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Fasting and Diabetes Mellitus: A Guide for Primary Care Physicians

By Dr. Nwamaka Osakwe

People with diabetes mellitus (DM) may choose to fast for spiritual, health, or other personal reasons. But fasting carries certain risks for people with DM. Therefore, clinicians should help patients who want to fast do so safely. This article can serve as a guide.

What is a fast?

A fast is abstinence from food or water or both for an extended period. For example, during Ramadan, Muslims abstain from food and drinks from dawn to dusk. People who also do intermittent fasting may fast for 16, 18, 20, or 24 hours. For example, a 16:8 intermittent fasting, known as time-restricted eating, involves fasting for 16 hours and eating within an 8 hours window.

During a fast, blood glucose levels fall, leading to a drop in insulin. The decrease in glucose and insulin levels triggers the body to increase glucagon and break down glycogen to form glucose. The body uses fatty acids as fuel when glycogen stores get used up.

What are the risks of fasting in people with DM?

The “main fear for fasting in patients with DM is the risk of hypoglycemia,” said Dr. Nicola Okech, a Consultant Endocrinologist with the Kenyatta University Teaching, Referral and Research Hospital, Nairobi, Kenya. The risk of hypoglycemia is notably higher in those on insulin and insulin secretagogues like sulfonylurea.

Hypoglycemia can lead to palpitations, tremulousness, seizures, loss of consciousness, and even death. In one study, a missed meal was the most common cause of hypoglycemia among patients with type 2 DM.

Another problem people with DM may encounter when fasting is “variations in glucose with occasional spikes of blood glucose,” continued Dr. Okech. These spikes in blood glucose levels are more common around the time the fast is broken. Dr. Okech explained that the spikes occur because most people break their fast with meals rich in carbohydrates and saturated fat.

The National Institute of Diabetes and Digestive and Kidney Diseases also points out that hyperglycemia may occur because people cut back on their medications. When people stop their medications, they risk developing hyperglycemia and even diabetic ketoacidosis. This emphasizes the need to educate patients on the importance of working with their healthcare team.

Dehydration is another potential challenge for people not taking fluids during their fast. This risk may be higher for patients on medications like sodium-glucose cotransporter 2 inhibitors and diuretics. If a person develops hyperglycemia, there’s a risk of osmotic diuresis and dehydration. People who have a fever have a higher risk of dehydration. Dehydration can cause hypotension, syncope, and falls.

Strategies for ensuring safe fasting in people with DM

People who want to go on a prolonged fast may need risk stratification. Patient safety is a critical factor, and Dr. Okech said not all patients would be eligible to fast. Many experts do not recommend fasting in the following cases.

  • Severe hypoglycemia in the last 3 months
  • Recurrent hypoglycemia or hypoglycemia unawareness
  • Diabetic ketoacidosis in the last 3 months
  • Hyperglycemic hyperosmolar state in the previous 3 months
  • A person on maintenance dialysis
  • Significant dementia
  • Acutely ill, intense physical activity, or pregnant
  • Children with type 1 diabetes

People who choose to fast can implement several strategies.

Pre-fast strategies

The Recommendations for the management of diabetes during Ramadan; Update 2020 state that it is crucial to review management plans and adjust medication regimens. A similar approach can be used for patients who also wish to fast for a prolonged duration for other reasons besides Ramadan.

Given the dangers, it is important to teach patients the symptoms of hypoglycemia, such as sweating, anxiety, and confusion. Patients should also be asked to stop a fast if there are symptoms of hypoglycemia, hyperglycemia, dehydration, or blood sugar less than 3.9mmol/L or greater than 16.6mmol/L.

Dr. Okech cautioned against initiating new drugs 4 weeks before the intended fast as this is insufficient time to assess their glucose-lowering effects and manage adverse events.

Besides adjusting the medications, the patient will benefit from nutritional therapy. A consultation with a dietitian would help the patient plan meals appropriately. A fasting period may also help motivated overweight or obese patients lose weight if adequately counseled.

Furthermore, patients should also be taught self-glucose monitoring. 

Close up of woman hands using lancet on finger to check blood sugar level by Glucose meter using as Medicine, diabetes, glycemia, health care and people concept.

Review medications

When reviewing medications, healthcare professionals must consider

  • The control of DM in the last 2-4 weeks
  • Duration of the fast
  • Mechanism of action of DM medication
  • Onset and duration of action of DM medication.

For example, sulfonylureas are insulin secretagogues with a significant risk of hypoglycemia. The duration of action lasts about 24-36 hours. Therefore, it is appropriate to stop the drug for at least 24 hours before a 24-hour fast, according to Martin Grajower, MD (as per NIDKK). Some experts recommend half the dose for a partial day fast, but this should be done with caution.

If the patient is going to eat during the 24-hour period (for example, Ramadan fast, 16:8 intermittent fasting), then the patient can continue with metformin, thiazolidinediones, and DPP4 inhibitors. For people on metformin only, the timing of the dose may be modified so that two-thirds of the daily dose is taken with the heavy meal. So, a patient on Ramadan fast receiving a total of 1,500mg of metformin should get 500mg during the predawn meal and 1000mg with the sunset meal. But if the person is undergoing a 24-hour fast, the last dose should be taken by the last meal.

For patients taking SGLT2 inhibitors, there are concerns about dehydration. Experts recommend taking the SGLT2 inhibitor with their meals when they break the fast. Patients should also increase fluid intake during the non-fasting period. It is essential that managing physicians review other medications that can cause dehydration. A patient on an SGLT2 inhibitor who feels unwell during the fast should be tested for ketones because of the risk of DKA.

Glucagon-like peptide-1 agonists have been used safely during fasts. However, weekly regimens may be more straightforward for patients. It’s best to titrate to a tolerable dose about 4-8 weeks before a prolonged fast begins.

People with type 2 DM embarking on a 24-hour fast may need to reduce basal insulin to a third or half of the usual dose taken at the usual time. For Ramadan, the basal insulin may be reduced by 20% and taken with the predawn meal.

Adjusting the patient’s drug regimen may be tricky. As such, you may want to work with a specialist. If physical access is unavailable, consider arranging a virtual consultation for the patient.

Blood Glucose Monitoring

Monitoring blood glucose during the fasting period is important, said Dr. Okech. She further explained that blood glucose monitoring allows for better control over blood glucose and avoids variability. This monitoring should include self-monitoring of blood glucose and regularly scheduled visits to the doctor. 

The frequency of self-monitoring is individualized. For example, people with a higher risk of hypoglycemia, such as those on insulin or insulin secretagogues, may need more frequent monitoring. In addition, monitoring should be more frequent at the beginning of the fast after a dose adjustment to ensure the adjusted dose is optimal.

Post-fast strategies

Dr. Okech said, “the trend is to break the fast with high-carb meals, mostly of high glycemic index.” She suggests that patients with diabetes should avoid this trend as they lead to poor glucose control. Instead, she advised patients to make appropriate food choices such as vegetables, high-fibre carbs, proteins, and unsaturated fat.

Because religious fasts may end with festivities and high food consumption, portion control is crucial.

After a prolonged fast, physicians should review weight, blood pressure, HbA1c, and medications.


Managing a patient during a fast is highly individualized. However, there are tools and digital apps that can help. Below are resources you may find helpful.

Useful Resources

Fasting safely with diabetes

24-hour fasting with diabetes: A guide to physicians

Recommendations for management of diabetes during Ramadan

Clinical management of intermittent fasting in patients with DM

Diabetes and Ramadan International Alliance

Nwamaka Osakwe, FWACP, MBBS, is an award-winning physician who loves writing about health and wellness. You can reach her here

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