What is the Fast 800 diet?
The Fast 800 diet, created by Dr Michael Mosley, is the most intensive phase of the Blood Sugar Diet and comes with claims of rapid weight loss and reversal of Type 2 Diabetes. Followers of the 'Fast 800' are encouraged to consume only 800 calories (or approx. 3347 kilojoules) per day for up to 8 weeks. After this, followers are encouraged to “transition” to a new iteration of his popular ‘5:2 diet’, aiming for 800 calories per day for 2 days a week, with a 14-hour fast overnight and to follow a low-carb, Mediterranean style diet pattern for the remaining 5 days.
What the diet says you can eat?
The 'Fast 800' diet recommends recipes which are low in refined carbohydrates and high in healthy fats. It has based its recipes on the Mediterranean style diet which has extensive scientific research demonstrating its health benefits. The 'Fast 800' diet includes optional meal replacement shakes, which are around 200 calories (or approx 836 kilojoules) and increase satiety. Here are some examples of daily meal plans with recipe ideas for Breakfast, Lunch and Dinner all within the 800 calorie limit.
What are the potential benefits of the Fast 800 diet?
The 'Fast 800' claims are backed by two recent studies, DiRECT and DROPLET. Both studies prescribed ~800 calories per day using total meal replacement formula in the initial phase followed by a period of of 800 calories from food. Both studies compared interventions to usual-care/best practice controls with intervention groups achieving greater weight loss and more cases of ‘diabetes remission’ compared to controls.
Another claim of the 'Fast 800' is that rapid weight loss is better than gradual weight loss. However, recent research reports it is absolute weight loss, not the rate of weight loss that is associated with better health outcomes. You may arrive there sooner on the 'Fast 800' but it is likely more difficult to sustain.
In addition, few studies have actually examined the long-term effects of intermittent fasting on humans for longer than six months. Much of the early support for intermittent fasting was based on findings in animal studies which suggested fasting could help reduce the risk of diabetes, cardiovascular disease and certain types of cancer.
One of the longer studies in humans took place over a 50-week period with 150 participants split into three groups. One group followed the ‘5:2’ intermittent fasting diet, while the second group reduced daily calorie intake by approximately 20%. A third control group were advised to stick to their usual lifestyle. Key findings showed that both the ‘5:2’ intermittent fasting and daily calorie restriction both led to significant weight and fat loss compared to the control group. However, the 5:2 intermittent fasting diet was no more effective than conventional dieting for losing weight. Furthermore, there was no differences between the three groups on any circulating biomarkers including insulin sensitivity (risk of diabetes indicated by ability to control blood glucose) and cholesterol.
What are the downsides and risks of Fast 800 diet?
- May be difficult to stick to: like with all energy-restricted diets including intermittent fasting, you may have increased feelings of hunger and this will be difficult to tolerate in the long term.
- Intermittent fasting may lead to malnourishment if you already have a poor diet or take it to the extreme.
- May not be suitable for people with a history of eating disorders, or with specific health conditions such as diabetes.
The bottom line:
There is limited research to support the Fast 800 diet as being superior to other equivalent energy-restricted diets. While the evidence suggests that an intermittent fasting diet may be an effective way to lose weight, it is unlikely to be more effective for weight loss than traditional methods of dieting. There are elements of the Fast 800 diet which attempt to overcome the issues relating to intermittent fasting diets and make it easier to stick to in the long term (i.e., inclusion of shakes). More research is needed to understand the long-term effects of this diet. Please seek advice from your doctor before embarking on this or any other restrictive diet, as it may not be suitable for everyone.