What are the Rules for the Fast 800 Diet?

October 12, 2025 •

2 min read

According to a study published in The Lancet, a low-calorie diet of 800 calories per day, similar to the Fast 800, can lead to significant weight loss and type 2 diabetes remission. This guide breaks down what are the rules for the Fast 800 diet, a popular program developed by Dr. Michael Mosley that focuses on flexibility and long-term health improvements.

Quick Summary

The Fast 800 is a flexible weight-loss and health program with three phases: The Very Fast 800 (800 calories daily), The New 5:2 (800 calories twice a week), and the Way of Life (maintenance). Each phase emphasizes a low-carb, Mediterranean-style diet rich in protein, fiber, and healthy fats.

Key Points

Three Phases: The Fast 800 includes The Very Fast 800 (800 calories daily for up to 12 weeks), The New 5:2 (800 calories twice per week), and the Way of Life (maintenance).
Mediterranean-Style Eating: The program is built on a moderately low-carb, Mediterranean diet rich in protein, healthy fats, fiber, and vegetables.
Food Restrictions: Avoid processed foods, refined carbohydrates like white bread, sugary snacks, and low-fat products.
Intermittent Fasting: The plan uses intermittent fasting to trigger metabolic changes, especially in the 5:2 approach and during the initial rapid weight loss phase.
Hydration is Key: Drinking plenty of water and herbal teas is essential, especially during the 800-calorie days, to combat potential side effects like headaches and tiredness.
Medical Supervision: The intense, rapid weight loss phase should only be undertaken under medical guidance, particularly for those with underlying health issues like diabetes.
Long-Term Strategy: The ultimate goal is to transition to the Way of Life phase, which emphasizes sustainable, healthy eating habits without strict calorie counting.

The Three Phases of the Fast 800

The Fast 800 diet is structured into three adaptable phases, moving from initial weight loss to long-term maintenance, all based on Mediterranean eating principles.

Phase 1: The Very Fast 800

This phase is designed for rapid weight loss and improving metabolic health.

Caloric Intake: 800 calories per day.
Duration: Typically 2 to 12 weeks, or until a healthy BMI is reached. Medical supervision is recommended, especially with health conditions like type 2 diabetes.
Diet: Focus on whole, nutrient-dense foods following a moderately low-carb, Mediterranean style. Meal replacement shakes can be used, but whole foods are prioritized.

Phase 2: The New 5:2

This intermittent fasting phase supports gradual weight loss and metabolic health.

Caloric Intake: 800 calories on two non-consecutive days each week.
Other Days: Maintain a moderately low-carb, Mediterranean-style diet.
Flexibility: Choose fasting days that fit your schedule.

Phase 3: The Way of Life

This is the long-term phase for maintaining weight and a healthy lifestyle.

Caloric Intake: No calorie counting required.
Eating Pattern: Continue a sensible, moderately low-carb, Mediterranean-style diet.
Goal: Make healthy eating a sustainable habit.

Core Dietary Principles and What to Eat

The Fast 800 emphasizes a Mediterranean-style diet across all phases.

Foods to include:

Lean protein (fish, chicken, eggs, tofu, legumes, dairy).
Healthy fats (olive oil, nuts, seeds, avocados).
Non-starchy vegetables.
Fiber-rich whole grains and legumes in moderation.
Low-sugar fruits (berries, apples).
Plenty of water, herbal teas, and black coffee.

Foods to avoid or minimize:

Processed foods and refined carbohydrates.
Added sugars and sugary drinks.
Low-fat products high in sugar.
Excessive sweet fruits.
Snacking, especially on fasting days.

Comparative Analysis of Fast 800 Phases


Feature	The Very Fast 800	The New 5:2	The Way of Life
Calorie Intake	800 calories daily.	800 calories on two non-consecutive days.	No calorie restriction, sensible portions.
Pace	Rapid weight loss.	Gradual weight loss.	Weight maintenance.
Duration	Up to 12 weeks, medical supervision advised.	Can be followed long-term.	Designed for life.
Diet Style	Moderately low-carb, Mediterranean.	800-calorie Mediterranean on fast days; regular Mediterranean on others.	Low-sugar, Mediterranean.
Who It's For	Significant weight loss, blood sugar management (under guidance).	Gradual weight loss or maintenance.	Sustainable healthy lifestyle after reaching goal weight.

Important Considerations and Safety

The Fast 800 is not suitable for everyone, including those with a history of eating disorders, pregnant or breastfeeding women, and individuals with certain medical conditions. Potential initial side effects like headaches or fatigue can often be managed with increased fluid intake. Consulting a healthcare professional before starting is strongly recommended. Benefits can include weight loss, improved blood sugar, and better metabolic health when followed safely.

Conclusion

The rules for the Fast 800 diet involve a flexible, three-phase approach moving from intensive 800-calorie daily restriction to intermittent fasting and finally a sustainable Mediterranean-style eating pattern. The plan emphasizes nutrient-dense whole foods and minimizing processed items and refined carbs to promote weight loss and lasting health. It is designed as a long-term lifestyle change rather than a short-term diet.

Frequently Asked Questions

The Fast 800 has flexible rules depending on the phase, but the core principle involves consuming 800 calories on fasting days. In the rapid weight loss phase (The Very Fast 800), this is done daily for up to 12 weeks. In the maintenance phase (The New 5:2), it's done two days per week.

The diet is based on a moderately low-carb, Mediterranean-style eating plan. You should focus on nutrient-dense foods such as lean protein, vegetables, healthy fats (like olive oil and nuts), and fiber.

The Fast 800 is not suitable for everyone. Individuals who should avoid or use extreme caution include pregnant or breastfeeding women, people with a history of eating disorders, and those with certain medical conditions like type 1 or poorly controlled type 2 diabetes.

While not strictly mandatory, exercise is a core principle of the Fast 800 program. The plan encourages incorporating guided workouts, ranging from low-impact options like yoga to high-intensity interval training (HIIT), to complement the dietary changes and improve overall health.

The Very Fast 800 phase, which involves consuming 800 calories daily, should be followed for a minimum of two weeks and a maximum of 12 weeks. It is an intensive, short-term approach intended to kickstart weight loss.

Snacking is discouraged on fasting days because it can spike insulin levels. If necessary, small, nutrient-dense snacks like non-starchy vegetables, a piece of cheese, or a handful of nuts are recommended.

The Way of Life is the final and long-term maintenance phase of the Fast 800. It involves continuing a sensible, low-carb, Mediterranean-style diet without strict calorie counting, focusing on sustainable healthy habits for life.