Know the contraindications before attempting FMD

The FMD is not universally safe — understand who should not do it before starting.

Why it works

The FMD produces significant metabolic and hormonal shifts: insulin falls, glucagon rises, and the body transitions to partial fat-burning. In populations dependent on insulin (type 1 diabetes), at risk from undernutrition (underweight, malnutrition), or with histories of disordered eating, these shifts can produce dangerous hypoglycemia, accelerated catabolism, or psychological harm. Medications that require food for absorption or that affect glucose metabolism further complicate the protocol.

How to do it

1. Do not begin FMD without physician clearance if: you have diabetes (any type), are underweight (BMI < 18.5), are pregnant or breastfeeding, have a history of anorexia or bulimia.
2. Disclose all medications to a physician — metformin, insulin, and many other drugs interact with fasting states.
3. If you develop severe dizziness, fainting, or persistent heart rate irregularities during the 5 days, stop and eat.
4. Consider working with a registered dietitian for the first cycle if you have any metabolic health conditions.

Evidence

Contraindication lists are based on clinical judgment and known metabolic physiology rather than RCT-derived safety data on adverse events. The clinical trial protocol excluded these populations for these reasons. (clinical)

Safety data beyond the 71-person pilot trial is primarily from the commercial ProLon product’s adverse event reporting, which is not published in peer-reviewed form.

Common mistake

Dismissing contraindications because "it’s just eating less." A 5-day 800-calorie protocol is a clinically significant metabolic intervention with real risks in vulnerable populations.