Can fasting make you feel weird? Understanding the side effects

October 18, 2025 •

4 min read

According to research published in the National Library of Medicine, initial side effects like headaches, lethargy, and mood swings are common among individuals adopting intermittent fasting. So, if you've ever asked, "Can fasting make you feel weird?", you're not alone, and the answer is yes, especially as your body adjusts to its new eating pattern.

Quick Summary

It is normal to feel off when beginning a fasting regimen as your body shifts its primary fuel source from glucose to fat. These temporary symptoms, often called the "keto flu," are caused by metabolic changes, electrolyte imbalances, and dehydration, but can be managed with proper hydration and nutrition. Most people experience an improvement in how they feel within a few weeks as their body adapts.

Key Points

Metabolic Shift: Feelings of being 'weird' are a normal part of your body transitioning from burning glucose to utilizing fat for energy, causing a temporary energy dip.
Electrolyte Imbalance: Increased fluid loss during fasting can deplete essential electrolytes like sodium, potassium, and magnesium, leading to headaches, fatigue, and dizziness.
Manageable Side Effects: Most common fasting side effects are temporary and can be managed effectively with simple strategies like staying hydrated and replenishing electrolytes.
Adaptation is Key: The body typically adapts to a new fasting routine within 2 to 4 weeks, after which the uncomfortable symptoms usually resolve and energy levels stabilize.
Listen to Your Body: While mild symptoms are normal, it is important to stop fasting and seek medical advice if you experience severe or persistent discomfort, or have underlying health conditions.
'Keto Flu' Connection: The transient, flu-like symptoms experienced during early fasting are comparable to those felt when beginning a ketogenic diet, often referred to as the "keto flu".

For many, embarking on a fasting journey, particularly with intermittent fasting, can lead to some unexpected and unsettling physical and mental sensations. It’s normal to feel a bit strange during the initial adaptation phase, a period often referred to as the “keto flu" because of its symptom similarity to the metabolic changes experienced when starting a ketogenic diet. These feelings of unease—including headaches, fatigue, and irritability—are a direct result of your body transitioning its primary fuel source away from glucose and toward stored fat. Understanding the science behind these temporary side effects can help you better manage them and navigate your fasting journey with confidence.

The Physiological Triggers of Feeling 'Weird'

Several key physiological changes contribute to the discomfort experienced during early fasting. As your body depletes its readily available glucose stores, it begins to seek alternative energy sources, a shift that can cause a temporary shock to the system.

Metabolic Transition and Hypoglycemia

When you stop eating, your blood sugar levels naturally drop. For those accustomed to a constant supply of carbohydrates, this can be a significant change. Your body enters a state of mild hypoglycemia, or low blood sugar, while it begins to produce ketones from fat for energy. Before the brain fully adapts to running on ketones, this energy gap can lead to brain fog, poor concentration, and irritability.

Electrolyte Imbalance and Dehydration

In the early stages of fasting, particularly with lower insulin levels, your body can flush out large amounts of water and essential electrolytes like sodium, potassium, and magnesium. This process, called natriuresis, can cause significant fluid loss, leading to dehydration. Dehydration and electrolyte imbalances are primary culprits behind headaches, dizziness, fatigue, and muscle cramps experienced by many new fasters.

Caffeine and Sugar Withdrawal

If your daily routine included regular consumption of caffeinated beverages or sugary foods, fasting can trigger a withdrawal. Both caffeine and sugar have powerful effects on your brain and body chemistry. Abruptly removing them from your diet can cause side effects like headaches and mood swings, which can be easily mistaken as symptoms of fasting itself. Gradually reducing your intake of these substances before starting a fast can significantly lessen the severity of withdrawal symptoms.

Managing the Fasting Adjustment Period

Fortunately, most of the negative side effects of fasting are temporary and can be effectively managed with some simple strategies. The key is to support your body through the transition and listen to its signals.

Strategies to Ease Discomfort

Prioritize hydration: Drink plenty of water throughout the day, even more than you think you need. Since you're not getting fluids from food, it's crucial to proactively replenish. Herbal tea and black coffee are also acceptable.
Replenish electrolytes: To combat the electrolyte flushing, consider adding a pinch of salt to your water or using a low-calorie electrolyte supplement during longer fasts. Sources of magnesium and potassium, like leafy greens during your eating window, are also beneficial.
Start gradually: Don't jump into an extended fast if you're a beginner. Start with shorter fasting windows, such as a 12-14 hour overnight fast, and slowly increase the duration as your body adapts.
Break the fast mindfully: After a fasting period, break your fast with a small, nutrient-dense meal containing protein, healthy fats, and fiber. Avoid feasting on large, sugary, or processed meals, which can cause digestive issues and energy crashes.
Adjust exercise intensity: During the first few weeks, consider reducing the intensity of your workouts. Light activities like walking or yoga are great ways to stay active without over-exerting yourself while your body adapts.

Fasting Experiences: Initial vs. Adapted


Feature	Initial Adaptation Phase (First 1-4 weeks)	Adapted Phase (After a few weeks)
Energy Source	Transitioning from glucose to ketones; temporary energy gap.	Efficiently using fat and ketones for fuel, resulting in stable energy.
Common Symptoms	Headaches, fatigue, irritability, dizziness, brain fog.	Symptoms typically subside or resolve completely as the body becomes more efficient.
Hunger Levels	Intense hunger and cravings, especially on fasting days.	Appetite hormones stabilize, leading to reduced overall hunger.
Fluid/Electrolytes	Increased fluid and electrolyte loss through urination.	Electrolyte balance stabilizes; easier to maintain hydration.
Metabolic Flexibility	Low, as the body is unaccustomed to metabolic switching.	High, with the body easily transitioning between glucose and fat utilization.
Mental State	Can experience mood swings, anxiety, and poor concentration.	Often reports of improved mental clarity, focus, and mood.

Conclusion: Navigating the Weirdness Toward Wellness

Yes, the initial phase of fasting can indeed make you feel weird, but it's a completely normal part of the process as your body undergoes a profound metabolic shift. For most healthy individuals, these side effects—such as headaches, mood changes, and fatigue—are temporary and subside as the body becomes "fat-adapted". By focusing on proper hydration, managing electrolyte levels, easing into your fasting schedule, and listening to your body, you can smooth the transition and minimize discomfort. It’s important to remember that fasting is not for everyone, and if you experience severe or persistent symptoms, it is advisable to stop and consult with a healthcare professional before proceeding further. The potential long-term benefits, such as improved energy and mental clarity, often outweigh the initial short-term challenges.

For more detailed guidance on managing the initial stages of fasting, consult reputable resources like the article on Healthline.

Frequently Asked Questions

The weird feelings are primarily caused by your body shifting from its usual energy source of glucose to using stored fat. This metabolic transition can temporarily cause low blood sugar, leading to symptoms like fatigue, headaches, and irritability.

Most people find that their body adapts to a new fasting regimen within two to four weeks. During this period, initial side effects like hunger, headaches, and low energy levels tend to subside as your metabolism adjusts.

Yes, replenishing electrolytes is crucial, especially during longer fasts. Electrolyte imbalances can cause headaches, dizziness, and muscle cramps, so consuming water with added salt or an electrolyte supplement can help prevent these issues.

Yes, headaches are a common side effect of fasting, especially in the early stages. They can be triggered by low blood sugar, dehydration, and caffeine or sugar withdrawal. Staying well-hydrated is a key strategy for prevention.

The 'keto flu' is a collection of flu-like symptoms, including headaches, fatigue, and irritability, that occur as your body adjusts to burning fat for fuel. Since fasting also triggers this metabolic switch, it can cause similar temporary symptoms.

You should stop fasting and consider consulting a healthcare professional if you feel unwell or experience severe or persistent symptoms. Red flags include extreme weakness, fainting, vision changes, or pain that doesn't improve with hydration or rest.

Yes, bad breath, or halitosis, can occur while fasting. It is often caused by the increased production of acetone as your body breaks down fat for energy, a process that can also lead to dry mouth due to dehydration.

To prevent fatigue, ensure you are getting enough electrolytes and staying properly hydrated. Easing into fasting gradually, adjusting exercise intensity, and getting enough rest can also help manage energy levels.