Is a 36-hour fast safe for your health? Doctors reveal metabolic changes, electrolyte dangers and who should avoid it

A 36-hour fast has become a widely discussed practice among fitness enthusiasts who want rapid improvements in their health and metabolic wellbeing. Despite its rising popularity, Dr Sudhir Kumar, a senior neurologist at Apollo Hospitals in Hyderabad, cautions that this extended form of intermittent fasting is not appropriate for everyone. In a recent X post, he explained that although some healthy adults may tolerate such long fasting windows, it should not be viewed as a universally safe method. His post focuses on potential benefits, overlooked risks and the essential precautions that anyone considering this form of fasting must keep in mind.

During a typical 36-hour fast, individuals consume only water, plain tea or black coffee without sweeteners. Dr Kumar described this approach as a longer version of intermittent fasting, where the body goes without solid food for an extended period. He noted that while some people feel mentally sharper during the fast, others may struggle depending on their daily routine, underlying health conditions and activity levels.

How a 36-hour fast boosts insulin sensitivity and mental clarity

Dr Kumar explained that certain healthy individuals may experience positive effects when practising a 36-hour fast. He highlighted that extended fasting periods can improve insulin sensitivity and help regulate calorie intake. Some degree of autophagy, the body’s natural process of removing damaged cells, may also be activated, although he emphasised that evidence in humans remains limited and should be interpreted cautiously.

Many people report clearer focus and sharper concentration during the fasting period. Dr Kumar mentioned that this is likely due to the body shifting into fat-burning mode and producing ketones, which can act as an alternative energy source for the brain. He also observed that such fasts may help individuals recognise their true hunger cues and reduce unnecessary snacking, contributing to improved eating habits in the long term.

Why long fasts can pose serious health risks

Although online discussions often highlight the supposed advantages, Dr Kumar warned that extended fasts also carry important health risks. He emphasised that drinking only water for long periods can lead to dehydration and imbalances in electrolytes because essential minerals such as sodium and potassium are not replenished, potentially triggering weakness and dizziness in vulnerable individuals.

He also noted that low blood sugar may occur, causing symptoms such as dizziness, irritability and headaches. This risk increases in individuals who continue heavy physical activity during the fast. Elevated cortisol levels may disrupt sleep, leading to restlessness at night. Dr Kumar further cautioned that frequent long fasts may contribute to muscle loss, especially when regular eating days do not provide sufficient protein. Digestive issues such as acidity or gastritis may also worsen when tea or coffee are consumed on an empty stomach.

People who should avoid a 36-hour fast

According to Dr Kumar, extended fasting is unsuitable for certain groups. Individuals with diabetes face a higher risk of hypoglycaemia, while people with hypertension who take diuretics may experience dangerous fluid and salt imbalances. Those with very low BMI or individuals dealing with eating disorders or food-related anxiety should also avoid this practice. Pregnant and breastfeeding women are advised not to attempt such prolonged fasts due to increased nutritional demands.

He also clarified that the idea of fasting as a detox solution is misleading. The liver and kidneys constantly filter waste products and maintain internal balance, so the body does not require fasting to remove toxins in the way many online trends claim.

Key precautions for anyone attempting a 36-hour fast

For healthy adults who still wish to experiment with this form of fasting, Dr Kumar outlined several essential precautions. He recommended choosing electrolyte-rich water instead of plain water to maintain mineral levels throughout the fasting period. Intense workouts should be avoided, as they increase the risk of fatigue, low blood sugar and dehydration. Breaking the fast should be done slowly, beginning with light foods rich in protein to minimise digestive discomfort. He also advised that long fasts should not be repeated frequently, and for most healthy individuals, once a month is generally considered the upper safe limit.




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