Nutrition Diet: What are the disadvantages of fasting?

October 22, 2025 •

4 min read

While intermittent fasting is often praised for its potential benefits, studies show that many people experience unpleasant side effects, with some reporting headaches and fatigue during the initial weeks. Understanding what are the disadvantages of fasting is crucial for making an informed decision about your diet and overall health.

Quick Summary

Fasting, particularly when prolonged or improperly managed, can lead to significant side effects such as metabolic disturbances, mood changes, and nutritional deficiencies. It is not suitable for everyone.

Key Points

Initial Discomfort is Common: New fasters often experience side effects like headaches, fatigue, and irritability as their body adjusts to fuel changes.
Metabolism Can Slow Down: Extended fasting or frequent periods of calorie restriction may cause a reduction in your resting metabolic rate, hindering long-term weight management.
Risks for Vulnerable Populations: Fasting is not safe for everyone, especially pregnant women, people with diabetes, and those with a history of eating disorders.
Nutrient Deficiencies are Possible: Without careful planning, prolonged fasting can lead to an inadequate intake of essential vitamins and minerals, risking malnutrition.
Potential for Disordered Eating: The restrictive nature of fasting can trigger a negative relationship with food and may increase the likelihood of binge-eating.
Electrolyte Imbalances are a Concern: The body can lose important electrolytes like sodium and potassium during a fast, which requires careful management to avoid dehydration.

Common Short-Term Side Effects

Many people who are new to fasting experience common, temporary side effects as their bodies adjust to the new eating pattern. These are typically most pronounced during the first few weeks.

Headaches and Dizziness: Headaches are a very common complaint, often caused by low blood sugar (hypoglycemia), caffeine withdrawal, or electrolyte imbalances. Dizziness and lightheadedness can also occur, particularly during longer fasts.
Fatigue and Low Energy: Feeling sluggish or having low energy is another frequent initial side effect as the body shifts from burning glucose to burning fat for fuel. Most people experience a burst of energy once ketosis kicks in, but fatigue can be a barrier for new fasters.
Irritability and Mood Swings: Low blood sugar can cause irritability and other mood disturbances. Some studies have found that women, in particular, may experience heightened irritability during fasting periods.
Digestive Issues: Constipation, diarrhea, nausea, and bloating are all possible digestive side effects. This can be due to changes in diet and altered digestion patterns.
Sleep Disturbances: Insomnia and other sleep-related issues, such as difficulty falling or staying asleep, are reported by some fasters.
Bad Breath: Often referred to as 'keto breath,' this is caused by the body's use of fat for fuel, which produces acetone as a byproduct.

Metabolic and Physiological Impacts

Beyond the initial discomfort, fasting, especially if prolonged, can have deeper physiological consequences that are important to consider.

Slowed Metabolism: Extended periods of calorie restriction can lead to a reduction in resting metabolic rate (RMR) as the body attempts to conserve energy. This can make weight loss more difficult over time and may lead to weight regain after stopping the fast.
Hormonal Imbalances: Fasting can disrupt normal hormone production, especially those controlling hunger, metabolism, and stress. In women, this can lead to irregular menstrual cycles and fertility issues. Stress hormones like cortisol can also increase.
Nutrient Deficiencies and Malnutrition: If fasting periods are not properly balanced with nutrient-dense meals, there is a risk of developing vitamin and mineral deficiencies. This is especially true for prolonged or repeated fasts.
Muscle Loss: In the absence of sufficient protein and energy intake, the body can begin to break down lean muscle tissue for fuel, especially during prolonged fasts. This is more common if an individual's diet is not well-balanced and protein-rich during eating windows.
Dehydration and Electrolyte Imbalances: During a fast, the body releases excess water and salts, which can lead to dehydration if fluid and electrolyte intake are not managed. This can become a serious health concern, particularly with prolonged water-only fasts.
Increased Inflammation and Platelet Activation: Some research suggests that prolonged fasting can trigger an acute inflammatory response and increase platelet activation, which may raise cardiometabolic risks for certain individuals.

Psychological and Social Drawbacks

Fasting is not just a physical process; it also has mental and social components that can be challenging for some people.

Disordered Eating Patterns: For those susceptible to eating disorders, fasting can be a 'slippery slope' that fosters a restrictive mindset and an unhealthy relationship with food. It can increase the risk of binge-eating during non-fasting periods.
Social Isolation: Many social events and traditions revolve around food. Fasting can make participation in these activities difficult and may lead to feelings of isolation.
Difficulty Sticking with the Plan: Maintaining a strict fasting schedule can be challenging, especially for those with busy or variable schedules. The hunger and fatigue can make consistency difficult, leading to a high drop-out rate.

Comparison of Short-Term vs. Long-Term Fasting Disadvantages


Disadvantage	Short-Term Fasting (e.g., 16/8)	Prolonged Fasting (e.g., 48+ hours)
Physical Symptoms	Common (headaches, fatigue, hunger, irritability)	More intense and prolonged symptoms. Higher risk of dizziness, nausea, and dehydration.
Metabolism	Possible initial dip, but generally less severe	Significant reduction in resting metabolic rate (RMR).
Nutrient Deficiencies	Low risk if diet is well-balanced during eating windows	Higher risk of deficiencies if not supplemented carefully.
Muscle Loss	Minimal risk with adequate protein and exercise	Increased risk of lean muscle tissue breakdown.
Hormonal Changes	Mild fluctuations in insulin and hunger hormones	Significant disruption to hunger, stress, and sex hormones.
Disordered Eating Risk	Can trigger restrictive behaviors in susceptible individuals	Higher potential for a binge-restrict cycle and amplified disordered eating behaviors.
Sustainability	Easier to adapt for many people	Can be very difficult and unsustainable long-term.

Who Should Avoid Fasting

Fasting is not a one-size-fits-all approach and is explicitly discouraged for several groups to prevent serious health complications.

Pregnant or Breastfeeding Women: These individuals have increased energy and nutrient needs for their and their baby's health.
Individuals with a History of Eating Disorders: Fasting can be a trigger for relapse or the development of a disordered relationship with food.
People with Diabetes or Blood Sugar Issues: Fasting can cause dangerously low blood sugar levels (hypoglycemia) and requires medical supervision.
Children and Teenagers: Young people are in a crucial developmental stage and require consistent nutrition for proper growth.
Individuals Taking Certain Medications: Some medications, especially for blood pressure or heart conditions, can cause electrolyte imbalances during fasting. It is vital to consult a doctor first.

Conclusion

While fasting is a popular dietary approach with perceived benefits, understanding the potential drawbacks is essential for health and safety. The side effects, ranging from temporary discomforts like headaches and irritability to more severe risks such as metabolic slowdown and hormonal disruptions, highlight the complexity of this eating pattern. Moreover, fasting is not suitable for everyone, particularly those with pre-existing medical conditions, a history of disordered eating, or those who are pregnant. Anyone considering a fast should prioritize safety by consulting a healthcare professional. Ultimately, focusing on a balanced, nutrient-dense diet remains a sustainable and safe path for long-term health for most people, as noted by organizations like the Mayo Clinic.

Frequently Asked Questions

The most common side effects reported by those new to intermittent fasting include headaches, fatigue, dizziness, irritability, hunger pangs, and digestive issues like constipation or bloating.

Yes, prolonged periods of calorie restriction and fasting can cause your metabolism to slow down as your body attempts to conserve energy, a process known as adaptive thermogenesis.

Fasting should be avoided by pregnant and breastfeeding women, children and adolescents, individuals with diabetes, people with a history of eating disorders, and those taking certain medications.

If not combined with adequate protein intake and exercise, some studies suggest that fasting, particularly prolonged fasts, might lead to the loss of lean muscle mass.

Yes, fluctuations in blood sugar and stress hormones during fasting can lead to mood disturbances, such as increased irritability, anxiety, and difficulty concentrating.

The risk of nutrient deficiencies and malnutrition increases with fasting, especially with longer durations or if nutrient-dense foods are not consumed during eating windows.

Yes, bad breath, sometimes called 'keto breath,' is a known side effect caused by the production of acetone as a byproduct when the body breaks down fat for fuel.