The Fast 800, a very low-calorie and intermittent fasting diet popularized by Dr. Michael Mosley, is designed for rapid weight loss and improving metabolic health in suitable candidates. However, the restrictive nature of consuming only 800 calories per day, even for a short period, makes it unsuitable and dangerous for many people. Several key health conditions and life stages act as strong contraindications for this diet, requiring individuals to seek alternative weight management strategies or follow the program only under strict medical supervision.
Medical Conditions and Health Status
Type 1 Diabetes and Certain Medications
For individuals with Type 1 diabetes, the Fast 800 is not recommended due to the high risk of severe complications, including hypoglycemia (dangerously low blood sugar) and diabetic ketoacidosis (DKA). The diet is particularly dangerous for those on insulin or certain diabetes medications known as SGLT2 inhibitors ('flozins'), as it can mask symptoms of DKA. Even individuals with Type 2 diabetes who are on medication must be closely monitored by a healthcare professional, as medication doses often need to be adjusted as blood sugar improves.
Heart, Kidney, and Liver Conditions
Anyone with a history of significant cardiac events, such as a heart attack or stroke within the past three months, should not attempt this diet. It is also contraindicated for those with uncontrolled heart disease, uncontrolled hypertension, or kidney failure. The severe caloric restriction can put undue stress on these vital organs and exacerbate existing conditions.
Psychiatric Disorders and Eating Disorders
Individuals with a history of eating disorders, such as anorexia, bulimia, or binge eating disorder, should strictly avoid the Fast 800. The highly restrictive nature of the diet can trigger a relapse of disordered eating patterns. Furthermore, people with significant psychiatric conditions or substance abuse issues should also not undertake this diet without specific medical guidance.
Pregnancy, Breastfeeding, and Fertility Treatment
The nutritional demands of pregnancy and breastfeeding are high, making an 800-calorie diet entirely unsuitable and potentially harmful to both mother and baby. Anyone undergoing fertility treatment should also not do the fast 800, as severe calorie restriction can negatively impact reproductive hormones.
Lifestyle and Age Considerations
Underweight Individuals
People who are already underweight or have a low body mass index (BMI) should never follow a very low-calorie diet. The purpose of the Fast 800 is weight loss, and for those already at a healthy weight, the diet poses significant risks, including muscle mass loss and severe nutrient deficiencies.
Children and Adolescents
Children and adolescents under 18 should not follow the Fast 800. Their growing bodies require a consistent and sufficient intake of nutrients and energy for proper development. Restrictive diets can impede growth and cause long-term health problems.
Frail or Post-Surgery Individuals
Those who are frail, unwell, or recovering from recent significant surgery (within the last six months) are advised against this diet. The body needs adequate energy and nutrients to heal and recover, and an 800-calorie diet is insufficient for this purpose.
Potential Side Effects and Risks
Beyond specific contraindications, the Fast 800 carries a number of risks and potential side effects, even for those who might be medically cleared. These include:
- Nutrient Deficiencies: The severe calorie limit makes it extremely difficult to obtain all necessary vitamins and minerals from whole foods alone.
- Fatigue and Low Energy: A drastic reduction in calories can lead to persistent feelings of tiredness and lack of energy.
- Metabolism Slowdown: When the body receives significantly fewer calories, it can enter a "starvation mode," which slows metabolism to conserve energy, making subsequent weight loss harder.
- Muscle Mass Loss: Without enough calories and protein, the body may break down muscle tissue for energy, rather than just fat.
- Increased Risk of Gallstones: Rapid weight loss, a feature of VLCDs, is a known risk factor for developing gallstones.
- Negative Relationship with Food: The highly restrictive nature can foster an unhealthy relationship with food, increasing the likelihood of binge eating once the diet ends.
Comparison of Suitable vs. Unsuitable Scenarios
| Factor | Potentially Suitable (With GP Supervision) | Unsuitable (Avoid) |
|---|---|---|
| Health Status | Overweight or obese individuals with Type 2 diabetes (on medication), pre-diabetic | Underweight, Type 1 diabetes, heart/kidney/liver disease, uncontrolled hypertension, history of stroke/heart attack |
| Life Stage | Not applicable | Pregnant, breastfeeding, undergoing fertility treatment, children under 18 |
| Mental Health | Healthy individuals seeking rapid weight loss | History of eating disorders, significant psychiatric disorders, substance abuse |
| Medication | Taking certain medications (e.g., blood pressure, warfarin) | Taking insulin, SGLT2 inhibitors ('flozins') |
| General Condition | Generally healthy, but overweight | Frail, unwell, or recovering from recent surgery |
Conclusion: Always Prioritize Medical Advice
Given the significant list of contraindications and potential risks, the most important takeaway is that the Fast 800 is not a universal solution. While it offers a structured and rapid approach for some, it is explicitly dangerous for others. Before embarking on any very low-calorie or intermittent fasting regimen, a thorough consultation with a healthcare professional is non-negotiable. This medical oversight ensures the diet is appropriate for your individual health status, helping you avoid severe complications and pursue a safe, sustainable path to better health. For more general guidance on healthy eating, refer to the NHS Eatwell Guide.
