What happens to your body when you cut out carbs?

October 12, 2025 •

4 min read

Carbohydrates are the body's main source of fuel, with the brain depending on glucose from carbs as its primary energy source. When you dramatically reduce or eliminate them, your body is forced to adapt, leading to a cascade of physiological changes. The answer to 'What happens to your body when you cut out carbs?' involves both immediate withdrawal symptoms and significant long-term metabolic shifts.

Quick Summary

Drastically reducing carbohydrate intake forces the body to burn alternative fuel, transitioning into ketosis. This metabolic change can trigger short-term withdrawal symptoms, including fatigue and headaches, while potentially offering benefits like weight loss and improved blood sugar control. Long-term health effects vary and require careful consideration.

Key Points

The 'Keto Flu' is real: In the first few days, your body adjusts to the absence of glucose, causing symptoms like headaches, fatigue, and irritability.
Ketosis shifts your fuel source: Your body burns stored fat for energy, producing ketones as fuel when carbs are cut, leading to potential fat loss.
Electrolyte balance is critical: Rapid water loss in the beginning can deplete electrolytes like sodium and potassium, so staying hydrated is key.
Nutrient deficiencies are a long-term risk: Excluding entire food groups can lead to a lack of fiber, vitamins, and minerals over time.
Weight loss is often rapid initially: The swift loss of water weight and increased satiety can lead to a quick reduction on the scale.
Prioritize nutrient-dense foods: To mitigate health risks, focus on replacing carbs with healthy fats, lean proteins, and low-carb vegetables.

The Initial Shock: The "Keto Flu"

When you first cut out carbohydrates, your body undergoes a period of adaptation known as the "keto flu". The body, which is accustomed to running on glucose from carbs, suddenly finds its primary fuel source depleted. This forces it to seek an alternative, leading to a range of withdrawal-like symptoms that typically last a few days to a couple of weeks.

During this phase, your body rapidly depletes its glycogen stores. Since glycogen holds onto a significant amount of water, you will experience a rapid loss of water weight and increased urination. This fluid loss can also lead to a drop in electrolytes, particularly sodium, potassium, and magnesium, exacerbating many of the flu-like symptoms.

Symptoms of the keto flu include:

Headaches and brain fog
Fatigue and weakness
Irritability and mood swings
Nausea and upset stomach
Constipation or diarrhea
Muscle cramps

The Metabolic Shift to Ketosis

After the initial withdrawal phase, your body enters a state of nutritional ketosis. With minimal glucose available, the liver starts converting fat into compounds called ketones, which are used as a new, alternative fuel source for the body and brain.

This is the core metabolic change that drives fat loss on a very low-carb diet. By burning fat for fuel, the body can reduce fat stores. Many people report feeling more sustained energy and mental clarity once they adapt to this new metabolic state. One interesting side effect of ketosis is "keto breath," a metallic or fruity smell caused by acetone, one of the ketones, being expelled through the breath.

Potential Benefits and Drawbacks

Cutting out carbs is often associated with weight loss, and research shows that low-carb diets can be effective for short-term weight reduction. The initial drop in water weight is followed by fat loss, often accompanied by a natural reduction in appetite, which helps with overall calorie intake. For individuals with type 2 diabetes, a low-carb diet can lead to better blood sugar control and reduced reliance on medication in the short term.

However, there are notable drawbacks and risks. Long-term restrictive diets can lead to nutritional deficiencies, as many carb-rich foods like fruits, vegetables, and whole grains are key sources of fiber, vitamins, and minerals. This can lead to digestive issues like constipation and may affect gut microbiome health. Some studies also link long-term low-carb diets high in animal fats to potential increases in LDL ("bad") cholesterol and an elevated risk of heart disease.

Short-term vs. Long-term Effects of Cutting Carbs


Feature	Short-Term Effects (Days to Weeks)	Long-Term Effects (Months to Years)
Metabolism	Transitioning into ketosis, burning glycogen first.	Stable ketosis, burning fat for fuel.
Energy Levels	Fatigue, weakness, and brain fog (keto flu).	Often sustained energy and mental clarity.
Weight	Rapid water weight loss, followed by fat loss.	Slower, more consistent fat loss.
Appetite	Can fluctuate, but often decreases as body adapts.	Better appetite control and reduced cravings.
Nutrient Intake	Risk of electrolyte imbalance and dehydration.	Potential for vitamin and mineral deficiencies.
Digestive Health	Constipation or diarrhea are common.	Risk of low fiber intake and altered gut flora.
Health Risks	Generally minimal short-term risks for healthy individuals.	Potential increase in LDL cholesterol, kidney stone risk.

How to Cut Carbs in a Healthy Way

If you choose to follow a low-carb diet, it's crucial to do it in a way that minimizes risk and maximizes nutritional value. Focus on replacing refined carbs and sugars with nutrient-dense, whole foods.

Best practices for a healthy low-carb diet:

Prioritize healthy fats: Incorporate sources like avocados, nuts, seeds, and olive oil to keep you full and satisfied.
Include non-starchy vegetables: Fill your plate with leafy greens, broccoli, cauliflower, and peppers for essential fiber, vitamins, and minerals.
Choose lean proteins: Opt for chicken, fish, eggs, and lean cuts of beef to maintain muscle mass.
Stay hydrated: Drink plenty of water and replenish electrolytes, especially during the initial phase, to combat dehydration and muscle cramps.
Consider nutrient supplementation: Given the restriction of certain food groups, talk to a healthcare provider about potential vitamin deficiencies.
Avoid processed low-carb foods: Many processed foods marketed as "low-carb" contain unhealthy additives and preservatives.

Conclusion

Cutting out carbs triggers a significant metabolic shift, moving the body from a glucose-burning state to one that utilizes fat for fuel. While this can lead to effective short-term weight loss and improved metabolic markers for some, it comes with a period of adjustment known as the "keto flu." The long-term effects are more complex and require careful attention to nutrition to avoid deficiencies and potential health risks. Before making any drastic dietary changes, consulting a healthcare professional is highly recommended to determine the best approach for your individual needs.

For more information on the nuances of low-carb diets, a comprehensive review of comparative studies is available on the Cochrane Library.

Frequently Asked Questions

Ketosis is a metabolic state where your body burns fat for fuel instead of carbohydrates. It occurs when your carb intake is low and your liver begins producing ketones from fat to serve as an alternative energy source.

Symptoms of the 'keto flu' typically last a few days to a couple of weeks. The duration and severity can vary based on individual factors, hydration, and electrolyte balance.

Initially, a rapid drop in glycogen and water can make it seem like you've lost muscle mass. However, a well-formulated low-carb diet that includes adequate protein can help preserve lean body mass while burning fat.

Bad breath, sometimes described as fruity or metallic, is a common side effect of ketosis. It is caused by the release of acetone, a type of ketone, through your breath.

A strictly zero-carb diet is not widely recommended by health experts, as it can lead to nutrient deficiencies and potential long-term health risks. A balanced, low-carb approach that includes healthy vegetables and fats is generally safer.

Focus on nutrient-dense foods like leafy greens, broccoli, cauliflower, lean proteins (chicken, fish), healthy fats (avocado, nuts, olive oil), and some dairy.

In the short term, low-carb diets can be effective for managing blood sugar levels and weight for people with type 2 diabetes. However, adjustments to medication may be necessary, and you should always consult a healthcare team.

Constipation is often due to a lack of fiber. Ensure you are getting enough fiber from non-starchy vegetables, and stay well-hydrated. Discussing supplementation with a doctor can also be an option.