What is the fasting-mimicking diet 500 calories? A comprehensive guide

October 15, 2025 •

5 min read

Developed at the University of Southern California, the fasting-mimicking diet (FMD) is a scientifically formulated, short-term plan that triggers fasting-like effects while allowing a specific, low-calorie nutrient intake. The diet typically involves a 5-day cycle with calories reduced to approximately 500-800 on days 2-5, following a higher intake on day one.

Quick Summary

The fasting-mimicking diet (FMD) is a specific five-day, low-calorie, and low-protein dietary plan that induces a fasting state. This method promotes cellular renewal and metabolic health benefits, including weight loss and reduced inflammation, without complete food restriction.

Key Points

Definition: The FMD is a structured, 5-day low-calorie, low-protein diet designed to replicate the health benefits of fasting without complete food restriction.
Calorie Structure: After a higher intake on day one, the diet restricts calories to approximately 500-800 on days 2-5 to trigger a fasting-like state.
Mechanism: The diet works by influencing nutrient-sensing pathways to induce metabolic changes like ketosis and cellular repair processes such as autophagy.
Key Benefits: Reported benefits include weight loss (especially visceral fat), improved metabolic markers like blood pressure and glucose, and reduced inflammation.
Safety and Risks: Mild side effects like fatigue and headaches are possible. The diet is not suitable for everyone and requires medical consultation, particularly for individuals with pre-existing health conditions.
DIY Implementation: A DIY FMD can be created using plant-based foods rich in healthy fats but must be done with extreme care to ensure proper nutrition and avoid complications.

What is the fasting-mimicking diet 500 calories?

The fasting-mimicking diet (FMD) is a specific nutritional program created by researchers at the University of Southern California (USC) Longevity Institute, led by Dr. Valter Longo. Instead of a water-only fast, which can be difficult and carries risks, the FMD involves consuming a carefully balanced, low-calorie, plant-based diet for a limited period, typically five days. The program is specifically designed to provide essential nutrients while keeping the body in a fasting-like state, a biological trick to maximize the benefits of fasting while minimizing the side effects.

A typical FMD cycle, such as the commercially available ProLon program, involves a higher calorie intake on day one (around 1,100 calories) and then drops significantly to 500–800 calories for the next four days. These calories come from a very specific macronutrient composition, emphasizing healthy fats and complex carbohydrates while keeping protein low. This carefully calibrated approach is what distinguishes it from simple, unstructured calorie restriction.

How the FMD induces fasting effects

The FMD works by manipulating the body's nutrient-sensing pathways. By restricting calories and, crucially, specific macronutrients like protein, the diet keeps certain pathways—such as Insulin-like Growth Factor 1 (IGF-1) and mTOR—inactive. This sends a signal to the body that food is scarce, shifting its metabolic state towards ketosis (burning fat for fuel) and activating cellular repair and renewal processes.

Key biological processes triggered by FMD:

Autophagy: Often referred to as the body's internal 'house cleaning' process, autophagy involves the recycling of old and damaged cellular components. The FMD has been shown to accelerate this process, which may contribute to enhanced cellular function and longevity.
Ketogenesis: When carbohydrate and calorie intake is low, the body switches its primary fuel source from glucose to ketones, derived from fat. This metabolic state, known as ketosis, is a hallmark of fasting and is associated with various health benefits.
Stem cell regeneration: Research has indicated that periodic FMD cycles may promote stem cell regeneration in multiple systems of the body, potentially leading to the replacement of old, worn-out cells with newer, healthier ones.

Potential health benefits of the 500-calorie FMD cycle

Clinical studies on the FMD have shown promising results in various areas of health:

Weight loss and reduced visceral fat: The low-calorie nature of the FMD leads to weight loss. Studies have specifically highlighted a reduction in visceral fat, the dangerous fat stored around abdominal organs.
Improved metabolic health: FMD has been linked to improvements in multiple markers of metabolic health, including reduced blood pressure, lowered fasting glucose, and better cholesterol profiles (total and LDL cholesterol).
Reduced inflammation: Chronic inflammation is a driver of many age-related diseases. The FMD has demonstrated the ability to lower markers of inflammation, such as C-reactive protein (CRP).
Support for healthy aging: By promoting cellular repair and reducing risk factors for age-related diseases, the FMD offers potential anti-aging and longevity benefits.

FMD vs. other fasting methods


Feature	Fasting-Mimicking Diet (FMD)	Intermittent Fasting (e.g., 5:2)	Water-Only Fasting
Caloric Intake	Low calorie (~500-800 for most days of the cycle), specific macronutrient ratio	Very low calorie (500-600) on 2 non-consecutive days; normal eating on 5 days	Zero calories for the duration of the fast
Duration	Short-term cycle (typically 5 consecutive days), repeated periodically	Continuous, on a weekly basis (e.g., 2 days/week)	Varies, often 24+ hours, with medical supervision for extended fasts
Food Type	Plant-based, nutrient-rich foods (soups, nuts, olives) in specific amounts	Lean protein, vegetables, fruits (for the 500-calorie days)	Water and calorie-free fluids only
Benefits	Weight loss (visceral fat), metabolic health improvements, cellular rejuvenation, longevity	Weight loss, improved insulin sensitivity, general health markers	Potentially significant metabolic changes, cellular repair, but higher risk
Supervision	Recommended, especially for specific health goals or conditions	Generally considered safe for healthy individuals; minimal supervision needed	Crucial for extended periods, as it carries more significant risks

How to approach a 500-calorie FMD cycle

Disclaimer: It is essential to consult a healthcare professional before starting any restrictive diet, especially one as low in calories as the FMD. This is particularly true for individuals with pre-existing health conditions or those on medication.

For a do-it-yourself (DIY) approach, the key is to replicate the specific macronutrient composition of the clinical FMD while maintaining low calories. A DIY plan must be carefully crafted to avoid malnutrition and ensure a high intake of micronutrients from whole foods.

Sample DIY 500-calorie day (for days 2-5):

Breakfast: Herbal tea (unsweetened) with a small handful of almonds (approx. 10 almonds).
Lunch: A cup of vegetable broth-based soup with leafy greens (kale, spinach).
Dinner: Steamed broccoli and cauliflower with a tablespoon of olive oil.
Snacks: Limited amounts of specific low-sugar fruits like a few berries or a small piece of cucumber.

Important guidelines:

Stay hydrated: Drink plenty of water throughout the day. Herbal teas are also recommended.
Watch electrolytes: Ensure adequate sodium and potassium intake to prevent electrolyte imbalances.
Avoid intense exercise: Stick to light movement, such as walking, to conserve energy.

Risks and safety precautions

While studies suggest that the FMD is safe for healthy individuals, it is not suitable for everyone. Potential risks include:

Common side effects: Mild to moderate fatigue, headaches, dizziness, and weakness are the most frequently reported symptoms.
Not for everyone: Pregnant or breastfeeding women, individuals with a history of eating disorders, those underweight, or people with certain medical conditions (like diabetes, kidney, or heart disease) should avoid this diet without strict medical supervision.
Nutritional deficiencies: Long-term or improperly executed low-calorie diets can lead to nutrient deficiencies and potential muscle loss. This is why the FMD is a short-term, periodic program, not a continuous diet.

Conclusion

The fasting-mimicking diet, with its 500-calorie core cycle, offers a science-backed approach to reaping the benefits of fasting without abstaining from food entirely. By carefully controlling caloric and macronutrient intake, it triggers metabolic shifts, promoting weight loss, cellular rejuvenation, and reduced inflammation. While the FMD has shown promising results in clinical trials, it is a restrictive and specialized regimen. As with any significant dietary change, it should be approached with caution, knowledge, and, ideally, in consultation with a qualified healthcare provider to ensure safety and effectiveness. For those interested in deeper research, the work of Dr. Valter Longo at the USC Longevity Institute is foundational.

Frequently Asked Questions

The fasting-mimicking diet is designed to be completed in periodic cycles, not continuously. Clinical studies often use three monthly 5-day cycles, with cycles repeated every few months or as needed for maintenance.

No, the FMD is not suitable for everyone. People who are pregnant, breastfeeding, underweight, or have certain medical conditions like diabetes, kidney disease, or a history of eating disorders should not attempt this diet without close medical supervision.

The FMD is plant-based and focuses on healthy fats and complex carbs while keeping protein low. Foods include vegetable-based soups, nuts, olives, leafy greens, berries, and herbal teas. Processed foods, animal products, and refined sugars are avoided.

Yes, studies have shown that FMD can lead to weight loss, particularly a reduction in body and visceral fat. It is designed to preserve lean muscle mass while promoting fat burning.

Common side effects, which are usually mild to moderate, can include fatigue, weakness, headaches, and dizziness. Staying hydrated and following the diet's guidelines can help manage these symptoms.

While both involve low-calorie days, they are different. The 5:2 diet typically restricts calories to 500-600 on two non-consecutive days each week. The FMD is a more specific and structured 5-day cycle with a precise macronutrient composition designed to induce deeper cellular changes.

Medical supervision is highly recommended, especially if you have pre-existing health conditions or are on medication. A healthcare provider can assess your suitability for the diet and ensure it is executed safely.