Extended Fasting: 36-Hour to 7-Day Fasts

What Counts as Extended Fasting

Definitions vary, but a workable scale:

• 16–24 hours: Daily intermittent fasting. Not extended.
• 24–36 hours: Long intermittent fast (for example, Eat Stop Eat or a 36-hour fast).
• 36–72 hours: Extended fasting. Larger metabolic changes; requires preparation, electrolyte attention and a refeeding plan. Check with your healthcare provider first.
• Beyond 72 hours: Prolonged fasting. Should only be done under medical supervision; refeeding risks increase.

The Approximate Timeline

The stages below are rough guides based on typical changes in blood sugar, insulin and ketones. Individual variation is large and depends on your last meal, activity level, body composition and diet (people who eat low-carb may reach ketosis sooner). No timer can tell you exactly where your body is.

0–8 hours: Postprandial

Insulin is still raised from your last meal, and the body mainly uses glucose from that meal.

8–12 hours: Drawing on glycogen

Insulin falls. The liver breaks down glycogen (stored glucose) to keep blood sugar steady, and fat use begins to increase.

12–18 hours: Glycogen depletion

Liver glycogen runs lower and fat burning increases. Ketone levels start to rise slightly. Waves of hunger are common, often around usual mealtimes.

18–24 hours: Moving toward ketosis

Ketone levels continue to rise, and many people reach mild ketosis somewhere around this point or later. The liver increasingly makes new glucose from amino acids and glycerol (gluconeogenesis) to supply tissues that need it.

24–36 hours: Mostly fat-fueled

Ketones keep rising and growth hormone increases. Many people report that hunger becomes less intense after the first day, though this varies. The kidneys excrete more sodium and water, so fluid and electrolyte intake becomes more important; light-headedness on standing is common.

36–48 hours: Established ketosis

The body is now running mainly on fat and ketones. Some people feel stable energy; others feel tired, cold or irritable. Sleep often becomes lighter.

48–72 hours: Prolonged ketosis

Ketone levels are higher still, and growth hormone remains elevated. Autophagy is often mentioned at this stage, but it cannot be measured at home and its timing in humans is not well established (see autophagy and fasting). Many people hit a difficult patch in this window, with fatigue, headaches or dizziness; these are often related to fluids and electrolytes, but can also be a sign to stop.

Beyond 72 hours

Risks keep increasing, including electrolyte disturbances and refeeding syndrome, and there is little human evidence of additional benefit. Some animal research has looked at effects on immune and stem cells, but these findings have not been shown to translate into health benefits in people. Fasts beyond 72 hours should only be done under medical supervision.

Before You Consider One

Extended fasting is far more demanding than daily intermittent fasting. At a minimum, anyone considering it should be:

• A healthy adult who has checked with their healthcare provider
• Comfortable with daily 16:8 or 18:6 for at least a few months, and with at least one 24-hour fast
• Not taking medications that need to be taken with food or that affect blood sugar, blood pressure or fluid balance
• Free of current or past disordered eating
• Able to rest, avoid driving if unwell, and plan a careful refeeding day
• Clear about why they are doing it, rather than chasing a number or a streak

People give many reasons for extended fasts, such as metabolic health, autophagy, weight loss or discipline. The evidence that extended fasts offer benefits beyond those of shorter, safer fasting schedules or ordinary calorie reduction is limited. For most goals, a daily time-restricted schedule is a lower-risk option.

Who Should Not

Add to the standard fasting contraindications (pregnancy, breastfeeding, eating disorder history, underweight, children — see safety guide):

• Anyone with diabetes, particularly on insulin, sulfonylureas or SGLT2 inhibitors, unless supervised by their diabetes team
• Anyone with a history of severe hypoglycemia
• People with heart disease or heart rhythm problems, kidney disease, liver disease, or low blood pressure
• Anyone taking blood pressure medication, diuretics, lithium, or other medications affected by fluid and electrolyte changes, unless their prescriber agrees
• Older adults and people who are frail or have low muscle mass
• Anyone who has not built up to extended fasting through shorter protocols
• People with active gout (extended fasts can trigger flares)
• Anyone using extended fasting as compensation for binge eating — this is a sign to seek different support, not to fast longer

Preparing and Fasting Safely

Preparation (the 24 hours before)

• Eat normally — don’t pre-load. A massive last meal makes the early hours harder, not easier.
• Make the final meal a balanced one with protein and vegetables rather than a sugary or refined-carbohydrate meal.
• Hydrate well.
• Plan the refeeding meal in advance and buy the food beforehand.
• Choose a quieter period: avoid fasting before long drives, demanding physical work, or when you'll be alone and far from help.
• Tell someone you live with or see regularly that you are fasting.

During the fast

• Water: drink regularly through the day, typically around 2–3 liters, more in hot weather or if you sweat. Pale-yellow urine is a simple guide; avoid forcing large volumes.
• Plain coffee and tea are fine in moderation. Many people find caffeine hits harder while fasting.
• Electrolytes become important after the first day. See the next section.
• Light movement such as walking is fine. Skip heavy or intense training.
• Get up slowly from sitting or lying down to reduce dizziness.
• Track basic markers: how you feel, energy on a 1–10 scale, and blood pressure and heart rate if you have a home monitor.

When to end the fast early

• Palpitations, a racing or irregular heartbeat, or chest pain (seek medical help)
• Fainting, severe dizziness or confusion (seek medical help)
• Vomiting, or being unable to keep fluids down
• Muscle cramps, weakness or headaches that don’t settle with fluids and electrolytes
• Low blood pressure readings or dizziness every time you stand
• Feeling unwell in any way that worries you
• Any of the warning signs in our safety guide

Ending early is not failure. Stopping because you noticed warning signs is the right decision.

Electrolyte Management

During a fast, the kidneys lose more sodium and water than usual, and you are not getting electrolytes from food. Low sodium intake is a common reason people feel dizzy, weak or headachy on longer fasts. The amounts below are commonly used by healthy adults during extended fasts; they are general information, not a prescription:

• Sodium: Many people use roughly 3–5 g of sodium per day, spread across the day (about 1¼ to 2 teaspoons of table salt dissolved in water or broth).
• Potassium: Some people add a small amount of potassium, for example from a potassium chloride salt substitute. Potassium supplements can be dangerous for people with kidney disease or those taking certain blood pressure medications (such as ACE inhibitors, ARBs or potassium-sparing diuretics). Do not take them without your healthcare provider's agreement, and never take large single doses.
• Magnesium: Some people take a magnesium supplement. The recommended upper limit for supplemental magnesium in adults is 350 mg per day; higher amounts often cause diarrhea. People with kidney disease should not take magnesium without medical advice.

Practical approach: sip salted water or plain broth through the day rather than taking salt all at once. If you have high blood pressure, heart failure or kidney disease, do not increase salt without speaking to your healthcare provider.

For more detail, see the electrolyte guide.

The Refeeding Plan

How you end an extended fast matters. Eating too much too quickly can cause stomach upset and, less commonly, refeeding syndrome, a serious condition caused by rapid shifts in electrolytes. The risk is low for healthy, well-nourished adults after short fasts, but it rises with longer fasts, low body weight, and in people who were eating little before the fast. For fasts of up to about 72 hours in healthy adults, the gradual approach below is a sensible default.

The first meal

• Small. Roughly a third of your normal meal size.
• Easy to digest: broth, eggs, avocado, cooked vegetables, or a small amount of fish or chicken.
• Avoid large amounts of raw vegetables, high-fiber foods, sugary foods and alcohol at first.
• Eat slowly. Stop before you feel full.
• Wait 2–4 hours before the next meal.

The first 24 hours after the fast

• 3–4 small meals rather than 1–2 large ones.
• Protein at each meal.
• Keep eating a normal amount of salt; refeeding moves electrolytes into cells and can lower blood levels.
• Reintroduce carbohydrates gradually, particularly after fasts of 3 days or more.
• Seek medical help if you develop swelling, breathlessness, palpitations, confusion or severe weakness after refeeding.

For fasts longer than 72 hours

Fasts of this length should be medically supervised, and refeeding should be planned with that team. In hospital settings, people at high risk of refeeding syndrome are restarted on food slowly over several days, with blood tests to monitor electrolytes such as phosphate, potassium and magnesium.

For a deeper guide on the first meal, see how to break a fast properly.

How Often Is Reasonable

There is no research establishing a safe frequency for extended fasts, so a conservative approach makes sense:

• 36–48 hour fasts: Occasional rather than routine, with enough normal eating in between to recover fully.
• 72-hour fasts: Infrequent, and only after discussing with your healthcare provider.
• Longer than 72 hours: Only under medical supervision.

Frequent extended fasting brings increasing costs, such as muscle loss, possible effects on bone and menstrual cycles, and an unhealthy relationship with food. The fast should serve a goal, not become one.

Real Risks to Know About

Refeeding syndrome

Uncommon after short fasts in healthy, well-nourished adults, but the risk increases with longer fasts and in people who are underweight or were eating little beforehand. When food, especially carbohydrate, is reintroduced, phosphate, potassium and magnesium move rapidly into cells and blood levels can fall to dangerous levels, causing heart rhythm problems, breathing difficulty or seizures. Gradual refeeding reduces the risk; longer fasts need medical supervision.

Low blood sugar (hypoglycemia)

Uncommon in healthy adults because the liver maintains blood glucose, but a real danger for people on insulin or sulfonylureas, who should not fast without supervision from their diabetes team.

Postural hypotension

Blood pressure can fall, especially on standing, causing dizziness or fainting. Stand up slowly and keep up fluids and salt. People on blood pressure medication are at higher risk; never change medication without your prescriber's input.

Gout flares

Uric acid levels rise during fasting, and people with a history of gout can have flares.

Gallstone-related events

Long gaps between meals and rapid weight loss are associated with gallstone problems in susceptible people. Pain in the upper right abdomen, especially after the refeeding meal, needs medical evaluation.

Disordered eating triggering

For anyone with prior disordered eating, extended fasts can re-activate restriction patterns. The protocol can feel rewarding in ways that are not actually serving health. Watch for signs that fasting is becoming a way to control or punish rather than a tool.

Cumulative effects

For a healthy adult, the risks of a single, well-managed 48-hour fast are generally low. Repeating extended fasts often, over long periods, has not been well studied. If you fast this way repeatedly, review your health and blood tests with your healthcare provider, and pay attention to your relationship with food.

Frequently Asked Questions