Understanding What are the Downsides of Intermittent Fasting?

October 4, 2025 •

5 min read

While intermittent fasting is often promoted for weight loss and metabolic improvements, a significant percentage of participants in studies drop out due to negative side effects. This makes it essential to understand what are the downsides of intermittent fasting before you commit to the regimen.

Quick Summary

Intermittent fasting carries potential health risks and side effects, including hunger, fatigue, headaches, nutrient deficiencies, and the promotion of disordered eating patterns. Certain populations are at higher risk and should exercise caution.

Key Points

Initial Side Effects: Many people experience headaches, fatigue, dizziness, and intense hunger pangs when first starting intermittent fasting.
Risk of Disordered Eating: The restrictive nature of IF can trigger or exacerbate disordered eating patterns, including bingeing, especially in susceptible individuals.
Hormonal Impact: Intermittent fasting can increase stress hormone (cortisol) levels, which may disrupt hormonal balance and menstrual cycles, particularly in women.
Nutrient Deficiencies: Due to a compressed eating window, there is an increased risk of failing to consume a sufficient quantity and variety of nutrient-dense foods.
Uncertain Long-Term Effects: The long-term safety and efficacy of intermittent fasting are still not fully understood due to a lack of extensive human research.
Not for Everyone: Certain populations, including those who are pregnant, breastfeeding, diabetic, or have a history of eating disorders, should avoid IF.

Common Initial Side Effects

When beginning an intermittent fasting (IF) regimen, many people report experiencing a range of unpleasant side effects as their body adjusts. These initial reactions are often linked to fluctuations in blood sugar and the shift in eating patterns. The most frequently cited side effects include:

Hunger and Cravings: It is common to experience intense hunger pangs and cravings, especially during the first few weeks. For some, this intense hunger never fully subsides and can be a major factor in discontinuation.
Headaches and Dizziness: Headaches, often described as mild but persistent, along with dizziness, are commonly reported. This can be a symptom of low blood sugar or dehydration, which occurs when fluid intake isn't adequately maintained during fasting periods.
Fatigue and Low Energy: Many individuals experience a decrease in energy levels, especially when starting IF. This can impact daily activities and physical exercise, making it difficult to adhere to the plan.
Irritability and Mood Changes: The combination of hunger and low energy often leads to irritability or mood swings. The term "hangry"—a portmanteau of hungry and angry—is a real and frequent complaint among new fasters.
Digestive Issues: Altered eating schedules can disrupt digestive processes, leading to constipation, bloating, and nausea. Ensuring sufficient fiber and hydration is crucial for mitigating these effects.

Potential for Binge Eating and Disordered Patterns

One of the more serious downsides of intermittent fasting is the potential to trigger or worsen disordered eating behaviors, particularly for those with a history of such issues. The restrictive nature of fasting can set up an unhealthy cycle of restriction followed by binging.

All-or-Nothing Mindset: The strict timeframes of IF can lead to an all-or-nothing mindset. After a prolonged fasting period, some individuals may feel an urge to overeat or binge during their eating window, negating any potential calorie deficit created by the fasting phase.
Ignoring Hunger Cues: Intermittent fasting fundamentally requires ignoring the body's natural hunger cues for a set period. This can weaken a person's ability to listen to their body and eat intuitively, which is a key component of a healthy relationship with food.
Orthorexia Risk: For some, IF can lead to an obsessive focus on "correct" eating patterns and timing, a pattern seen in orthorexia. This fixation can negatively impact a person's overall well-being and mental health.

Hormonal and Metabolic Impacts

While IF can have a positive effect on some hormones, it can also disrupt others, especially in women. The body's stress response can be significantly taxed by prolonged fasting.

Hormonal Fluctuations: Fasting, particularly when aggressive, can increase cortisol (the stress hormone) levels. Elevated cortisol can negatively impact reproductive health in women, potentially causing irregular menstrual cycles. This can be particularly concerning for women over 35 or those in perimenopause.
Nutrient Deficiencies: Limiting food intake to a small window makes it challenging to consume a wide variety of nutrient-dense foods, increasing the risk of vitamin and mineral deficiencies. A poor nutrient intake during eating windows can lead to problems like hair loss and fatigue.
Metabolic Adaptation: While some evidence suggests a slight boost in metabolism with short-term fasting, prolonged calorie restriction can eventually lead to a reduced basal metabolic rate. This can make weight loss plateaus or even weight regain more likely over time.

Comparison with Standard Calorie Restriction

Research has explored whether intermittent fasting is superior to continuous calorie restriction (CCR). A recent study found that intermittent energy restriction diets were not superior to CCR diets for weight loss, although IF did show small but significant decreases in waist circumference and fat-free mass. However, the long-term effectiveness and sustainability remain unclear.


Feature	Intermittent Fasting (IF)	Continuous Calorie Restriction (CCR)
Eating Pattern	Restricts when you eat (time-restricted windows or fasting days).	Restricts how much you eat daily by counting calories.
Hunger Management	Can lead to intense hunger and cravings during fasting periods.	Consistent, moderate hunger levels managed throughout the day.
Nutrient Intake	Risk of nutrient deficiency if eating window is too short or diet is not varied.	Easier to ensure adequate nutrient intake with multiple meals and proper planning.
Disordered Eating Risk	Higher risk of triggering binge-restrict cycles and an unhealthy relationship with food.	Lower risk, but still possible depending on the individual's history and relationship with food.
Sustainability	High dropout rates in some studies due to difficulty adhering long-term.	Some find daily calorie counting tedious, but may be more sustainable for others.
Long-Term Effects	Largely unknown; limited human research available.	More established understanding of long-term effects on weight and health.

Who Should Avoid Intermittent Fasting?

IF is not safe or recommended for everyone. Consultation with a doctor is essential before starting, especially for individuals with pre-existing conditions or certain life stages. High-risk groups include:

People with Diabetes: IF can dangerously affect blood sugar levels and is not recommended for those with diabetes, especially type 1.
Pregnant or Breastfeeding Women: These individuals have increased caloric and nutrient needs and should not restrict their intake.
Individuals with a History of Eating Disorders: The restrictive nature of IF can be a significant trigger for relapses or the development of disordered eating.
Older Adults: Older adults, especially those at risk of bone loss or falls, may be vulnerable to excessive weight loss and potential nutrient imbalances from IF.
People on Specific Medications: Some medications must be taken with food. Skipping meals could interfere with proper medication absorption and increase the risk of side effects.

Conclusion

While intermittent fasting has gained popularity for its potential short-term weight loss benefits, it is crucial to recognize and evaluate what are the downsides of intermittent fasting. The initial side effects, including fatigue, headaches, and hunger, can be challenging. More importantly, IF poses risks such as promoting disordered eating, impacting hormonal balance, and potentially leading to nutrient deficiencies if not carefully managed. Furthermore, the long-term health effects are not yet well-understood, with limited human studies available. For many, focusing on a sustainable, balanced diet of nutrient-dense foods may be a safer and more effective path to long-term health than adhering to strict fasting schedules. Anyone considering IF should speak with a healthcare professional to ensure it is the right and safe option for their individual circumstances.

For more information on dietary choices, consult with a registered dietitian or visit a reliable health resource like the Harvard T.H. Chan School of Public Health.

Frequently Asked Questions

Yes, headaches are a commonly reported side effect, especially when beginning intermittent fasting. They are often attributed to low blood sugar levels and dehydration.

Individuals who are pregnant or breastfeeding, have a history of eating disorders, are diabetic, or are on medication that requires regular food intake should avoid intermittent fasting.

There is a risk of nutrient deficiencies with intermittent fasting, particularly if the eating window is too short or the diet lacks variety, making it hard to get adequate vitamins and minerals.

Yes, the restriction associated with intermittent fasting can trigger overeating or bingeing during the eating window, potentially leading to an unhealthy binge-restrict cycle.

For women, particularly those over 35, intermittent fasting can increase cortisol levels and disrupt the balance of reproductive hormones, potentially causing irregular menstrual cycles or other issues.

No, the long-term health effects of intermittent fasting are still largely unknown. Most studies are short-term and have involved small sample sizes, limiting what is known about its impact over extended periods.

Studies have found that intermittent fasting is not necessarily more effective for weight loss than continuous calorie restriction, especially in the long term. Sustainability and individual adherence are key factors.