What a Lack of Carbohydrates Causes in the Body

January 8, 2026 •

4 min read

The body primarily uses carbohydrates as its main source of energy. Restricting this vital macronutrient forces the body to seek alternative fuel, which can trigger a cascade of short- and long-term side effects.

Quick Summary

A lack of carbohydrates forces the body into ketosis, leading to various short-term symptoms like fatigue and headaches, and potential long-term issues including nutrient deficiencies and health risks.

Key Points

Initial Fatigue and 'Keto Flu': A sudden lack of carbohydrates can cause weakness, headaches, and brain fog as the body shifts from glucose to ketone fuel.
Nutrient Deficiencies: Eliminating carb-rich foods like fruits and whole grains can lead to deficiencies in essential vitamins, minerals, and fiber.
Digestive Issues: A significant reduction in fiber, found abundantly in complex carbs, often results in constipation or other gastrointestinal discomforts.
Impact on Brain Function: The brain relies primarily on glucose for fuel, and without it, cognitive function can temporarily decrease until it adapts to using ketones.
Dehydration and Electrolyte Loss: Restricting carbohydrates can act as a diuretic, leading to fluid and electrolyte imbalances that exacerbate fatigue and headaches.
Long-Term Health Risks: Sustained, severe carb restriction may contribute to increased LDL cholesterol, potential kidney issues, and damage to the gut microbiome.
Muscle Catabolism: In the absence of glucose, the body may break down muscle protein for conversion into glucose, a process known as gluconeogenesis.

The Body's Primary Fuel Source and the Switch to Ketosis

Carbohydrates are the body's most readily available source of energy, broken down into glucose that fuels the brain and muscles. When carbohydrate intake is severely restricted, the body's glycogen stores are depleted within a few days. To maintain energy, the liver begins to break down stored fat, producing chemicals called ketones that the body can use for fuel. This metabolic state is known as ketosis, and while sometimes intentionally pursued for weight loss, it can trigger significant physiological changes and uncomfortable side effects.

The Short-Term Effects: The 'Keto Flu'

For many people, the transition into ketosis is accompanied by a collection of symptoms often dubbed the 'keto flu'. These side effects typically appear within a few days of drastically cutting carbs and can last for a week or more while the body adapts. These symptoms are often the result of electrolyte imbalances and dehydration, as reduced insulin levels cause the kidneys to excrete more sodium and water.

Common short-term effects include:

Fatigue and Weakness: The most reported side effect, as the body and brain adjust from a glucose-dependent energy system.
Headaches: Often caused by electrolyte loss and dehydration, as the body sheds water weight.
Brain Fog: A temporary decrease in cognitive function, such as difficulty concentrating and mental clarity issues, which occurs as the brain adapts to using ketones.
Irritability and Mood Swings: Hormonal shifts and lower serotonin levels have been linked to mood changes during the initial phase of carb restriction.
Bad Breath: Also known as 'keto breath,' this is caused by the release of acetone, a type of ketone, through the breath.
Digestive Issues: Constipation or diarrhea are common due to a drop in dietary fiber intake, as many high-fiber foods are also carbohydrate-rich.
Nausea and Upset Stomach: The introduction of ketones into the body can lead to digestive discomfort for some individuals.

Long-Term Consequences of Severe Carbohydrate Restriction

While the initial 'keto flu' is temporary, consistently depriving the body of carbohydrates can lead to more serious and lasting health concerns. These long-term risks are often associated with extreme or poorly managed low-carb diets, especially those that lack sufficient nutrient-dense vegetables and fruits.

Nutrient Deficiencies: Many nutritious foods, like fruits, starchy vegetables, and whole grains, are restricted on very low-carb diets. This can lead to deficiencies in essential vitamins and minerals, including calcium, potassium, magnesium, and certain B vitamins.
Potential Kidney and Liver Issues: High protein and fat intake can increase the metabolic load on the liver and kidneys. While healthy individuals may tolerate this, it can worsen existing conditions or, in rare cases, lead to kidney stone formation.
Cardiovascular Health Risks: Some low-carb diets encourage high consumption of animal fats, which are high in saturated fats. This can lead to an increase in LDL ('bad') cholesterol, raising the risk of heart disease. However, the effect varies depending on the specific fats consumed.
Impact on Gut Health: A lack of dietary fiber, especially from complex carbohydrates, can negatively alter the gut microbiome. This can decrease the beneficial bacteria that support immune function and digestive health.
Muscle Loss: While the body adapts to burning fat, it can also break down muscle tissue to convert amino acids into glucose for the brain. While a well-formulated diet can mitigate this, excessive muscle loss is a risk during periods of prolonged carb restriction.

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


Symptom Category	Short-Term Effects (First weeks)	Long-Term Effects (Months/Years)
Energy & Performance	Fatigue, weakness, decreased endurance.	Potential long-term impairment of high-intensity exercise capacity due to low muscle glycogen.
Mental Acuity	Brain fog, difficulty concentrating, irritability.	Risk of cognitive decline if nutritional deficiencies or glucose deprivation persist.
Nutritional Health	Electrolyte imbalance, dehydration.	High risk of vitamin, mineral, and fiber deficiencies.
Digestive Health	Constipation or diarrhea due to low fiber.	Altered gut microbiome, potential for chronic constipation.
Organ Health	Stress on liver and kidneys as metabolic pathways shift.	Increased risk of kidney stones and potential strain on liver.
Metabolic Health	Rapid water weight loss, ketosis.	Potential for elevated LDL ('bad') cholesterol and long-term impact on heart health.

Conclusion: The Importance of a Balanced Approach

A deficiency of carbohydrates can cause a range of physiological and metabolic changes. While some individuals may experience initial benefits like rapid weight loss and reduced blood sugar, these are often accompanied by unpleasant side effects like the 'keto flu'. Prolonged or extreme restriction of carbohydrates carries notable health risks, including nutrient deficiencies, potential harm to organ systems, and an altered gut environment. The body is adaptable and can survive without dietary carbohydrates for extended periods by entering ketosis. However, the crucial role of complex carbohydrates in providing sustained energy, fiber for gut health, and essential vitamins and minerals should not be overlooked. A balanced approach that incorporates healthy, fiber-rich carbs is generally recommended for sustained well-being. For those considering a very low-carb or ketogenic diet, consulting a healthcare professional is crucial to manage risks and ensure proper nutrient intake. You can find more authoritative health information on the topic by visiting the Cleveland Clinic.

Potential Health Implications of Carb Restriction

It is essential to understand that not all low-carb diets are created equal. Restricting processed foods and simple sugars is widely considered beneficial. However, eliminating complex, nutrient-dense carbohydrates like whole grains, fruits, and vegetables removes crucial sources of fiber, vitamins, and minerals that are vital for overall health. The long-term risks associated with severe carb restriction emphasize the importance of dietary balance. This includes not just carbohydrate intake, but also sourcing healthy fats and proteins and ensuring sufficient micronutrient and fiber consumption, perhaps through supplementation, if necessary. The initial side effects can be managed with adequate hydration and electrolyte replacement, but the long-term metabolic and nutritional impacts require careful, ongoing consideration and medical supervision, particularly for those with pre-existing health conditions.

Frequently Asked Questions

Without carbohydrates, your brain, which relies on glucose, experiences a temporary period of 'brain fog' as it transitions to using ketones for fuel. Long-term, consistent glucose deprivation can affect concentration and memory.

Yes, fatigue is one of the most common and immediate side effects of carbohydrate deficiency. It is caused by the body's and brain's initial struggle to adapt to an alternative fuel source (ketones) instead of its preferred glucose.

The 'keto flu' is a cluster of temporary symptoms, including headaches, fatigue, nausea, and irritability, that some people experience as their body adapts to burning fat for energy instead of carbs.

Yes, low-carb diets can cause digestive problems like constipation or diarrhea. This is largely due to the reduced intake of dietary fiber, which is found in many carbohydrate-rich foods like fruits, vegetables, and whole grains.

In a controlled, therapeutic context, dietary ketosis can be safe and even beneficial for some conditions. However, in uncontrolled Type 1 diabetes, it can escalate into a life-threatening state called ketoacidosis. Long-term health effects of nutritional ketosis are still being researched, but potential risks include kidney stones, bone density issues, and nutrient deficiencies.

You can mitigate low-carb side effects by ensuring you stay well-hydrated and replenish electrolytes like sodium, potassium, and magnesium, which are lost as insulin levels drop. Easing into the diet gradually can also help.

Yes, during a prolonged carbohydrate deficit, the body may break down muscle protein to produce glucose for the brain through a process called gluconeogenesis. While this can be mitigated, it is a potential risk.