Why You Should Not Fast: Understanding the Risks and Contraindications

Recent surveys indicate that nearly 1 in 10 Americans have tried intermittent fasting, but for many people, this trend is dangerous. While promoted for weight loss and other benefits, there are critical reasons why you should not fast, especially if you have underlying health issues or are in a vulnerable population.

Quick Summary

Examines the reasons fasting is unsafe for specific groups, including those with chronic diseases, eating disorders, or compromised immune systems. It also details the common side effects and health risks associated with the practice for certain individuals.

Key Points

• Diabetes Risk: Fasting can cause dangerous blood sugar fluctuations and potentially life-threatening hypoglycemia in people with diabetes, especially type 1.

• Eating Disorder Concerns: Fasting should be avoided by anyone with a history of disordered eating, as it can trigger restrictive behaviors and disrupt recovery.

• Pregnancy and Breastfeeding: Pregnant and breastfeeding women have increased nutritional needs, and fasting can compromise their health and the baby's development or milk supply.

• Nutrient Deficiencies: Prolonged or improperly managed fasting can lead to nutrient and electrolyte imbalances, causing fatigue and other health issues.

• Medical Clearance Needed: Individuals with heart conditions, kidney disease, or on certain medications must consult a doctor before fasting to avoid serious health risks.

• Negative Side Effects: Common side effects include fatigue, headaches, irritability, and dizziness, which can significantly impact daily functioning.

• Not for Children: Fasting is not recommended for children and adolescents whose bodies require consistent nourishment for proper growth.

Who Should Not Fast: Key Health Contraindications

Fasting, in its various forms, has gained popularity for its supposed health benefits, from weight loss to improved metabolic function. However, it is not a universally safe practice. Numerous medical and physiological reasons exist for why you should not fast, and for some, it can be extremely dangerous. Consulting a doctor before starting any form of fast is critical for those with pre-existing conditions or other risk factors.

Diabetes and Blood Sugar Regulation

For individuals with diabetes, especially type 1, fasting can pose severe risks. The body's inability to produce or properly use insulin means that going without food can lead to life-threatening fluctuations in blood sugar levels.

• Hypoglycemia (low blood sugar): A dangerous drop in blood sugar can cause dizziness, confusion, seizures, and even coma. For those on insulin or certain oral medications, this risk is significantly heightened.
• Hyperglycemia (high blood sugar): A rebound effect after fasting can cause dangerously high blood sugar levels. This can lead to diabetic ketoacidosis in type 1 diabetics, a serious medical emergency.

Pregnant and Breastfeeding Women

Nutritional needs increase significantly during pregnancy and breastfeeding, and fasting can compromise both the mother's and baby's health.

• Nutrient and calorie deficits: A developing fetus requires a constant supply of nutrients and energy. Restricting calories can lead to malnutrition, negatively affecting fetal growth and development.
• Dehydration and low milk supply: Breastfeeding mothers need extra fluids to maintain adequate milk production. Fasting can cause dehydration, which may reduce milk supply and harm the infant.

History of Disordered Eating

For individuals with a history of or active eating disorders, such as anorexia, bulimia, or binge eating disorder, fasting is highly discouraged.

• Triggers restrictive patterns: Fasting reinforces restrictive eating behaviors and can create an unhealthy and obsessive relationship with food.
• Risk of relapse: For those in recovery, adopting a fasting regimen can be a serious trigger, making relapse more likely.
• Binge-restrict cycle: The deprivation inherent in fasting can lead to uncontrollable binge eating during re-feeding periods, a damaging cycle for mental and physical health.

Fasting vs. Healthy Eating: A Comparison

Aspect	Fasting (for at-risk groups)	Healthy, Balanced Diet
Blood Sugar	High risk of dangerous fluctuations and hypoglycemia.	Supports stable blood glucose levels and insulin sensitivity.
Nutrient Intake	Can lead to nutrient deficiencies and malnutrition.	Ensures adequate vitamins, minerals, and macronutrients for health.
Energy Levels	Often causes fatigue, lethargy, and weakness.	Provides sustained energy throughout the day for daily activities.
Mental Health	Can trigger anxiety, mood swings, and disordered eating.	Promotes a healthy and sustainable relationship with food and body.
Social Impact	Can disrupt social activities centered around food and mealtimes.	Easily integrates into social events and communal dining.

The Risks of Prolonged Fasting and Dehydration

Any form of prolonged fasting can increase the risk of severe dehydration, especially in warmer climates or for individuals with kidney issues. Dehydration places a strain on the kidneys and can lead to electrolyte imbalances, which affect heart function and overall health. Symptoms like dizziness, headaches, and weakness are common side effects of dehydration during a fast. In extreme cases, dehydration can lead to kidney stones or even kidney failure.

Other Vulnerable Populations

Certain other groups should be extremely cautious or avoid fasting entirely without direct medical supervision.

• Elderly individuals: With weaker immune systems and pre-existing conditions, older adults are more susceptible to the adverse effects of fasting.
• Children and adolescents: Fasting is not recommended for anyone under 18. Their bodies require a constant, stable intake of energy and nutrients for proper growth and development.
• Athletes or people with physically demanding jobs: High-intensity activity requires a consistent fuel source. Fasting can lead to fatigue, poor performance, and potentially dangerous drops in blood sugar, especially in hot environments.
• Individuals on certain medications: Some medications, especially those for blood pressure, diabetes, or heart conditions, must be taken with food. Skipping meals could interfere with their effectiveness or cause harmful side effects.

Conclusion

While fasting may be an effective strategy for some, it is far from a one-size-fits-all approach to health. The reasons why you should not fast often outweigh the potential benefits for vulnerable populations, including those with chronic medical conditions like diabetes, kidney disease, or heart problems, as well as pregnant and breastfeeding women, and individuals with a history of eating disorders. For these groups, prioritizing balanced nutrition and stable eating patterns is the safer and more effective path to long-term wellness. Always consult a healthcare provider to determine if fasting is appropriate for your specific health needs before beginning any restrictive diet. A comprehensive evaluation can help you understand the risks and ensure your approach to health is both safe and sustainable. The potential for severe side effects like dehydration, malnutrition, and dangerous blood sugar levels is a serious consideration that should not be taken lightly.

Potential side effects of fasting

• Fatigue: A common symptom, especially in the early stages, as the body adjusts to the lack of fuel.
• Headaches: Often caused by low blood sugar and caffeine withdrawal, headaches are a frequent complaint during fasting.
• Irritability and mood swings: Changes in blood sugar and hormones can affect mood and lead to increased irritability.
• Dizziness and lightheadedness: Can result from low blood sugar or dehydration, particularly in those not used to fasting.
• Dehydration: A significant risk, especially with longer fasts, if fluid and electrolyte intake are not managed carefully.
• Constipation: Altered eating patterns and reduced fiber intake can lead to digestive issues.
• Malnutrition: Prolonged or severe fasting can result in deficiencies in essential vitamins and minerals.
• Orthorexia: The restrictive nature of fasting can foster an unhealthy obsession with "pure" or "clean" eating.

Authority link

For more information on the risks associated with intermittent fasting and eating disorders, consult resources from The Emily Program, which specializes in treatment and education regarding eating disorders.

Fasting exemptions in religious contexts

• Sickness: For many religious fasts, including Ramadan, individuals who are ill are exempt from fasting until they recover.
• Travel: Those on a long or exhausting journey are often excused from fasting, with the expectation that they make up the missed days later.
• Pregnancy and menstruation: Both pregnant women and women who are menstruating are typically exempt to protect their health and that of their baby.
• Old age: Elderly people for whom fasting would be a severe hardship are excused and may offer an alternative form of charity.

Who else should not fast?

• Underweight individuals: Those who are malnourished or underweight can suffer serious health consequences from further calorie restriction.
• People with severe anemia: Fasting can exacerbate fatigue and weakness in individuals with severe anemia.
• Cancer patients: Individuals undergoing chemotherapy or radiation require consistent nutrition and hydration, making fasting dangerous.

The importance of a balanced perspective

It's important to approach the topic of fasting with a balanced perspective. While studies highlight potential benefits for some populations, particularly in controlled settings, the very real risks for others cannot be ignored. The current body of research on long-term fasting in humans is limited, and many studies are short in duration or conducted on animals. This emphasizes the need for caution and medical supervision, especially for anyone with a pre-existing health concern. The focus should always be on sustainable, healthy eating patterns rather than restrictive practices that may lead to harm.

Considerations beyond physical health

Beyond the physical risks, fasting can have a profound impact on mental and emotional well-being. The obsession with a restricted eating window can lead to anxiety and a preoccupation with food. For many, ignoring the body's natural hunger cues is an unsustainable and potentially damaging practice. Food is a social and cultural cornerstone, and fasting can lead to social isolation when individuals feel unable to participate in communal meals.

Conclusion: Your Health First

In conclusion, the decision to fast should be made with careful consideration of one's individual health status and potential risks. For many people, especially those with underlying medical conditions, a history of disordered eating, or those who are pregnant or breastfeeding, the risks associated with fasting far outweigh any perceived benefits. A balanced, healthy diet remains the safest and most effective strategy for promoting long-term health and well-being. Always consult a medical professional before starting a fasting regimen to ensure it is the right and safe choice for you.

Sources:

• The Emily Program. "Is Intermittent Fasting Dangerous?" The Emily Program Blog, June 25, 2024. https://emilyprogram.com/blog/is-intermittent-fasting-dangerous/
• Dalili Medical. "Diseases that prevent fasting in Ramadan." Dalili Medical, March 9, 2025. https://www.dalilimedical.com/en/article-3686/Diseases-that-prevent-fasting-in-Ramadan
• ABC News. "Is intermittent fasting right for you? Who should avoid it?" ABC News, August 3, 2021. https://www.abc.net.au/news/2021-08-04/intermittent-fasting-who-is-it-right-for/100337294
• The Daily Independent. "Heart, Diabetes, and Blood Pressure Patients Should Consult Doctors Before Fasting: Dr. Abdul Hameed Ghori." The Daily Independent, February 27, 2025. https://www.dailyindependent.com.pk/2025/02/27/heart-diabetes-and-blood-pressure-patients-should-consult-doctors-before-fasting-dr-abdul-hameed-ghori/
• Cleveland Clinic. "Intermittent Fasting Explained: Benefits and How To Do It Safely." Cleveland Clinic Health Essentials, August 9, 2024. https://health.clevelandclinic.org/intermittent-fasting-4-different-types-explained
• Johns Hopkins Medicine. "Intermittent Fasting: What is it, and how does it work?" Johns Hopkins Medicine, Undated. https://www.hopkinsmedicine.org/health/wellness-and-prevention/intermittent-fasting-what-is-it-and-how-does-it-work
• Islamic Relief. "What are Missed Fasts in Islam and Who Has to Make Them Up?" Islamic Relief Worldwide, Undated. https://islamic-relief.org/what-are-missed-fasts-in-islam-and-who-has-to-make-them-up/
• Ministry of Health (Israel). "Taking Medicines During a Fast." Gov.il, February 6, 2023. https://www.gov.il/en/pages/medications-while-fasting

Additional citations

• Zero Longevity Science. "Should You Avoid Fasting?" Zero Longevity Science, April 12, 2019. https://zerolongevity.com/blog/should-you-avoid-fasting/
• EMC Healthcare. "Is Fasting Beneficial or Risky for the Kidneys?" EMC Healthcare, March 11, 2025. https://www.emc.id/en/care-plus/is-fasting-beneficial-or-risky-for-the-kidneys
• NIH. "Intermittent Fasting: Benefits, Side Effects, Quality of Life, and..." PubMed Central, February 7, 2023. https://pmc.ncbi.nlm.nih.gov/articles/PMC9998115/
• Healthline. "8 Health Benefits of Fasting, Backed by Science." Healthline, April 23, 2025. https://www.healthline.com/nutrition/fasting-benefits
• NIH. "Muslim patients in Ramadan: A review for primary care physicians." PubMed Central, July 24, 2017. https://pmc.ncbi.nlm.nih.gov/articles/PMC5525471/
• Sahyadri Hospital. "Who Should Not Fast During Ramadan?" Sahyadri Hospital Blog, March 9, 2025. https://sahyadrihospital.com/blog/who-should-not-fast-during-ramadan
• Mayo Clinic Health System. "Intermittent fasting for weight loss." Mayo Clinic Health System, June 17, 2022. https://www.mayoclinichealthsystem.org/hometown-health/speaking-of-health/intermittent-fasting-fad-or-solution
• ZOE. "Is Fasting Good For You? Here's What The Science Says." ZOE, April 18, 2024. https://zoe.com/learn/is-fasting-good-for-you
• NIH News in Health. "To Fast or Not to Fast." NIH News in Health, December 2019. https://newsinhealth.nih.gov/2019/12/fast-or-not-fast
• Healthline. "48-Hour Fast: How-To, Benefits, and Downsides." Healthline, April 5, 2024. https://www.healthline.com/nutrition/48-hour-fasting

Frequently Asked Questions

Is it safe for diabetics to fast?

No, fasting is generally unsafe for individuals with diabetes, particularly those with type 1 or those on medication. It can cause dangerous swings in blood sugar levels, including severe hypoglycemia, and should only be considered with strict medical supervision.

Can fasting trigger an eating disorder?

Yes, for people with a history of disordered eating, fasting can be a significant trigger that promotes restrictive behaviors, creates an unhealthy relationship with food, and increases the risk of relapse.

What are the risks of fasting during pregnancy?

During pregnancy, fasting can lead to dehydration and malnutrition, which can compromise the baby's development and the mother's health. Pregnant women should not fast without a doctor's explicit approval.

Why is fasting dangerous for people with kidney problems?

For individuals with pre-existing kidney conditions, fasting increases the risk of dehydration and electrolyte imbalances. This can strain the kidneys and potentially worsen function or lead to kidney stones.

Can fasting cause dehydration?

Yes, especially during prolonged fasts or in hot environments, not consuming enough fluids can lead to severe dehydration. This can cause headaches, dizziness, and stress the body's organs.

Should children and teenagers fast?

No, fasting is not recommended for children and adolescents. Their bodies are still developing and require consistent energy and nutrients for proper growth, which fasting can disrupt.

Does fasting interfere with medication?

Yes, many medications require a steady food intake to work effectively and safely. People on blood pressure, diabetes, or other critical medications should not fast unless explicitly advised and monitored by a doctor.

What are some common side effects of fasting?

Common side effects include headaches, fatigue, irritability, dizziness, and digestive issues like constipation. These symptoms often occur as the body adjusts to the caloric restriction.