effect of insulin falling and the rising counter-regulatory hormones including increased sympathetic tone, noradrenaline, cortisol and growth hormone, in addition to glucagon. These all have the effect of pushing glucose from liver storage into the blood. This is normal. If you are not eating, you want to use some stored glucose. The question is this – if you are not eating, and your blood glucose went up, where did that glucose come from? It can only have come from your own body (liver). So, it’s a natural phenomenon, and the fasting now allows your body to use some of the glucose for energy.
Dr. Jason Fung
When combining keto and 5:2 intermittent fasting on fasting days, do you still follow the macro guidelines for keto?
On the two days of fasting, should the 500 calories consumed be of the same macro proportions as followed on the normal keto days? Or can you consume 500 calories of any food without going out of ketosis?
Andrea
I don’t generally recommend people to count calories. Fasting means not eating anything at all, so there is no ‘counting macros’. Dr. Mosley’s 5:2 diet allows 500 calories of whatever you like. But that’s his diet, not mine. I advocate intermittent fasting, with no calories.
Dr. Jason Fung
Veggies or fat or protein
I was taught to eat my salad first, then the meal… but I read that you advised to break a longer fast with ¼ cup of Macadamia nuts, and an hour later a salad. He talked about the veggies doing their thing in the upper intestine, and the fat and protein in the lower. My question is which should I eat first to receive the best benefit. I eat once a day while fasting at least 24 hours. When 24 hours comes past 8 pm, I wait until the next day around lunch, then move ahead one hour each day and repeat.
William
I think you should eat the salad first. There is some data to suggest that eating the veggies first reduces insulin.
Dr. Jason Fung
Will fasting for long periods of time activate the famine/fat protocols in the body?
Hi Dr Fung,
I’m very interested in intermittent fasting and keto for weight loss, I am however concerned about the “starvation” factor – not so much physiologically but also mentally. I’ve just finished reading Jon Gabriel’s book where he talks about fat programs in your body and the importance of not “dieting” or denying yourself food because it activates the fat programs. I’m worried that fasting will “switch on” the “famine/fat program” and there will be more weight gain in the long run. Can you help me understand how fasting will not activate the “fat/famine” protocol in my mind and body?
Kerren
Yes, the psychological part of fasting is hard. Fasting is supposed to be a part of everyday life – that is why we have the word ‘break-fast’. Most people think they can’t do it, but the truth is that people have fasted throughout history. We only think we can’t. As with anything else, it is about getting the right education and the right support group. From a physiological standpoint, fasting allows you to burn body fat, which is a storage form of food for your body when no food is available. This is what most people want.
Dr. Jason Fung
More
Intermittent fasting for beginners
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77 comments
Please work with your doctor for specifics, but generally speaking, below 90 is not too low. If you are on medication please read this.
https://www.dietdoctor.com/low-carb/with-diabetes-medications
Check out this article on how to reverse diabetes naturally https://www.dietdoctor.com/diabetes
Lower blood sugar does not necessarily indicate fat burning.
Not necessarily since the A1c will average out the whole day across the 3 month testing window.
Please check in with your doctor. The vertigo or some sort of system stress on your body may have affected your blood sugar but you should see your doctor for thorough diagnosis and treatment.
Thanks
Please check in with your doctor.
Is it normal that Uric Acid also increases after a fast? Can you please confirm on this matter as I seem to have mild symptom of gout attack after 2 days of OMAD?
Here is our guide regarding gout.
https://www.dietdoctor.com/low-carb/side-effects/gout
Thank you for your materials! Greatly appreciated!! I am relatively new to intermittent fasting. Its mostly 16/8 but have recently introduced an occasional 48hr fast and other times 'one meal a day'. Been doing I.F. for about 5-6 weeks and have lost 15#. Recently introduced brisk walks as well. This week, I started tracking my glucose, ketone, and GKI with a meter. My question is this, my ketones seem to hover in the "mild ketosis" range, however, my glucose appears to be vary greatly from reading to reading. One morning it was 81 (before starting my last 48 hr), but then about 36 hours into it, my glucose reading was 111. After breaking the fast, the next morning it was at 104. My question is this.... should I be concerned (see my doctor) about these fluctuations (even during fasting windows) or is this part of the liver/body trying to adapt to the more recent aggressive fasting cycles (more one meal a day with soon cycling in a 48 hr once a week or so)???
Diet Doctor recommends medical supervision for fasts longer than 24-36 hours.
I am not diagnosed with Diabetes but before I started doing IF and Lowcarb lifestyle, I know that my glucose was already high (I was OW).
Now that I have been practicing IF and Lowcarb for 6 months, I have noticed that my glucose in the morning (while in a fasted state for 16 hrs) is quite high, its at 105 to 113 while my postprandial glucose will be from 80 to 90.
Is there something I can do to improve my glucose while fasting? Is my postprandial glucose ok? Please help.
These articles may be helpful for you!
https://www.dietdoctor.com/low-carb/fasting-blood-glucose-higher
https://www.dietdoctor.com/low-carb/the-dawn-effect
I 've been doing a water fast for seven days to lower glucose since becoming insulin resistant. It has done an amazing job! On the seventh day of my water fast my blood glucose was 5.3 mmol/l in the late evening, but by the next morning it was 6.5 mmol/l. I slept approximately four hours during the night which may be related to the higher glucose, nevertheless, I would expect the liver to have used all the stored glycogen already, how long does it typically take to deplete glycogen in the liver, and if it's not from the liver where is the increase in glucose coming from? Thanks for providing more details!
Hi, Jean! We don't recommend extended fasts longer than 36 hours without a doctor's supervision. Please check in with your physician with any questions or concerns regarding your glucose levels.
Thanks for the reply, is it possible to explain in general, how long does it typically takes to deplete glycogen in the liver when fasting? For example, if doing a 36 hours fast is it expected that the liver would still have some glycogen, and if not what are the possible reasons?
Please check with your doctor, especially if you are on any medication. Fasting when taking medicine for diabetes can be dangerous.