Why Fasting Makes You Irritable and Angry

author avatar Dr. Eric Berg 03/28/2024

When people start fasting or a ketogenic diet, they can often become very irritable. This is because shifting from burning glucose to utilizing ketones as energy may often cause temper fluctuations, including irritability.

Learn why these mood shifts occur during fasting periods and what practical solutions you can use to manage them.

Understanding the Body's Fuel Sources

The human body operates on a complex fuel system, utilizing three primary sources: sugar (glucose), ketones, and fatty acids. Each source is vital to maintaining our health and well-being.

With intermittent fasting, the body switches energy away from glucose, which typically acts as the primary fuel. This easily accessible energy is obtained from carbohydrates in everyday foods like bread or pasta.

Nonetheless, alternative energy sources become paramount in sustaining essential bodily functions in instances of decreased carbohydrate intake, such as those encountered during intermittent fasting.

Ketones and Fatty Acids: The Alternative Fuels

When glucose levels are low due to decreased carb consumption or extended fasting periods, your body resorts to burning stored fat into molecules known as ketones - this metabolic state is called ketosis.

Ketone bodies serve efficiently, especially for brain function, since fats cannot be used directly for energy.

Fatty acids act as another backup fuel that originates from dietary fats or internal fat stores when the food supply dwindles significantly.

Most cells can utilize these except those within the brain because, unlike glucose and ketones, they cannot cross the blood-brain barrier effectively.

Making Shift Towards Using Ketone Bodies

A diet devoid of carbs triggers a shift towards increased usage of ketone bodies instead of heavily relying upon only one form, i.e., glucose, which takes time, leading you to experience 'keto flu' symptoms, including fatigue and irritability, while the adjustment phase lasts.

It also includes gradually increasing dependence on both fatty acid utilization and production until complete adaptation occurs, with the majority (about 60-75%) of daily total energy requirements coming from burning fats.

The Metabolic Switch to Ketosis

A metabolic switch occurs when the fasting person reduces their carbohydrate and sugar intake.

The body utilizes ketones and fatty acids as its primary energy source instead of glucose when the individual reduces their carbohydrate and sugar intake during fasting.

This transition is not instantaneous; during this phase, roughly 40% of our energy derives from ketones, while about 60% comes from fatty acids. The precise ratio can fluctuate based on diet composition and individual metabolism.

The Role of Insulin Resistance in Glucose Absorption

In blood sugar regulation, insulin plays an integral role by promoting glucose absorption into cells for energy production. However, when insulin resistance occurs often due to prolonged high-sugar diets, the effectiveness of insulin decreases significantly.

A person with insulin resistance attempting intermittent fasting may find that their cells struggle more to absorb glucose effectively even when ample amounts are in the bloodstream, especially if they're following a low-carb or ketogenic diet, which inherently limits available glucose supplies.

Consequently, transitioning smoothly into ketosis could be challenging since bodies have become accustomed to utilizing large quantities of readily available fuel (glucose).

Navigating through Insulin Resistance During Fasting Periods

If you are facing challenges related to insulin resistance while trying out intermittent fasting or adopting a ketogenic lifestyle, don't lose hope.

A well-structured approach can help manage potential obstacles such as mood swings caused by low blood sugar levels or feelings of restlessness brought on by changes in your body's biological clock.

Evidence suggests that keto-adaptation improves cellular sensitivity towards insulins over time, aiding smoother transitions into periods without food consumption.

This process has been known to alleviate some adverse effects associated with dietary change, including causing common mood disturbances among those new to these practices.

Brain illustration

The Brain's Fuel Preferences

Our brain, a power-hungry organ, requires around 20% of the body's total caloric intake. Interestingly, it has a selective preference for its energy source compared to other organs in our bodies.

The brain's primary energy sources are glucose and ketones, the former from carbohydrates or our liver during periods of low carbohydrate intake and the latter when glucose availability is restricted. Glucose is derived from carbohydrates we consume or produce by our liver during fasting or low carbohydrate consumption.

At the same time, ketones come into play when glucose availability is limited, such as during prolonged fasting, rigorous exercise, or ketogenic diets, significantly reducing carbohydrate intake.

Fatty Acids

Fatty acids are an excellent fuel source for most body tissues but not so much for the brain due to 'the blood-brain barrier.'

This protective shield restricts access to certain nutrients, including fatty acids, ensuring only specific molecules gain entry into your grey matter.

This restriction leaves neurons dependent primarily on glucose and secondarily on ketones under circumstances where carbs are running low, like intermittent fasting person experiences or those following a ketogenic diet regimen.

Mood Changes During Fasting: A Biochemical Perspective

Gaining insight into how different fuels affect neural functioning can help us understand why mood changes might occur during transition phases where you shift from using glucose predominantly to relying more heavily on ketone bodies - such situations include starting keto diet regimens or practicing intermittent fasting regularly.

Your neurons have evolved to use both these sources efficiently; however, switching between them isn't always smooth sailing, especially if you've been consuming high amounts of carbohydrates previously.

A key point here is that once fully adapted, many people report improved mental clarity, possibly owing to increased mitochondrial efficiency achieved through regular utilization of ketone bodies.

Strategies for Managing Mood Changes During Ketosis Transition

The journey towards adopting a ketogenic lifestyle or intermittent fasting can be challenging, especially when mood swings set in. Nevertheless, some methods have been successful in controlling these alterations.

Prioritizing Hydration to Avoid Irritability

Hydration is critical during the early stages of transitioning into ketosis, as dehydration often exacerbates feelings of irritability and restfulness. Drinking plenty of water throughout the day becomes crucial at this stage.

Beyond just drinking water, though, incorporating electrolyte-rich beverages like coconut water or low-sugar sports drinks could also help replenish essential minerals lost through increased urination, which is common among individuals starting a keto diet.

Sleep: A Vital Component In Regulating Moods

Adequate sleep plays an integral role in managing moods while adapting to ketosis. Research shows lack of it leads to increased irritability, which may amplify dietary transition effects such as entering ketosis.

To ensure you get enough quality sleep, establish consistent bedtime routines, including reducing noise levels before bedtime and limiting exposure to light-emitting devices before sleeping.

Mindful Stress Management Techniques

Anxiety tends to heighten perceived irritability brought by new eating patterns; hence, employing mindful stress management techniques is paramount during the period.

Navigating Through Mental Health Challenges

If your moods remain unstable despite implementing the above strategies, seeking professional assistance should not be overlooked.

Mental health professionals offer tailored advice based on individual circumstances and potential medication adjustments necessary.

24-Hour Fasting Mood Challengers

Exploring the potential mood challenges of a 24-hour fast? While a 24-hour fast can offer various health benefits, including improved metabolism and mental clarity, some individuals may experience mood fluctuations during fasting.

Hunger, low energy levels, and changes in blood sugar can contribute to feelings of irritability or mood swings. It's essential to be aware of these potential challenges and to listen to your body.

Staying well-hydrated, consuming nutrient-dense foods during eating windows, and ensuring that the fasting approach aligns with your individual needs can help mitigate mood challenges and make the fasting experience more manageable.


Mood fluctuations during fasting or ketogenic diets are common due to the body's shift from glucose to ketones as the primary fuel source.

Understanding this transition is essential for managing irritability effectively. Strategies such as staying hydrated, prioritizing sleep, and practicing mindful stress management can help alleviate mood changes during fasting periods.

Seeking professional assistance is recommended if mood disturbances persist.

With knowledge about why irritability occurs during the transition towards a low-carb lifestyle, you can better manage your moods and support your overall health.

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