Understanding the Symptoms of Low Carbohydrates

November 17, 2025 •

4 min read

According to Healthline, some individuals report experiencing a collection of flu-like symptoms when they first start a very low-carb diet. The symptoms of low carbohydrates, commonly referred to as the 'keto flu,' occur as the body adapts to using fat for fuel instead of glucose.

Quick Summary

The body's adjustment to a low-carbohydrate diet can cause symptoms such as fatigue, headaches, and brain fog, as its primary fuel source shifts from glucose to fat. Other side effects can include constipation, irritability, and muscle cramps.

Key Points

Fatigue and Weakness: Reducing carbohydrates, the body's primary energy source, often leads to a temporary dip in energy, feeling sluggish or weaker than usual.
Headaches and Brain Fog: Changes in blood glucose and electrolyte levels during the metabolic switch can trigger headaches and mental fogginess.
Digestive Issues: Low fiber intake from restricting carbs can cause constipation, while the body's adaptation can lead to nausea and other stomach upsets.
Muscle Cramps: Increased fluid and electrolyte loss in the initial stages can result in painful muscle cramps, which can be mitigated with proper hydration and electrolyte replacement.
Bad Breath: The production of ketones as the body burns fat for fuel can cause a distinct, often fruity or metallic, breath odor.
Irritability and Cravings: Hormonal shifts and the body's adjustment can cause mood changes and intense cravings for carbohydrates.
How to manage symptoms: The key to managing these effects involves staying hydrated, replacing electrolytes, eating sufficient fat, getting adequate rest, and potentially transitioning to the diet more gradually.

What are the short-term symptoms of a low-carbohydrate diet?

When transitioning to a low-carb eating plan, the body shifts from burning glucose (from carbohydrates) to ketones (from fat) for energy. This metabolic adaptation, known as ketosis, can trigger a temporary set of side effects, often referred to as the 'keto flu'. These symptoms typically appear within the first few days and can range in severity from mild to debilitating, but they usually resolve as the body becomes more efficient at burning fat for fuel.

Some of the most common short-term symptoms of low carbohydrate intake include:

Fatigue and Lethargy: Carbohydrates are the body's preferred source of energy. A drastic reduction can cause a noticeable drop in energy levels and a general feeling of weakness while the body adapts to a new fuel source.
Headaches: Fluctuating blood glucose and insulin levels during the initial phase can trigger headaches. Staying well-hydrated and replenishing electrolytes can often help.
Brain Fog: The brain is highly sensitive to glucose levels. As the body switches to using ketones for fuel, some people experience a temporary period of mental fogginess, difficulty concentrating, and irritability.
Nausea and Upset Stomach: Changes in nutrient intake and electrolyte balance can cause stomach issues such as nausea, vomiting, or diarrhea.
Constipation: Many high-carbohydrate foods like whole grains, fruits, and legumes are also rich in fiber. Eliminating or severely restricting these can lead to a significant drop in fiber intake, resulting in constipation.
Muscle Cramps: The initial stages of a low-carb diet often lead to increased fluid and electrolyte excretion. A deficiency in minerals like sodium, potassium, and magnesium can cause painful muscle cramps, especially in the legs.
Bad Breath: As the body produces ketones, one of them, acetone, is released through the breath. This can cause a fruity or metallic odor, sometimes known as 'keto breath'.

Long-term implications and considerations of low-carb dieting

While many of the initial discomforts fade, sustained carbohydrate restriction may present other challenges and require careful planning to ensure nutritional needs are met.

Comparison of Potential Short-Term vs. Long-Term Low-Carb Effects


Symptom	Short-Term (Initial Adaptation)	Long-Term (Sustained Restriction)
Energy Levels	Significant fatigue and lethargy as the body switches fuel sources.	Energy often stabilizes and may even increase for some, but can be lower during high-intensity exercise.
Cognitive Function	Temporary 'brain fog' and difficulty concentrating.	Can lead to improved mental clarity and focus for some, though others may experience mood swings.
Nutrient Intake	Risk of electrolyte imbalance and dehydration due to fluid loss.	Potential for vitamin and mineral deficiencies, such as B1, C, folate, calcium, magnesium, and iron, due to food restrictions.
Digestive Health	Constipation or diarrhea are common as the gut microbiota adjusts to changes in fiber intake.	Potential for chronic constipation if fiber intake remains low. Possible impact on the gut microbiome.
Cardiovascular Health	Temporary heart palpitations due to dehydration and electrolyte imbalance.	Conflicting research, with some studies linking long-term low-carb diets to higher LDL ('bad') cholesterol.

Addressing nutrient deficiencies and other side effects

To mitigate potential side effects, a well-formulated low-carb diet is crucial. Ensuring a proper intake of electrolytes, drinking plenty of water, and consuming a variety of low-carb, nutrient-dense foods can make the transition smoother and more sustainable.

To ensure optimal gut health and fiber intake, focus on non-starchy vegetables, which provide fiber without a high carb load. If experiencing constipation, fiber supplements or high-fiber foods like flaxseed may be beneficial.

How to manage and minimize low-carb symptoms

For those experiencing significant discomfort, easing into the new diet more slowly may be a viable option. Rather than an immediate drastic cut, gradually tapering carbohydrate intake can give the body more time to adapt without the shock to the system.

In addition, avoiding strenuous exercise during the initial adaptation period can help conserve energy and reduce fatigue. Light activities like walking or yoga are often a better choice until your energy levels stabilize. If symptoms persist or are severe, seeking guidance from a healthcare professional or registered dietitian is always recommended. For more detailed nutritional information, resources like Medicine LibreTexts offer extensive knowledge on metabolic functions, including the body's use and storage of carbohydrates.

Conclusion

While transitioning to a low-carb diet can cause a range of temporary and unpleasant side effects, most people can successfully manage them with careful planning and supportive measures. Symptoms such as fatigue, headaches, constipation, and bad breath are primarily linked to the body's metabolic shift during the initial phase. For many, these resolve within a few weeks. However, being mindful of potential long-term risks like nutritional deficiencies and ensuring adequate hydration and electrolyte intake is important for a sustainable approach to low-carb eating.

Frequently Asked Questions

The 'keto flu' is a colloquial term for the set of flu-like symptoms experienced by some people when they first start a very low-carbohydrate, or ketogenic, diet. It is caused by the body's adaptation to using fat for fuel instead of glucose.

For most people, the uncomfortable symptoms of the keto flu are temporary and resolve within a few days to a couple of weeks. The duration can vary from person to person, and some may have a more difficult adaptation period.

Yes, dehydration is a common cause of many low-carb symptoms, including headaches, fatigue, and muscle cramps. When carbohydrate intake is reduced, the body depletes its glycogen stores, which are bound to water, leading to increased fluid excretion.

To prevent muscle cramps, which are often caused by electrolyte imbalance, ensure you are adequately hydrated and replenish lost minerals like sodium, potassium, and magnesium. Broth, avocados, and leafy greens are good sources.

Constipation on a low-carb diet is often due to reduced fiber intake. To help, increase your water consumption and focus on eating plenty of non-starchy vegetables and high-fiber, low-carb options like flaxseed.

Yes, low-carb diets can cause bad breath, or 'keto breath,' due to the body producing ketones as it burns fat for energy. Acetone, one type of ketone, is expelled through the lungs, causing a distinct odor.

It is generally recommended to avoid strenuous exercise during the initial phase of a low-carb diet. Your body is under stress as it adapts, and intense workouts can worsen symptoms like fatigue and muscle soreness. Opt for lighter activities instead.