What Happens During the First Week of a Carnivore Diet?

January 6, 2026 •

4 min read

Approximately 50% of individuals transitioning to a carnivore diet experience a temporary adaptation phase known as the 'keto flu'. Understanding what happens during the first week of a carnivore diet is key to navigating this initial adjustment period successfully.

Quick Summary

The first week of a carnivore diet involves significant metabolic changes, as the body shifts from burning carbs to fat for fuel. This often triggers a temporary period of fatigue, cravings, and digestive changes, which subside with proper hydration and electrolyte management.

Key Points

Keto Flu Symptoms: Expect temporary fatigue, headaches, and irritability as your body adjusts to burning fat instead of carbs.
Electrolyte Importance: Increased water and salt loss in the first week requires proactive electrolyte replenishment to prevent muscle cramps and dizziness.
Digestive Changes: Bowel movements may become irregular, experiencing either diarrhea or constipation as the gut microbiome and fat intake adjust.
Hydration is Key: Drink plenty of water and add salt to combat dehydration, which is common during the initial phase.
Appetite Fluctuations: Intense cravings may occur initially, but high-protein and fat meals often lead to increased satiety and reduced hunger over time.
Rest and Patience: Allow your body time to adapt. Avoid strenuous exercise and prioritize rest during the first week.
Embrace Fat: Eat fatty cuts of meat to provide your body with the necessary fuel to ease the transition and maintain energy.

Your Body's Metabolic Shift

The fundamental change that occurs in the first week of a carnivore diet is a major metabolic shift. Your body, previously reliant on carbohydrates and glucose for energy, must now adapt to using fat and protein. This process is called fat adaptation or ketosis, and it can take several days to initiate and weeks to fully optimize. This transition is the primary reason for most of the initial symptoms you will experience.

The 'Carnivore Flu' Experience

Many people transitioning to a carnivore diet report a set of symptoms similar to the influenza virus, colloquially known as the 'carnivore flu' or 'keto flu'. These symptoms are not caused by an actual infection but are a direct result of your body's adaptation process. They can range from mild discomfort to more severe, flu-like feelings. The most common symptoms include:

Fatigue and low energy: Your body's primary fuel source is gone, and it hasn't become efficient at burning fat yet. Expect to feel sluggish, especially in the first few days.
Headaches and dizziness: This is often caused by dehydration and a loss of electrolytes as your kidneys excrete more water when insulin levels drop.
Brain fog and irritability: The brain is also adjusting to a new fuel source. This can lead to temporary fogginess, mood swings, and a feeling of being 'off'.
Cravings: Your body's addiction to sugar and carbohydrates will trigger cravings. These are mental hurdles that subside as your body and brain adjust to the new diet.

Electrolyte Imbalance and Management

One of the most critical factors during the first week is managing electrolytes. When you cut out carbs, your insulin levels drop significantly, signaling your kidneys to flush out more sodium and water. This loss can throw your electrolyte balance out of whack, leading to many of the keto flu symptoms. To prevent this, proactive replenishment is essential.

Here are some strategies for maintaining electrolyte balance:

Liberally salt your food: Add more salt to your meals than you are used to. Using a quality salt, like sea salt or Himalayan pink salt, provides essential minerals.
Drink bone broth: Bone broth is rich in naturally occurring electrolytes and minerals. A cup a day can help replenish what is lost and soothe your system.
Consider supplements: If food and broth aren't enough, especially if you exercise, a sugar-free electrolyte supplement may be necessary to balance sodium, potassium, and magnesium.

Digestive System Adjustments

Removing fiber and suddenly increasing fat and protein intake will inevitably impact your digestive system. Anecdotal reports show people experience one of two extremes during the first week: diarrhea or constipation.

Diarrhea: This can occur as your gut microbiome adjusts to the absence of fiber and the sudden influx of fats. It can also be a sign of too much fat too quickly.
Constipation: On the other hand, constipation may happen because a pure carnivore diet produces very little indigestible waste. Bowel movements may become less frequent, which is normal. Hydration and adequate fat intake are key to preventing discomfort.

Comparison of First Week vs. Adaptation


Symptom	First Week Experience	Post-Adaptation Experience
Energy	Often low, can feel fatigued and sluggish as body transitions from carbs to fat.	Typically stable and sustained throughout the day, without the energy crashes from blood sugar fluctuations.
Mental State	May experience brain fog, irritability, and mood swings due to metabolic changes.	Many report improved mental clarity, focus, and stable moods.
Cravings	Strong cravings for sugar, carbs, and processed foods as the body withdraws.	Cravings significantly diminish or disappear as appetite hormones regulate.
Digestion	Unpredictable bowel movements, potentially diarrhea or constipation, as the gut adjusts.	Regulated, predictable bowel movements that are less frequent than on a high-fiber diet.
Weight	Initial rapid weight loss, primarily water weight due to glycogen depletion.	Consistent, sustainable fat loss as the body becomes efficient at burning its own fat stores for energy.

Tips for a Smoother Transition

The first week is the most challenging, but with the right approach, you can significantly mitigate the discomfort.

Prioritize Hydration and Salt: Drink plenty of water and salt your food liberally from day one. This is the single most important step for preventing headaches and muscle cramps.
Eat Enough Fat: Do not be afraid of fat. Eating fatty cuts of meat like ribeye, pork belly, and eggs will provide the necessary fuel for your body and increase satiety, helping to reduce cravings.
Prioritize Rest: Your body is working hard to adapt. Avoid strenuous exercise and ensure you get adequate sleep to support recovery and adaptation.
Keep it Simple: Especially in the first week, focus on simple, well-cooked animal products. Ground beef and eggs are often recommended as they are easy to digest. Once you are adapted, you can experiment with more variety.

Conclusion

The first week of a carnivore diet is a period of significant, but manageable, physiological change. The symptoms, often called the 'carnivore flu,' are a sign that your body is successfully transitioning from burning carbohydrates to fat for energy. By focusing on proper hydration, ensuring adequate electrolyte intake, and listening to your body, you can navigate this initial phase with minimal discomfort. The temporary challenges often pave the way for increased energy, mental clarity, and improved health that many people report after adapting to this way of eating.

Frequently Asked Questions

Your body is switching its primary energy source from carbohydrates to fat, a process called fat adaptation. This metabolic shift temporarily reduces your energy levels until your body becomes more efficient at using fat for fuel.

Headaches often result from dehydration and electrolyte loss, while cravings stem from carbohydrate withdrawal. Both are common during the initial detox phase as your body adjusts.

Yes, it is common to experience changes in bowel movements. The elimination of fiber and a sudden increase in fat can cause temporary digestive upset, which typically resolves as your gut adapts.

Focus on staying well-hydrated and increasing your electrolyte intake, especially sodium. Liberal salting of food and drinking bone broth or salted water can help manage symptoms.

To ease the transition, stick to simple, fatty cuts of meat like ground beef or ribeye, along with eggs. Ensure you are eating to satiety and not under-eating.

It is best to stick to light activity like walking or yoga. Your energy levels will likely be low, and strenuous exercise can exacerbate symptoms. Listen to your body and prioritize rest.

The initial adaptation phase can last anywhere from a few days to a few weeks, with symptoms generally subsiding by the second or third week as your body becomes fat-adapted.