The Fast 800 diet, popularized by Dr. Michael Mosley, involves a very low-calorie intake of 800 calories per day, typically for a short period, often followed by a more sustainable intermittent fasting plan. While effective for rapid weight loss and improving metabolic health markers like blood sugar levels, it is not without its challenges. The restrictive nature of the diet, particularly during the intensive initial phase, can trigger several common side effects as your body adapts to the significantly lower energy intake.
Common Short-Term Side Effects of the Fast 800
Fatigue and Low Energy
Reduced calorie intake is the most immediate cause of fatigue and decreased energy levels, especially during the initial days of the plan. Your body, accustomed to a higher caloric load, needs time to adjust to using stored fat for energy. This lethargy can be accompanied by 'brain fog' as your cognitive functions adapt to lower glucose levels. As your body shifts its metabolic state, energy levels often improve, but it can be a difficult hurdle to overcome at the start.
Headaches and Dizziness
Headaches are one of the most frequently reported side effects of fasting and caloric restriction, often presenting as a mild to moderate, non-pulsating pain. Common causes include dehydration, caffeine withdrawal, and low blood sugar levels as the body enters a state of mild ketosis. Dizziness or lightheadedness, particularly when standing up quickly, can also occur due to low blood pressure (hypotension) or reduced blood sugar.
Increased Hunger and Cravings
Though the diet is designed to suppress the hunger hormone ghrelin once the body enters ketosis, intense hunger pangs and cravings are common in the first few days. The body is simply reacting to the sudden and significant reduction in food. For those following the 'real food' version of the plan, managing these hunger pains can be a significant challenge, especially compared to those using meal replacement shakes which are designed to aid satiety.
Digestive Issues: Constipation or Diarrhea
Changing food intake dramatically can disrupt bowel habits. Constipation is often caused by dehydration and the reduced bulk of food being consumed. Conversely, some individuals may experience diarrhea or general gastrointestinal discomfort as their digestive system adapts. Ensuring adequate hydration and consuming high-fiber vegetables is important to combat these issues.
Mood Swings and Irritability
Calorie restriction can significantly impact mood, leading to irritability, anxiety, and general 'crankiness'. This is a normal physiological and psychological response to hunger and changes in blood sugar and energy levels. It’s important to be aware of these potential mood changes and to communicate them to family or close friends who can offer support.
Bad Breath (Keto Breath)
As the body begins to burn fat for energy (a process called ketosis), it produces ketones, which are released from the body through urine and breath. A side effect of this metabolic state can be a noticeable, unpleasant odor on the breath, often described as fruity or metallic.
Managing and Mitigating Side Effects
Practical Tips for the Early Stages
Here are some practical strategies to help manage the initial side effects of The Fast 800:
- Hydrate extensively: Drink 2-3 litres of calorie-free fluids, such as water and herbal teas, throughout the day to combat dehydration, headaches, and constipation. Sometimes, adding a little extra salt to your diet can also help with low blood pressure.
- Prioritize nutrient-dense foods: Ensure your 800 calories come from protein, fiber, and healthy fats found in a Mediterranean-style diet to maximize satiety and nutrient intake.
- Consider supplementation: A high-quality multivitamin can help safeguard against potential micronutrient deficiencies, which are a real risk on such a low-calorie diet.
- Ease into the diet: Instead of an abrupt start, consider gradually reducing your calorie intake or beginning with an easier intermittent fasting method like the 5:2 approach to let your body adjust more gently.
- Plan ahead: Prepare your meals and have low-calorie snacks on hand to manage hunger pangs and avoid impulse eating.
- Listen to your body: If side effects are severe or persistent, it's best to break the fast and consult a medical professional. The diet should not cause significant or prolonged distress.
Long-Term Risks and Considerations
Nutritional Deficiencies
Prolonged adherence to a very low-calorie intake can make it difficult, if not impossible, to meet your body's full nutritional requirements through food alone. Without proper supplementation and medical monitoring, this could lead to a range of issues from weakened immunity and muscle loss to reduced bone strength.
Potential for Muscle Loss
While The Fast 800 aims to burn fat, a side effect of severe calorie restriction, especially without sufficient protein and resistance exercise, is the loss of lean muscle mass. This can negatively impact metabolism in the long run.
Relationship with Food and Regain Risk
Very restrictive diets can negatively impact a person's relationship with food, potentially masking or triggering disordered eating patterns. Furthermore, a significant risk with any rapid weight loss method is that the weight is quickly regained after the diet ends. The long-term maintenance phases are designed to mitigate this, but success depends on individual adherence and psychological factors.
Comparison of Short-Term vs. Long-Term Effects
| Aspect | Short-Term Side Effects (Initial 12 weeks) | Long-Term Side Effects/Risks (beyond 12 weeks) |
|---|---|---|
| Commonality | Very common and expected as the body adjusts. | Less common, but can be more serious if not managed. |
| Severity | Usually mild to moderate, manageable for most. | Can be severe, including malnutrition and health complications. |
| Primary Cause | Low calories, dehydration, and adjustment to metabolic changes. | Sustained nutrient deficit, muscle loss, and psychological impact. |
| Duration | Typically resolves as the body adapts within days or weeks. | Risk increases with extended duration on the restrictive phase. |
| Best Mitigation | Hydration, planning, and listening to your body. | Medical supervision, balanced nutrient intake, and gradual transition to maintenance. |
Who Should Avoid The Fast 800?
This diet is not suitable for everyone, and it is crucial to consult a medical professional before starting, especially if you have pre-existing conditions. It is explicitly not recommended for the following groups:
- Pregnant or breastfeeding women: Nutrient needs are higher during these periods.
- Individuals with a history of eating disorders: The restrictive nature can exacerbate or trigger these conditions.
- Underweight individuals: Can lead to an unhealthy and dangerous level of weight loss.
- Type 1 diabetics: The severe calorie restriction can cause dangerously low blood sugar levels (hypoglycemia).
- Type 2 diabetics on certain medications (e.g., insulin or 'flozin' drugs): Requires careful medical monitoring to avoid complications.
- People with specific medical conditions: Including heart disease, kidney failure, or recent cardiac events.
- Children and teenagers: Essential for proper growth and development.
Conclusion: Weighing the Risks and Rewards
While the Fast 800 offers the potential for significant and rapid weight loss, it is a demanding dietary plan that comes with a range of side effects. Common short-term issues like fatigue, headaches, and hunger are normal but require proactive management through hydration and proper nutrition. More serious long-term risks, such as nutritional deficiencies and a negative relationship with food, necessitate strict medical supervision and a careful transition to a maintenance phase. The Fast 800 is not a universal solution and should only be undertaken by suitable candidates under the guidance of a healthcare provider. Understanding what are the side effects of fast 800 and how to manage them is the first step toward a safe and successful weight loss journey.
For more detailed scientific background on the principles behind The Fast 800, you can review the DiRECT trial results published in The Lancet.
