This is what happens to your body after 3 days of water fasting

Water fasting, which involves drinking only water for several days, has recently emerged as a quick way to reset the metabolism, detoxify, and lose weight rapidly. But what really happens inside your body after 72 hours without food? Dr Sunil Rana, Associate Director and Head of Internal Medicine at Asian Hospital, notes that the changes are significant, stressful, and not always beneficial.

By the time a person reaches the 72-hour mark of water fasting, the body has exhausted its readily available fuel sources. “The liver’s glycogen stores are usually depleted within the first 24 hours, leading to a steady fall in blood glucose levels,” explains Dr Rana. To compensate, counter-regulatory hormones such as glucagon, cortisol, and growth hormone rise, triggering fat breakdown.

“The liver converts free fatty acids released from fat tissue into ketone bodies, which begin to supply energy to the brain and muscles—a state known as nutritional ketosis,” he adds.

While blood sugar levels may stabilise at a lower range in healthy individuals, the body is under metabolic strain. “Ketosis at 72 hours is more pronounced, but the body is still adapting, and this transition can be physiologically stressful—especially for people with low nutritional reserves,” he cautions.

Is water fasting healthy? (Photo: Freepik)

Proponents of prolonged fasting often claim benefits, including cellular repair, detoxification, and metabolic reset. But Dr Rana urges caution. “There is some scientific evidence that short-term fasting can activate autophagy, a cellular recycling process, but most robust data comes from animal studies or fasting-mimicking diets—not prolonged water-only fasting in humans,” he explains.

In humans, the extent to which autophagy is meaningfully activated within 72 hours—and whether it translates into long-term health benefits—remains unclear. Any improvement in metabolic flexibility is often temporary and usually reverses once normal eating resumes,” says Dr Rana.

On the other hand, the risks are well documented. Hypoglycaemia, dehydration, dizziness, electrolyte imbalance, and low blood pressure are common. “Without medical supervision, the physiological stress of a three-day water fast often outweighs its unproven benefits, making it an unreliable and potentially unsafe health practice,” he adds.

After three days without food, the strain on vital systems becomes more evident. Fluid and sodium loss lead the kidneys to conserve water, but dehydration can still develop. “Electrolyte imbalances—particularly low sodium, potassium, and magnesium—are among the most immediate concerns, as they can affect heart rhythm, nerve signalling, and muscle function,” Dr Rana explains.

Although the brain adapts to using ketones, many people experience headaches, irritability, and reduced concentration. Muscle breakdown also begins early. “When glucose is scarce, the body uses amino acids from muscle to maintain essential glucose production. While ketosis reduces this need somewhat, muscle loss still occurs,” says Dr Rana. Although vital muscles such as the heart and diaphragm are relatively well-preserved, skeletal muscle loss after 3 days can be clinically significant.

Dr Rana emphasises that water fasting is unsafe for several groups. “People with diabetes—especially those on insulin or sulfonylureas—chronic kidney disease, heart disease, gout, eating disorders, or those who are underweight should not attempt prolonged fasting,” he warns. Pregnant or breastfeeding women, older adults, and individuals on blood pressure or anticoagulant medications are also at increased risk, he adds.

For those seeking metabolic or weight-related benefits, safer, evidence-backed options exist. “Time-restricted eating, such as 14–16 hour daily fasting, offers many metabolic advantages with far fewer risks,” Dr Rana notes.

“Sustainable metabolic health comes from gradual dietary changes, regular physical activity, adequate sleep, and consistency—not extreme fasting,” he adds.

DISCLAIMER: This article is based on information from the public domain and/or the experts we spoke to. Always consult your health practitioner before starting any routine.