Understanding Fasting: What Days Shouldn't You Fast? And Who Should Avoid It?

January 14, 2026 •

4 min read

According to a 2023 review, while intermittent fasting has been linked to numerous health benefits, it is not appropriate or safe for everyone, and certain medical conditions and life stages pose significant contraindications. Understanding these risks is crucial for anyone considering a fasting regimen.

Quick Summary

This guide details the critical factors and circumstances that make fasting unsafe. It covers specific medical conditions, vulnerable demographics, and situational risks that should prevent individuals from fasting without professional medical guidance. Critical indicators for breaking a fast are also outlined to ensure safety.

Key Points

Diabetics are at high risk: People with Type 1 diabetes and poorly controlled Type 2 diabetes face severe risks of hypoglycemia and ketoacidosis from fasting.
Certain medical conditions are contraindications: Chronic kidney disease, liver disease, active ulcers, and cancer treatment are serious conditions that make fasting unsafe.
Pregnant and breastfeeding women should avoid fasting: The increased caloric and nutrient needs during pregnancy and lactation make fasting risky for both mother and baby.
Fasting is not for children, teens, or the elderly: These demographics have critical nutritional needs for development and frailty, respectively, making fasting unsuitable.
Avoid fasting with a history of eating disorders: Fasting can act as a trigger, potentially leading to a relapse or worsening of disordered eating patterns.
Listen to your body and medical guidance: If you feel unwell, dizzy, or experience signs of dehydration, break your fast immediately. Always consult a healthcare professional before starting.

While fasting has become a popular topic in the wellness space, promoted for potential benefits like weight management and metabolic health, it's essential to recognize that it is not a one-size-fits-all approach. For certain individuals and in specific situations, fasting can be dangerous and detrimental to health. A blanket recommendation ignores the nuances of individual health and the real risks posed to vulnerable populations. A foundational principle of safe fasting is to always consult a healthcare provider before making significant dietary changes, especially if underlying health issues are present.

Medical and Chronic Health Conditions

People with pre-existing medical conditions should approach fasting with extreme caution or avoid it entirely, as it can worsen symptoms and cause complications.

Diabetes and Blood Sugar Concerns

Fasting is particularly risky for individuals with diabetes. Those with Type 1 diabetes, or those with poorly controlled Type 2 diabetes, face a heightened risk of blood sugar fluctuations. This can lead to:

Hypoglycemia (low blood sugar): A dangerous drop in blood sugar levels, especially for those on insulin or certain medications.
Diabetic Ketoacidosis: A life-threatening condition where the body produces excessive acid (ketones) due to insufficient insulin.

Kidney and Liver Health

Dehydration is a significant risk with fasting, which can be particularly damaging for those with chronic kidney disease. Individuals on dialysis or with pre-existing kidney infections are strongly advised against fasting. Similarly, patients with liver cirrhosis or other liver diseases may find fasting exacerbates their condition.

Cardiovascular and Digestive Issues

For those with heart conditions or high blood pressure, fasting, especially if it leads to dehydration, can alter blood pressure levels and heart function. Individuals with active peptic ulcers or severe acid reflux may find fasting irritates their digestive system and worsens symptoms.

Cancer Treatment

Cancer patients undergoing chemotherapy or radiation require consistent nutrition to maintain strength and muscle mass. Fasting can deplete their energy reserves and should be avoided unless specifically advised by their oncology team.

Vulnerable Population Groups

Certain demographic groups have specific nutritional requirements that make fasting unsuitable for their stage of life or health status.

Fasting During Pregnancy and Lactation

Pregnant women have increased caloric needs to support the health and development of their baby. Fasting can increase the risk of preterm labor and result in lower birth weight. For breastfeeding mothers, fasting can decrease milk supply and affect the baby's nutrition. In the early months of exclusive breastfeeding, fasting is not recommended at all.

Fasting for Youth and Elderly

Children and teenagers are in critical developmental stages and require a consistent supply of nutrients for proper growth and development. Fasting is not advised for anyone under the age of 18. Conversely, older adults, especially those who are frail or have chronic illnesses, are at a higher risk of complications from fasting, including falls, fractures, and electrolyte imbalances.

Psychological Health and Eating Disorders

For individuals with a history of or current eating disorders, such as anorexia, bulimia, or binge eating disorder, fasting is highly inadvisable. It can trigger or exacerbate disordered eating patterns and create a cycle of restriction and bingeing. The psychological stress and obsession with food that can accompany fasting can be a major setback in recovery.

Situational and Timing-Based Considerations

Even for generally healthy people, there are specific situations where fasting should be avoided.

When You Are Feeling Ill

If you have an active infection, fever, or are otherwise unwell, fasting can hinder your body's ability to fight the illness. The body needs energy to fuel its immune response. Signs like dizziness, extreme weakness, or signs of dehydration (dark urine, dry mouth) are clear indicators to break a fast.

Interaction with Medications

Many medications must be taken with food to be absorbed correctly or to prevent stomach upset. If your medication schedule requires regular meals, fasting is not an option. Always consult a pharmacist or doctor to understand medication requirements.

High Stress or Intense Activity

During periods of high stress, intense exercise, or significant travel that disrupts your circadian rhythm, fasting can put additional strain on the body. It can increase the risk of hypoglycemia and affect fluid balance, making it best to postpone a fast.

Fasting Risk Comparison Table


Factor	High-Risk Group	Caution Advised	Low-Risk Group (Consult Doctor First)
Medical Condition	Type 1 Diabetes, Active Cancer, Chronic Kidney/Liver Disease, Active Peptic Ulcers	Well-controlled Type 2 Diabetes, High Blood Pressure (with medical supervision)	Generally healthy adults with no chronic conditions
Life Stage	Children, Teenagers, Frail Elderly	Pregnant or Breastfeeding women (depending on stage and health)	Healthy adults (with doctor's approval)
Psychological Health	Active Eating Disorder, History of Disordered Eating	Prone to anxiety or obsessive behavior around food	Healthy individuals with a stable relationship with food
Situational	Active Illness, Dehydration, Medications needing food	High Stress, Intense Exercise, Long-distance Travel	Stable, low-stress environment and health

Conclusion

While fasting can offer therapeutic benefits for some, it is not a universally safe practice. The decision to fast should be an informed one, made in close consultation with a healthcare provider, especially if you fall into any of the high-risk categories. Situational factors like illness, stress, or medication schedules must also be considered. Prioritizing safety over a popular trend is the most responsible approach to your health. Remember, a diet that supports your overall well-being is always better than one that puts you at risk.

For more information on safe and healthy eating patterns, consider exploring resources from reputable health organizations.

Frequently Asked Questions

It is not recommended to fast if you have poorly controlled Type 2 diabetes or take specific medications that regulate blood sugar, as it poses a high risk of hypoglycemia. If your condition is well-managed, you may be able to fast, but only with strict medical supervision and guidance from your doctor.

No, fasting is not recommended during pregnancy. Pregnant women have increased caloric and nutrient needs to support the baby's health. Fasting can increase risks such as preterm labor and low birth weight, and it's essential to prioritize consistent nutrition for both the mother and developing fetus.

Individuals with a history of or current eating disorders should not fast. Fasting can be a significant trigger that exacerbates disordered eating patterns, potentially leading to relapse and negative psychological effects.

You should break your fast immediately if you feel unwell. Signs such as severe dizziness, extreme weakness, or symptoms of low blood sugar or dehydration indicate that fasting is harming your body. Prioritize your health and seek medical advice.

Fasting is generally not recommended for older adults, especially those who are frail or have chronic health conditions. They are more susceptible to complications like dehydration, falls, and electrolyte imbalances. Any fasting attempt should be closely monitored by a healthcare provider.

Many medications must be taken with food for proper absorption and to prevent side effects. If you are on any regular medication, particularly those requiring mealtime intake, you should not fast without explicit approval and a revised schedule from your doctor.

Yes, even healthy individuals should avoid fasting during periods of illness, intense stress, or extreme exercise. These situations put extra strain on the body, making it more vulnerable to issues like dehydration and hypoglycemia.