The 5 day fast explained

The 5 day fast: what it does and what it costs

The 5 day fast represents a significant escalation from intermediate fasting. Four points explain the physiology and the real risks.

1. The physiological state at 5 days

By 5 days of fasting the body is in deep nutritional ketosis with blood ketones reaching 2 to 5 mmol/L. Brain energy comes 60 to 70 percent from ketones with the remainder from glucose produced via gluconeogenesis. Growth hormone is substantially elevated. T3 thyroid hormone has fallen meaningfully (15 to 30 percent reduction). Insulin is at minimum sustained level. Significant autophagy upregulation occurs in animal models with uncertain magnitudes in humans. Muscle protein breakdown for gluconeogenesis is suppressed to preserve lean mass. The body is in classic prolonged fasting metabolism.

2. The Valter Longo Fasting Mimicking Diet alternative

Valter Longo developed the Fasting Mimicking Diet as a modified 5 day protocol that produces some fasting metabolic effects with reduced medical risk. The protocol provides specific plant-based foods totalling around 800 kcal on day 1 and 500 kcal on days 2 to 5. Total of around 2900 kcal over 5 days from specific compositions of fats, proteins and carbohydrates. The ProLon commercial product is one branded version. Trials suggest periodic FMD (a few times per year) may produce cardiometabolic benefits with better safety than water fasting. Most published 5 day fasting research uses FMD not water fasting.

3. The real medical risks of water fasting for 5 days

The British Dietetic Association does not recommend self-directed extended water fasting. Specific risks include refeeding syndrome on completion (covered by NICE Clinical Guideline 32 - electrolyte shifts can cause respiratory failure or cardiac arrhythmia), severe electrolyte imbalance during the fast itself, gallstone formation (1992 Festi study documented elevated risk with very low calorie protocols), gout flares (uric acid rises during ketosis), hypoglycaemia, cardiac arrhythmia in susceptible individuals, significant lean mass loss, severe dehydration and rarely death. These risks are not theoretical: 5 day fasts have caused medical emergencies and deaths in unsupervised settings.

4. The weight loss reality

The dramatic scale numbers during a 5 day fast are misleading. Typical 3 to 7 kg loss is mostly water (glycogen-bound water releases when glycogen depletes), gastrointestinal content and modest fat loss (around 1 to 1.5 kg actual body fat). Most of the weight rebounds within days of refeeding. The 5 day fast does not produce more sustainable fat loss than equivalent caloric deficit through other approaches. Pursuing 5 day fasts primarily for weight loss is misguided.

How to approach a 5 day fast responsibly

Five steps if you are seriously considering a 5 day fast.

Consider whether you need to do this at all

Most fasting goals (weight loss, metabolic improvements) are better achieved through sustainable daily intermittent fasting. The 5 day fast is not a shortcut. The metabolic adaptations of extended fasting are not necessary for most health outcomes. Consider what you actually want and whether a 5 day fast is the right tool for it. For most people the answer is no.

Consider FMD rather than water fasting

The Fasting Mimicking Diet (ProLon or self-directed equivalent) provides much of the metabolic effects with substantially reduced medical risk. If you want the experience and effects of extended fasting, FMD is the safer evidence-based option. Self-directed water fasting for 5 days carries materially higher risk than FMD.

Get medical input before attempting

Discuss with your GP. Bloods (electrolytes, kidney function, glucose) before the fast. Confirmation you have no contraindications. Plan for monitoring during the fast. Plan for refeeding supervision. This is not optional for safe practice. Self-directed without medical input is not recommended.

Build up through shorter fasts first

Never attempt a 5 day fast without prior experience of 24, 36 and 48 hour fasts. Your body needs to know fasting metabolism and you need to know how you respond. First-time 5 day fasts attempted without preparation produce high rates of medical complications and abandonment.

Refeed properly

The refeeding period (2 to 3 days) is as important as the fast. Refeeding syndrome causes serious medical events including death. Start very small (bone broth, then small protein-vegetable meals), gradually increase over 2 to 3 days, avoid refined carbohydrates initially, supplement electrolytes if guided by medical advice. Skipping careful refeeding undermines the entire protocol.

Who should never attempt a 5 day fast

Absolute contraindications for 5 day water fasting include:

• History of eating disorders. Contraindicated.
• Pregnancy or breastfeeding. Contraindicated.
• Type 1 diabetes or insulin dependent type 2 diabetes. Risk of ketoacidosis and hypoglycaemia is significant. Contraindicated outside specialist medical settings.
• BMI under 20 or unintended weight loss. Refeeding syndrome risk is high.
• Children, adolescents or adults under 18. Contraindicated.
• Adults over 65 or with cardiovascular disease. Risk of cardiac arrhythmia, electrolyte imbalance, falls. Contraindicated outside specialist settings.
• History of gallstones or gout. Significant flare risk.
• Kidney or liver disease. Contraindicated.

For others 5 day fasting should be approached with medical input and ideally supervision. The British Dietetic Association does not recommend self-directed extended fasting.

More on extended fasting

Several pages cover extended fasting safety and physiology. Our piece on extended fasting and health risks explained covers the full risk picture. Refeeding after extended fasting covers the critical recovery period. And the 48 hour fast explained covers the more accessible step below.