top of page


  • Alexander Olaussen

How Long Should We Fast For? An Evidence-Based Guide to Fasting Duration

Updated: Nov 14, 2023

The topic of fasting has gained considerable attention in the health and longevity community. It promises benefits ranging from weight loss to cellular repair. However, there are still debates about how long one should fast for optimal health. A key question to consider when you are asking how long to fast for is; “why am I fasting?”, “what is the purpose behind this fast?”. Is it weight loss, mental clarity, autophagy, cellular repair, or all of the above? This blog post aims to shed light on this issue based on available evidence.

Grounded in Evidence

Intermittent Fasting: 16/8, 18/6, or 20/4

  • Duration: 16-20 hours of fasting, 4-8 hours of eating.

  • Evidence: Several studies have shown that intermittent fasting can lead to weight loss, improve metabolic markers, and offer cardiovascular benefits.

  • Caveat: Some individuals may experience low energy or reduced performance during fasting periods. Pregnant women and those with certain medical conditions should consult a healthcare provider. Ensure adequate water and salt intake during periods of fast to offset some of these symptoms.

24-Hour Fast: Eat-Stop-Eat

  • Duration: 24-hour fast, once or twice a week.

  • Evidence: Research suggests that 24-hour fasting can produce acute changes beneficial for health, such as reductions in insulin levels and increased autophagy.

  • Caveat: Extended fasting may not be suitable for everyone, especially those who are pregnant, elderly, or have certain medical conditions. Note: that although this is called 24-hour fast, if you go from dinner on Monday till breakfast on Wedesnday, for instance, you have in fact done 36 hours of fasting.

5:2 Diet

  • Duration: 5 days of normal eating, 2 non-consecutive days of reduced-calorie intake (around 500-600 calories).

  • Evidence: Studies have demonstrated that the 5:2 diet can lead to weight loss and improved metabolic health.

  • Caveat: The reduced-calorie days may be challenging to adhere to and may not be suitable for those with certain medical conditions.

Multi-Day Fasting

  • Duration: Fasting for 2-7 days.

  • Evidence: Longer fasting periods have been shown to induce deeper states of autophagy and may have anti-aging benefits.

  • Caveat: Extended fasting should only be done under medical supervision. It may cause nutrient deficiencies if done repeatedly without proper planning.

Speculative Insights

Fasting Mimicking Diet (FMD)

  • Duration: 5 days of restricted caloric intake that mimics the effects of fasting.

  • Evidence: Although promising, more research is needed to fully understand the long-term effects and benefits of FMD.

Dry Fasting

  • Duration: Fasting without water intake for 16-24 hours.

  • Evidence: There is limited scientific evidence supporting the safety and efficacy of dry fasting.

Prolonged Fasting (>7 Days)

  • Duration: More than one week.

  • Evidence: The data is inconclusive, and health risks like nutrient deficiencies and muscle wasting are potential concerns.


  • This NEJM paper from 2019 is key reading for anyone interested in this space.

De Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. New England Journal of Medicine. 2019 Dec 26;381(26):2541-51.
  • The conclusion reads:

    • Although we do not fully understand the specific mechanisms, the beneficial effects of intermittent fasting involve metabolic switching and cellular stress resistance. However, some people are unable or unwilling to adhere to an intermittent fasting regimen. By further understanding the processes that link intermittent fasting with broad health benefits, we may be able to develop targeted pharmacologic therapies that mimic the effects of intermittent fasting without the need to substantially alter feeding habits.

Randomised Controlled Trials

  • Lots of articles supporting the safety and efficacy of IF.

  • Check this post for a complete and continually updating overview.

Framework for decision

This is how I think of IF in clinical practice


The optimal fasting duration can vary depending on individual health conditions, lifestyle, and goals. While evidence supports the benefits of intermittent fasting and shorter fasts (24-hour and 5:2 diet), extended fasting periods require medical supervision. Remember, fasting is not a one-size-fits-all solution and should be tailored to individual needs for maximum efficacy and safety.

Recent Posts

See All

Tirzepatide: GLP-1 Agonist

Background: Tirzepatide is a relatively new medication that has been making waves in the world of diabetes management and metabolic health. Tirzepatide is a once-weekly injectable medication developed

RCTs on Intermittent Fasting

Background: There is a lot of evidence supporting IF Below is a collection of all RCTs on IF Evidence: Updated 11th October 2023 RSS feed Efficacy and Safety of Intermittent Fasting in People With Ins


bottom of page