QUOTE from Dr Fung (emphasis mine):
What if I have diabetes?
SPECIAL CARE MUST be taken if you are diabetic or are taking diabetic medications. (Certain diabetic medications, such as metformin, are used for other conditions such as polycystic ovarian syndrome.) Monitor your blood sugars closely and adjust your medications accordingly. Close medical follow-up by your physician is mandatory. If you cannot be followed closely, do not fast.
Fasting reduces blood sugars. If you are taking diabetic medications, or especially insulin, your blood sugars may become extremely low, which can be a life-threatening situation. You must take some sugar or juice to bring your sugars back to normal, even if it means you must stop your fast for that day. Close monitoring of your blood sugars is mandatory. Low blood sugar is expected during fasting, so your dose of diabetic medication or insulin may need to be reduced. If you have repeated low blood sugars, it means that you are over-medicated, not that the fasting process is not working. In the Intensive Dietary Management Program, we often reduce medications before starting a fast in anticipation of lower blood sugars. Since the blood sugar response is unpredictable, close monitoring with a physician is essential. 
NOTE – What does it say?
When a person is fasting no nutrients are brought into the digestive system by mouth. The body begins to burn glucose in the bloodstream and supply glucose through the conversion of glycogen which is stored in the liver. When available glucose is used up the body turns to fat for energy. This is a process that is controlled within the human body. Diabetic medications force blood sugar to lower levels and can, therefore, interfere with the natural mechanism of the body controlling blood sugar. While I believe that it is unlikely for me, given the medications I take, diabetic medication could lower blood sugar levels to a point of danger to the human body in a person like myself with Type 2 diabetes.
Very low blood sugar is known as insulin shock, and it can be deadly. Wikipedia reports: A commonly used “number” to define the lower limit of normal glucose is 70 mg/dl (3.9 mmol/l) … Symptoms of diabetic hypoglycemia … can be mild, moderate or severe, depending on how low the glucose falls and a variety of other factors. It is rare but possible for diabetic hypoglycemia to result in brain damage or death. Indeed, an estimated 2–4% of deaths of people with type 1 diabetes mellitus have been attributed to hypoglycemia.
In North America a mild episode of diabetic hypoglycemia is sometimes termed a “low” or an “insulin reaction,” and in Europe a “hypo”, although all of these terms are occasionally used interchangeably in North America, Europe, Australia and New Zealand. A severe episode is sometimes also referred to as “insulin shock”…. Hypoglycemia is a true medical emergency, which requires prompt recognition and treatment to prevent organ and brain damage. [https://en.wikipedia.org/wiki/Diabetic_hypoglycemia]
For these reasons, it is important that my doctor be informed of my fasting practices and provide me with advice to help my blood sugar levels to remain above 70 at a minimum during fasting, as the typical lower limit of normal glucose is 70. This can include permission for me to decrease diabetic medication under my Physician’s advice during the time of fasting. Insulin shock is a particular danger to type 1 diabetics.
Typical symptoms of hypoglycemia include tremor, sweating, and palpitations; these are caused by the release of adrenaline (epinephrine) into the bloodstream when the body detects hypoglycemia. It is important to note that some diabetics do not display symptoms of hypoglycemia; this is why regular testing while fasting is important.
I believe that if I test myself frequently, up to four times a day, I can become familiar with how my body reacts to low blood sugar levels and respond appropriately. I also believe that it makes sense to eat something whenever my blood sugar is within low but normal levels. So I may break the fast if I test blood sugar at lower than 100. As a type 2 diabetic, if my blood sugar is above 130 I do not need to worry about insulin shock because I do not have hypoglycemia! As a type 2 diabetic, my normal blood sugar levels are frequently high; fasting will lower them.
I will break a fast if tested blood sugar is below 90.
If blood sugars are consistently normal (<130) in the morning, I will decrease nightly insulin by 5 units a night until the morning test is > 130, with the support of my physician.
My endocrinologist’s current advice to me: If my blood sugar is below 150 at bedtime, do not take evening insulin (Lantus). If above 150, take 20 units – half my previous presciption; he also put me back on Invokana.
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SOURCE – Footnotes:
 Quote from The Obesity Code: Unlocking the Secrets of Weight Loss by Dr. Jason Fung; Greystone Books (March 3, 2016). Page 252. Kindle location: 3981.
Photo Fasting-a-glass-of-water-on-an-empty-plate, via Wikimedia Commons
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Key: Fung-03 Shipped: 05/04/2021. Updated 01/11/2022.