Short term vs long term fasting explained

Four key differences between short and long term fasting

Short and long term fasting are different practices with different goals, mechanisms and risks. Four key distinctions clarify the comparison.

1. Physiological depth: how deeply you enter fasting metabolism

Short term fasts (12 to 36 hours) capture the early fasting phase. Glycogen depletes through the first 12 to 24 hours. Fat oxidation rises. Insulin falls. Mild ketosis may appear toward the end of 24 to 36 hour fasts. Growth hormone rises modestly. Long term fasts (3 to 7 days) reach deeper metabolic states. Sustained ketosis develops with blood ketones reaching 2 to 5 mmol/L. Growth hormone rises substantially (5 to 10 fold by 48 hours). Autophagy activation appears more pronounced in animal studies. Brain shifts substantially to ketone fuel.

2. Sustainability: daily practice versus periodic events

Short term fasting is designed for daily or near-daily practice over months to years. The cumulative effect of consistent practice drives outcomes. Long term fasting is by nature episodic: a 5 day fast cannot be done daily. Extended fasts are events maybe undertaken a few times per year. Most fasting benefits in well-studied trials come from sustained short term protocols not periodic extended ones. The Valter Longo Fasting Mimicking Diet attempts to capture some extended fast benefits in a 5 day protocol done a few times per year.

3. Weight loss patterns: gradual versus dramatic-but-temporary

Short term fasting produces modest sustained weight loss over months. Daily 16:8 typically produces 1 to 5 kg over 12 to 16 weeks. Long term fasting produces dramatic short-term loss (4 to 8 kg in a 5 day fast) but most is water and glycogen which return within days of refeeding. True body fat loss from a single extended fast is modest (1 to 2 kg). The dramatic scale appearance after extended fasts is misleading. Sustainable weight loss happens through daily practice not periodic extreme events.

4. Medical risks: minimal versus significant

Short term fasting in healthy adults carries minimal medical risk. The contraindications apply (eating disorders, pregnancy, type 1 diabetes, BMI under 18.5, under 18s) but otherwise short term fasts can be self-directed safely. Extended fasting carries real medical risks including refeeding syndrome, electrolyte imbalance, gallstones, severe dehydration, gout flares, cardiac arrhythmia and others. The British Dietetic Association does not recommend self-directed extended fasting. Medical supervision is appropriate.

Which approach suits which goal

Different goals match different approaches. Five common goals and how to think about them.

For weight loss

Daily short term fasting wins. Sustained 16:8 or similar over months produces sustainable loss. Extended fasts can be a starting punctuation but the work happens through daily practice afterwards. Repeated extended fasting is not sustainable and not necessary for weight loss.

For metabolic health (insulin sensitivity, blood pressure, lipids)

Daily short term fasting. The 2018 Sutton early time-restricted eating trial and similar work documented metabolic benefits with daily practice. Early time-restricted eating (eating window in the morning portion of the day) may have specific advantages. Periodic extended fasting can add to daily practice but daily practice is the core.

For deeper metabolic states (sustained ketosis, growth hormone)

Extended fasting reaches these states. Whether the deeper states translate to better health outcomes is less clear. The Fasting Mimicking Diet (5 days a few times per year) attempts to capture these effects with reduced risk through partial caloric intake.

For autophagy

Theoretically extended fasting produces more autophagy than short fasting. Direct human evidence is limited. Animal evidence suggests autophagy continues rising through 48 to 72 hours of fasting. The clinical importance of greater autophagy is not established. Pursuing extended fasting purely for autophagy without other benefits is not well supported.

For longevity and disease prevention

The evidence is preliminary for both approaches. Animal data suggests both daily caloric restriction and intermittent fasting extend lifespan. Human data for longevity is weaker. Reasonable approach: daily intermittent fasting at sustainable intensity, possibly with annual or semi-annual supervised extended fast if you tolerate one and find value in it. Do not assume more is better.

Risk differences between short and long fasts

Different risks at different durations require different precautions.

• Short term fasting (under 36 hours) in healthy adults: standard contraindications apply but minimal acute medical risk. Self-directed practice generally safe.
• 24 to 48 hour fasts: mild electrolyte attention needed, hydration matters, otherwise low risk in healthy adults.
• 3 to 5 day fasts: real risks including electrolyte imbalance, gallstones, gout flares, hypoglycaemia in some individuals. Medical input recommended.
• 5 to 7 day fasts: significant medical risks. Refeeding syndrome risk on completion. Medical supervision strongly recommended.
• Over 7 day fasts: should not be self-directed. Supervised programmes only. Multiple serious risks accumulate.

Standard contraindications apply across all durations: eating disorder history, pregnancy or breastfeeding, type 1 diabetes or insulin dependent type 2 diabetes, BMI under 18.5, children, adolescents and adults under 18. Anyone on medications or with significant medical conditions should discuss any fasting plan with their GP first.

Drill into specific durations

Several pages cover specific fast lengths. Our piece on the 24 hour fast explained covers the entry point to longer fasting. The 48 hour fast explained covers the typical demanding intermediate fast. And extended fasting and health risks explained covers the risks of longer durations.