What is an FMD Fast? Understanding the Fasting Mimicking Diet

January 4, 2026 •

3 min read

According to research from the University of Southern California, a periodic FMD fast can significantly reduce visceral fat, lower risk factors for age-related diseases, and improve metabolic markers. This specific, plant-based diet is designed to trigger the body's self-renewal processes while still allowing for a limited intake of food.

Quick Summary

The Fasting Mimicking Diet (FMD) is a short-term dietary plan, typically lasting five days, structured with low-calorie, plant-based foods to induce a fasting-like state in the body, which helps to trigger cellular renewal and enhance metabolic health.

Key Points

FMD Definition: The Fasting Mimicking Diet is a periodic, plant-based diet that uses a low-calorie, low-protein structure to induce a fasting-like state.
Cellular Renewal: A key mechanism of FMD is autophagy, a process where the body cleanses and recycles old and damaged cells for rejuvenation.
Structured Protocol: FMD is typically followed in a five-day cycle, with higher calories on the first day and lower intake for the subsequent four days.
Potential Benefits: Benefits include weight and visceral fat reduction, improved metabolic markers (blood pressure, glucose), and potential anti-aging effects.
Clinical Development: The FMD was developed and clinically tested by Dr. Valter Longo and colleagues at the University of Southern California.
Important Warnings: The diet is not suitable for everyone and requires consultation with a healthcare provider, especially for those with health conditions or on medication.

The Fasting Mimicking Diet (FMD), developed at the University of Southern California, is a dietary program designed to replicate the health benefits of fasting while allowing for some nutrient intake. It controls calorie, protein, and sugar intake to induce a fasting state, activating cellular cleanup and rejuvenation. A typical five-day FMD cycle may offer benefits like weight management and improved metabolic health. The program aims to be more manageable than complete food deprivation.

The Science Behind the FMD Fast

An FMD fast works by manipulating macronutrient intake to trigger specific metabolic responses. The diet keeps calories low to prevent the activation of food-sensing pathways that stop regenerative processes.

Key Cellular Processes Activated by FMD

Ketosis: Reduced carbohydrates lead the body to burn fat for energy.
Autophagy: This cellular mechanism removes damaged components, promoting regeneration and anti-aging. FMD accelerates this process.
Stem Cell Regeneration: FMD may promote stem cell regeneration, contributing to health and longevity.

A Typical 5-Day FMD Protocol

A standard five-day cycle involves a specific calorie breakdown:

Day 1: Around 1,100 calories to ease into fasting.
Days 2–5: About 700–800 calories daily, with low protein and sugar and high healthy fats, to fully enter the fasting-mimicking state.

The meals are plant-based and often come in kits like ProLon.

FMD-Approved Food Examples:

Vegetable soups
Nut bars
Herbal teas
Olives
Supplements like chicory

Benefits of an FMD Fast

Research suggests potential benefits:

Weight Loss: Reduction in overall body weight and visceral fat, while preserving muscle.
Metabolic Health: Improvements in blood pressure, fasting blood sugar, and cholesterol.
Anti-Aging: Promotion of cellular rejuvenation and reduced inflammation linked to potential reduction in biological age.
Cellular Regeneration: Autophagy helps clear old cells and promotes new ones.

FMD Fast vs. Other Fasting Methods

Comparing FMD with other methods highlights its unique approach.


Feature	FMD Fast	Water Fasting	Intermittent Fasting (e.g., 16:8)
Food Intake	Specific low-calorie plant-based food.	Only water and non-caloric liquids.	Food intake within a time window.
Duration	Typically 5 days.	Usually 24-72 hours.	Daily or weekly cycles.
Protocol	Specific, structured, often with kits.	No formal protocol.	Flexible timing for eating windows.
Main Goal	Mimic fasting, cellular renewal, longevity.	Ketosis and autophagy via food restriction.	Weight loss, metabolic health via time restriction.
Side Effects	Mild fatigue, headaches, hunger.	Intense hunger, fatigue, dizziness, potential deficiencies.	Variable; fatigue or irritability.

Important Considerations and Potential Risks

An FMD fast is generally safe for healthy individuals but requires consulting a healthcare provider, especially with pre-existing conditions.

Who Should Avoid an FMD Fast?

Pregnant/breastfeeding women.
Individuals with eating disorders.
Underweight or malnourished people.
Those with diabetes, kidney, or heart disease.
Anyone on certain medications.

Common side effects like fatigue, headaches, and hunger are usually mild. Staying hydrated can help manage these. Note that only proprietary kits like ProLon are backed by clinical studies.

Conclusion

An FMD fast offers a strategic approach to gaining fasting benefits without complete restriction. Its structured, low-calorie regimen triggers cellular renewal processes like autophagy and can improve metabolic health markers. While more manageable than water-only fasting, it's a potent intervention requiring careful consideration and healthcare guidance. For those seeking cellular rejuvenation, metabolic improvement, and weight management, periodic FMD presents a compelling option. More information is available from resources like the USC Longevity Institute, where FMD was developed.

Frequently Asked Questions

An FMD fast, or Fasting Mimicking Diet, is a dietary program designed to replicate the physiological effects of fasting through a low-calorie, plant-based meal plan over a specific period, typically five days.

Unlike a water fast, which restricts all food and caloric beverages, FMD allows a limited intake of specific foods designed to keep calorie and protein levels low enough to trick the body into a fasting state.

Key benefits include weight loss, particularly a reduction in visceral fat, improved markers of metabolic health like blood sugar and cholesterol, and the promotion of cellular rejuvenation through autophagy.

Common and typically mild side effects reported by users include hunger, fatigue, headaches, and weakness, particularly during the initial days of the fast.

Pregnant or breastfeeding individuals, those who are underweight or malnourished, and people with certain medical conditions like diabetes, kidney disease, or eating disorders should avoid the FMD fast.

The diet consists of specific plant-based foods such as vegetable-based soups, nut bars, olives, and herbal teas, often provided in a pre-packaged kit like ProLon.

The frequency can vary based on health goals, but the developer, ProLon, often recommends completing three cycles once a month for the first three months, followed by cycles once every few months.