Why am I so tired on Fast 800?

November 17, 2025 •

4 min read

According to a 2023 study published in the journal Nutrients, lethargy and fatigue were among the most frequently reported side effects by individuals practicing intermittent fasting and low-calorie diets. If you're wondering, "Why am I so tired on Fast 800?", it's a very common experience as your body adjusts to significant changes in fuel sources and calorie intake.

Quick Summary

The fatigue experienced on the Fast 800 diet is a common and usually temporary side effect resulting from your body adapting to a new metabolic state. Key causes include the 'keto flu' transition, dehydration, electrolyte imbalances, and insufficient nutrient intake. Addressing hydration, balancing electrolytes, and ensuring high-quality nutrients are crucial for managing energy levels.

Key Points

Metabolic Adjustment: Initial fatigue on the Fast 800 is primarily caused by your body shifting from burning carbohydrates to burning fat for energy during ketosis.
Dehydration and Electrolytes: Restricted calories and carbs cause increased water and electrolyte excretion, leading to dehydration and mineral imbalances that cause tiredness.
Nutrient Deficiencies: Not consuming enough high-quality protein, healthy fats, or micronutrients like B vitamins and iron can deplete energy reserves.
Lifestyle Management: Reducing high-intensity exercise and prioritizing adequate sleep can help manage energy levels during the diet's intensive phase.
Monitor Your Body: The initial fatigue should be temporary; if it persists or is severe, it may signal an issue that requires medical attention.

Understanding the Fast 800's Impact on Your Body

Embarking on the Fast 800 diet, particularly the initial Very Fast 800 phase of consuming only 800 calories a day, is a significant metabolic shift for your body. For many years, your body has likely relied on a steady stream of glucose from carbohydrates for energy. When this primary fuel source is drastically reduced, your body must transition to burning fat for fuel, a process called ketosis. This transition, while beneficial for fat loss, doesn't happen overnight and is the primary reason many people feel so tired on Fast 800.

The 'Keto Flu' and Metabolic Adaptation

The term 'keto flu' is frequently used to describe the initial set of side effects when transitioning into a state of ketosis. Your body and brain are accustomed to using glucose, and when that fuel is restricted, they must learn to burn ketones instead. This metabolic switch can cause symptoms like fatigue, headaches, irritability, and 'brain fog'. The good news is that these symptoms are temporary, often subsiding within a few days or weeks as your body becomes more 'fat-adapted' and efficient at using ketones for energy.

Dehydration and Electrolyte Imbalance

One of the most common and easily rectifiable causes for feeling tired on the Fast 800 is dehydration and a subsequent imbalance of electrolytes. Restricting calories and carbohydrates causes your body to excrete more water and essential electrolytes, like sodium, potassium, and magnesium. Without adequate replenishment, you can experience symptoms such as fatigue, muscle cramps, and headaches. Staying properly hydrated with at least 2-3 litres of calorie-free fluid per day is crucial. A little extra salt, such as adding broth, can help replenish sodium, while increasing intake of leafy greens and nuts can boost potassium and magnesium.

Inadequate Nutrient and Macronutrient Intake

Even within a low-calorie framework, the quality of your calories matters immensely. The Fast 800 diet is based on a Mediterranean-style eating plan, emphasizing protein, fiber, and healthy fats. A deficiency in any of these can lead to low energy:

Protein: High-quality protein is vital for muscle maintenance and keeps you feeling full longer, preventing hunger-driven fatigue. If you're not getting enough protein (around 60g a day is often recommended), your body may feel run down.
Healthy Fats: Fats are your new primary fuel source in ketosis. Skimping on healthy fats from sources like olive oil, avocados, and nuts can leave your energy tank running on empty.
Vitamins and Minerals: Micronutrient deficiencies can occur on a very low-calorie diet if not carefully planned. B vitamins, iron, and magnesium are especially critical for energy production. Consider speaking to a healthcare professional about supplements to ensure you are meeting your needs.

Lifestyle Factors and Overtraining

Your energy levels are not solely determined by your diet. The Fast 800 can be intense, and other lifestyle factors can amplify feelings of tiredness. It's not the time to push yourself to the limit with high-intensity exercise.

Overtraining: Heavy or intense exercise, especially endurance training, can be too much for a very low-calorie diet. Strenuous activity should be temporarily reduced or avoided during the initial phase to prevent excessive fatigue. Gentle exercise like walking or yoga is a better choice.
Sleep Deprivation: Lack of sleep is a well-known cause of fatigue, and initial diet changes can sometimes disturb sleep patterns. Prioritizing adequate rest is essential for your body to recover and adapt.
Stress: High stress levels can impact your body's energy regulation. Combining dietary stress with mental or emotional stress can further deplete your reserves.

Fast 800 vs. Other Low-Calorie Approaches


Feature	Fast 800 (Intensive Phase)	Standard Low-Calorie Diet (LCD)	Notes
Calories	Strictly 800 calories/day	Typically 1,400-1,900 calories/day	Fast 800's severity accelerates metabolic changes but increases fatigue risk.
Carb Profile	Moderately low-carb, Mediterranean-style	Varies widely, often higher in carbs	Fast 800 pushes for fat adaptation, causing temporary 'keto flu' fatigue.
Fuel Source	Shifts from glucose to fat (ketones)	Primarily remains glucose-based	The metabolic switch on Fast 800 is the direct cause of early fatigue.
Nutrient Emphasis	High protein, high fiber, healthy fats	Can be deficient if not well-planned	Fast 800 focuses on nutrient density, crucial for energy.
Typical Fatigue Profile	Acute, intense initial fatigue (keto flu)	More gradual, chronic fatigue	Fast 800 fatigue is often tied to adaptation, LCD fatigue to general deficiency.
Sustainability	Short-term (max 12 weeks) before transitioning	Often intended for longer-term	Fast 800 transitions to a less restrictive 5:2 or 'Way of Life' plan to sustain energy.

Conclusion: Navigating Fatigue for Success

It's clear that experiencing fatigue on the Fast 800 diet, especially during the initial phase, is a very common side effect. The primary cause is your body's metabolic adjustment, the so-called 'keto flu', as it shifts from burning glucose to burning fat. Other major culprits include dehydration, electrolyte imbalances, and insufficient intake of key nutrients like protein and healthy fats. However, these challenges are often temporary and manageable. By focusing on proper hydration, supplementing electrolytes, ensuring nutrient-dense meals, and adjusting your exercise and sleep routines, you can effectively mitigate the tiredness. If fatigue persists beyond the initial adaptation period, or if you have underlying health conditions, it's essential to consult a healthcare professional. Remember that the intensive phase of Fast 800 is designed for a short duration, and transitioning to a more sustainable maintenance plan is key for long-term health and energy levels. For more information on safely implementing dietary changes, consider reliable resources like the NHS, which offers guidance on low-calorie diets and general wellness.

Frequently Asked Questions

Yes, it is very normal to feel tired, especially during the initial phases. This is largely due to your body adapting to a new, low-calorie diet and shifting its primary energy source from glucose to fat via ketosis.

For most people, the initial fatigue, often referred to as the 'keto flu,' is temporary and resolves within a few days to a couple of weeks as the body becomes more efficient at using ketones for energy.

Yes, absolutely. The low-carb nature of the diet causes your body to shed more water and electrolytes. If you don't actively increase your fluid intake, you can become dehydrated, which is a major contributor to fatigue.

Focus on nutrient-dense foods rich in healthy fats, protein, and fiber. Examples include avocados, nuts, lean meats, and green leafy vegetables. These provide sustained energy and help manage hunger.

While exercise is beneficial, it's wise to reduce the intensity and frequency of workouts during the initial, high-fatigue phase. Opt for lighter activities like walking or yoga and return to more strenuous exercise once your energy levels stabilize.

Increase your intake of sodium by adding a pinch of salt to your water or meals, and eat potassium-rich foods like leafy greens. Magnesium supplements can also be considered after consulting a doctor.

While some fatigue is normal, you should contact a healthcare provider if it is severe, persistent, or accompanied by other concerning symptoms like extreme dizziness, nausea, or prolonged weakness. Prolonged side effects can indicate that the diet is not suitable for you.