Religious fasting is practiced in many faith traditions and may involve temporary abstinence from food, water, or selected foods. For people with diabetes mellitus, fasting alters meal timing and glucose homeostasis, increasing the risk of hypoglycaemia, hyperglycaemia, dehydration, diabetic ketoacidosis, and hyperosmolar hyperglycaemic state. Safe fasting requires individualized risk assessment, patient education, medication adjustment, regular self-monitoring of blood glucose, and close follow-up.

Patients with poorly controlled diabetes or those at high risk of acute metabolic complications should be advised not to fast.

Types of fasting

Normal fast

The individual abstains from all food for a limited period but is allowed to drink water.

Total fast

Complete abstinence from both food and water.

This type of fasting should not exceed three days and is not recommended for people with diabetes, particularly those receiving insulin or insulin secretagogues, because of the high risk of dehydration and hypoglycaemia.

Partial fast

The individual abstains from selected foods, beverages, or one or more meals during the fasting period.

For patients treated with insulin, partial fasting is generally the safest option.

Patients who should avoid fasting

Fasting is generally discouraged in patients with:

• Poor glycaemic control

• Recurrent or severe hypoglycaemia

• Hypoglycaemia unawareness

• Recent diabetic ketoacidosis

• Recent hyperosmolar hyperglycaemic state

• Acute illness or active infection

• Advanced chronic kidney disease

• Severe cardiovascular disease

• Pregnancy requiring insulin therapy

• Frail elderly individuals living alone

• Brittle type 1 diabetes mellitus

Possible complications during fasting

Patients should be educated about symptoms that require immediate medical attention.

Hypoglycaemia

Symptoms include:

• Sweating

• Tremor

• Palpitations

• Hunger

• Dizziness

• Confusion

• Blurred vision

• Loss of consciousness

Hyperglycaemia

Symptoms include:

• Excessive thirst

• Polyuria

• Fatigue

• Blurred vision

Dehydration

Symptoms include:

• Dry mouth

• Weakness

• Dizziness

• Hypotension

• Syncope

Diabetic ketoacidosis

Symptoms include:

• Nausea

• Vomiting

• Abdominal pain

• Rapid breathing

• Fruity breath odour

• Altered consciousness

Pre-fasting assessment

Patients intending to fast should ideally be reviewed 1–2 months before the fasting period.

Assessment should include:

• Review of glycaemic control

• HbA1c

• Blood glucose

• Blood pressure

• Renal function

• Current medications

• Previous episodes of hypoglycaemia

• Presence of diabetes complications

• Individual risk assessment

• Patient education regarding safe fasting

General management

Patients who choose to fast should receive individualized counselling.

General recommendations include:

• Patients with poor glycaemic control should be advised not to fast.

• Total fasting is not recommended for people with diabetes.

• Adequate hydration should be maintained during non-fasting hours.

• Partial fasting is preferred for patients receiving insulin.

• Vigorous physical activity should be avoided during fasting.

• Patients should have ready access to healthcare services during the fasting period.

• Blood glucose results should guide medication adjustment throughout fasting.

Self-monitoring of blood glucose

Self-monitoring of blood glucose (SMBG) is essential for all people with diabetes who elect to fast.

Recommended frequency includes:

Patients should be informed that finger-prick blood glucose testing and insulin injections do not invalidate religious fasting.

When the fast should be terminated

Patients should immediately break the fast if any of the following occur:

• Recurrent hypoglycaemia

• Severe hypoglycaemia

• Marked hyperglycaemia

• Positive urine ketones

• Acute illness

• Intercurrent infection

• Symptoms of dehydration

• Altered mental status

• Loss of consciousness

Pharmacological management

Patients treated with oral hypoglycaemic agents

Most patients can safely fast with appropriate medication adjustment.

Continue at the usual dose

The following medicines generally do not require dose reduction:

• Metformin

• Alpha-glucosidase inhibitors

• Thiazolidinediones

These medicines should be taken with meals.

Sulphonylureas

Patients taking:

• Glibenclamide

• Gliclazide

• Glipizide

• Glimepiride

should take the medicine at the time of breaking the fast (sunset meal) rather than before dawn to reduce the risk of hypoglycaemia.

Patients treated with insulin

Once-daily insulin

Patients receiving once-daily bedtime insulin can usually continue the usual bedtime dose.

Twice-daily short- and intermediate-acting insulin

Before the pre-dawn meal

Administer:

• The usual evening dose of short-acting insulin

Do not administer the intermediate-acting insulin.

Before the sunset meal

Administer:

• The usual morning dose of short-acting insulin

• The usual morning dose of intermediate-acting insulin

Basal-bolus regimen

Patients using basal-bolus therapy should:

• Continue the usual short-acting insulin before the pre-dawn meal.

• Continue the usual short-acting insulin before the sunset meal.

• Continue the usual basal (intermediate-acting or long-acting) insulin at approximately 10:00 pm.

Frequent SMBG is essential to allow appropriate insulin dose titration throughout the fasting period.

Recommended treatment according to fasting type

Non-pharmacological management

Patients should be advised to:

• Eat a balanced pre-dawn meal rich in complex carbohydrates.

• Avoid excessive intake of sugary foods when breaking the fast.

• Maintain adequate fluid intake during non-fasting hours.

• Avoid strenuous physical activity during fasting.

• Continue healthy dietary habits.

• Carry a source of rapidly absorbable glucose at all times.

• Inform family members about symptoms requiring urgent assistance.

Prevention

Safe fasting can be promoted through:

• Individual risk assessment before fasting

• Pre-fasting medical review

• Medication adjustment

• Patient education

• Frequent blood glucose monitoring

• Adequate hydration

• Appropriate meal planning

• Prompt termination of fasting when medically indicated

• Post-fasting clinical review

Outcome

Most patients with well-controlled type 2 diabetes mellitus can fast safely with appropriate medical supervision, individualized medication adjustment, regular blood glucose monitoring, and adherence to dietary recommendations. Patients with poorly controlled diabetes or those at high risk of acute metabolic complications should be advised against fasting, as the risks of hypoglycaemia, hyperglycaemia, dehydration, and diabetic ketoacidosis outweigh the potential benefits. Early counselling, careful planning, and close follow-up improve safety and reduce fasting-related complications.