Water fasting, the practice of consuming only water for several days, is gaining popularity as a quick method for weight loss, detoxification, and metabolic reset. But what truly occurs inside your body after enduring 72 hours without any food? According to medical experts, the changes are profound, stressful, and often not as beneficial as claimed.
The Body Enters Survival Mode: Metabolic Shift at 72 Hours
By the time you complete three days of a water-only fast, your body has exhausted its most accessible energy reserves. Dr Sunil Rana, Associate Director and Head of Internal Medicine at Asian Hospital, explains that the liver's glycogen stores are typically depleted within the first 24 hours. This leads to a steady decline in blood glucose levels.
In response, the body releases counter-regulatory hormones like glucagon, cortisol, and growth hormone. These hormones initiate the breakdown of fat stores. "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," Dr Rana adds.
While blood sugar may stabilise at a lower level in healthy individuals, the body is under significant metabolic strain. Ketosis becomes more pronounced at the 72-hour mark, but the adaptation process is physiologically stressful, especially for those with low nutritional reserves.
Examining the Claims: Benefits vs. Documented Risks
Advocates for prolonged fasting often cite benefits like cellular repair (autophagy), detoxification, and metabolic reset. However, Dr Rana urges caution. He points out that while some scientific evidence from animal studies suggests short-term fasting can activate autophagy, robust data from human studies on prolonged water-only fasting is lacking.
"In humans, the extent to which autophagy is meaningfully activated within 72 hours—and whether it translates into long-term health benefits—remains unclear," he states. Any perceived improvement in metabolic flexibility is usually temporary and reverses once normal eating patterns resume.
Conversely, the risks are well-documented and immediate:
- Hypoglycaemia (low blood sugar)
- Dehydration and electrolyte imbalances
- Dizziness and low blood pressure
- Headaches, irritability, and poor concentration
"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," Dr Rana concludes.
Organ Strain, Muscle Loss, and Who Must Avoid It
After 72 hours without food, the strain on vital systems intensifies. Fluid and sodium loss force the kidneys to conserve water, yet dehydration remains a real threat. Electrolyte imbalances—particularly in sodium, potassium, and magnesium—can disrupt heart rhythm, nerve function, and muscle activity.
Muscle breakdown also begins early in the fast. "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," explains Dr Rana. The loss of skeletal muscle mass after three days can be clinically significant.
Dr Rana strongly advises that specific groups should never attempt a prolonged water fast:
- People with diabetes (especially on insulin or sulfonylureas), chronic kidney disease, or heart disease.
- Individuals with a history of gout, eating disorders, or those who are underweight.
- Pregnant or breastfeeding women, older adults, and those on medications for blood pressure or blood thinning.
For those seeking metabolic or weight-related benefits, safer, evidence-backed alternatives exist. Time-restricted eating, such as a 14–16 hour daily fasting window, offers many metabolic advantages with far fewer risks.
"Sustainable metabolic health comes from gradual dietary changes, regular physical activity, adequate sleep, and consistency—not extreme fasting," Dr Rana emphasises. Always consult a healthcare practitioner before making any drastic changes to your diet or fasting routine.
