Sonam Wangchuk's 16-day fast: What happens to the human body at every stage

Sonam Wangchuk's hunger strike entered its 16th day, raising fresh concerns about his health. Reports indicate he has lost significant weight and his blood sugar has dropped, highlighting the physiological toll that prolonged fasting can take on the human body.

While every person's response to fasting depends on factors such as body composition, hydration, electrolyte intake and underlying health conditions, decades of research on human metabolism provide a clear understanding of how the body adapts when food is unavailable.

FIRST 24 HOURS: THE BODY USES STORED CARBOHYDRATES

During the first day without food, the body continues to rely primarily on glucose circulating in the bloodstream and glycogen, the stored form of carbohydrate found in the liver and muscles.

As blood glucose begins to fall:

• Insulin levels decrease.
• Glucagon levels increase.
• The liver breaks down glycogen to maintain normal blood sugar.

This phase is known as the post-absorptive state. Most healthy adults have enough liver glycogen to supply glucose for about 12–24 hours, although the exact duration varies.

Common symptoms may include hunger, headache, irritability and fatigue.

Dr Amit Prakash Singh, Consultant – Internal Medicine at CK Birla Hospital, Delhi, says the body initially uses glucose from the last meal before drawing on glycogen stored in the liver to maintain blood sugar. "Water and sodium are lost rapidly during this phase, leading to an early drop in body weight. Hunger is usually at its peak, and many people experience headache, fatigue, irritability and increased thirst," he explains.

DAYS 2–3: THE BODY SWITCHES TO FAT AS ITS MAIN FUEL

Once liver glycogen becomes largely depleted, the body undergoes a major metabolic shift.

Fat stored in adipose tissue is broken down into fatty acids, while the liver converts some of these fatty acids into ketone bodies. Ketones gradually become an important fuel for the brain, reducing its dependence on glucose.

During this stage:

• Fat breakdown (lipolysis) increases significantly.
• Ketone production rises.
• Some muscle protein is broken down to produce glucose for tissues that cannot use ketones, such as red blood cells.

Rapid weight loss during these days is common, but much of it reflects the loss of water associated with depleted glycogen stores rather than body fat alone.

According to Dr Singh, once glycogen stores are exhausted, the body begins producing glucose from amino acids and glycerol while rapidly increasing fat breakdown. "Muscle protein is temporarily broken down to supply amino acids, causing weakness, dizziness, reduced exercise capacity and the characteristic fruity odour of ketones on the breath," he says.

DAYS 4–8: KETOSIS BECOMES THE DOMINANT METABOLIC STATE

By the end of the first week, the body is largely adapted to nutritional ketosis.

Most organs are now obtaining a significant portion of their energy from fat and ketones. The brain also derives much of its energy from ketones, reducing, but not eliminating, the need to break down muscle protein.

Physiological changes include:

• Continued fat oxidation.
• Lower insulin levels.
• Reduced glucose requirements.
• Increased dependence on ketones for brain function.

Many people also experience lower blood pressure during this period because fasting reduces sodium retention and body water.

Dr Singh says this is the stage where fat becomes the body's primary fuel. "The brain increasingly relies on ketones, reducing the need for glucose and slowing muscle breakdown. Hunger often decreases noticeably, but the metabolic rate falls to conserve energy. Blood pressure and heart rate may decrease, and people commonly experience fatigue, cold intolerance, constipation and reduced physical strength," he explains.

DAYS 8–12: THE BODY CONSERVES ENERGY

As fasting continues, the body attempts to preserve its remaining energy stores.

The metabolic rate gradually decreases, helping reduce overall energy expenditure. Fat remains the primary fuel source, but muscle protein continues to be broken down at a slower rate than during the early stages of fasting.

Doctors are also becoming increasingly concerned about:

• Sodium depletion
• Potassium deficiency
• Magnesium deficiency
• Dehydration
• Low blood pressure

These electrolyte disturbances can affect muscle function, nerve signalling and heart rhythm.

Dr Singh notes that although fat continues to provide most of the body's energy, gradual muscle loss persists. "The immune system becomes less efficient, wound healing slows, and deficiencies of potassium, magnesium and phosphate may develop if they are not replaced. Standing may cause dizziness because of low blood pressure, and physical endurance declines further," he says.

DAY 12 ONWARD: THE RISK OF MEDICAL COMPLICATIONS INCREASES

Beyond two weeks without adequate nutrition, prolonged fasting carries substantially greater health risks.

Although the body continues to rely heavily on stored fat, muscle protein breakdown persists because certain tissues still require glucose.

Potential complications include:

• Progressive muscle loss
• Electrolyte imbalances
• Low blood sugar (hypoglycaemia)
• Dehydration
• Low blood pressure
• Abnormal heart rhythms (arrhythmias)
• Fainting due to reduced blood flow to the brain

Doctors also monitor closely for refeeding syndrome, a potentially life-threatening condition that can occur when nutrition is restarted too quickly after prolonged starvation. During refeeding, rapid shifts in electrolytes, particularly phosphate, potassium and magnesium, can lead to serious cardiac and neurological complications.

"After nearly two weeks of fasting, prolonged starvation begins to affect multiple organs," says Dr Singh. "Muscle wasting becomes more pronounced, including loss of heart and respiratory muscle. Dehydration, electrolyte disturbances, low blood pressure, kidney injury, infections and potentially life-threatening cardiac rhythm abnormalities are becoming increasingly likely."

He adds that reintroducing food after such a prolonged fast must be done carefully. "Rapid feeding can trigger refeeding syndrome, a dangerous condition caused by sudden shifts in electrolytes that may lead to heart failure, respiratory failure or fatal arrhythmias. Nutrition should be restarted gradually under medical supervision," he cautions.

WHY DOCTORS CLOSELY MONITOR PROLONGED FASTS

Medical experts distinguish prolonged fasting from intermittent fasting or short-term fasts lasting 24–72 hours.

Extended fasts require careful medical supervision because prolonged calorie deprivation can lead to serious metabolic and cardiovascular complications. Regular monitoring of body weight, blood pressure, blood glucose and electrolyte levels is considered essential during prolonged therapeutic or protest fasts.

While the human body is remarkably capable of adapting to periods without food by switching from carbohydrates to fat and ketones, these adaptations are survival mechanisms, not indications that the body can safely go without nutrition indefinitely.

As fasting extends beyond two weeks, the risk of complications increases significantly, which is why healthcare professionals closely monitor individuals undertaking prolonged fasts.