What are the side effects of not eating enough carbohydrates?

November 18, 2025 •

3 min read

According to the National Academy of Medicine, a minimum of 130 grams of carbohydrates daily is recommended for basic bodily functions. When you fail to consume enough, the body's primary energy source is depleted, leading to a cascade of negative side effects that affect both physical and mental well-being.

Quick Summary

A lack of carbohydrates can lead to unpleasant short-term symptoms like fatigue, headaches, and digestive issues, often referred to as the 'keto flu'. Long-term consequences can include nutrient deficiencies, mood changes, and potential heart health concerns from replacing carbs with unhealthy fats.

Key Points

Keto Flu: Drastically cutting carbs can cause temporary headaches, fatigue, nausea, and irritability known as the 'keto flu'.
Electrolyte Loss: Initial water and glycogen loss leads to dehydration and an imbalance of electrolytes, causing muscle cramps and lightheadedness.
Digestive Problems: Restricting high-fiber carbs like whole grains and fruits often results in constipation and digestive discomfort.
Bad Breath (Halitosis): The body's production of ketones during ketosis can cause a distinct, fruity-smelling breath.
Nutrient Deficiencies: Long-term avoidance of carbs can lead to deficiencies in essential vitamins, minerals, and fiber.
Reduced Physical Performance: Athletes may experience a significant drop in high-intensity exercise capacity due to insufficient glycogen stores.
Brain Fog: The brain's reliance on glucose can lead to mental fog and reduced concentration during the adaptation phase.

The Initial Shock: The "Keto Flu" and Electrolyte Imbalances

When the body is deprived of its preferred fuel source, glucose from carbohydrates, it undergoes a metabolic shift into ketosis, where it starts burning fat for energy. This transition period can trigger a cluster of short-term symptoms known as the "keto flu.". This occurs as your body, and especially your brain, adapts to using ketones for fuel. The initial depletion of glycogen stores also causes significant water and electrolyte loss, which contributes heavily to the onset of symptoms.

Common symptoms during this phase include:

Headaches: Often caused by electrolyte imbalances and dehydration.
Fatigue and Lethargy: Reduced energy levels are a direct result of the body scrambling for fuel.
Irritability and Mood Swings: Fluctuations in brain fuel can impact mood regulation and lead to irritability.
Dizziness and Nausea: Some individuals experience lightheadedness and stomach upset.
Muscle Cramps: Lack of electrolytes like magnesium and potassium, which are lost with water, can cause painful muscle cramps.

The Digestive and Metabolic Consequences

Insufficient carbohydrate intake can also wreak havoc on your digestive system. Many high-carb foods, like whole grains, fruits, and legumes, are rich in dietary fiber. Drastically cutting these from your diet can lead to significant digestive upset.

The Impact of Low Fiber

A diet low in carbohydrates often means a diet low in fiber. Fiber is essential for promoting regular bowel movements and a healthy gut microbiome. Without it, you may experience:

Constipation: This is one of the most frequently reported side effects of low-carb diets.
Abdominal Discomfort: Bloating and gas can become more common due to changes in gut flora.

The Role of Ketosis

While ketosis is a natural metabolic process, it is not without side effects. One of the more peculiar and socially awkward symptoms is bad breath, often described as fruity or metallic. This is caused by acetone, a type of ketone, which the body releases through the breath.

Long-Term Health Risks

While many short-term side effects subside as the body adapts, long-term restriction of carbohydrates can pose more serious health risks if not managed properly.

Nutrient Deficiencies

Many carbohydrate-rich foods are also excellent sources of essential vitamins and minerals. Restricting them can lead to deficiencies in important nutrients such as:

B-vitamins: Crucial for energy metabolism and brain function.
Vitamin C: Important for immune function and antioxidant protection.
Potassium and Magnesium: Key electrolytes for nerve and muscle function.
Fiber: Necessary for digestive health.

Potential for Heart and Kidney Issues

For those on very low-carb diets, especially those high in animal fats and protein, there is concern about the potential for negative long-term effects on heart and kidney health. Some studies suggest potential links to cardiac contractile function impairment, heart arrhythmias, and an increased risk of kidney stones. However, the quality and type of fats and proteins consumed are major factors.

Low-Carbohydrate vs. Balanced Diet: A Comparison


Feature	Low-Carbohydrate Diet	Balanced Diet (Including Healthy Carbs)
Energy Source	Primarily fat, producing ketones	Primarily glucose from carbohydrates
Nutrient Variety	Often limited, risking deficiencies	Wide and diverse range of vitamins and minerals
Fiber Intake	Typically low, potentially causing constipation	Adequate, promoting healthy digestion
Satiety Signals	High fat/protein intake may prolong fullness	Fiber and complex carbs provide sustained energy
Initial Side Effects	'Keto flu' (fatigue, headaches, irritability)	No specific "flu"; stable energy levels
Long-Term Risks	Potential for nutrient deficiencies, heart and kidney issues	Balanced approach minimizes nutrient gaps

Impact on Mental and Physical Performance

The brain's primary fuel source is glucose. While it can adapt to run on ketones, the initial transition can cause mental fog and reduced cognitive function. Furthermore, athletes or highly active individuals may notice a significant drop in performance due to limited glycogen stores, which are vital for high-intensity exercise.

Conclusion: Finding the Right Balance

Not eating enough carbohydrates can trigger a range of side effects, from the temporary but unpleasant "keto flu" to more serious long-term health concerns. The key is to find a sustainable approach to eating. Rather than completely eliminating carbohydrates, focus on consuming healthy, complex carbs from sources like whole grains, vegetables, and fruits. Consulting a healthcare professional or registered dietitian is always recommended to ensure your diet is balanced and meets your body's specific needs.

Optional Link

For more detailed information on nutrient recommendations, see the resources provided by the National Institutes of Health (NIH).

Note: Any dietary change should be discussed with a healthcare provider, especially if you have pre-existing health conditions.

Frequently Asked Questions

The 'keto flu' is a collection of symptoms like headaches, fatigue, and nausea experienced during the first week of a very low-carb diet. You can minimize symptoms by staying hydrated, consuming enough electrolytes (sodium, potassium, magnesium), and ensuring adequate fat and protein intake.

Yes, a low-carb diet can cause bad breath, often described as fruity. This is because your body produces ketones for energy, and acetone, a type of ketone, is released through your breath.

Long-term low-carb diets, especially those high in saturated fats, may pose a risk to heart health. It is crucial to replace carbohydrates with healthy fats and lean proteins. Consulting a doctor is recommended to monitor your health markers.

Yes, for high-intensity exercise, your body relies on glycogen, which comes from carbs. Low carb intake limits these stores, potentially reducing endurance and peak performance, especially during the adaptation period.

Constipation is a common side effect because many high-carb foods like whole grains, legumes, and fruits are also primary sources of dietary fiber. Reducing these can lead to less bulk in your stool.

The brain's preferred fuel is glucose. While it can adapt to use ketones, the transition can cause temporary brain fog and impaired concentration. The liver can produce glucose from protein and fat, but this process is less efficient.

Health authorities recommend a minimum of 130 grams of carbohydrates per day for basic functions, but individual needs can vary based on activity level, health, and other factors.