What is The Fast 800 for 2 weeks? Your Guide to Kickstarting Weight Loss

November 17, 2025 •

5 min read

According to studies, a rapid weight loss approach using an 800-calorie diet can be highly effective for weight reduction and improving metabolic health. The Fast 800 for 2 weeks is the intensive, initial phase of this program, designed to trigger fast fat loss and reset eating habits.

Quick Summary

The Fast 800's 2-week phase, also known as 'The Very Fast 800', is an intensive 800-calorie daily diet based on a Mediterranean-style eating plan. It combines calorie restriction with low-carb, high-protein foods to induce mild ketosis, targeting rapid weight loss and visceral fat reduction.

Key Points

Intensive Phase: The Fast 800 for 2 weeks is the initial, rapid weight loss phase, involving 800 calories per day to kickstart results.
Ketosis Trigger: The significant calorie restriction helps induce mild ketosis, where the body burns fat for fuel.
Mediterranean-Style Focus: Meals consist of high-protein, high-fiber, healthy fats, and non-starchy vegetables to maximize nutrition within the calorie limit.
Medical Supervision: It is crucial to consult a healthcare professional before starting, especially if you have existing health conditions like type 2 diabetes.
Not for Everyone: This diet is unsuitable for pregnant or breastfeeding women, children, and those with a history of disordered eating.
Transition is Key: The 2-week period is a reset, and long-term success depends on transitioning to a more sustainable maintenance plan, like the 5:2 diet.
Manage Side Effects: Initial side effects like fatigue and headaches can occur, and staying hydrated is essential to help mitigate them.

The Fast 800 diet, popularized by the late Dr. Michael Mosley, offers a structured path to weight loss and improved metabolic health through various approaches. The most intensive of these is "The Very Fast 800," which involves consuming 800 calories a day, and is often followed for an initial two-week period to kickstart the weight loss process. This phase is not a long-term solution but a powerful reset for those with a significant amount of weight to lose, or those needing a rapid start. It operates on the principle that restricting calories, while focusing on nutrient-dense foods, can lead to significant fat reduction, especially around the midsection.

How the 2-Week Fast 800 Works

By limiting calorie intake to 800 per day, the body is forced to use stored fat for energy. This process can lead to a metabolic state called mild ketosis, where the body begins burning fat rather than glucose for fuel. The diet emphasizes consuming high-quality, whole foods based on a Mediterranean-style pattern to ensure nutrient intake is optimized despite the caloric deficit.

The diet structure focuses on:

Prioritizing Protein and Fiber: A high intake of protein keeps you feeling full, helping to manage hunger and maintain muscle mass during rapid weight loss.
Consuming Healthy Fats: Sources like olive oil, nuts, and seeds provide satiety and essential nutrients, balancing the low-carbohydrate intake.
Eating Plenty of Non-Starchy Vegetables: These low-calorie, high-fiber foods can be eaten freely and provide essential vitamins and minerals.
Avoiding Processed Foods and Sugar: To control blood sugar spikes and prevent fat storage, refined carbohydrates and sugary items are eliminated.

Potential Benefits of the Intensive 2-Week Plan

While challenging, the two-week rapid weight loss phase can yield significant benefits:

Rapid Weight Loss: Many people experience substantial and quick weight loss, which can be highly motivating.
Improved Blood Sugar Control: Studies, including the DiRECT trial which used an 800-calorie diet, have shown significant improvements in blood sugar levels and even remission of type 2 diabetes in some participants.
Reduced Visceral Fat: The diet targets dangerous visceral fat, the fat stored around abdominal organs, which is linked to metabolic disease.
Enhanced Metabolic Health: The overall focus on whole, unprocessed foods can lead to improved cholesterol and insulin sensitivity.

Important Considerations and Potential Risks

This intensive phase is not suitable for everyone and requires caution. Potential risks and side effects include:

Fatigue and Headaches: The body adjusting to a low-calorie intake can lead to initial tiredness, headaches, and irritability.
Hunger: Despite emphasizing filling foods, the significant calorie restriction can be difficult to sustain due to hunger.
Nutrient Deficiencies: Without careful planning and potentially supplements, prolonged restriction could lead to deficiencies, though shakes are available to help mitigate this.
Not for Everyone: The diet is explicitly not recommended for pregnant or breastfeeding women, children, those with a history of eating disorders, or individuals on certain medications like insulin. Always consult a healthcare professional before starting.

A Sample 2-Week Fast 800 Meal Plan

Meals should be balanced and high in protein, fiber, and healthy fats within the 800-calorie limit. Here is a sample structure drawing on typical Mediterranean-style options:

Week 1
- Day 1: Breakfast: Scrambled eggs with spinach (approx. 150 kcal). Lunch: Grilled chicken salad with lemon vinaigrette (approx. 300 kcal). Dinner: Baked salmon with steamed broccoli (approx. 350 kcal).
- Day 2: Breakfast: Greek yogurt with berries (approx. 200 kcal). Lunch: Tuna salad with avocado and tomatoes (approx. 350 kcal). Dinner: Stir-fried tofu with mixed vegetables (approx. 250 kcal).
- Day 3: Breakfast: Chia seed pudding with unsweetened almond milk and berries. Lunch: Lentil soup with mixed greens. Dinner: Roasted eggplant with chickpeas and tahini sauce.
- Day 4: Breakfast: Omelette with mushrooms and tomatoes. Lunch: Baked cod with roasted Brussels sprouts. Dinner: Chicken stir-fry with bell peppers and cauliflower rice.
- Day 5: Breakfast: Avocado toast on whole-grain bread (small portion). Lunch: Sardines with mixed greens and olive oil vinaigrette. Dinner: Turkey meatballs with zucchini.
- Day 6: Breakfast: Cottage cheese bowl with mushrooms and avocado. Lunch: Prawn and noodle salad. Dinner: Chicken schnitzel with coleslaw.
- Day 7: Breakfast: Poached eggs with mushrooms and spinach. Lunch: Lebanese salad with grilled chicken. Dinner: Baked fish with mint yogurt dressing.
Week 2
- Day 8: Repeat Day 1's meal plan or similar Mediterranean-style meals to maintain simplicity.
- Day 9: Repeat Day 2's meal plan.
- Day 10: Repeat Day 3's meal plan.
- Day 11: Repeat Day 4's meal plan.
- Day 12: Repeat Day 5's meal plan.
- Day 13: Repeat Day 6's meal plan.
- Day 14: Repeat Day 7's meal plan.

Very Fast 800 vs. New 5:2 Comparison

The Fast 800 offers several approaches depending on goals. The initial 2-week period falls under 'The Very Fast 800' phase. After this, people can transition to a less intensive phase like 'The New 5:2'.


Feature	The Very Fast 800 (First 2 weeks)	The New 5:2 (Longer-term)
Intensity	High, rapid weight loss	Moderate, steady weight loss
Daily Calories	800 calories every day	800 calories, two non-consecutive days per week
Duration	2 to 12 weeks, as advised	Sustainable long-term
Goal	Kickstart weight loss, metabolic reset	Ongoing weight loss, maintenance, metabolic health
Structure	Strict calorie count daily	Calorie count on 2 days, moderate-low carb on 5 days
Flexibility	Less flexible, highly structured	Highly flexible, adaptable to lifestyle

Transitioning to a Sustainable Lifestyle

Upon completing the 2-week phase, the goal is to prevent weight regain by transitioning to a more sustainable eating pattern. Most followers move to a modified 5:2 approach or the "Way of Life," which involves a Mediterranean-style diet with no strict calorie counting. This gradual shift helps consolidate healthy habits and maintain the progress made during the intensive initial period. The Fast 800 website provides ample resources for this transition, including recipes and guidance for long-term success.

Conclusion

The 2-week Fast 800 program is an intensive, but effective, way to jumpstart significant weight loss and improve metabolic health. By adhering to the 800-calorie Mediterranean-style plan, individuals can quickly shed weight and improve biomarkers related to chronic diseases like type 2 diabetes. However, due to its restrictive nature, it should only be followed for a short duration, ideally under medical supervision, before transitioning to a more flexible and sustainable long-term plan. For more information and resources on all phases of the diet, visit the official website.

Frequently Asked Questions

After the intensive 2-week period, it is recommended to transition to a more sustainable eating pattern, such as 'The New 5:2' plan (800 calories two days a week) or the Mediterranean-style 'Way of Life' approach for long-term maintenance.

No, the intensive Fast 800 approach is not suitable for everyone. It is not recommended for pregnant or breastfeeding women, Type 1 diabetics, those with a history of eating disorders, or certain medical conditions. Always consult a healthcare professional before starting.

The diet focuses on high-quality, whole foods based on a Mediterranean pattern, including vegetables, lean proteins (fish, eggs, chicken), healthy fats (olive oil, nuts, seeds), and minimizing refined carbohydrates and sugar.

Due to the caloric restriction, micronutrient intake can be low, especially with a 'real food' plan. The Fast 800 program suggests that supplements may be needed, and meal replacement shakes are fortified with vitamins and minerals.

Common side effects during the initial phase include headaches, fatigue, constipation, and hunger. These often subside as your body adapts. Staying well-hydrated is crucial.

Weight loss can vary significantly among individuals based on starting weight and metabolic factors. Clinical studies using the 800-calorie diet have shown an average weight loss of around 9.5kg over eight weeks, indicating rapid initial loss is possible.

Yes, gentle exercise is encouraged. However, intense exercise should be approached with caution during the initial, calorie-restricted phase due to reduced energy levels. The program often includes guided workouts suitable for different fitness levels.

Meal replacement shakes are an optional tool offered by The Fast 800 program, designed to help with satiety and nutritional completeness during the intensive phases, though the emphasis is on whole foods.