Why do I feel so tired on a low-carb diet?

October 7, 2025 •

4 min read

According to a study published in Frontiers in Nutrition, many people starting a ketogenic diet experience flu-like symptoms, including fatigue and decreased energy, as their bodies adapt to a new metabolic state. This phenomenon is a key reason you might be asking: Why do I feel so tired on a low-carb diet?

Quick Summary

The initial fatigue experienced on a low-carb diet often results from the body's metabolic adjustment, electrolyte loss, dehydration, and a temporary fuel switch from glucose to fat.

Key Points

Metabolic Adjustment: Your body is switching from burning glucose to fat (ketosis), a transition that can cause temporary fatigue known as the "keto flu".
Electrolyte Loss: Cutting carbs causes your body to shed water and vital electrolytes like sodium, potassium, and magnesium, leading to dehydration and fatigue.
Inadequate Fuel: Not eating enough calories or healthy fats can leave your body under-fueled, especially when fat becomes the main energy source.
Micronutrient Deficiencies: Restricting certain food groups can result in a lack of essential vitamins and minerals crucial for energy production.
Gradual Transition: Easing into a low-carb diet rather than an abrupt switch can help minimize the severity of withdrawal symptoms like fatigue.
Hydration and Replenishment: Actively drinking water and consuming electrolyte-rich foods or supplements is key to overcoming initial low-carb fatigue.

The Initial Shock: Adapting to Ketosis

When you dramatically cut your carbohydrate intake, your body must find a new primary fuel source. Normally, your body runs on glucose, which it derives from carbs. By restricting carbs, you deplete your glycogen (stored glucose) stores, forcing your liver to start breaking down fat for energy instead. This process produces molecules called ketones, and the metabolic state is known as ketosis.

For many, this metabolic shift does not happen instantly or smoothly. The transition period can cause a set of symptoms colloquially known as the "keto flu" or "carb flu".

Symptoms of the keto flu include:

Headaches
Brain fog and difficulty concentrating
Irritability
Nausea
Muscle cramps
Fatigue and general weakness

These symptoms typically appear within the first few days and can last for a week or two as your body becomes "fat-adapted," meaning it has become efficient at using fat and ketones for fuel.

The Culprit: Electrolyte Imbalance and Dehydration

One of the most significant contributors to early low-carb fatigue is the loss of water and electrolytes. As your carbohydrate intake drops and your body's glycogen stores are used up, your body excretes the stored water that was bound to the glycogen. This increased urination flushes out essential minerals known as electrolytes, particularly sodium, potassium, and magnesium.

An imbalance of these electrolytes can cause a variety of symptoms that lead to fatigue, including:

Dizziness and lightheadedness
Muscle weakness and cramping
Headaches
Low blood pressure

To combat this, it is crucial to focus on rehydrating and replenishing lost electrolytes. While many turn to supplements, it is often possible to get what you need from keto-friendly foods.

Keto-friendly electrolyte sources:

Sodium: Salting your food, drinking bone broth, or having a cup of broth.
Potassium: Avocados, spinach, mushrooms, and salmon.
Magnesium: Leafy greens, nuts (almonds, macadamia), and seeds (pumpkin, chia).

Hidden Calorie and Nutrient Gaps

Sometimes, fatigue on a low-carb diet is not just about the carb withdrawal; it's about inadequate overall nutrition. Some dieters mistakenly equate low-carb with low-calorie, inadvertently putting themselves in a caloric deficit that is too severe. If your body isn't getting enough fuel, it will naturally conserve energy, leading to constant tiredness.

On a low-carb diet, fat is your primary energy source, and it's important to consume enough healthy fats to stay energized and satisfied.

Healthy fat sources include:

Avocados and avocado oil
Olives and olive oil
Nuts and seeds
Fatty fish like salmon and mackerel
Grass-fed butter and ghee

Furthermore, by eliminating entire food groups like fruits, whole grains, and legumes, you risk micronutrient deficiencies. A lack of key vitamins (like B-vitamins for energy) and minerals (like iron and magnesium) can directly contribute to fatigue. A well-planned diet that includes a wide variety of low-carb vegetables and other nutrient-dense foods is essential for mitigating this risk.

Fatigue Factors: Adaptation vs. Fat-Adapted State

Understanding the difference between the initial adaptation phase and the subsequent fat-adapted state can help you manage your expectations and symptoms. The initial fatigue is a temporary side effect of a major metabolic shift.


Feature	Adaptation Phase (First 1-4 Weeks)	Fat-Adapted State (Post-Adaptation)
Primary Fuel Source	Transitioning from glucose to fat and ketones.	Efficiently uses fat and ketones for energy.
Energy Levels	Often low, with symptoms like fatigue, brain fog, and irritability.	Stable and potentially increased energy levels throughout the day.
Fluid/Electrolytes	Rapid loss of water and electrolytes, increasing risk of imbalance.	Fluid and electrolyte levels stabilize as the body adjusts.
Carb Cravings	Strong cravings for sugar and high-carb foods.	Reduced cravings for sugar and a feeling of fullness.
Physical Performance	Decreased endurance, especially during high-intensity exercise.	Restored or improved physical performance and endurance.

Overcoming Low-Carb Fatigue: A Practical Guide

If you are experiencing fatigue on your low-carb diet, here are some actionable steps to take:

Hydrate Adequately: Drink plenty of water throughout the day. Your urine should be clear or pale yellow. Water is not enough, though; you need to replenish lost minerals.
Increase Electrolyte Intake: Actively consume sodium, potassium, and magnesium. Use mineral-rich salts, enjoy bone broth, and incorporate electrolyte-rich foods like avocados and spinach into your meals. You can also try a homemade keto-ade with water, salt, and potassium-rich cream of tartar.
Eat Enough Fat and Calories: Do not go into severe calorie restriction. Ensure your meals contain enough healthy fats to provide a steady supply of energy. Listen to your hunger cues and eat until you are satisfied.
Gradual Carb Reduction: Instead of cutting carbs abruptly, try a more gradual reduction. This can help ease your body into the metabolic shift and reduce the severity of withdrawal symptoms.
Prioritize Sleep: Fatigue and irritability can worsen without adequate rest. Aim for 7-8 hours of quality sleep per night to support your body's adaptation process.
Modify Exercise Intensity: During the initial adaptation phase, avoid strenuous exercise. Stick to lighter activities like walking, yoga, or stretching until your energy levels stabilize.
Consider Supplements: If dietary changes are not enough, consider supplementing with magnesium or a sugar-free electrolyte powder. Consult a doctor or dietitian for guidance before starting new supplements.

Conclusion

Feeling tired on a low-carb diet is a common and often temporary experience. By understanding the root causes—primarily metabolic adaptation, electrolyte loss, and inadequate nutrient intake—you can take targeted steps to overcome the fatigue. Hydrating properly, replenishing electrolytes, and ensuring you eat enough healthy fats are critical for managing the transition. If your fatigue is severe or lasts longer than a few weeks, it is wise to consult a healthcare professional to rule out any underlying issues. With careful planning, you can navigate the initial challenges and begin to enjoy the more stable energy levels that many low-carb adherents report.

For more information on the keto flu and how to manage its symptoms, you can visit a reputable health resource like Healthline.

Frequently Asked Questions

The 'keto flu' is a term for the flu-like symptoms, including fatigue, headaches, and irritability, that some people experience when first starting a low-carb or ketogenic diet. It's caused by the body's metabolic adjustment as it shifts from using carbs for energy to using fat.

For most people, the initial fatigue from adapting to a low-carb diet lasts for a few days to a couple of weeks. It typically resolves as the body becomes more efficient at using ketones for energy.

Yes. When you reduce carb intake, your body flushes out excess water, leading to dehydration and the loss of electrolytes. This fluid and mineral imbalance can cause fatigue, dizziness, and muscle cramps.

On a low-carb diet, you should pay special attention to replenishing sodium, potassium, and magnesium, as these are often lost through increased urination during the initial phase.

Yes. If you cut carbs but don't increase your intake of healthy fats to compensate for the lost energy, you may enter a caloric deficit that is too large, resulting in fatigue.

Potentially. Since low-carb diets restrict food groups like fruits, starchy vegetables, and grains, they can sometimes lead to deficiencies in micronutrients like B vitamins, vitamin C, and iron, which can affect energy levels.

For some people, a gradual reduction in carbohydrate intake is a more manageable approach. It can help ease the body into the metabolic shift and reduce the intensity of fatigue and other "keto flu" symptoms.