What Happens If You Cut Out Carbs?

November 8, 2025 •

4 min read

According to StatPearls, very low-carb diets like the ketogenic diet can trigger a metabolic state called ketosis, where the body burns fat for energy instead of glucose. But what happens if you cut out carbs entirely, and is this practice sustainable or healthy long-term? This comprehensive guide explains the physiological effects, both positive and negative, that occur when you severely restrict carbohydrate intake.

Quick Summary

When you restrict carbohydrate intake, your body transitions from using glucose to burning fat for fuel, a process known as ketosis. This can cause short-term side effects like headaches, fatigue, and irritability, often referred to as the 'keto flu'. While it may lead to weight loss and improved blood sugar, long-term risks include potential nutrient deficiencies and heart health concerns.

Key Points

Initial 'Keto Flu': A sharp decrease in carbs can cause temporary fatigue, headaches, and irritability as your body adjusts to using fat for energy.
Energy Shift: The body transitions from burning glucose to burning ketones, which are produced from breaking down stored fat.
Weight Loss: Initial weight loss is largely water weight due to the depletion of glycogen stores, with fat loss occurring later as the body enters ketosis.
Nutrient Deficiency Risk: Eliminating whole grains, fruits, and legumes can lead to insufficient intake of essential fiber, vitamins, and minerals.
Impact on Heart Health: Depending on your replacement diet, long-term effects can range from improved triglyceride levels to potentially increased LDL cholesterol if unhealthy fats are consumed.
Digestive Changes: The lack of fiber from carb-rich foods can lead to constipation and other gastrointestinal issues.

The Immediate Impact: The 'Keto Flu' and Energy Shift

When you first begin to cut out carbs, your body experiences a significant change in its primary energy source. Carbohydrates, which are converted into glucose, are the brain and body's preferred fuel. As your carb reserves (glycogen) are depleted, you may experience a cluster of short-term side effects known as the 'keto flu'.

These temporary symptoms arise as your body adapts to using ketones from fat for energy instead of glucose.

Fatigue and Weakness: Without its primary fuel source, your body initially feels sluggish and low on energy, especially during physical activity.
Headaches and Brain Fog: Your brain depends on glucose, and the switch to an alternative fuel can cause mental fogginess and headaches until it adapts.
Irritability and Mood Swings: Changes in brain chemistry, including a potential drop in serotonin levels, can lead to mood disturbances.
Cravings: The body's initial reliance on glucose can trigger strong cravings for sugary or starchy foods.
Constipation: A severe reduction in fiber, typically found in whole grains, fruits, and starchy vegetables, is a major cause of digestive issues.
Bad Breath: The production of ketones can cause a distinct, often fruity or acetone-like, smell on your breath.

Potential Short-Term Benefits and Changes

Beyond the initial discomfort, many people report several short-term benefits that motivate them to continue with a low-carb diet.

Rapid Water Weight Loss: In the first few days, the body flushes out water along with glycogen stores, resulting in noticeable, but mostly temporary, weight loss.
Reduced Appetite: Increased protein and fat intake, coupled with the appetite-suppressing effect of ketones, can lead to a feeling of greater satiety and reduced hunger.
Improved Blood Sugar Control: For individuals with type 2 diabetes or insulin resistance, reducing carbohydrate intake can significantly improve blood sugar regulation.
Mental Clarity and Focus: While the initial phase brings 'brain fog,' many people report increased mental clarity and sustained energy once their body is fully adapted to ketosis.

Long-Term Effects: Considerations and Risks

While short-term results can be motivating, the long-term effects of severely restricting carbohydrates require careful consideration. A balanced approach is crucial to avoid potential risks.

Comparison Table: Short-Term vs. Long-Term Effects


Feature	Short-Term (1-3 weeks)	Long-Term (Months+)
Energy Levels	Initial fatigue, 'keto flu'	Sustained energy, increased focus
Weight	Rapid water weight loss	Slower fat loss
Nutrient Intake	Risk of electrolyte imbalance, dehydration	Potential deficiencies in fiber, vitamins, and minerals
Heart Health	Often improves blood triglycerides and HDL	Risks may increase with high intake of saturated fats
Digestion	Increased risk of constipation	May lead to persistent GI issues due to lack of fiber
Hormonal Health	Potential for hormonal disruptions as the body adjusts	Cortisol (stress hormone) may increase in response to low energy availability

Potential Long-Term Risks

Nutrient Deficiencies: Eliminating carb-rich foods like fruits, vegetables, and whole grains can lead to inadequate intake of fiber, vitamins (like B and C), and essential minerals.
Heart Health Concerns: If the diet is replaced with large quantities of unhealthy saturated fats, it could increase LDL ('bad') cholesterol, despite improvements in triglycerides and HDL ('good') cholesterol.
Kidney Damage: High protein intake without adequate hydration and nutrient balance can potentially strain the kidneys over a long period, especially in individuals with pre-existing kidney conditions.
Bone Density: Long-term carbohydrate restriction, particularly in very low-calorie diets, has been linked to potential issues with bone mineral density.
Sustainability Issues: The restrictive nature of cutting out carbs is difficult for many people to maintain indefinitely, often leading to a cycle of deprivation and overindulgence.

Key Considerations for Following a Low-Carb Diet

If you decide to cut out carbs, consider a moderated approach rather than complete elimination. It is essential to focus on high-quality replacements and ensure nutritional adequacy.

Best practices include:

Prioritizing healthy fats and proteins: Sources like avocados, nuts, seeds, olive oil, fatty fish, and lean meats should replace unhealthy, high-fat processed foods.
Incorporating low-carb, high-fiber vegetables: Non-starchy vegetables such as leafy greens, broccoli, and cauliflower provide essential vitamins and fiber to support digestion and overall health.
Staying hydrated and managing electrolytes: Drinking plenty of water and ensuring you get enough sodium, potassium, and magnesium can help mitigate 'keto flu' symptoms.
Gradual transition: Instead of an abrupt change, consider gradually reducing carbs to allow your body time to adjust, which can lessen the severity of side effects.

Conclusion: A Balanced Perspective

What happens if you cut out carbs depends largely on your individual health, the quality of your diet, and the duration of the restriction. While a low-carb diet can lead to effective weight loss and blood sugar control, especially in the short term, it is not without risks. Potential long-term concerns, including nutrient deficiencies and heart health issues, highlight the importance of careful planning and monitoring. A balanced approach that focuses on replacing refined carbs with nutrient-dense, whole foods is often a more sustainable and healthier strategy than complete elimination. Before making significant changes, it is always best to consult a healthcare professional or registered dietitian.

For more information on the effectiveness and safety of low-carbohydrate diets, you can consult research from the National Institutes of Health.

Frequently Asked Questions

The 'keto flu' is a common term for the temporary side effects experienced when transitioning to a very low-carb diet. Symptoms include headaches, fatigue, brain fog, and irritability, and typically subside within a few days to a couple of weeks.

Yes, many people experience rapid initial weight loss when cutting carbs, primarily due to losing water weight as glycogen stores are depleted. Long-term, weight loss depends on maintaining a caloric deficit and how your body responds to burning fat for fuel.

Completely eliminating carbs is not recommended for everyone. Carbohydrates are the body's main energy source and are rich in important nutrients and fiber. A total exclusion can lead to nutrient deficiencies and side effects, and is difficult to sustain long-term.

Carbohydrates are the brain's preferred fuel. When you cut carbs, the brain must adapt to using ketones for energy, which can cause initial 'brain fog' and mental fatigue. Once adapted, some people report improved clarity, but the long-term impact on mood and function varies.

When reducing carbs, focus on replacing them with healthy fats and proteins. Examples include lean meats, fish, eggs, avocados, nuts, seeds, and non-starchy vegetables like leafy greens, broccoli, and cauliflower.

The adjustment period varies for each individual. The initial 'keto flu' symptoms typically last from a few days to a few weeks. Full metabolic adaptation, where your body becomes more efficient at burning fat for energy, can take several weeks.

While low-carb diets can improve markers like triglycerides and HDL cholesterol, the risk of heart problems depends on the type of fats consumed. A diet high in unhealthy saturated fats can increase LDL ('bad') cholesterol. Choosing healthy fats is crucial for cardiovascular health.