Is There a Downside to Fasting? Risks, Side Effects, and Who Should Be Cautious

October 29, 2025 •

4 min read

While intermittent fasting has gained significant popularity for its potential health benefits, some studies report a range of adverse effects, from common discomforts to more serious health concerns. So, is there a downside to fasting? Understanding the full picture is crucial for anyone considering this dietary approach.

Quick Summary

This article explores the potential risks and side effects of fasting, including common discomforts like headaches and more serious long-term health concerns. It also identifies individuals who should approach fasting with caution or avoid it entirely.

Key Points

Common Side Effects: Fasting can cause temporary headaches, fatigue, and irritability as your body adjusts to changes in blood sugar and hormone levels.
Who Should Avoid: Fasting is not safe for everyone; high-risk groups include pregnant women, people with diabetes, and those with a history of eating disorders.
Metabolic Risks: Prolonged or severe fasting may lead to muscle mass loss and potentially cause nutrient deficiencies if not carefully managed.
Cardiovascular Concerns: Preliminary research has suggested a link between long-term time-restricted eating and increased cardiovascular mortality, highlighting a need for further study.
Disordered Eating: For susceptible individuals, the restrictive nature of fasting can exacerbate or trigger disordered eating behaviors.
Safe Fasting: To minimize downsides, stay hydrated, focus on nutrient-dense meals during eating windows, and always consult a doctor before beginning.

Potential Short-Term Side Effects

Starting a fasting regimen can cause an adjustment period for your body, leading to a variety of temporary, but often unpleasant, side effects. As your body shifts from using glucose for energy to burning stored fat, you may experience symptoms similar to a caffeine or sugar withdrawal. These side effects typically diminish within the first few weeks as your body adapts.

Hunger and Cravings: It's no surprise that abstaining from food leads to hunger. While ghrelin, the hunger hormone, may decrease over a long fast, initial cravings can be intense and challenging to manage.
Headaches and Lightheadedness: Many people report headaches during the early days of fasting. These can be caused by low blood sugar, caffeine withdrawal, or electrolyte imbalances.
Fatigue and Low Energy: Feeling tired and weak is common as your body adjusts. This can be related to lower blood sugar levels and the initial release of excess water and salt.
Irritability and Mood Changes: Low blood sugar can also lead to mood disturbances, including increased irritability and anxiety.
Dehydration: The initial diuretic effect of fasting means your body excretes more water and salt. If you don't actively replace fluids and electrolytes, dehydration can occur.
Digestive Issues: Constipation and bloating are possible as a result of reduced food intake. Some people may also experience nausea or acid reflux.
Sleep Disturbances: Issues with falling or staying asleep have been reported by some fasters, particularly during the adjustment phase.

Who Should Avoid Fasting?

While intermittent fasting may be safe for many healthy adults, it is not suitable for everyone. Certain groups are at a higher risk of adverse effects and should avoid fasting unless under strict medical supervision.

Individuals with a History of Eating Disorders: Restrictive eating patterns can trigger or exacerbate disordered eating behaviors and an unhealthy relationship with food.
Pregnant or Breastfeeding Women: These stages require increased energy and nutrient intake to support the mother and baby. Fasting is generally not recommended.
Children and Adolescents: Growing bodies have high energy demands for proper development, making fasting potentially harmful.
Individuals with Diabetes or Blood Sugar Issues: Fasting can cause dangerous fluctuations in blood sugar levels. Those with Type 1 diabetes, in particular, face a high risk of unsafe hypoglycemia.
People Taking Certain Medications: Some drugs, including blood pressure medication and insulin, must be taken with food. Fasting could interfere with dosage and effectiveness.
The Elderly: Older individuals, especially those at risk for bone loss or falls, may face increased risks with fasting.

Deeper Metabolic and Long-Term Concerns

Beyond the initial discomforts, some research raises more serious questions about the long-term metabolic impacts of fasting.

Muscle Mass Loss: While often praised for fat loss, some studies show that fasting, particularly more restrictive methods, can lead to a loss of lean muscle mass. A decrease in muscle can negatively impact metabolism over time.
Increased Cardiovascular Risk: Preliminary research presented at an American Heart Association meeting indicated a potential link between long-term time-restricted eating (under 8 hours) and a 91% higher risk of death from cardiovascular disease. While this abstract is not yet peer-reviewed and has limitations, it highlights an area requiring more long-term investigation.
Hormonal Disruption: For women, some studies suggest potential issues with reproductive function due to energy restriction, though human research is limited. Hormonal imbalances can affect menstrual regularity and overall well-being.
Nutrient Deficiencies: If not managed carefully with a nutrient-dense diet during eating windows, a restrictive eating schedule could lead to inadequate intake of essential vitamins and minerals, resulting in malnutrition over time.
Increased Systemic Inflammation: A supervised prolonged water-only fast study surprisingly found an acute inflammatory response and increased biomarkers of platelet activation. This suggests that for individuals with pre-existing conditions, fasting may carry unexpected cardiometabolic risks.

Fasting: Benefits vs. Downsides


Feature	Potential Benefits	Potential Downsides
Weight Management	Can lead to weight loss by reducing overall calorie intake and promoting fat burning.	Can lead to muscle mass loss and potential for a binge-restrict cycle.
Metabolic Health	May improve insulin sensitivity and blood sugar levels.	Risk of hypoglycemia, especially for diabetics, and potential for metabolic slowdown.
Cardiovascular Health	Can improve blood pressure and cholesterol levels in some individuals.	Some studies suggest an increased risk of cardiovascular mortality with long-term, time-restricted eating.
Ease of Adherence	Simplifies meal planning by focusing on when to eat rather than what.	High dropout rates reported in some studies due to difficulty maintaining the regimen long-term.

The Importance of a Balanced Approach

For those who choose to fast and are medically cleared to do so, it is crucial to focus on nutritional quality during eating periods. Filling eating windows with nutrient-dense foods like whole grains, lean proteins, healthy fats, fruits, and vegetables is vital for overall health and to prevent deficiencies. A Mediterranean-style diet is often recommended as a healthy blueprint. Starting gradually and listening to your body's signals can help mitigate common side effects. However, the emerging research on long-term impacts suggests that a balanced approach, considering both what and when you eat, remains key.

Conclusion

While fasting is celebrated for a range of health benefits, the potential downsides, risks, and side effects are significant and should not be overlooked. For certain individuals, including those with pre-existing health conditions or a history of eating disorders, fasting is not recommended and may be dangerous. Common side effects like headaches and fatigue are frequent during the initial phase. Longer-term concerns regarding muscle loss, cardiac health, and systemic inflammation highlight the need for more research and caution. Ultimately, the safest and most sustainable approach to a nutrition diet emphasizes quality food choices and is personalized to one's individual health status and needs. As the BBC notes, a focus on what you eat may be more important than the clock.

For more information on intermittent fasting, Johns Hopkins Medicine offers extensive resources.

Frequently Asked Questions

The most common short-term side effects include hunger, headaches, fatigue, dizziness, irritability, and digestive issues like constipation or bloating. Many of these subside as the body adapts.

Yes, fasting can be harmful for people with diabetes, especially Type 1, due to the risk of dangerously low blood sugar (hypoglycemia). It should only be attempted under close medical supervision.

Intermittent fasting is a restrictive eating pattern that can be a trigger for disordered eating behaviors in susceptible individuals. Health experts, particularly those treating eating disorders, often warn against it.

Yes, during the initial phase of fasting, the body can release large amounts of water and salt. This can lead to dehydration if fluids and electrolytes are not properly replenished.

Yes, if not done correctly, particularly with prolonged or very restrictive fasts, the body may break down muscle tissue for energy. Some studies on shorter intermittent fasting methods have also noted muscle loss.

Long-term risks are still being researched, but concerns include a potential link to increased cardiovascular mortality with prolonged time-restricted eating and increased inflammatory responses from prolonged fasting.

Children and adolescents, pregnant or breastfeeding women, the elderly, those with a history of eating disorders, and people with certain medical conditions like diabetes or kidney disease are advised to avoid fasting.

To safely break a fast, start with small, easily digestible, nutrient-rich foods such as broth, fruits, or lean protein. Avoid large, heavy meals, and don't overeat.