Intermittent Fasting: Cardiovascular and Metabolic Benefits and Risks

Cardiovascular Benefits

The American College of Cardiology recommends an 8-12 hour eating window as the optimal balance between metabolic benefits and cardiovascular safety 1
Blood pressure reductions occur with time-restricted feeding, affecting both systolic and diastolic measurements 2, 3
Triglycerides decrease by 16-42% with intermittent fasting regimens 2
Total cholesterol and LDL cholesterol decrease, particularly in metabolically unhealthy individuals 2, 3
Insulin sensitivity improves with accompanying decreases in fasting glucose 2, 3
An analysis of 20,000 U.S. adults found that restricting eating to less than 8 hours daily significantly increased cardiovascular disease mortality risk compared to eating over 12-16 hours, both in the general population and those with existing cardiovascular disease 2, 3

The European Society of Cardiology identifies specific high-risk cardiac conditions where intermittent fasting must be avoided: Acute coronary syndrome 2, Advanced heart failure 2, Recent percutaneous coronary intervention or cardiac surgery 2, Severe aortic stenosis 3, Poorly controlled arrhythmias 3, Severe pulmonary hypertension [3] [2, 1, 3]

Patients with type 1 diabetes are at very high risk and should be strongly advised against fasting 4, 5
Patients with type 2 diabetes controlled by diet alone have quite low fasting risk, but should distribute calories over two to three smaller meals during the non-fasting interval to prevent postprandial hyperglycemia 4, 5, 6
Sulfonylureas carry inherent hypoglycemia risk during fasting and require individualized, cautious use 6, 4, 7
Metformin alone may be safe for patients to fast because hypoglycemia risk is minimal, with dosing adjustment to two-thirds of total daily dose immediately before the sunset meal, and one-third before the predawn meal 6, 4, 5
Patients on glitazones have low hypoglycemia risk and usually require no dose change 6, 4, 5
Patients with type 2 diabetes on insulin face similar risks to type 1 diabetes, though hypoglycemia incidence is lower, and require significant dose reduction and multiple daily glucose checks 6, 7

Fluid restriction and dehydration during fasting increase thrombotic event risk, particularly in older patients with hypertension and dyslipidemia who are often on anticoagulation, necessitating adequate hydration during non-fasting periods and awareness of dietary changes affecting warfarin stability 6, 5

Screen for absolute contraindications (cardiac conditions listed above) 2, 1, 3
Assess diabetes status and medications, with strong advice against fasting for type 1 diabetes patients unless willing to do intensive glucose monitoring and aggressive insulin reduction 4, 5
Establish 8-12 hour eating window, never shorter than 8 hours 1, 3
Ensure adequate hydration during non-fasting periods, especially in patients on anticoagulation 6, 5
Monitor closely during first 3-4 weeks with frequent glucose checks if diabetic 8

For patients on once-daily sulfonylureas, the entire dose should be shifted to before the sunset meal, while for twice-daily sulfonylureas, half the usual morning dose should be taken at predawn and the full dose at sunset, according to the American Diabetes Association 9
Patients with type 1 diabetes are at very high risk and should be strongly advised against fasting, as recommended by the American Diabetes Association 9
For type 2 diabetes patients on premixed insulin 70/30 twice daily, the usual morning dose should be taken at sunset and half the evening dose at predawn, as suggested by the American Diabetes Association 9
Excessive insulin reduction risks hyperglycemia and diabetic ketoacidosis, creating a narrow therapeutic window, according to the American Diabetes Association 9
For patients on metformin, dosing should be adjusted to two-thirds of the total daily dose immediately before the sunset meal and one-third before the predawn meal, as recommended by the American Diabetes Association 9
Patients on glitazones usually require no dose change during fasting, according to the American Diabetes Association 9
Once-daily dosing of glitazones can continue unchanged during fasting, as suggested by the American Diabetes Association 9

Pregnant women with any form of diabetes should avoid fasting due to high morbidity and mortality risk to both fetus and mother, as recommended by the American College of Obstetricians and Gynecologists 9
Very elderly patients with type 2 diabetes requiring insulin for many years should avoid fasting, according to the American Geriatrics Society 9
Patients with a history of recurrent hypoglycemia or hypoglycemia unawareness should avoid fasting, as recommended by the American Diabetes Association 9

Patients must receive pre-Ramadan assessment and education on physical activity, meal planning, glucose monitoring, and medication timing, as recommended by the American Diabetes Association 9
Close follow-up is essential during the first 3-4 weeks with frequent glucose checks if diabetic, according to the American Diabetes Association 9

No medication changes are needed for patients on diet and exercise control only, but they should modify timing and intensity of exercise to avoid hypoglycemia, as recommended by the American Diabetes Association 9
Ensure adequate fluid intake during non-fasting hours, according to the American Diabetes Association 9

In adults with diabetes who fast, basal insulin dose should be reduced to 70 % of the pre‑fasting dose (a 30 % reduction) and administered in the evening before the sunset meal; fasting glucose should be monitored closely, and the dose further adjusted by 2 units if more than half of the readings are consistently above or below the target range. 10

For fasting individuals with diabetes, rapid‑ or short‑acting insulin should not be administered at bedtime; instead, the total daily rapid‑acting insulin dose should be allocated (approximately 40 % of the total daily insulin) and split between the predawn (Suhoor) and sunset (Iftar) meals. 10