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Short Summary
Main recommendation: Do not be alarmed if blood glucose rises during fasting, especially first thing in the morning, because this dawn phenomenon is a normal release of stored glucose; but if you take diabetes medications, talk to your doctor about adjusting them when you change your diet. Dr. Fung says fasting itself does not usually cause dangerously low blood sugar in people not taking diabetes medications, because the body converts stored glycogen, body fat, and in extreme cases protein into glucose.
Key Points
- The dawn phenomenon is a surge of counter-regulatory hormones (noradrenaline, the sympathetic nervous system, growth hormone, and cortisol) driven by circadian rhythm that releases stored glucose into the blood around waking.
- In normal people the morning rise is very small, while some people with type 2 diabetes see blood glucose climb very high in the morning without eating.
- Fasting does not cause low blood sugar in people who take no diabetes medications, since the liver's glycogen, body fat, and gluconeogenesis keep glucose stable.
- Low blood sugar risk during fasting comes mainly from diabetes medications, because the body uses up its glucose while the medication keeps lowering it.
- Dr. Fung says ketones become a major fuel for the brain during fasting, which he believes explains why fasted people felt normal even at very low glucose levels.
Details Worth Noting
- Concrete instructions: If you take medication for diabetes and change your diet or fast, speak to your doctor about adjusting your medication before or as you change your diet. If your blood glucose tends to go too low while fasting, reduce your diabetes medications under medical guidance. If blood glucose is too low, you may need to stop the medications completely. If your blood glucose rises during fasting, continue fasting and keep going until that blood glucose comes back down, since the rise reflects glucose your body is releasing from its own stores.
- Dr. Fung describes a study of men fasted for 30 to 60 days whose baseline fasted glucose was about 4.5-5.5; three of them were given 20 units of insulin over 24 hours, driving glucose down to about 9, and they remained asymptomatic.
- Dr. Fung cites a 1972 study in which nine people were fasted for 60 days and then given intravenous insulin; he says all participants felt normal even as glucose became almost unmeasurable in many cases.
- The speaker notes that lowering diabetes medication doses reflects improving diabetes, so it is a positive outcome rather than a problem.
- The speaker presents these points as explanation and does not offer them as professional medical advice.
Bottom Line
A rising blood glucose reading during fasting is usually the body's normal stored-glucose release rather than a sign of harm, while anyone on diabetes medication needs medical guidance because fasting can push glucose too low.
