What happens if a person is deficient in carbohydrates?

October 13, 2025 •

4 min read

The human brain alone consumes approximately 120 grams of glucose daily, accounting for roughly 20% of the body's total energy expenditure. When a person is deficient in carbohydrates, the body's primary energy source is cut off, triggering a cascade of metabolic and physiological changes that affect both physical and mental well-being.

Quick Summary

Carbohydrate deficiency forces the body to burn fat for fuel, a metabolic state known as ketosis. This can lead to short-term symptoms such as fatigue, headaches, and digestive issues, as well as potential long-term complications affecting mood, performance, and nutrient intake. The shift away from the body's preferred fuel source has wide-ranging health implications.

Key Points

Metabolic Shift to Ketosis: Without sufficient carbohydrates, the body burns fat for fuel, a state known as ketosis.
Initial 'Keto Flu' Symptoms: The body's transition to burning ketones can cause temporary fatigue, headaches, brain fog, and bad breath.
Potential Nutrient Deficiencies: Eliminating carb-rich foods can lead to a lack of fiber, vitamins, and minerals found in whole grains, fruits, and vegetables.
Impaired Performance: Athletes, in particular, may experience reduced stamina and impaired muscle recovery, as carbohydrates are the optimal fuel for high-intensity exercise.
Mood Regulation and Cognitive Effects: Insufficient carbohydrates can negatively impact mood, cause irritability, and impair cognitive function due to the brain's reliance on glucose and neurotransmitter synthesis.
Long-Term Health Risks: Extended low-carb diets may increase the risk of cardiovascular issues, kidney strain, and affect gut health.
Balanced Approach is Best: For most people, a balanced diet focusing on nutrient-dense complex carbohydrates is more beneficial for sustained energy and overall health than severe restriction.

The Role of Carbohydrates

Carbohydrates are one of the three main macronutrients, alongside fats and proteins, and serve as the body's primary and most efficient energy source. Upon consumption, carbohydrates are broken down into glucose, which is absorbed into the bloodstream. This glucose powers our cells, organs, and muscles, with the brain being particularly dependent on a steady supply. Any excess glucose is converted into glycogen and stored in the liver and muscles for future use. Adequate carbohydrate intake is fundamental not only for energy but also for cognitive function, physical performance, and overall metabolic health. When dietary intake of carbohydrates is severely restricted, the body's entire metabolic process is forced to adapt, leading to a host of symptomatic and long-term health effects.

The Shift to Ketosis

When carbohydrate intake is insufficient, the body exhausts its stored glycogen reserves within a day or two and must find an alternative fuel source. It begins to break down stored fat into molecules called ketones, which the body and brain can use for energy. This metabolic process is known as ketosis, which is distinct from the life-threatening diabetic ketoacidosis. While the body can function on ketones, the transition can be jarring and is often accompanied by an initial set of unpleasant symptoms collectively referred to as the 'keto flu'.

Common Short-Term Side Effects

The initial days or weeks of carbohydrate restriction can be a challenging period of adjustment, during which a person might experience the following symptoms:

Fatigue and Lethargy: With the body's preferred fuel (glucose) in short supply, energy levels plummet as it adapts to burning fat. This can make even simple daily tasks feel exhausting.
Headaches and Brain Fog: The brain is the body's largest consumer of glucose. Its adjustment to using ketones can cause a period of mental fogginess, reduced concentration, and headaches.
Bad Breath: As ketones are produced and excreted, a byproduct called acetone is released, resulting in a distinctively fruity or sweet-smelling breath.
Digestive Issues: Many healthy carbohydrate sources, such as whole grains, fruits, and vegetables, are also rich in fiber. Restricting these foods can lead to a significant drop in fiber intake, causing constipation.
Irritability and Mood Swings: Stable blood sugar levels help regulate mood. The initial fluctuations and the body's stress response to a lack of glucose can lead to irritability and moodiness.
Dehydration and Mineral Imbalances: Low-carb diets can act as a diuretic, causing the body to excrete more water and important electrolytes like sodium and potassium, leading to dehydration and muscle cramps.

Long-Term Health Consequences of Carbohydrate Deficiency

Sustained carbohydrate deficiency, especially without careful nutritional planning, can lead to more serious and long-lasting health concerns beyond the initial adaptation period.

Nutrient Deficiencies: Eliminating broad food groups, particularly whole grains, fruits, and starchy vegetables, can result in a significant loss of crucial nutrients like B vitamins, Vitamin C, magnesium, and fiber.
Impaired Athletic Performance: Carbohydrates are vital for fueling high-intensity exercise and replenishing muscle glycogen stores post-workout. Chronic deficiency can lead to reduced stamina, poorer performance, and impaired recovery.
Potential Kidney Issues: High-protein, low-carb diets may increase the strain on the kidneys over time, particularly for individuals with pre-existing kidney conditions.
Cardiovascular Risks: While some studies show low-carb diets can improve triglyceride and HDL levels, others report increases in LDL ('bad') cholesterol, which can heighten the risk of heart disease.
Mental Health Implications: Research suggests that consistently low carbohydrate levels may negatively impact serotonin levels in the brain, a neurotransmitter that helps regulate mood, potentially increasing the risk of anxiety and depression.
Disrupted Gut Health: Fiber-rich carbohydrates are essential for a healthy gut microbiome. Restricting them can negatively alter gut bacteria, impacting overall digestive and immune function.

Comparison of Acute vs. Chronic Effects


Feature	Acute (Short-Term) Effects	Chronic (Long-Term) Effects
Energy	Rapid decrease, causing fatigue and weakness.	Stabilizes as the body adapts to ketones, but lower overall energy for high-intensity activity.
Cognitive Function	'Keto flu' symptoms like brain fog and difficulty concentrating.	Potential for sustained negative impact on mood due to serotonin disruption.
Weight	Initial rapid weight loss, primarily due to water loss as glycogen stores deplete.	Potential for sustained weight loss, but also risk of muscle loss if protein isn't prioritized.
Digestion	Common onset of constipation due to lack of fiber.	Persistent gut health issues due to changes in the microbiome.
Nutrient Intake	Risk of electrolyte imbalance, leading to cramps and dehydration.	Higher risk of broader micronutrient deficiencies.
Athletic Performance	Significantly reduced capacity for high-intensity exercise.	Impaired muscle recovery and reduced overall athletic endurance.

Conclusion: Finding the Right Balance

While low-carbohydrate diets have gained popularity, it is clear that completely eliminating or severely restricting them without a specific medical reason and supervision can lead to numerous health complications. For most individuals, the key to optimal health lies in a balanced approach, focusing on the quality of carbohydrates consumed rather than just the quantity. Opting for nutrient-dense, complex carbohydrates like whole grains, fruits, vegetables, and legumes provides sustained energy and essential fiber and micronutrients, helping to avoid the negative consequences of true carbohydrate deficiency. Before making drastic dietary changes, it is always recommended to consult with a healthcare professional to ensure your nutritional needs are met in a way that supports your long-term health. The goal should not be to simply cut carbs, but rather to make smart, informed choices that fuel your body efficiently and sustainably.

One resource with further information is WebMD's slideshow on the topic: What Happens When You Stop Eating Carbs.

Frequently Asked Questions

Ketosis is a metabolic state where the body burns fat for fuel instead of glucose. It is a natural process that can occur with a low-carb diet or fasting and is generally not dangerous. It should not be confused with diabetic ketoacidosis (DKA), a life-threatening condition in people with uncontrolled diabetes.

Initial symptoms often appear within the first few days or weeks of severely restricting carbohydrates, as the body transitions from using glucose to ketones for energy.

While low-carb diets can lead to initial rapid weight loss, much of this is water weight. Long-term weight loss is primarily dependent on maintaining a consistent caloric deficit, and effectiveness can be similar to other diets over time.

Healthy or complex carbohydrates are found in whole grains, legumes, fruits, and vegetables. These foods provide sustained energy, fiber, and essential nutrients, unlike simple carbs from refined sugars.

Yes, a diet lacking in carbohydrates can impact your mood. The brain's reliance on a steady glucose supply and carbohydrates' role in producing mood-regulating neurotransmitters like serotonin can lead to irritability, anxiety, and depression when intake is low.

Yes, bad breath is a common side effect of carbohydrate deficiency. As the body breaks down fat into ketones for fuel, a byproduct called acetone is released, which causes a fruity or sweet odor on the breath.

For athletes, particularly those in high-intensity or endurance sports, a carbohydrate deficiency can significantly reduce performance. It can lead to depleted glycogen stores, reduced stamina, and slower recovery.

No, a low-carb diet is not safe for everyone. It is not recommended for most children, pregnant or breastfeeding individuals, or those with kidney disease or a history of eating disorders. It is crucial to consult a doctor before starting a restrictive diet.