Is there anything negative about fasting? Understanding the Downsides

October 27, 2025 •

4 min read

While many studies highlight the potential health benefits of fasting, a 2024 review found that intermittent fasting may be linked to certain adverse effects, including hunger, headaches, and fatigue. These and other potential drawbacks raise the question: is there anything negative about fasting?

Quick Summary

Fasting can cause negative side effects like headaches, fatigue, and irritability, particularly when first starting. Risks including dehydration, electrolyte imbalances, and nutrient deficiencies can occur, especially during longer fasts. Certain individuals, such as those with a history of disordered eating or underlying medical conditions, should avoid fasting. Concerns also exist about potential long-term metabolic slowdown and a higher risk of developing disordered eating patterns.

Key Points

Initial Discomfort: Headaches, fatigue, and irritability are common, especially when first starting, but usually subside as the body adapts.
Dehydration and Electrolyte Imbalance: A significant risk during extended fasting is the loss of fluids and essential minerals, which can cause dizziness, muscle cramps, and dangerous heart issues.
Risk of Disordered Eating: The restrictive nature of fasting can trigger or worsen eating disorders like binge eating, bulimia, and orthorexia by creating an unhealthy relationship with food.
Nutrient Deficiencies: With a restricted eating window, it can be challenging to consume all necessary vitamins and minerals, potentially leading to malnutrition over time.
Who Should Avoid Fasting: Pregnant or breastfeeding women, adolescents, and individuals with a history of eating disorders or certain medical conditions like diabetes should avoid fasting unless medically supervised.
Social Interference: The rigid schedules of fasting can interfere with social gatherings and family meals, potentially impacting one's mental and social well-being.
Potential for Overeating: Some individuals experience an intense urge to overeat after a fast, which can undermine weight loss goals and contribute to a binge-restrict cycle.
Metabolic Concerns: There is debate over whether prolonged fasting can lead to a metabolic slowdown, making sustained weight management more difficult.

Common Side Effects of Fasting

For many, especially those new to fasting, the initial phase can bring noticeable and unpleasant side effects. Most of these tend to subside as the body adapts, but it is important to be aware of them.

Physical Discomfort and Changes

Headaches: Often caused by hypoglycemia (low blood sugar), dehydration, or caffeine withdrawal, headaches are a frequently reported side effect during the initial days of fasting.
Fatigue and Low Energy: As the body shifts its energy source from glucose to stored fat, feelings of sluggishness and low energy are common. This typically improves as the body becomes more efficient at burning fat for fuel.
Dizziness: Lightheadedness can result from changes in blood pressure or blood sugar levels, particularly when standing up quickly. This is often linked to dehydration and electrolyte imbalance.
Constipation and Digestive Issues: A significant reduction in food intake can lead to changes in bowel movements, with constipation being a common complaint.
Bad Breath: Fasting can lead to a condition known as "keto breath" caused by the breakdown of ketones for energy. While harmless, it can be socially undesirable.

Serious Health Risks and Who Should Avoid Fasting

While milder side effects are often temporary, more serious health risks exist for certain individuals or with more extreme fasting methods. It's crucial for specific populations to avoid fasting or to do so only under strict medical supervision.

Potential Health Dangers

Dehydration: The body releases large amounts of water and salts during the initial stages of fasting. Without sufficient fluid and electrolyte replenishment, dehydration can occur, leading to serious issues like muscle cramps, dizziness, and confusion.
Electrolyte Imbalance: Extended fasting can deplete essential minerals like sodium, potassium, and magnesium. This can lead to muscle weakness, fatigue, and, in severe cases, dangerous irregular heart rhythms.
Nutrient Deficiencies: Restricting eating windows or prolonged fasting can make it challenging to consume a wide range of nutrient-dense foods. This can lead to deficiencies in essential vitamins, minerals, and proteins, weakening the immune system and causing fatigue.
Metabolic Slowdown: Although controversial, some evidence suggests that prolonged calorie restriction can lower the basal metabolic rate, independent of weight loss. This can make weight regain more likely once a normal eating pattern resumes, a phenomenon known as "yo-yo" dieting.

Fasting vs. Disordered Eating

For some, fasting can represent a slippery slope into disordered eating. It is important to recognize the warning signs and understand the psychological risks involved.

Preoccupation with Food: Fasting, particularly intermittent fasting, can increase an individual's focus on food and eating times, potentially fueling obsessive thoughts and anxiety.
Risk of Binge Eating: Depriving the body of food can lead to a powerful biological drive to overeat when the eating window opens. This can trigger a binge-restrict cycle, which is a symptom of eating disorders like binge eating disorder (BED) and bulimia nervosa.
Ignoring Hunger Cues: Fasting requires overriding the body's natural hunger signals. Over time, this can lead to a disconnection from internal cues of hunger and fullness, which is a hallmark of disordered eating.
Negative Impact on Mental Health: The stress of fasting and the subsequent mood disturbances can negatively affect mental health, especially for those with a pre-existing history of anxiety or depression.

Comparison of Potential Fasting Downsides


Aspect	Initial Fasting Symptoms	Extended or Improper Fasting Risks
Physical Effects	Headaches, fatigue, hunger pangs, mood swings, nausea.	Dehydration, severe electrolyte imbalances, malnutrition, loss of muscle mass.
Metabolic Effects	Transition period as the body switches fuel sources.	Potential metabolic slowdown, increased risk of rebound weight gain, insulin resistance (in some contexts).
Mental/Emotional Effects	Irritability, decreased concentration, anxiety, sleep disturbances.	Increased risk of developing disordered eating patterns, obsession with food, social withdrawal related to meal schedules.
Who is Affected	Most people who are new to fasting.	Individuals with underlying medical conditions, history of eating disorders, pregnant/breastfeeding women, adolescents.

Conclusion: Making an Informed Decision

While fasting can offer significant health benefits for some individuals, it is not without potential negative consequences. Initial side effects are common and often temporary, but more serious risks like dehydration, electrolyte imbalance, and nutrient deficiencies can occur with longer or improperly managed fasts. The risk of promoting or exacerbating disordered eating patterns is a particularly concerning aspect for vulnerable individuals. Moreover, fasting is not universally safe and should be approached with caution by pregnant or breastfeeding women, adolescents, and those with certain medical conditions like diabetes or a history of eating disorders. Ultimately, the best approach is to consult with a healthcare professional to determine if fasting is appropriate and safe for your specific health profile. The potential downsides of fasting must be weighed carefully against the perceived benefits, and personal well-being should always take precedence over the pursuit of a dietary trend. For many, a balanced and consistent eating pattern may be a more sustainable path to health than restrictive fasting cycles.

Disclaimer: The information in this article is for educational purposes only and should not be considered medical advice. Always consult a healthcare professional before starting any new diet or fasting regimen.

Frequently Asked Questions

The most common side effects include headaches, fatigue, irritability, hunger pangs, bad breath, and digestive issues like constipation. These symptoms are often most prominent during the initial days as your body adapts.

No, fasting is not safe for everyone. People who are pregnant, breastfeeding, underweight, or have a history of eating disorders should not fast. Those with medical conditions like diabetes, heart disease, or kidney problems should only consider fasting under strict medical supervision.

Yes, fasting can lead to dehydration, especially with longer fasts. Your body releases a significant amount of water and salt initially, and if you don't adequately replace fluids and electrolytes during your eating window, you are at risk. Symptoms can include dizziness, lethargy, and muscle cramps.

Concerns exist about muscle loss during fasting, particularly prolonged or water-only fasts. While some studies suggest minimal and reversible muscle loss occurs in the early stages, newer research indicates that the body has mechanisms to protect muscle mass during shorter fasts, especially when combined with nutrient-dense meals and resistance exercise during eating periods.

The restrictive nature of fasting can override natural hunger and fullness cues, potentially leading to a disconnection from your body's signals. This preoccupation with food and eating schedules can pave the way for a binge-restrict cycle and foster anxiety around eating.

There is evidence suggesting that prolonged calorie restriction, including certain fasting protocols, can lead to a reduction in the basal metabolic rate. This metabolic slowdown could make it harder to maintain weight loss over the long term and can contribute to a cycle of weight loss and regain.

Warning signs include persistent and severe headaches, extreme fatigue, irregular heartbeat, constant hunger, mood disturbances, preoccupation with food, and withdrawal from social activities involving meals. If you experience any of these, it's advisable to stop fasting and consult a healthcare professional.