What do 0 carbs do to your body?

November 4, 2025 •

3 min read

Over one-third of American adults follow some form of low-carbohydrate diet, with some attempting to get as close to zero carbs as possible. A zero-carb approach forces your body to undergo significant metabolic shifts, using fat for fuel instead of its primary energy source, glucose. While this can offer some benefits, it also presents notable side effects and long-term health risks.

Quick Summary

The body, deprived of carbohydrates, switches its primary fuel source from glucose to ketones via ketosis, leading to various short-term side effects and long-term nutritional concerns. The process impacts energy levels, brain function, and digestive health.

Key Points

Ketosis Induction: Without carbohydrates, your body enters a metabolic state called ketosis, converting fat into ketones for energy.
Initial Side Effects: The transition often causes 'keto flu' symptoms like fatigue, headaches, and brain fog as the body adjusts to burning fat.
Digestive Health Impacts: The lack of dietary fiber from fruits and vegetables can cause significant constipation and alter gut health.
Nutrient Deficiencies: A zero-carb diet risks severe deficiencies in essential vitamins, minerals, and antioxidants typically found in plant foods.
Long-Term Health Risks: Sustained zero-carb intake can increase cardiovascular risk from high saturated fat intake and may strain the kidneys.
Medical Consultation is Key: Given the risks, consulting a healthcare professional is essential before starting or maintaining a zero-carb diet.

The Metabolic Shift to Ketosis

When you completely eliminate carbohydrates from your diet, your body no longer has access to its preferred fuel source: glucose. This forces it into a metabolic state called nutritional ketosis. In ketosis, your liver breaks down stored fat to produce molecules called ketones, which then become the body's new primary energy source, including for the brain.

The 'Keto Flu' and Initial Side Effects

The first few weeks of a zero-carb diet can be challenging as your body adapts. This transition period is often referred to as the 'keto flu.' Symptoms arise as your body adjusts to using fat and ketones for energy instead of glucose.

Fatigue and Brain Fog: Without readily available glucose, energy levels can plummet. The brain, which typically relies heavily on glucose, may experience temporary fogginess or difficulty concentrating until it fully adapts to ketone utilization.
Headaches and Dizziness: Electrolyte imbalances, particularly a loss of sodium and potassium, can cause headaches, dizziness, and muscle cramps. This is often due to increased urination that flushes out electrolytes along with water.
Constipation: A zero-carb diet severely restricts fibrous foods like whole grains, fruits, and many vegetables. This lack of dietary fiber can lead to significant constipation.
'Keto Breath': As the body breaks down fat into ketones, it releases acetone as a byproduct, which can result in noticeably bad breath with a sweet or fruity odor.

Impact on Digestion and Nutrient Intake

Eliminating all carbohydrates means cutting out entire food groups, which has a ripple effect on your digestive system and overall nutrient profile. A zero-carb diet, often resembling a carnivore diet, can lead to a severe lack of fiber, antioxidants, and certain vitamins.

Comparison: Zero-Carb vs. Balanced Diet


Feature	Zero-Carb Diet	Balanced Diet
Primary Fuel Source	Ketones from fat breakdown	Glucose from carbohydrates
Digestion & Fiber	Very low fiber, can cause constipation and impact gut health	High in fiber from whole grains, fruits, and vegetables; promotes regular digestion and healthy gut flora
Micronutrient Profile	Potential for deficiencies in Vitamin C, B vitamins, and certain minerals due to food restriction	Broad and varied, supplying a wide range of essential vitamins and minerals
Energy Levels	Initial fatigue followed by potentially stable, but lower, sustained energy	Consistent energy levels through regular glucose metabolism; can experience crashes with refined sugar intake
Sustainability	Highly restrictive and difficult to maintain long-term due to limited food choices	Offers a wider range of food options, making it more sustainable and flexible for most people
Heart Health	High in saturated fats from animal products; risks of increased LDL cholesterol need monitoring	Promotes lean proteins, healthy fats, and a variety of plant foods, which can reduce cardiovascular risk

Potential Long-Term Risks

While some people report short-term benefits like weight loss and reduced inflammation, a complete lack of carbohydrates is not well-researched for long-term safety. Potential long-term consequences include:

Cardiovascular Strain: High intake of saturated fats from an animal-based zero-carb diet can raise LDL ('bad') cholesterol, increasing the risk of heart disease.
Kidney Issues: A very high protein intake puts added strain on the kidneys to filter waste products. For those with pre-existing kidney conditions, this can worsen the issue and increase the risk of kidney stones.
Osteoporosis: Restricting nutrient-dense plant foods may lead to deficiencies that impact bone health over time, potentially increasing the risk of osteoporosis.
Nutrient Deficiencies: The severe restriction of fruits, vegetables, and whole grains makes it extremely difficult to get adequate amounts of fiber, Vitamin C, potassium, and magnesium, among others.
Social and Mental Health: The highly restrictive nature of the diet can make it socially isolating and mentally challenging to maintain, potentially leading to disordered eating behaviors.

Conclusion

When you eliminate all carbohydrates, your body enters ketosis, a metabolic state that burns fat for fuel. While this can lead to rapid initial weight loss and improved blood sugar control for some, it is not without significant side effects. The short-term transition can bring on the 'keto flu,' while the long-term sustainability and nutritional adequacy are highly questionable due to the severe dietary restrictions. Ultimately, a balanced, whole-foods-based diet that includes healthy carbohydrates is recommended for most individuals for sustainable health.

It is crucial to consult a healthcare professional or registered dietitian before beginning a zero-carb diet, as it is a drastic change with uncertain long-term effects. For more information on dietary guidelines, consider resources like the Dietary Guidelines for Americans issued by the U.S. Department of Health and Human Services and U.S. Department of Agriculture.

Frequently Asked Questions

For most people, a diet with zero carbohydrates is not recommended or considered safe long-term. It is extremely restrictive and can lead to nutrient deficiencies, digestive issues, and potential cardiovascular or kidney strain.

Initially, many people experience a drop in energy and fatigue as the body adapts to burning fat instead of its preferred fuel, glucose. After adapting to ketosis, energy levels may stabilize, but often at a lower overall capacity compared to a balanced diet.

Yes, 'keto flu' symptoms, including headaches, fatigue, irritability, and nausea, are common during the first few weeks of a zero-carb or very-low-carb diet as the body transitions into ketosis.

The brain normally runs on glucose. With no carbs, it switches to using ketones for fuel, a process that can cause initial brain fog. While the brain can use ketones, it still requires a small amount of glucose produced by the liver.

Yes, it is very likely to cause constipation. By eliminating fruits, vegetables, and grains, a zero-carb diet removes virtually all sources of dietary fiber, which is essential for healthy digestion.

A zero-carb diet is an extreme version of a ketogenic diet. While both are low-carb, keto typically allows for a minimal amount of carbohydrates (e.g., 20-50g per day), whereas a zero-carb diet aims for complete elimination.

The long-term effects are not well-studied, but potential risks include increased LDL cholesterol, elevated risk of kidney stones, osteoporosis, and significant nutrient deficiencies due to the lack of food variety.