I have the privilege of being the Medical Director at the Mission Medical Clinic, a Christian clinic for the working poor in Colorado Springs. Because we care for our patients’ physical, emotional, and spiritual needs, and because so many of our patients are diabetic, the question of how to fast when one is a diabetic comes up all the time. In addition, our patients need to fast before some lab tests or procedures.
So here’s my advice as I try to suggest a commonsense approach and consider the patient’s meds — all based upon the advice of the experts at Prescriber’s Letter:
For patients taking sulfonylureas, insulin, Prandin, and Starlix (which are the biggest culprits for hypoglycemia):

• For fasting labs or procedures, I suggest trying to get them done by 9 AM.
• I tell patients to hold their oral hypoglycemic or SHORT-acting or MIXED insulin until they eat.
• But I also explain it’s usually okay to take LONG-acting insulin … IF they get the labs done early and eat shortly after.

• For longer fasts for surgery, etc, I suggest not taking oral hypoglycemics the evening before and during the fast.
• Also I suggest holding short-acting insulins …and consider reducing the dose of long-acting or basal insulin by one-third to one-half.

• I caution type 1 diabetics to NOT stop their basal insulin … they still need some insulin during fasting to prevent diabetic ketoacidosis.

For patients taking metformin, Actos, Byetta, Victoza, Januvia, Onglyza, and Tradjenta (since these usually don’t usually cause hypoglycemia on their own):

• For fasting labs, it’s usually not necessary to stop these drugs … except Byetta since it’s taken before meals.
• For longer fasts, suggest holding these other oral meds too.

For Muslims, Ramadan is a different story because patients fast during the day and eat only at night for a month. See the recommendations from Prescriber’s Letter below. In addition, these patients should consult with their doctor in advance and consider switching some once-daily morning meds to the evening instead.
Diabetics (type 1 or type 2) who are fasting can always keep a watch on their finger stick blood sugars. I tell my patients to stop any fast if their finger stick glucose falls below 70 mg/dL … or even below 90 mg/dL in patients at higher risk for hypoglycemia. Also, I advise them to have some glucose or other carbohydrate on hand if necessary.
Finally, I discourage religious fasting for patients with poorly controlled diabetes, acute illness, or during pregnancy.
Here are more details from Prescriber’s Letter for “Fasting During Religious Holidays:”

Fasting for Religious Holidays
In a number of religions, fasting is performed. While the period of fasting varies, patients with diabetes must understand how to adjust their medications and monitor blood glucoses in order to avoid hypoglycemia and hyperglycemia.
It is important to note that many religions exempt patients with medical conditions or pregnancy from fasting.
However, some patients insist on fasting and should be educated about how to do so safely.
In general, if significant hypoglycemia develops (usually defined as a blood glucose of 70 mg/dL to 90 mg/dL, depending on the patient), the fast should be broken immediately and a carbohydrate should be consumed.
In the Jewish religion, fasting is performed on one to six holy days. Some of these fasts are only during daytime hours, while others last up to 25 hours.
On these days, restriction of intake of all food and liquid (especially on Yom Kippur) is observed.
For daytime only fasts in patients with type 2 diabetes, patients should eat a normal meal on the evening before the fast and can therefore take their short-acting medications (short- or rapid-acting insulin, mealtime meds) with this evening meal. Evening doses of intermediate or basal insulin should be reduced by about 20%.
For patients taking sulfonylureas, the bedtime dose on the day before the fast should be omitted.
In patients with type 1 diabetes, the evening doses of intermediate-acting or basal insulin should be reduced by 20% taken on the day before the fast.
On the day of the fast, patients with type 2 diabetes should not take any medications. For those taking insulin, blood glucoses should be monitored frequently, and short-acting insulin should be used for blood glucose values greater than 250 mg/dL.
Patients with type 1 diabetes should be instructed to take one-half to one-third of their intermediate-acting or basal insulin. Short-acting insulin should only be used for blood glucose values greater than 250 mg/dL.
After the fast, all medications and meals should be resumed, as usual.
In extended 25 hour fasts such as on Yom Kippur and Ninth of Av, patients should be instructed to follow the same instructions as those with the daytime fast, except evening doses of intermediate-acting or basal insulin should be reduced to one-half to one-third of the usual dose in patients with type 1 and type 2 diabetes.
In the event of hyperglycemia (blood glucose concentrations exceed 250 mg/dL), short- or rapid–acting insulin should be given.
After the fast, all medications and meals should be resumed, as usual.
Fasting during the holy month of Ramadan is a situation which requires education and planning.
It is important to note that fasting during Ramadan for patients with diabetes is discouraged and these patients are medically exempt, but many patients will insist on fasting.
Ramadan is a lunar-based month during which patients who are Muslim fast from predawn to after sunset for 29 to 30 consecutive days. Because the dates of Ramadan vary with the moon cycles, for the next decade, the fast will occur during the summer months in the Northern hemisphere. This will impact patients with diabetes who choose to fast because the summer months are a time when there are extended hours of daylight and therefore extended hours of fasting.
As with other reasons for fasting, patients who fast during Ramadan must check their blood glucose concentrations multiple times a day. Hypoglycemia and hyperglycemia are common during Ramadan.
Although there is very little published literature on the effects of fasting in patients with diabetes, one large, recent study has been published. In the population-based Epidemiology of Diabetes and Ramadan 1422/2001 (EPIDIAR) study, 12,243 patients with diabetes from 13 Islamic countries were studied.
Fasting during Ramadan increased the risk of severe hypoglycemia, requiring hospitalization 4.7-fold in patients with type 1 diabetes (from 3 events per 100 people per month to 14 events per 100 people per month) and 7.5-fold in patients with type 2 diabetes (from 0.4 events per 100 people per month to 3 events per 100 people per month).
These numbers are likely an underestimate, because patients who developed hypoglycemia but were cared for by family and friends and did not seek medical care were not included. Severe hypoglycemia was more common in patients in whom recent dosage changes of oral antidiabetic agents or insulin were made.
As with hypoglycemia, the incidence of hyperglycemia is increased during Ramadan.
In the EPIDIAR study mentioned above, there was a five-fold increase in hyperglycemia requiring hospitalization in patients with type 2 diabetes (from 1 event per 100 patients per month to 5 events per 100 people per month). In patients with type 1 diabetes, the incidence of severe hyperglycemia with or without ketoacidosis was 3-fold higher during Ramadan (from 5 events per 100 people per month to 17 events per 100 people per month). It has been proposed that the increases were a result of excessive reductions in antidiabetic medications.
Prior to the holy month of Ramadan, patients with diabetes who wish to fast should undergo a complete medical assessment including assessment of glycemic control, blood pressure, and lipids. Any changes to diet, exercise, or medication regimens should be made during this period so that the patient is on a stable and effective regimen when fasting begins. Extensive education regarding the importance of glucose monitoring during fasting and nonfasting hours, when to stop the fast, meal planning to avoid hypoglycemia and dehydration, appropriate meal choices to avoid postprandial hyperglycemia, and timing and intensity of physical activity is vital.
Management of patients with type 2 diabetes varies depending on their antidiabetes regimen. The information below summarizes adjustments to medications for patients with diabetes during Ramadan.
Adjusting Medications for Patients with Type 2 Diabetes who Fast During Ramadan
Patient Characteristics          

• Medication Adjustments      

Diet controlled

• Divide caloric intake over two to three smaller meals during nonfasting hours to avoid postprandial hyperglycemia.

Metformin

• Total dose of metformin unaffected, but the timing of ingestion should be altered.
• Take two-thirds of the total daily dose with the sunset meal, and remaining one-third with predawn meal.

Sulfonylureas

• Avoid chlorpropamide.
• Once daily sulfonylureas – take before sunset meal.
• Twice daily sulfonylureas – take usual dose with the sunset meal, one-half of morning dose with the predawn meal.
• Adjust dose based on blood glucose concentrations.

Alpha-glucosidase inhibitors — Acarbose (Precose, others), Miglitol (Glyset),

• Take only when ingesting a meal.

Meglitinides — Nateglinide (Starlix), Repaglinide (Prandin)

• Take only when ingesting a meal.

Exenatide (Byetta), pramlintide (Symlin)

• Take before sunset and predawn meals.

Dipeptidyl peptidase-4 inhibitors —  Linagliptin (Tradjenta), Saxagliptin (Onglyza); Sitagliptin (Januvia); Liraglutide (Victoza); Pioglitazone (Actos); Bromocriptine (Cycloset)

• No adjustment necessary.

Insulin

• Limited evidence suggests that rapid-acting insulin (aspart, glulisine, lispro) produces less hypoglycemia than short-acting insulin before meals.
• If using basal insulin (glargine, detemir), take 80% of dose and use regular dose of rapid-acting before meals.
• If using twice daily insulin or mixed insulin, consider switching to basal insulin, with rapid-acting insulin before meals; take usual dose at sunset meal and half usual dose at predawn meal.
• Finally, in patients with type 1 diabetes who fast during Ramadan, there are a number of potential regimens which can be utilized. Examples include:

• 70% of the pre-Ramadan insulin dose should be calculated and divided into 60% as basal insulin (glargine, detemir) with the evening meal, and 40% as rapid-acting (aspart, glulisine, lispro) divided in two doses. One dose of rapid-acting insulin should be given immediately prior to the sunset meal and the other dose prior to the predawn meal.
• In patients using 70/30 premixed insulin twice daily, give 100% of the usual morning dose prior to the sunset meal, and one-half of the usual evening dose at the predawn meal.
• As with all patients with diabetes, frequent blood glucose monitoring is mandatory, and the fast may need to be broken in the event of hypoglycemia during daylight hours.

Here are a couple of my other blogs on fasting:

• Regular fasting reduces heart risks
• Medical and Spiritual Considerations for Christian Fasting