Fast 800 Diet: What is the 800 Calorie Diet and is it sustainable?

The Fast 800 Diet is a structured weight loss programme that combines calorie restriction with principles of Mediterranean-style eating and intermittent fasting, aiming to support fat loss while protecting metabolic health. 

The insights provided in this article are provided by Vivere's Head of Nutrition, Yusra Serdaroglu Aydin, a registered dietitian with a background in nutrition, food engineering and culinary arts. Her approach is grounded in science and shaped by a strong understanding of personalised nutrition and the human microbiome.

The Fast 800 Diet is a short-term, low-calorie diet designed to kick-start weight loss, followed by more sustainable eating for weight loss maintenance and support for long-term health.

• What is the Fast 800 Diet? It is a structured programme created by Dr Michael Mosley that combines an 800-calorie phase with Mediterranean-style eating and intermittent fasting.

• Is the Fast 800 Diet safe? It can be appropriate for some adults when followed correctly and for a limited time, ideally with medical guidance.

• What can you eat on the Fast 800 Diet? The plan focuses on whole foods, including vegetables, healthy fats and high-quality protein, while avoiding sugar and refined carbohydrates.

• Is the 800-calorie phase sustainable long-term? No, the 800-calorie stage is designed to be short-term.

• Who should avoid the Fast 800 Diet? Certain groups, including pregnant women and people with eating disorders, should not follow this plan.

The Fast 800 Diet is a weight loss programme developed by Dr Michael Mosley that centres around eating 800 calories per day for a short period, prioritising nutrient-dense, whole foods.

Watch Dr Mosley explain the Fast 800 Diet during an interview appearance on the BBC Breakfast show:

Unlike traditional crash diets that focus on calorie reduction, the Fast 800 Diet is designed to be nutritionally balanced, helping to preserve muscle mass and support overall health during weight loss. The programme is rooted in the principles of the Mediterranean diet, which is widely associated with improved heart health, better blood sugar control and reduced inflammation.

By encouraging vegetables, healthy fats and lean proteins, the plan aims to deliver essential nutrients even during the low-calorie phase, making it a more considered approach than many rapid weight loss diets [1] [2] [3].

The Fast 800 Diet works through a phased approach that moves from rapid weight loss to long-term lifestyle change. Below is an outline of the stages of the Fast 800 Diet:

This stage involves consuming around 800 calories per day using low-carbohydrate, Mediterranean-style meals.

The Very Fast 800 is a short-term intervention, often lasting between two and twelve weeks, to promote quick fat loss and improve metabolic markers such as insulin sensitivity. Meals focus on vegetables, protein and healthy fats, helping to keep blood sugar levels stable and reduce hunger despite low-calorie intake.

This stage introduces intermittent fasting by eating normally (but maintaining the principles of a healthy diet) for five days a week and restricting calories on two non-consecutive days.

On fasting days, calorie intake is typically reduced to around 800 calories, while the remaining days focus on balanced, nutritious meals. This allows more flexibility and is often easier to maintain.

This final stage focuses on maintaining weight loss through long-term healthy eating habits and lifestyle changes.

The Way of Life stage encourages mindful eating, portion control and regular movement, alongside continued use of Mediterranean diet principles. It is designed to help people maintain results without calorie counting or restrictive rules.

The Fast 800 meal plan prioritises whole, minimally processed foods that provide maximum nutrition per calorie.

The diet encourages a range of nutrient-rich foods that support satiety and overall health.

Healthy fats are included to support heart health and hormone function. Examples include:

• Olive oil

• Nuts

• Seeds

• Oily fish like salmon

Proteins help preserve muscle mass and keep you feeling full. Examples include:

• Eggs

• Chicken

• Turkey

• Tofu

• Beans

Vegetables form the base of most meals, providing fibre, vitamins and minerals. Examples include:

• Leafy greens

• Broccoli

• Cauliflower

• Peppers

• Courgettes

Full-fat dairy is also allowed in moderation, offering calcium and protein without excessive sugar. Examples include:

• Greek yoghurt

• Some cheeses

Certain foods are limited or excluded to help control blood sugar and calorie intake.

• Carbohydrates, particularly refined carbohydrates such as white bread, pasta and rice , are avoided within the constraints of this diet, due to their impact on blood glucose levels.

• Added sugars in the form of sweets, cakes, biscuits and sugary drinks are excluded to reduce empty-calorie intake.

• Processed foods, including ready meals and ultra-processed snacks, are discouraged, as they are often high in salt, unhealthy fats and additives.

The strict 800-calorie phase of the Fast 800 Diet is not intended to be followed long term, but the overall programme is designed with sustainability in mind.

Remaining on the Very Fast 800 stage for too long can potentially lower metabolic rate, increase fatigue and lead to nutrient deficiencies if not carefully managed. It also does not allow for a realistic long-term approach to healthy eating, whereby it is accepted that less nutritious foods can form a normal part of healthy eating when consumed in moderation. The programme includes a transition to the 5:2 and Way of Life stages, which allow for higher calorie intake and dietary flexibility. 

The initial phase acts as a short-term reset, helping to kick-start weight loss, reduce insulin resistance and establish healthier eating patterns. Long-term sustainability comes from learning to cook fresh food, understanding portion control and considering adopting time-restricted eating, such as consuming all meals within a 10 to 12-hour window.

The Fast 800 Diet is not suitable for everyone, and certain people should avoid it altogether.

You should not follow this diet if:

• You are under 18 years old, as younger people need consistent energy and nutrients.

• You are pregnant or breastfeeding due to increased nutritional requirements.

• You are underweight or have a history of eating disorders, as they can worsen physical and mental health. 

• You are taking medication, especially for diabetes or blood pressure. It is essential to speak with your GP before starting, as rapid changes in calorie intake can affect medication.

"The Fast 800 Diet can be an effective tool for short-term weight loss when followed properly, but its real value lies in the transition towards healthier, more sustainable eating habits that support long-term wellbeing rather than quick fixes".

"The Fast 800 Diet shows some promise as a structured approach to weight loss, but the evidence around its long-term effectiveness is still limited, particularly when it comes to sustained results beyond the initial phases. Caution should be exercised for certain groups, including those with a history of eating disorders, as the restrictive nature of the diet may not allow some people to make changes they can maintain long-term. 

Much of the success comes from short-term outcomes and general principles such as calorie restriction and Mediterranean-style eating, rather than large-scale, long-term data on the diet itself.

That said, there are encouraging elements, especially its focus on whole foods, metabolic health and transitioning into more sustainable habits, suggesting it could be a useful tool for some people, but more robust research is needed to fully understand its long-term impact."

Vivere helps you take control of your health with personalised insights from state-of-the-art gut microbiome testing, nutritional guidance, science-backed supplements, NAD injections and expert support. Sign up today and start living better, for longer.

[1] The Fast 800

[2] What is The Fast 800 diet? - Patient

[3] What is the Fast 800 diet? | Good Food