Common Side Effects of Going Low Carb and How to Manage Them

January 2, 2026 •

4 min read

According to research published by the Mayo Clinic, a sudden, large drop in carbohydrate intake can cause short-term side effects like constipation, headaches, and muscle cramps. This initial phase, often called the 'keto flu,' signals your body's metabolic adjustment from burning glucose to burning fat for fuel.

Quick Summary

The transition to a low-carb diet can trigger a range of temporary side effects as the body adapts to a new fuel source. These symptoms, including headaches, fatigue, and digestive issues, are often manageable through proper hydration, electrolyte balance, and a diet rich in fiber-filled, keto-friendly foods.

Key Points

Keto Flu: The initial metabolic shift from glucose to fat burning can cause flu-like symptoms such as headaches, fatigue, and brain fog, which typically last a week or two.
Electrolyte Loss: The diuretic effect of a low-carb diet causes the body to excrete more water and vital electrolytes like sodium, potassium, and magnesium, leading to cramps and fatigue.
Digestive Changes: A reduced fiber intake from cutting carbohydrates can lead to constipation, while the digestive system adapts to processing higher fat content.
Temporary Hair Loss: Some individuals may experience temporary hair shedding, known as telogen effluvium, due to the stress of rapid weight loss and metabolic changes.
Nutrient Deficiencies: Restricting fruits and grains can lead to a lack of key vitamins and minerals if not carefully managed with nutrient-dense, low-carb food choices.
Elevated Cholesterol: A small number of people, known as 'lean mass hyper-responders,' may see a significant increase in LDL cholesterol on a low-carb diet, requiring careful monitoring.
Management is Key: Most side effects can be mitigated by ensuring adequate hydration, balancing electrolytes, increasing fiber from low-carb sources, and making dietary changes gradually.

The 'Keto Flu': A Common Initial Side Effect

One of the most frequently reported side effects of beginning a low-carb diet is a cluster of symptoms known as the 'keto flu'. These flu-like symptoms are a normal part of the body's metabolic shift into ketosis, the state where it burns fat for energy instead of glucose. Symptoms typically appear within the first few days and can last up to a week or two as your body adapts.

Common symptoms of the keto flu include:

Headaches
Fatigue and low energy levels
Irritability and mood swings
Brain fog and difficulty concentrating
Nausea, sometimes accompanied by vomiting

These side effects are primarily caused by dehydration and a loss of electrolytes, such as sodium, potassium, and magnesium, which occurs as the body's insulin levels drop. Low insulin prompts the kidneys to excrete more water and salt, which can lead to these uncomfortable symptoms.

Dealing with Digestive Issues

For many, a low-carb diet can lead to changes in bowel habits, most notably constipation or, less commonly, diarrhea. Constipation is a frequent complaint, largely due to a significant reduction in dietary fiber from fruits, legumes, and whole grains. The body's digestive system also needs time to adjust to a higher fat intake. If not properly addressed, this can cause discomfort and potential long-term issues like hemorrhoids. Conversely, some individuals may experience diarrhea as the body adapts to digesting higher amounts of fat.

Managing Constipation

Increase water intake: Staying well-hydrated is crucial as low-carb diets have a diuretic effect.
Boost fiber: Incorporate more fiber-rich, low-carb vegetables like leafy greens, broccoli, and cauliflower. Ground flaxseed or psyllium husks are also excellent sources.
Replenish magnesium: Magnesium supplements or magnesium citrate can help stimulate bowel movements.

Other Notable Short-Term Side Effects

Beyond the keto flu, several other short-term side effects can occur, often related to the same physiological changes.

Bad Breath: As the body enters ketosis, it produces ketones, including acetone. This is exhaled through the breath, causing a distinct, often fruity odor. For most, this is temporary and resolves as the body becomes more efficient at using ketones.
Muscle Cramps: A deficiency in electrolytes like magnesium and potassium can lead to painful muscle cramps, particularly in the legs. Ensuring adequate salt intake and consuming electrolyte-rich foods can prevent this.
Reduced Athletic Performance: As the body's glycogen stores are depleted, athletes may notice a temporary reduction in high-intensity exercise performance. It can take weeks or months for the body to become fully 'fat-adapted' and regain peak performance.

Potential Long-Term Side Effects

While short-term side effects are common and usually resolve, some potential long-term considerations are debated within the medical community.

Elevated LDL Cholesterol: For a subset of individuals, transitioning to a high-fat, low-carb diet can cause a significant rise in LDL ('bad') cholesterol, despite favorable changes in triglycerides and HDL ('good') cholesterol. Regular blood tests are recommended to monitor this.
Nutrient Deficiencies: Restricting entire food groups like fruits and whole grains can lead to deficiencies in essential vitamins and minerals, including vitamin B1, vitamin C, folate, calcium, magnesium, and iron. A varied diet focusing on nutrient-dense, low-carb foods and, if necessary, supplementation is key to prevention.
Kidney Stones and Renal Stress: The metabolic changes associated with a very low-carb diet can sometimes increase uric acid levels and alter the mineral balance in urine, potentially increasing the risk of kidney stones. Those with pre-existing kidney conditions should consult a doctor before starting the diet.
Hair Loss: Some people experience temporary hair thinning, a condition known as telogen effluvium, about 2 to 5 months into a low-carb diet. This is often triggered by significant metabolic stress, rapid weight loss, or nutrient deficiencies. It typically resolves within several months.

Side Effects: Low-Carb vs. Standard Diet

To better understand the effects, here is a comparison of typical experiences on a low-carb diet versus a standard, balanced diet.


Feature	Low-Carb Diet (Initial Phase)	Standard Balanced Diet (General)
Energy Source	Shifts from glucose to fat (ketones).	Primarily relies on glucose from carbohydrates.
Initial Adaptation	'Keto flu' symptoms (fatigue, headache, brain fog) are common.	No significant adaptation period; energy levels remain stable.
Digestive Health	Can cause temporary constipation due to reduced fiber.	Regular bowel movements are typical with adequate fiber.
Energy Levels	Initial fatigue, followed by increased and sustained energy.	Stable but can have 'crashes' from high sugar intake.
Nutrient Variety	Restricted food groups can lead to potential nutrient deficiencies.	Wide variety of food groups provides a broad spectrum of nutrients.
Electrolyte Balance	Often imbalanced, leading to muscle cramps and dehydration.	Generally balanced with a varied diet and proper hydration.

Conclusion

While a low-carb diet can offer benefits like weight loss and improved blood sugar control, it's essential to be aware of the potential side effects. Most initial symptoms, like the 'keto flu,' are temporary and can be managed effectively through proper hydration and electrolyte intake. Long-term concerns, such as nutrient deficiencies or changes in cholesterol, highlight the importance of a well-planned, nutrient-dense diet and medical supervision, particularly for those with pre-existing conditions. By understanding and proactively managing these side effects, individuals can navigate their low-carb journey more comfortably and safely. It is always wise to consult a healthcare professional before making significant dietary changes, especially if you have underlying health concerns.

For more detailed guidance on managing the transition, explore authoritative resources such as the Diet Doctor's comprehensive guide on avoiding low-carb and keto side effects.

Frequently Asked Questions

The 'keto flu' is a collection of temporary, flu-like symptoms experienced when transitioning to a low-carb diet, caused by electrolyte and fluid imbalance. To treat it, increase your intake of water, sodium (e.g., bone broth), and potassium and magnesium through supplements or low-carb foods.

Constipation is a common side effect of going low-carb, often due to a decrease in dietary fiber from restricting fruits and whole grains. Managing it involves drinking more water, increasing intake of low-carb fiber sources like leafy greens, and potentially using a magnesium supplement.

A fruity or sweet-smelling breath, known as 'keto breath,' is caused by the body producing and exhaling acetone, a type of ketone body, as it shifts into ketosis. This is usually a temporary symptom that subsides as your body adapts.

No, hair loss associated with a low-carb diet is typically temporary. It's often a form of telogen effluvium triggered by metabolic stress or nutrient deficiencies from rapid weight loss. Hair growth usually returns to normal within several months.

Some studies have shown a link between certain high-fat, low-carb diets and increased LDL cholesterol, which can be a risk factor for heart disease. However, the quality of the fats consumed is crucial, and overall cardiovascular risk can vary greatly among individuals.

Muscle cramps are often caused by a deficiency in minerals, particularly magnesium and potassium, due to increased water and electrolyte excretion. Preventing them requires ensuring adequate intake of these minerals through dietary sources or supplements.

The long-term effects of low-carb diets are a subject of ongoing research. Some potential risks include nutrient deficiencies, altered gut microbiota, and increased LDL cholesterol in certain individuals. It is recommended to work with a healthcare professional for long-term management.

Ketosis is a normal metabolic state where the body uses fat for energy and produces a moderate amount of ketones. Ketoacidosis is a dangerous, life-threatening condition involving extremely high ketone and blood sugar levels, most commonly seen in uncontrolled type 1 diabetes.