The most common intermittent fasting (IF) side effects include temporary hunger, fatigue, headaches, and changes in digestion or sleep patterns. For many people, these physical adjustments are short-term and typically ease within one to two weeks as the body adapts to the new eating schedule.
Many people follow IF for a range of personal goals. Like any dietary approach, it can come with an adjustment period and some physical changes worth knowing about. It is highly versatile, with different protocols to suit different goals and preferences. Intermittent side effects may vary from one person to the next. It helps to understand what the transition phase might look like, how to support your body, and when you should reconsider your approach.
Every fasting experience is personal. Your health status, activity level, and dietary preferences all influence how IF feels for you. It’s strongly advisable to speak with a healthcare provider before starting any fasting regimen, particularly if you take medication or have an existing health consideration.
What Are Intermittent Fasting Side Effects?
When you change when and how you eat, your body requires time to adjust to the new routine. The cons of intermittent fasting are often most noticeable during the initial transition period.
Understanding intermittent fasting side effects can help you prepare and make the experience more comfortable. The table below outlines the physical changes people often notice, who tends to experience them, and when they typically subside.
| Side Effect | Who Is Most Likely to Experience It | When It Typically Eases |
|---|---|---|
| Keto Flu (fatigue, brain fog) | Beginners coming from a diet high in refined carbohydrates | Within 1–2 weeks as the body adapts (1) |
| Low Blood Sugar Symptoms | Individuals with blood sugar variation or those new to fasting | After the first week or when adjusting the fasting window (2) |
| Dehydration | People who forget to drink water outside of meal times | Eases immediately with proper fluid and electrolyte intake |
| Hunger | Almost everyone starting a new fasting routine (2) | Usually diminishes after 3–5 days |
| Irritability and Mood Swings | Those experiencing hunger or sleep changes (2) | Eases as the body gets used to the new schedule |
| Digestive Issues | People jumping into very long fasts or eating large meals quickly (2) | Eases when breaking fasts gently and eating balanced meals |
| Fatigue | Individuals not consuming enough calories or fluids (2) | Eases within a week or with better hydration |
| Hormone Level Shifts | Women, or individuals with significant caloric deficits | Varies; requires monitoring and a balanced approach (3) |
| Bad Breath | People who are under-hydrated or entering ketosis (4) | Eases with consistent hydration and oral care |
| Poor Sleep | Those who eat heavy meals right before bed or fast too long (2) | Often improves after the first 1–2 weeks |
| Malnutrition | Individuals not eating nutrient-dense foods during their eating window | Eases when prioritizing a well-rounded, balanced diet |
| Medication Interactions | People taking medications that require food | Needs immediate consultation with a healthcare provider |
First Week of Intermittent Fasting: What to Expect
The first week of intermittent fasting side effects can feel challenging as your body gets used to a different eating schedule. Knowing what to expect day by day can help you stay prepared and adjust your routine as needed (5).
- Days 1–2: These are typically the hardest days. Hunger tends to peak during your fasting window, and you may experience mild headaches and fatigue as your body starts to adjust to the lack of continuous energy intake.
- Days 3–5: The body often begins adapting. Hunger levels frequently reduce, though some people report experiencing keto flu-like symptoms, such as sluggishness or brain fog.
- Days 6–7: By the end of the week, many people find the routine starts to feel more natural. Energy levels often stabilize, and the initial discomfort usually begins to fade.
| Day Range | Common Experiences | Tips |
|---|---|---|
| Days 1–2 | Peak hunger, potential headaches, mild fatigue | Drink plenty of water, consider adding a pinch of salt to your water, and take it easy. |
| Days 3–5 | Reduced hunger, potential brain fog, sluggishness | Focus on nutrient-dense meals during your eating window and prioritize restful sleep. |
| Days 6–7 | More stable energy, routine feels more natural | Continue staying hydrated and adjust your fasting window if you still feel significant discomfort. |
1. Keto Flu
The keto flu refers to a group of physical adjustments that can occur when people transition to an eating pattern that drastically reduces carbohydrates, or when they begin fasting (6).
It is thought to be associated with the sudden reduction of carbs and sugar from the diet, which can lead to shifts in fluid balance and other changes. Some people report experiencing fatigue, headaches, brain fog, nausea, and irritability during this phase (6). People who are new to fasting may experience similar adjustments, especially if their previous dietary habits involved frequent snacking or a high intake of processed foods.
To make this transition smoother when fasting, ensure you stay hydrated and maintain a balanced intake of electrolytes. You can also try starting with shorter fasting periods and gradually increasing the duration as your body gets used to the new schedule. Fortunately, these adjustments are usually temporary and tend to ease after a week or two. If your symptoms are severe or ongoing, stop fasting and talk to your doctor.
2. Low Blood Sugar
One reason people explore intermittent fasting is to support their overall metabolic wellbeing. However, fasting can sometimes lead to lower blood sugar levels, particularly for those who are sensitive to dietary changes (2).
Some people notice physical changes associated with blood sugar variation during fasting, including weakness or shakiness, sweating, or irritability. If blood sugar drops significantly and goes unaddressed, it can cause serious physical effects requiring medical attention. Anyone who experiences persistent or intense blood sugar-related symptoms should seek medical attention promptly.
If you have diabetes or prediabetes and are considering intermittent fasting, be sure to talk to your doctor first. Your healthcare provider can help you figure out if this approach is suitable for your specific needs. They might recommend checking your levels frequently or using a continuous glucose monitor to track variations throughout the day.
Due to the potential for significant blood sugar drops, fasting is generally not recommended for certain groups, including pregnant individuals, children, adolescents, and people taking specific medications, without professional guidance.
Read More: 10 Intermittent Fasting Mistakes People Make And How To Avoid Them
3. Dehydration
When you are fasting, a few changes take place in your body that can affect your fluid balance. If you are not taking in fluids from food, you are already consuming less water than usual. Additionally, your body releases more water as it uses up stored glycogen for energy.
Dehydration can affect how you feel physically and cognitively. You might experience headaches, fatigue, brain fog, or dizziness. Staying well hydrated during fasting periods is particularly important to support your body’s basic functions and overall wellbeing (7).
To help maintain hydration, make sure to drink plenty of water throughout your fasting window. When you break your fast, consider incorporating fluids that provide electrolytes, such as coconut water or bone broth. Adding a small pinch of salt to your drinking water can also support hydration.
If you are active during your fast, be extra mindful of your fluid intake. If you notice signs of dehydration while exercising, you might want to adjust your routine so that you work out after you have had a balanced meal and plenty of fluids.
4. Hunger
It is completely normal to experience hunger when you adjust to an intermittent fasting routine. After all, your body is getting used to going for an extended period without food.
Fortunately, this initial hunger is usually temporary. As your body adapts to the new schedule and becomes more efficient at using stored energy, many people find that their appetite levels out (2). If hunger remains a constant, overwhelming issue, it may mean your fasting window is too long or your eating window isn’t providing enough nourishment.
Persistent dizziness, headaches, or feeling faint during fasting are signs worth paying attention to and discussing with a healthcare provider. If you experience minor hunger, drinking unsweetened black coffee or tea can sometimes help. Some people find that a small amount of protein or fat helps manage hunger during fasting hours, though this approach is better suited to flexible fasting protocols than strict water-only fasting.
If hunger continues to disrupt your day, consider shortening your fasting window. Eating a balanced diet with plenty of protein and healthy fats during your eating window can also help you feel satisfied for longer.
5. Irritability And Mood Swings
Your mood may shift when you begin fasting. This is often associated with the combined effects of hunger, adjustments in sleep, and hydration levels.
Changes in blood sugar during fasting periods may be associated with shifts in mood for some people. When your body is adapting to a new energy source, it is common to feel a bit more irritable or sensitive than usual (2). Dehydration can also contribute to these feelings of discomfort (8).
To help support a balanced mood, make sure you are drinking plenty of fluids, prioritizing restful sleep, and eating a well-rounded diet during your eating window. Adding nutrient-dense foods, like those rich in omega-3 fatty acids, may also support overall wellbeing. Engaging in restorative practices, such as taking a warm bath, meditating, or going for a gentle walk, can help you manage stress. If mood changes start interfering with your daily life or relationships, consider dialing back your fasting regimen.
6. Digestive Issues
Intermittent fasting can influence your digestive system, depending on the length of your fast and the types of foods you consume when you break it.
Shorter fasting windows, such as 12 hours, often go unnoticed by the digestive system, and some individuals even feel it gives their digestion a helpful rest (9). However, if you are fasting for much longer periods, you may start to experience indigestion, heartburn, and constipation. This can happen as the digestive system adjusts to less frequent, but sometimes larger, meals.
If you do not consume enough dietary fiber during your eating window, you might notice slower digestion. To help keep things moving, focus on high-fiber foods like vegetables, fruits, and whole grains, and drink plenty of water. Additionally, breaking a long fast with very large or rich meals can overwhelm the digestive system, potentially leading to discomfort, nausea, or loose stools. Eating smaller meals slowly and starting with easily digestible foods, like soups or smoothies, can help ease your body back into the digestive process.
7. Fatigue
Feeling tired is a very common experience when you first start intermittent fasting. This temporary fatigue can stem from several factors, including the body adjusting to a new energy source, changes in sleep routines, and hydration levels (2).
When you go for longer periods without food, shifts in your blood sugar can influence your overall energy. Inadequate fluid intake can also make you feel sluggish and drained.
To help manage fatigue, focus on getting enough restful sleep and drinking water consistently throughout the day. When you do eat, prioritize a balanced diet that includes lean proteins, complex carbohydrates, and healthy fats to provide sustained energy. Listening to your body and taking time to rest can make a big difference. Black coffee or unsweetened tea can offer a gentle lift, but if fatigue persists, you may want to consider shortening your fasting window or fasting on fewer days of the week.
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8. How Intermittent Fasting Can Affect Hormone Levels
Intermittent fasting can influence various hormone levels in the body, as hormones respond directly to changes in energy intake, sleep, and stress. The body’s endocrine system is highly sensitive, and significant dietary shifts require an adjustment period.
Here are some ways intermittent fasting may interact with your hormone levels:
Cortisol
When you fast, the body can perceive the lack of food as a physical stressor. Cortisol helps the body regulate various functions, and prolonged elevation is an area of ongoing research interest (10). To help support balanced cortisol levels, make sure you are getting plenty of restorative sleep and engaging in calming practices like yoga or mindfulness.
Estrogen
Significant changes in body weight or composition may be associated with shifts in estrogen levels in some people (11). Individual experiences vary considerably, and a healthcare provider can offer personalized guidance. Eating a well-rounded diet with plenty of nutrients, and ensuring you are not losing weight too rapidly, can support overall hormonal wellbeing.
Progesterone
Some research has explored associations between body weight changes and progesterone levels, particularly in women. Individual experiences vary, and anyone with concerns about their menstrual cycle should speak with a healthcare provider (12). Prioritizing a balanced, nutrient-dense diet and maintaining a steady, comfortable weight are helpful approaches to supporting overall wellness.
Insulin
Intermittent fasting is often discussed for its role in supporting metabolic health. When you fast, your body uses stored energy, which may influence how your body responds to insulin over time (13).
However, it is important to approach fasting carefully, especially if you have a history of blood sugar variations. Eating large, carbohydrate-heavy meals immediately after a fast can cause quick shifts in blood sugar. It is highly recommended to speak with a healthcare provider to ensure your approach is suitable for your body.
IF Side Effects in Women
Some research suggests that women’s bodies may be particularly sensitive to signals of energy scarcity (14). As a result, women may notice changes in their menstrual cycles or energy levels more quickly than men when fasting. Adjusting the fasting window to be shorter or more flexible can often make the practice more comfortable and suitable for women.
9. Bad Breath
You might notice a change in your breath when you start an intermittent fasting routine. As your body adapts to using fat for energy instead of carbohydrates, it can produce compounds called ketones.
These ketones can sometimes give your breath a distinct, fruity, or metallic odor. While it is a normal part of the metabolic shift, it can be bothersome (4). Staying well hydrated is commonly cited as one of the most helpful approaches to managing breath changes during fasting.
Drinking plenty of water helps maintain saliva production. You can also chew sugar-free gum, use mints, or maintain good oral hygiene with a gentle mouthwash to help neutralize the odor.
10. Poor Sleep
Changes to your sleep routine are a frequently reported experience when starting a new fasting schedule. As your body adapts to different meal timings and energy sources, you might find it harder to wind down at night (2).
Some people find their sleep is affected during the IF adjustment period, particularly in the first 1–2 weeks. Fasting late into the day, or conversely, eating a very large meal right before bed, can both influence how comfortably you sleep.
To support better rest, make sure you are drinking enough water during the day, but taper off your fluid intake in the evening to avoid waking up. Limiting caffeine in the afternoon and establishing a calming bedtime routine can also make a significant difference. If sleep issues continue, adjusting your intermittent fasting schedule so your eating window aligns better with your natural rhythms may be beneficial.
Read More: Intermittent Fasting Snacks: 10 Plus Healthy Bitings That Will Help You Stay On Track
11. Malnutrition
Intermittent fasting focuses on when you eat, but what you eat remains incredibly important. If your eating window is short, there is a risk that you might not consume enough calories or essential nutrients to support your body’s needs.
If your diet during the eating window lacks variety, you may miss out on important vitamins, minerals, and macronutrients. For instance, focusing only on one food group while neglecting others can leave you feeling depleted and un-energized.
To support your wellbeing, focus on eating a balanced diet that includes a variety of vegetables, fruits, whole grains, lean proteins, and healthy fats. Planning your meals ahead of time can ensure you get the right mix of nutrients. If you are unsure whether your current diet is meeting your needs, discussing it with a healthcare provider or a registered dietitian is a great step.
12. Interaction With Certain Medications
Fasting can influence how your body processes and absorbs certain substances. If you take medications, altering your eating schedule requires careful consideration and professional guidance.
Some medications need to be taken with food to be absorbed properly or to prevent stomach upset. Additionally, fasting can influence blood sugar and blood pressure levels, which can interact with medications designed for these purposes.
It is important to speak with your healthcare provider before you start intermittent fasting if you are on any regular medications. They can help you determine if fasting is appropriate for you and provide personalized advice on how to time your meals and medications safely.
How To Reduce IF Side Effects
While the adjustment period can bring some discomfort, there are several practical steps you can take to make the transition into intermittent fasting much smoother.
Ease Into IF
Starting slowly is generally the most comfortable way to approach fasting. Instead of jumping straight into a long fast, try starting with a 12-hour fasting window overnight. Once your body feels comfortable with that, you can gradually extend the fasting period by an hour or two at a time.
Drink Plenty of Water
Staying hydrated is a cornerstone of any wellness routine, and it is especially important when fasting. Aim to drink water consistently throughout the day. Unsweetened herbal teas and black coffee are also great options, but try to avoid sugary beverages or excessive caffeine, which can affect your hydration levels.
Eat A Balanced, Nutrient-Dense Diet
When your eating window opens, prioritize high-quality foods. A diet rich in vegetables, fruits, healthy fats, and proteins helps ensure your body gets what it needs to thrive. Reducing your intake of highly processed foods can also help keep your energy levels steady.
Get Enough Sleep
Since fasting can sometimes influence your rest, make a conscious effort to support your sleep hygiene. Stick to a consistent bedtime, keep your room cool and dark, and try to avoid screens right before bed.
Take a Break
If intermittent fasting side effects are making you uncomfortable or interfering with your daily life, it is perfectly okay to take a step back. You can always try a more flexible schedule, or simply focus on eating balanced meals without strict time restrictions.
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What Are the Benefits of Intermittent Fasting?
While it is important to be aware of the adjustment phase, it is also helpful to understand why so many people incorporate intermittent fasting into their lives. The benefits of intermittent fasting go beyond a simple eating schedule (15).
- Weight Management: Many people find IF helps them manage their calorie intake over time. By restricting the hours during which they eat, people often naturally consume fewer calories without the need for meticulous tracking.
- Metabolic Support: Some research has explored associations between IF and blood sugar regulation. Giving the body a break from constant digestion may support how the body processes energy.
- Mental Clarity: Some people report improved focus and a clearer mind during their fasting hours, finding that they have more sustained energy for tasks once their body adapts.
- Flexibility and Convenience: Intermittent fasting benefits and side effects aside, one of the biggest draws is its simplicity. IF can work with many different dietary patterns, whether you prefer plant-based meals, low-carb options, or a standard balanced diet. It often reduces the time spent planning and preparing early meals.
Remember, individual experiences vary, and what feels highly beneficial for one person may feel different for another.
Intermittent Fasting Rules: What You Need to Know Before You Start
To get the most out of your routine and minimize discomfort, there are a few basic intermittent fasting rules you should keep in mind.
First, try to maintain a consistent fasting window. Consistency helps your body understand and adapt to the new rhythm. Second, hydration is key—drink water regularly throughout your fasting and eating periods. Third, use your eating window to nourish your body, not as a license to overeat or consume large amounts of highly processed foods. Finally, always break your fast gently. A massive meal right away can overwhelm your digestive system.
Here is a quick look at common intermittent fasting schedules:
| Method | Fasting Window | Eating Window | Notes |
|---|---|---|---|
| 12:12 | 12 hours | 12 hours | Excellent for beginners. Often achieved simply by not snacking after dinner. |
| 14:10 | 14 hours | 10 hours | A gentle step up, suitable for many people looking to extend their overnight fast. |
| 16:8 | 16 hours | 8 hours | One of the most popular and commonly-used schedules. |
| 5:2 | Normal eating 5 days | Calorie reduction on 2 days | Focuses on days rather than hours; requires careful meal planning on fasting days. |
Frequently Asked Questions
How long do the side effects of intermittent fasting last?
Most adjustment side effects ease within one to two weeks as the body adapts to the new routine. The length of the adjustment period can depend on your previous diet, hydration levels, and how long your fasting window is, but many people find that initial discomforts like hunger and fatigue are short-term.
Is intermittent fasting suitable for everyone?
Intermittent fasting is not suitable for everyone. It is important to consult a healthcare provider before starting, especially for people taking medications, those who are pregnant or breastfeeding, individuals with a history of eating issues, or those managing specific health conditions.
How long should you do intermittent fasting?
There is no universally recommended duration, as intermittent fasting is often practiced as an ongoing lifestyle choice rather than a temporary program. Some people choose to follow it long-term, while others use it for defined periods, so your individual comfort and lifestyle should guide your approach.
What are the risks of intermittent fasting?
The main potential considerations include dehydration, blood sugar variation, shifts in hormone levels, and nutrient shortfalls if the diet during the eating window is not well-balanced. Being mindful of these factors and listening to your body can help you navigate the practice more comfortably.
Is intermittent fasting different for women than for men?
Some research suggests women may experience hormone-related changes more prominently during intermittent fasting than men (14). Because women’s bodies can be highly sensitive to energy intake, maintaining a flexible schedule and consulting a healthcare provider is particularly relevant for women interested in fasting.
The Bottom Line
Understanding intermittent fasting side effects is the first step toward building a routine that works for your lifestyle. While physical adjustments like hunger, fatigue, and shifts in digestion are common when you first start, they typically ease as your body adapts to the new schedule.
Prioritizing hydration, getting plenty of rest, and eating a nutrient-dense diet during your eating windows can make a significant difference. However, more serious considerations apply for specific groups, and fasting is not suitable for everyone.
Every fasting experience is personal. Your health status, activity level, and dietary preferences all influence how IF feels for you. It’s strongly advisable to speak with a healthcare provider before starting any fasting regimen, particularly if you take medication or have an existing health consideration.
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