Intermittent Fasting Side Effects: What to Expect and How to Manage

Understanding Side Effects

This guide covers the side effects people most often notice when they start intermittent fasting (IF), why they may happen, practical ways to ease them, and the warning signs that mean you should stop and get medical advice. Many of the common effects, such as hunger, headaches, and tiredness, are mild and tend to ease as you get used to a new eating routine. Others are signs that fasting isn’t suiting you.

Intermittent fasting is not suitable for everyone. If you are pregnant or breastfeeding, under 18, underweight, have a history of an eating disorder, have diabetes (especially if you use insulin or sulfonylureas), or take regular medication, talk to your healthcare provider before trying it. See our fasting safety guide for more.

Why Side Effects Occur

The transition to intermittent fasting triggers multiple physiological changes:

• Fuel changes: During longer gaps between meals, the body relies more on stored fat once readily available glucose runs low
• Hormonal adjustments: Hunger-related hormones such as ghrelin are partly tied to your usual meal times
• Fluid and mineral shifts: Fasting tends to increase salt and water loss through the kidneys
• Lower intake: Eating less overall, or missing fluids you used to get from meals
• Habit changes: Breaking a long-standing routine takes time

Timeline of Adaptation

This is a rough pattern many people describe. Individual experiences vary a lot, and not everyone goes through every stage:

Individual Variation Factors

Side effect severity and duration depend on:

• Previous diet: High-carb diets may cause more intense adaptation
• Health conditions and medications: These can change how you respond and may need medical input
• Fasting protocol: Longer fasts typically cause more side effects
• Hydration status: Dehydration worsens symptoms
• Stress levels: High stress amplifies side effects
• Sleep quality: Poor sleep impairs adaptation
• Activity level: Exercise timing affects symptoms

Common Initial Side Effects

The "Fasting Flu"

“Fasting flu” is an informal term, not a medical diagnosis. Similar to what people call “keto flu”, some people feel generally unwell during the first days of a new fasting routine:

Bad Breath (Halitosis)

Often called “keto breath”, this can happen during longer fasts as the body makes more ketones. A dry mouth from drinking less also contributes:

Cold Sensitivity

Many people report feeling colder during fasting:

• Cause: Less heat is produced from digesting food, and eating less overall can lower heat production
• Calorie restriction: Larger or prolonged calorie deficits can modestly lower thyroid hormone levels
• Solutions: Dress warmly, drink warm beverages, move regularly, make sure you’re eating enough
• When to check: If feeling cold persists alongside tiredness or hair loss, talk to your doctor

Hunger and Appetite Changes

Understanding Hunger Waves

Hunger during a fast is usually not constant. For many people it comes in waves, often around the times they used to eat, and then fades:

Appetite Hormones in Flux

Ghrelin, often called the “hunger hormone”, tends to rise around the times you habitually eat. This is one reason hunger can be strongest at your old breakfast or lunch time early on. As your routine changes, many people find these hunger signals shift to match, although how quickly this happens varies from person to person.

Cravings vs. Hunger

Distinguishing between true hunger and cravings is crucial:

Increased Appetite When Eating

Some experience ravenous hunger during eating windows:

• Cause: Natural hunger after a longer gap without food
• Risk: Overeating and digestive discomfort
• Solution: Eat slowly, include protein and fibre, plan balanced meals in advance
• Watch for: Episodes that feel out of control or like bingeing are a reason to stop and seek support

Energy and Fatigue Issues

Initial Energy Dips

Tiredness is common in the first week or two:

Managing Fatigue

Strategies to maintain energy during adaptation:

• Gradual transition: Start with shorter fasts
• Strategic caffeine: Black coffee or tea during fasts
• Electrolyte balance: Sodium, potassium, magnesium
• Adequate calories: Don't create excessive deficit
• Quality sleep: 7-9 hours nightly (the general adult recommendation)
• Light movement: Gentle exercise, not intense
• Stress management: Meditation, breathing exercises

Exercise Performance Changes

Some people find workouts feel harder at first, especially high-intensity sessions done fasted. Many find this improves as they get used to the routine, particularly if they time hard sessions near meals. Research on intermittent fasting and exercise performance is mixed and mostly short-term.

See exercise and fasting for more.

Mental Clarity Fluctuations

Cognitive function may vary during adaptation:

• Brain fog: Common in first week
• Concentration issues: Difficulty focusing initially
• Memory lapses: Temporary forgetfulness
• Often improves: Many people report these ease as they adjust

Digestive System Changes

Constipation

One of the most common digestive complaints during IF:

Diarrhea

Less common but can occur, especially when breaking fasts:

• Causes: Eating too quickly, high fat intake, coffee on empty stomach
• Breaking fast poorly: Large, heavy meals can trigger
• Solutions: Break fast gently, eat slowly, reduce fat initially
• When to worry: If persistent or accompanied by pain

Bloating and Gas

Digestive discomfort may increase initially:

Acid Reflux and Heartburn

Some people notice more reflux, heartburn, or stomach discomfort when fasting:

• Empty stomach acid: Can cause discomfort
• Coffee consumption: May worsen on empty stomach
• Large meals: Overeating can trigger reflux
• Solutions: Avoid oversized meals, avoid trigger foods, don’t lie down right after eating, raise the head of the bed
• Talk to your doctor: If you have GERD, stomach ulcers, or symptoms that don’t settle

Nausea

Occasional nausea may occur, especially in mornings:

• Low blood sugar: Can trigger nausea
• Dehydration: Common cause
• Empty stomach: Some are sensitive
• Remedies: Ginger tea, peppermint, small amount of salt water

Headaches and Dizziness

Headache Types and Causes

Different types of headaches may occur during IF:

Dizziness and Lightheadedness

Common especially when standing quickly:

Managing Symptoms

General strategies for headaches and dizziness:

• Hydration: A glass of water on waking, then regular sips through the day
• Electrolytes: A pinch of salt or a calorie-free electrolyte drink can help some people; see our electrolytes guide and check with your doctor if you have heart, kidney, or blood pressure problems
• Gradual position changes: Avoid sudden movements
• Cool environment: Heat exacerbates symptoms
• Rest when needed: Don't push through severe symptoms

Mood and Cognitive Effects

Irritability and Mood Swings

Emotional regulation can be challenging during adaptation:

Anxiety and Stress Response

Some experience increased anxiety during fasting:

• Physiological stress: Fasting is a mild stressor
• Stress hormones: Cortisol and adrenaline can rise somewhat during fasting
• Worry about symptoms: Can create anxiety cycle
• Management: Breathing exercises, meditation, gentle yoga

Depression and Low Mood

Some people report feeling better on a fasting routine, while others notice low mood:

• Hunger and low energy: Both can affect mood
• Social isolation: Missing meal-based socializing
• Energy depletion: Fatigue affects mood
• Solutions: Ensure adequate calories, maintain social connections, get daylight

If you have depression, anxiety, or another mental health condition, talk to your healthcare provider before fasting, and stop if your symptoms get worse. See fasting and mood.

Cognitive Changes

Focus and concentration may dip at first:

Sleep Disruptions

Common Sleep Issues

Sleep patterns often change during IF adaptation:

Sleep Quality Changes

Experiences vary. Some people report better sleep once they adjust, particularly if they stop eating large meals late at night. Others sleep worse if they go to bed hungry. Research on intermittent fasting and sleep is limited and mixed. See fasting and sleep for more.

Sleep Optimization Strategies

• Consistent schedule: Same bedtime/wake time daily
• Eating window timing: Finish eating a couple of hours before bed, but don’t go to bed very hungry
• Dark, cool room: Optimal sleep environment
• Limit screens: Blue light affects melatonin
• Relaxation routine: Wind down before bed

Hormonal Effects

Women's Hormonal Considerations

Some women notice changes to their menstrual cycle, particularly with longer fasts or large calorie deficits:

Thyroid Function

Longer fasts and sustained calorie restriction can affect thyroid hormone levels:

• T3 reduction: Active thyroid hormone may decrease
• Reverse T3: May increase with aggressive fasting
• TSH changes: Usually minimal with moderate IF
• Symptoms to mention to your doctor: Cold intolerance, fatigue, hair loss
• Prevention: Adequate calories, avoid very long or frequent extended fasts
• If you take thyroid medication: See our thyroid guide and check timing with your doctor

Testosterone and Male Hormones

Evidence on intermittent fasting and male hormones is limited and mixed:

• Time-restricted eating: Some small studies have observed modest decreases in testosterone, often without obvious effects on muscle or strength
• Prolonged or severe restriction: Can lower testosterone more noticeably
• Growth hormone: Rises during fasting, but what this means in practice is unclear
• Libido changes: Reduced libido can be a sign of under-eating
• What helps: Eating enough overall, including adequate fat and protein, and resistance training

Stress Hormones

Cortisol and adrenaline changes during IF:

• Cortisol: May rise somewhat during fasting
• Adrenaline and noradrenaline: Rise during longer fasts, which may partly explain feeling alert or wired
• Too much at once: Very long fasts on top of poor sleep and high stress can leave you feeling worse
• Balance: It is important not to overdo fasting

Long-term Considerations

Potential Long-term Side Effects

While most side effects resolve, some may persist or develop over time:

Metabolic Adaptation

As with any approach that leads to weight loss, the body adapts over time:

• Lower energy needs: A smaller body uses less energy, and some additional slowdown can occur with sustained restriction
• Hunger: Appetite can increase during prolonged dieting
• Insulin sensitivity: May improve, particularly with weight loss, but responses vary
• What helps: Avoiding extreme deficits, keeping up protein and strength training, and taking breaks from dieting

Psychological Considerations

Long-term psychological effects to monitor:

• Food obsession: Can develop in some
• Social isolation: If too rigid with fasting
• Disordered eating: Risk in susceptible individuals
• Rigidity: Feeling anxious or guilty when you can’t stick to your schedule

If fasting starts to feel compulsive, or you notice bingeing, guilt around food, or restricting more and more, stop and speak to a healthcare professional.

When to Be Concerned

Red Flag Symptoms

Certain symptoms require immediate attention:

Signs to Stop or Modify IF

Consider stopping or adjusting your protocol if experiencing:

• Persistent fatigue beyond 4 weeks
• Hair loss or skin problems
• Amenorrhea (loss of period)
• Persistent insomnia
• Depression or anxiety worsening
• Binge eating episodes
• Significant muscle loss
• Chronic digestive issues
• Getting ill more often than usual

Special Populations at Higher Risk

Certain groups should be extra cautious:

• People with diabetes: Risk of low blood sugar, especially with insulin or sulfonylureas; medical supervision is needed
• People taking regular medication: Timing and dosing may need adjusting; see fasting and medications
• Heart or kidney conditions: Fluid and electrolyte concerns
• History of eating disorders: Fasting can trigger relapse and is generally not advised
• Pregnant or breastfeeding: Not recommended
• Under 18: Growing bodies have higher needs; see fasting and teenagers
• Underweight people: Not recommended
• Older adults: Higher risk of muscle loss and under-nutrition

Prevention and Management Strategies

General Prevention Principles

Minimize side effects with these strategies:

• Gradual progression: Start with 12:12, increase slowly
• Proper hydration: Drink regularly; needs vary with body size, climate, and activity
• Electrolyte balance: Sodium, potassium, magnesium
• Adequate nutrition: Don't create extreme deficits
• Quality sleep: 7-9 hours nightly
• Stress management: IF adds stress, balance elsewhere
• Listen to your body: Adjust when needed

Supplements

Most people don’t need special supplements for intermittent fasting; a varied diet in your eating window comes first. If you are considering supplements, check with a pharmacist or doctor, especially if you take medication:

Creating Your Management Plan

Develop a personalized approach:

1. Track symptoms in a journal
2. Identify patterns and triggers
3. Implement targeted solutions
4. Adjust fasting protocol if needed
5. Monitor improvements over time
6. Talk to a healthcare professional when concerned

When to Seek Professional Help

Consider consulting healthcare providers for:

• Pre-existing medical conditions
• Medication interactions
• Persistent concerning symptoms
• Hormonal issues
• Mental health concerns
• Nutritional guidance