What is the 5-day fasting like diet?

October 29, 2025 •

4 min read

According to research published in Nature Communications, a 5-day fasting-mimicking diet can improve markers of prediabetes, reduce body fat, and reverse signs of immune system aging. This structured, plant-based eating plan, known as the Fasting-Mimicking Diet (FMD), tricks your body into a fasting state while still allowing limited food intake.

Quick Summary

The 5-day Fasting-Mimicking Diet (FMD) is a specific, plant-based eating plan designed to achieve the benefits of prolonged fasting with limited calorie and nutrient intake. It is performed in periodic cycles over a month to support weight management, cellular rejuvenation, and metabolic health.

Key Points

What it is: The 5-day fasting like diet, or Fasting-Mimicking Diet (FMD), is a scientifically-formulated, plant-based meal plan designed to provide the benefits of prolonged fasting while still allowing limited food intake.
How it works: By restricting calories, protein, and carbs for five days, the FMD activates metabolic pathways associated with fasting, promoting cellular cleanup (autophagy) and switching the body to burn fat for fuel.
Key benefits: FMD can lead to weight loss (particularly visceral fat), improved metabolic health (better blood pressure, insulin sensitivity), reduced inflammation, and signs of cellular rejuvenation.
What to eat: The diet consists of specific plant-based foods like vegetable soups, broths, nuts, seeds, olives, and certain teas, while restricting animal protein, dairy, and refined carbs.
Who should avoid: FMD is not suitable for pregnant or breastfeeding women, individuals with a history of eating disorders, those with certain medical conditions, or anyone underweight. Always consult a doctor first.
FMD vs. Intermittent Fasting (IF): Unlike the frequent, daily restriction of IF, FMD is a short, periodic reset (5 days at a time) that provides a specific, pre-packaged food protocol, making it different from standard IF approaches.
Safety: Research indicates FMD is generally safe for healthy adults when followed correctly, though mild side effects like fatigue and headaches can occur.

What is the Fasting-Mimicking Diet (FMD)?

The Fasting-Mimicking Diet (FMD) is a researched-backed, plant-based dietary program that lasts for five consecutive days. Developed by Dr. Valter Longo at the University of Southern California, it's designed to provide the body with essential nutrients while keeping calories, protein, and carbohydrates low enough to trick the body into a prolonged fasting state. This triggers a cellular repair process called autophagy and metabolic changes similar to those seen in water-only fasts, but with less restriction and perceived hunger.

Unlike traditional intermittent fasting methods that involve daily or weekly fasting windows, FMD is performed in periodic cycles, typically for five days a month over a few consecutive months. The most well-known and clinically tested version is the proprietary ProLon program, which provides prepackaged, specific foods for the five-day period. However, a DIY approach with careful planning can also be followed.

How does the 5-day FMD work?

The FMD functions by manipulating nutrient-sensing pathways in the body. By providing a very specific low-calorie, low-protein, and low-carbohydrate, but high-healthy-fat food combination, the diet keeps these pathways from being activated. This signals the body to switch its metabolic state from using glucose for energy to burning stored fat and producing ketones, a process called ketosis.

The key metabolic events during the 5-day cycle include:

Day 1: Transitioning to the fasting state. Calorie intake is slightly higher than on other fasting days. Your body begins to deplete its glycogen stores.
Day 2: Increased fat burning. The body's reliance on fat stores for energy ramps up, initiating the production of ketones.
Day 3: Autophagy begins. Cellular clean-up, or autophagy, is triggered. The body removes and recycles old or damaged cell components.
Days 4-5: Cellular rejuvenation. Cellular repair and regeneration continue, with fat-burning and ketone production remaining high. This is the period of peak rejuvenation and metabolic benefits.

What can you eat on a 5-day fasting like diet?

The FMD is a plant-based diet. It emphasizes healthy fats and vegetables while restricting high-protein and high-carbohydrate foods.

Allowed foods typically include:

Soups and Broths: Clear vegetable broths and vegetable-based soups (e.g., mushroom, tomato).
Vegetables: Leafy greens, broccoli, cauliflower, carrots, celery, zucchini, and cucumbers.
Healthy Fats: Small portions of olives, avocados, and olive oil.
Nuts and Seeds: Limited amounts of almonds, walnuts, and kale crackers.
Fruits: Small servings of berries.
Beverages: Herbal teas and plenty of water. Black, decaf coffee is often permitted in small quantities.

Foods to avoid include:

Animal protein (meat, poultry, fish).
Dairy products.
Refined sugars and processed foods.
High-starch grains and starchy vegetables (like white rice and potatoes).

FMD vs. Intermittent Fasting


Feature	5-Day Fasting-Mimicking Diet (FMD)	Standard Intermittent Fasting (IF)
Timing & Duration	5 consecutive days, typically once a month for a few cycles.	Followed daily (e.g., 16:8) or for 2 non-consecutive days per week (5:2).
Food Intake	Specific, low-calorie (700-1100 kcal) meals from plant-based, low-protein, and high-fat sources.	Can involve skipping meals or limiting calorie intake on certain days; food choices can vary.
Flexibility	Less flexible during the 5-day cycle due to strict food requirements, but not a long-term daily routine.	More flexible for daily life, but requires consistent adherence to a specific eating window or calorie limit.
Mechanism	Designed to activate autophagy and deep cellular repair through nutrient-sensing pathways.	Primarily focuses on weight loss and metabolic improvement via calorie restriction and insulin management.
Primary Goal	Supports healthy aging, cellular rejuvenation, and targeted fat loss.	Weight management, improved metabolism, and other health benefits.

Potential benefits of the 5-day fasting like diet

Clinical studies have shown that periodic cycles of FMD can lead to several health benefits, including:

Weight Loss: Reduction in body weight, particularly abdominal fat, while preserving lean muscle mass.
Metabolic Health: Improvements in blood pressure, insulin sensitivity, and cholesterol levels.
Cellular Rejuvenation: The promotion of autophagy, the process by which the body clears out damaged cells, contributes to overall cellular health.
Reduced Inflammation: Lowered levels of inflammatory markers associated with aging and chronic disease.
Longevity: Some research suggests FMD may decrease risk factors for age-related diseases.

Important precautions and risks

While generally safe for healthy individuals, the FMD is not for everyone and should always be undertaken with caution and professional medical advice. Potential side effects can occur, including:

Fatigue and weakness
Headaches
Dizziness and lightheadedness
Hunger and irritability

The FMD is not recommended for:

Individuals who are pregnant or breastfeeding.
Those with a history of disordered eating.
People with underlying medical conditions, such as diabetes, kidney disease, or heart disease.
Anyone who is underweight or at risk of malnutrition.

It is crucial to consult a healthcare provider before starting any new dietary regimen, especially one that involves prolonged calorie restriction.

Conclusion

The 5-day fasting like diet, or FMD, offers a unique approach to harnessing the benefits of prolonged fasting without requiring complete food abstinence. By following a structured, plant-based, low-calorie meal plan for a short, periodic cycle, individuals may experience targeted weight loss, improved metabolic markers, and enhanced cellular health. The program is designed to be a periodic intervention, not a daily lifestyle change, making it more sustainable for some compared to continuous calorie restriction or other fasting methods. While the potential for cellular rejuvenation and healthspan extension is promising, it is important to understand the specific protocols, potential side effects, and health contraindications before beginning. As with any significant dietary change, professional medical guidance is essential to ensure a safe and effective experience. For those interested in the proprietary version, additional details can be found through official sources, such as the Prolon website.

Frequently Asked Questions

The main difference is that a 5-day FMD allows for a limited intake of specifically formulated, plant-based foods, whereas a water-only fast permits no food intake at all. The FMD is designed to provide nutrients while keeping the body in a fasting state, making it potentially easier to complete than a prolonged water-only fast.

The recommended frequency for a 5-day FMD varies depending on individual health goals. Dr. Valter Longo suggests completing 3 to 4 cycles per year for general healthy aging benefits, or monthly for a few months for weight loss or metabolic health goals, followed by less frequent cycles for maintenance.

Light to moderate exercise is generally acceptable, such as walking, yoga, or stretching. However, strenuous, high-intensity workouts are not recommended during the five-day period due to the reduced calorie intake, as this can place unnecessary stress on the body.

Yes, studies have shown that FMD can be effective for weight loss, particularly in reducing visceral fat while preserving lean muscle mass. The effects are often more pronounced when performed in several monthly cycles, as demonstrated by the ProLon program's clinical studies.

Yes, some common side effects include fatigue, headaches, dizziness, and mild weakness, especially in the initial days. These effects are usually mild to moderate and subside over time. It is crucial to monitor your body's response and stop if adverse effects are severe.

It is possible to create a do-it-yourself (DIY) version of the FMD using plant-based, low-calorie foods, but it requires careful planning to achieve the specific macronutrient ratios needed to trigger the fasting-like state. Commercial kits like ProLon are precisely formulated based on research to ensure the correct nutritional balance.

After the five days are over, a transition day is recommended to slowly reintroduce normal eating, starting with easily digestible foods like soups and broths before returning to a regular, healthy diet. This helps prevent potential digestive issues and maximizes the benefits of the fast.