What happens if you lack carbs? The surprising effects on your body

October 16, 2025 •

4 min read

Many people report feeling tired and irritable when they first reduce their carbohydrate intake. So, what happens if you lack carbs? Your body shifts from using glucose as its primary fuel to burning fat, triggering a complex metabolic and physiological response.

Quick Summary

The body switches its primary fuel source from glucose to ketones made from fat when carbohydrates are restricted. This transition can cause temporary side effects, nutrient deficiencies, and affect physical performance and mental clarity.

Key Points

Metabolic Fuel Switch: When carb intake is very low, the body depletes its glycogen stores and begins producing ketones from fat to use as fuel.
'Keto Flu' Symptoms: The initial transition can cause temporary side effects such as fatigue, headaches, brain fog, and irritability as the body adapts to burning fat.
Digestive Issues: Severe carb restriction often leads to low fiber intake, which can cause constipation and negatively affect the gut microbiome over time.
Nutrient Deficiencies: Eliminating carb-rich foods can lead to deficiencies in essential vitamins (B, C) and minerals (magnesium, potassium) found in whole grains, fruits, and vegetables.
Reduced Athletic Performance: Glycogen is the body's optimal fuel for high-intensity exercise, so a lack of carbs can decrease performance and increase fatigue during intense physical activity.
Long-Term Health Risks: Prolonged carb deficiency, especially with high saturated fat intake, is associated with an increased risk of heart disease, kidney issues, and nutrient deficiencies.
Requires Medical Supervision: Individuals with diabetes or kidney problems should consult a healthcare provider before attempting a low-carb diet due to potential risks like hypoglycemia or worsening kidney function.

Carbohydrates are the body's preferred source of energy. When you consume them, your digestive system breaks them down into glucose, which is used by your cells for fuel. Excess glucose is stored as glycogen in the liver and muscles for later use. This system works efficiently to power your daily activities, from walking to thinking. However, when you drastically cut carbohydrates from your diet, you force your body to adapt, initiating a chain of metabolic and physiological changes.

The Shift to Ketosis

When dietary carbohydrate intake is severely reduced, your body first uses up its stored glycogen reserves. This typically happens within 24 to 48 hours. Once these stores are depleted, your body, needing an alternative fuel source, enters a metabolic state called ketosis.

In ketosis, the liver begins to break down stored fat into molecules called ketones (specifically, acetoacetate, beta-hydroxybutyrate, and acetone). These ketones can then be used by your body and brain for energy. The brain, which relies heavily on glucose, can adapt to using ketones for a significant portion of its energy needs, although some glucose is still required and must be produced by the body through a process called gluconeogenesis.

Short-Term Side Effects: The 'Keto Flu'

The metabolic shift into ketosis isn't always a smooth process. Many people experience a cluster of short-term side effects, often referred to as the 'keto flu,' as their body adapts. These symptoms typically appear within the first few days to a week of severe carbohydrate restriction and can include:

Fatigue and Weakness: Carbohydrates provide a quick and readily available energy source. As your body transitions, it can cause a temporary dip in energy levels and a feeling of lethargy.
Headaches: Many individuals report experiencing headaches as their bodies adjust to lower blood sugar levels and the initial stages of ketosis.
Brain Fog: The brain may take time to become efficient at using ketones for fuel, leading to temporary difficulties with concentration and mental clarity.
Constipation: A diet lacking in carbohydrates, especially whole grains, fruits, and legumes, is often low in fiber. This lack of dietary fiber is a primary cause of constipation.
Bad Breath: As your body produces ketones for fuel, one of the compounds, acetone, is released through your breath, causing a distinct, often fruity or metallic, odor.
Irritability and Mood Swings: Restricting a major food group can affect mood, and the initial withdrawal from sugar can cause irritability.

Long-Term Concerns and Risks

While some short-term side effects subside, prolonged and severe carbohydrate restriction can lead to more serious, long-term health concerns. A diet extremely low in carbs is often restrictive in whole foods, which can result in significant nutritional deficiencies over time.

Potential Nutritional Deficiencies

When you cut out entire food groups, you inevitably lose the nutrients they provide. A severe lack of carbs often means missing out on:

Fiber: Essential for digestive health and regularity.
Vitamins: Such as B vitamins, vitamin C, and folate, which are abundant in fruits and grains.
Minerals: Important minerals like magnesium, potassium, and calcium can be depleted.

Cardiovascular and Kidney Health

Long-term, high-fat, very-low-carb diets may increase the intake of saturated fat, which could raise LDL ("bad") cholesterol levels in some individuals and increase the risk of heart disease. Additionally, severe carb restriction and increased protein intake can stress the kidneys over time, potentially leading to issues like kidney stones.

Gut Microbiome and Athletic Performance

The beneficial bacteria in your gut thrive on dietary fiber. A lack of fiber from a low-carb diet can negatively alter the gut microbiome, with long-term consequences that are not yet fully understood. For athletes, especially those engaged in high-intensity exercise, a consistent lack of carbohydrates can be detrimental. Carbs are the most efficient fuel for intense, short-burst activities, and low glycogen stores can lead to reduced performance and premature fatigue.

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

This table outlines the differences between the body's immediate reactions and the potential outcomes of prolonged carbohydrate restriction.


Feature	Short-Term Effects (First few weeks)	Long-Term Effects (Months to years)
Metabolic State	Shift into nutritional ketosis.	Sustained ketosis, burning fat for fuel.
Energy Levels	Often decrease, causing fatigue and lethargy.	May stabilize as the body adapts, but reduced performance during high-intensity exercise.
Mental Acuity	Temporary 'brain fog' and difficulty concentrating.	Can improve for some individuals, though mood swings or irritability may persist.
Digestive System	Constipation due to lack of fiber.	Altered gut microbiome and potential for chronic digestive issues.
Physical Performance	Reduced capacity for intense physical activity.	Sustained reduction in high-intensity exercise performance.
Nutrient Intake	Risk of electrolyte imbalances and dehydration.	Significant risk of deficiencies in fiber, vitamins, and minerals.
Overall Risk	Generally low for healthy individuals; requires careful hydration.	Increased risk of heart issues, kidney stones, and osteoporosis.

Finding a Balanced Approach

For many people, the healthiest approach is not the complete elimination of carbohydrates, but rather the consumption of high-quality, complex carbs in moderation. Incorporating nutrient-dense, fiber-rich sources like whole grains, fruits, and vegetables can provide steady energy without the side effects of extreme restriction. Consulting with a healthcare professional or a registered dietitian is highly recommended before making drastic changes to your diet, especially if you have pre-existing health conditions like diabetes.

Conclusion

Lacking carbs can trigger a metabolic state where the body burns fat for energy, but this transition comes with significant consequences. While short-term effects like the 'keto flu' are common, the long-term dangers, including nutritional deficiencies, heart concerns, and kidney issues, should not be ignored. A thoughtful, balanced dietary approach that includes nutrient-rich carbohydrates is the safest and most sustainable path to long-term health and wellness. For more information on the ketogenic diet, a type of very-low-carb diet, you can explore resources like the Harvard T.H. Chan School of Public Health (The Nutrition Source).

Frequently Asked Questions

The 'keto flu' is a collection of temporary symptoms, such as headaches, fatigue, brain fog, and nausea, that can occur when the body first transitions from burning glucose to ketones for fuel during a very low-carb diet.

Carbohydrates are the body's preferred energy source. When you restrict them, your body needs time to adapt to using fat for fuel. This transitional period can cause a temporary drop in energy levels and overall weakness.

Yes, chronic and severe carb restriction can be harmful. It can lead to nutritional deficiencies, stress the kidneys, potentially raise 'bad' cholesterol levels, and negatively impact the gut microbiome.

Yes, it can. While low-carb diets might be suitable for endurance activities, they can significantly impair performance during high-intensity exercise, as the body lacks the glycogen stores necessary for quick energy bursts.

The brain normally runs on glucose. When carbs are restricted, the brain must adapt to using ketones for fuel. This shift can initially cause 'brain fog' and difficulty concentrating, though some report improved mental clarity once fully adapted.

Ketones are molecules produced by the liver from fat when there is not enough glucose for energy. The body produces them as an alternative fuel source to power the brain and other organs during periods of carbohydrate restriction.

If you choose to limit carbs, focus on consuming nutrient-dense, low-carb foods such as leafy greens, non-starchy vegetables, and nuts. However, long-term restriction often requires a vitamin and mineral supplement to ensure nutritional adequacy.