What Happens If You Don't Eat Any Carbs? The Metabolic Shift and Side Effects

October 7, 2025 •

3 min read

According to research on low-carbohydrate diets, the metabolic changes that occur when carb intake is drastically reduced can lead to significant weight loss in the short term, although this is often accompanied by side effects. To comprehend what happens if you don't eat any carbs?, it is vital to explore the body's physiological adaptations, the transition period, and the long-term nutritional consequences.

Quick Summary

Eliminating carbohydrates forces the body to enter a metabolic state called ketosis, where it burns fat for fuel instead of glucose. This shift triggers a range of physical and mental symptoms, often referred to as the 'keto flu,' and poses a risk of nutrient deficiencies if not managed correctly.

Key Points

Metabolic Shift to Ketosis: Eliminating carbs forces your body to burn fat for fuel, producing ketones that serve as an alternative energy source for the brain and body.
The 'Keto Flu' Transition: Initial carb restriction causes temporary symptoms like fatigue, headaches, and brain fog as the body adjusts to ketosis.
Risk of Nutrient Deficiencies: A zero-carb diet cuts out many nutritious foods, increasing the risk of missing out on essential vitamins, minerals, and fiber.
Potential for Digestive and Organ Issues: Lack of fiber can cause constipation, and high protein intake can potentially stress the kidneys over time.
Reduced Athletic Performance: Without the primary energy source from carbohydrates, endurance and high-intensity athletic performance may be negatively impacted.
Focus on Protein and Fats: A no-carb diet relies heavily on meats, fish, eggs, and fats, while severely restricting or eliminating grains, fruits, and starchy vegetables.
Sustainability Concerns: The highly restrictive nature of a zero-carb plan makes it difficult to maintain long-term, and professional guidance is often recommended.

The body primarily relies on carbohydrates as its main and preferred source of energy. When carbohydrate intake is significantly reduced or eliminated, the body is deprived of its primary fuel, triggering a metabolic shift towards using fat as an alternative energy source. This state is known as nutritional ketosis. While a very low-carb approach like the ketogenic diet can be used medically for conditions such as epilepsy, its application for general health requires careful consideration.

The Metabolic Shift to Ketosis

Within days of reducing or eliminating carbs, the body's glycogen stores are depleted. The liver then begins to break down fat into ketone bodies, which serve as fuel for the brain and other tissues. This process is central to a ketogenic diet. During this transition, the brain primarily uses ketones, although some glucose is still needed, which the liver produces from protein and fat through gluconeogenesis.

Short-Term Side Effects of Eliminating Carbs

The initial phase of carbohydrate elimination often involves symptoms known as the "keto flu". These symptoms typically last one to two weeks and are part of the body's adjustment to burning fat for fuel. Staying hydrated and maintaining electrolyte balance can help manage these effects.

Common short-term side effects include:

Fatigue and Brain Fog: Reduced energy and difficulty concentrating are common as the body adapts.
Headaches and Dizziness: These can result from sugar withdrawal and electrolyte imbalances.
Bad Breath (Halitosis): A fruity or metallic breath odor is a sign of ketosis due to acetone release.
Constipation: Reduced fiber intake from eliminated grains and fruits can cause constipation, which may be improved by consuming non-starchy vegetables and sufficient fluids.
Muscle Cramps: Loss of fluids and electrolytes can lead to muscle cramps.

Long-Term Health Risks and Nutritional Deficiencies

The long-term effects of a zero-carb diet are not fully understood and carry risks.

Nutrient Deficiencies: Excluding nutrient-rich foods like whole grains, fruits, and legumes can lead to deficiencies in essential vitamins, minerals, and antioxidants.
Kidney Health Concerns: High protein intake may increase uric acid and the risk of kidney stones.
Heart Health: High intake of saturated fat may raise LDL cholesterol, a risk factor for heart disease.
Digestive Issues: Lack of fiber can negatively impact gut health.
Reduced Athletic Performance: Depleted glycogen stores can hinder performance in intense exercise.

Comparing Carbohydrate Metabolism to Ketosis


Feature	Normal Carbohydrate-Based Diet	Zero-Carb Ketogenic Diet
Primary Fuel Source	Glucose from carbohydrates	Ketones from fat breakdown
Energy Source for Brain	Predominantly glucose	Primarily ketones, with some glucose from gluconeogenesis
Glycogen Stores	Stored in liver and muscles for energy reserve	Depleted within days, leading to water weight loss
Symptom Profile	Generally stable energy, no "flu" symptoms	"Keto flu" common during adaptation phase
Satiety	Varies based on carb type	Increased satiety due to higher fat and protein intake
Nutritional Profile	Diverse nutrients from fruits, grains, veggies	Risk of nutrient deficiencies from restricted food groups
Long-Term Risks	Related to excess processed carbs and sugar	Potential issues with kidneys, heart, and gut health

What to Eat and What to Avoid

Foods to Focus On (for a Zero-Carb approach):

Meats and Fish: Various types of meat and fish.
Eggs: A good source of protein and fat.
Healthy Fats and Oils: Olive oil, avocado oil, coconut oil, and butter.
Cheese: Full-fat cheeses.
Some Non-Starchy Vegetables (limited): Certain leafy greens and cauliflower are often included despite having some carbs.

Foods to Eliminate or Severely Restrict:

Grains: Bread, pasta, rice, oats, and cereal.
Fruits: Most fruits are excluded, except perhaps small amounts of very low-carb berries in some variations.
Starchy Vegetables: Potatoes, corn, and peas.
Legumes: Beans, lentils, and chickpeas.
Sugar and Sweets: Candy, pastries, sodas, and baked goods.

Conclusion: Weighing the Risks and Rewards

Eliminating carbs is a highly restrictive diet that significantly changes the body's metabolism. While it can lead to quick initial weight loss, it often comes with uncomfortable side effects and potential long-term risks, including nutrient deficiencies and concerns for kidney and heart health. Maintaining this diet long-term is challenging. Although dietary carbohydrates are not strictly essential for adults, as the body can produce glucose, a zero-carb diet requires careful management to avoid malnutrition. Most health experts recommend a more balanced diet including nutrient-dense, high-fiber carbohydrates. It is advisable to consult a healthcare professional before making drastic dietary changes.

For more details on the ketogenic diet, a less extreme form of very low-carb eating, information is available from the National Center for Biotechnology Information (NCBI).

Frequently Asked Questions

You may experience significant short-term weight loss, but much of this is initially water weight due to the depletion of glycogen stores. Fat loss can occur later, but the long-term effectiveness of a zero-carb diet versus other calorie-restricted diets is often debated.

The 'keto flu' refers to a group of flu-like symptoms, including headaches, fatigue, and irritability, that occur during the initial adaptation to ketosis. Staying well-hydrated and ensuring adequate electrolyte intake from sources like bone broth and salts can help manage these symptoms.

Yes, long-term risks include potential nutrient deficiencies from the exclusion of major food groups, concerns for kidney and heart health (especially with high animal fat intake), and negative effects on the gut microbiome.

Carbohydrates are the body's preferred fuel for intense exercise. Without them, muscle glycogen stores are depleted, which can lead to reduced endurance and impaired performance.

By cutting out fruits, grains, and some vegetables, you risk deficiencies in fiber, B vitamins, vitamin C, folate, magnesium, and iron.

The brain typically uses glucose as its primary fuel. While it can adapt to use ketones derived from fat for most of its energy needs, some parts still require glucose, which the liver produces through gluconeogenesis.

For individuals with pre-existing conditions, particularly related to the kidneys, liver, or heart, a no-carb diet can be risky and may exacerbate existing issues. Medical supervision is highly recommended.