What Happens to Your Brain When You Don't Eat Enough Carbs?

December 30, 2025 •

3 min read

The human brain uses around 20% of the body's energy. When carb intake is low, the brain adapts its primary energy source, triggering changes in mood and memory.

Quick Summary

Restricting carbs forces the brain to use ketones for energy, potentially causing 'brain fog' as it adjusts. This metabolic shift can impact cognitive function and mood, though long-term effects vary.

Key Points

Initial Brain Fog: Reducing carbs can cause temporary cognitive impairments like brain fog, fatigue, and memory issues during the initial adaptation phase, known as the 'keto flu'.
Fuel Shift to Ketones: The brain switches from using glucose, its primary fuel, to utilizing ketones derived from fat when carbohydrate intake is consistently low.
Mood Regulation Affected: Fluctuations in serotonin, a mood-regulating neurotransmitter, can occur during the carb-reduction phase, potentially leading to increased irritability or anxiety.
Variable Cognitive Performance: While some studies show temporary memory impairment on low-carb diets, others suggest improved attention and focus after the brain fully adapts to using ketones.
Electrolytes are Crucial: Maintaining adequate levels of electrolytes like sodium, potassium, and magnesium is vital for minimizing 'keto flu' symptoms, including cognitive ones.
Adaptation is Possible: After the initial adjustment period, many individuals report sustained mental clarity and energy as the brain stabilizes on ketone metabolism.
Nutrient-Dense Carbs Still Matter: Even on a low-carb diet, prioritizing nutrient-rich complex carbs and healthy fats is important for overall brain health and nutrient intake.

The "Keto Flu" and Brain Fog

The start of a low-carb diet can bring the "keto flu". As the brain's glucose stores are used up, and the liver makes more ketones, people may feel tired or have headaches.

"Brain fog" is a common cognitive side effect during this change. It includes:

Trouble focusing
Slower thinking
Memory problems
Mental fatigue

This happens because the brain has to learn how to use ketones for fuel. Drinking enough water and balancing electrolytes are crucial to lessen these symptoms.

Shifting to Ketone Metabolism

As the body gets used to less carbs, the metabolism changes. The liver starts making ketones from body fat. The brain then uses these ketones for energy. After a few weeks, the brain can get up to 75% of its energy from ketones. This offers a steady fuel supply, avoiding blood sugar swings.

Glucose vs. Ketones: Brain Fuel Comparison


Feature	Glucose	Ketones	References
Primary Source	Carbohydrates	Fat (via the liver)
Energy Delivery	Fast, but can cause spikes and crashes	Steady and sustained
Storage	Limited glycogen stores in liver and muscles	Ample fat stores in the body
Initial Adaptation	None (default fuel)	May cause temporary brain fog
Oxidative Stress	Can lead to higher levels	May result in lower levels
Neuroprotective Effects	Essential for function	May have specific neuroprotective effects

Mood and Neurotransmitters

Carbs affect mood by influencing the neurotransmitter serotonin. Carbs increase tryptophan, which helps make serotonin in the brain. Reducing carbs can disrupt serotonin production, possibly leading to mood changes like irritability, anxiety, or depression. Some studies show that people on low-carb diets had lower serotonin levels over time. However, some report better moods when fully adapted to using ketones as energy.

Effects on Cognitive Performance

Studies on the long-term cognitive effects of low-carb diets show mixed results. Some studies have found memory impairments during the initial stages.

Memory Impairment: One study showed that people on low-carb diets performed worse on memory-based tasks compared to those on low-calorie, high-carb diets, especially during the first few weeks. This effect was reversed when carbohydrates were reintroduced. The brain's reliance on glucose, particularly for demanding memory tasks, may be a contributing factor.
Improved Attention and Focus: On the other hand, some evidence suggests that adapted individuals on low-carb diets may experience improved attention and faster response times in vigilance tasks, with reduced self-reported confusion. This may be due to the steady energy supply from ketones, which helps avoid the mental fatigue associated with fluctuating glucose levels.

Adapting to Less Carbs

To help the brain adjust to less carbs:

Choose Complex Carbs: If you're not going extremely low-carb, pick whole grains, like oatmeal and beans.
Use Electrolytes: Low-carb diets can cause loss of minerals. Use foods or supplements with potassium, magnesium, and sodium to avoid fatigue and cramps. Broth is an easy source of sodium and fluids.
Eat Nutrient-Rich Foods: Eat plenty of leafy greens, healthy fats, and proteins.
Reduce Carbs Slowly: This gives the brain time to adapt and may lessen 'keto flu' symptoms.

Conclusion: Navigating the Metabolic Shift

When carb intake decreases, the brain changes from using glucose to using ketones. This can cause initial challenges, like 'brain fog' and mood swings. However, many people later find mental clarity and stable energy. Long-term cognitive effects vary. It's wise to speak with a healthcare provider before making diet changes. More on the brain's response to diet can be found on the National Institutes of Health website.(https://pmc.ncbi.nlm.nih.gov/articles/PMC12209867/)

Frequently Asked Questions

Brain fog is a period of reduced mental sharpness, trouble concentrating, and slower thinking that many people experience when they first cut carbs significantly. It occurs as the brain adjusts to using ketones instead of glucose.

Yes, it can. Reducing carbs can affect serotonin production in the brain, which may lead to mood swings, irritability, and anxiety, especially at first. However, some people report more stable moods once they adapt.

The initial adaptation, or 'keto flu,' usually lasts a few days to a couple of weeks. Full adaptation, where the brain efficiently uses ketones, can take several weeks.

Yes, for the most part. The brain can use ketones for a large part of its energy needs, up to 75% or more, when carbs are limited. The liver also produces a small amount of glucose through gluconeogenesis.

Research provides mixed findings on long-term cognitive effects. Some studies show temporary memory impairment during the transition, but the long-term impact can vary. Some users report improved focus, while others may experience differences in specific types of memory tasks.

To ease symptoms, stay hydrated and maintain electrolyte levels (sodium, potassium, magnesium). Slowly reducing carbs can also help. Eating nutrient-rich, low-carb foods and healthy fats is beneficial.

A zero-carb diet is not typically recommended, as it removes many healthy foods. The body can produce the necessary glucose, but it is important to consult a healthcare professional for personalized guidance.