Intermittent Fasting with Long Fasts : Dangerous or Beneficial ?

Intermittent fasting provides many health benefits, and most of them can be enhanced with long fasts.
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Intermittent fasting provides many health benefits, and most of them can be enhanced with long fasts. Research suggests that long fasts are beneficial for weight loss, promote autophagy, stimulate growth hormones and improve blood sugar control. But long fasts also have some downsides.

Written by
Jill Lebofsky
15+yr Women's Wellness Expert, Holistic Menopause Support, Intermittent Fasting, Midlife, Essential Oil, Author, Speaker

Throughout history, people have fasted for multiple days, for religious reasons, due to famine, or not being able to find food. As intermittent fasting has gained popularity in recent years, the effects of fasting for longer periods have been studied more closely.

Intermittent fasting long fasts vs. prolonged/extended/water fasting

Glass of water.

Most people who do intermittent fasting refrain from eating for 12-18 hours daily. However, some will fast longer, up to 48 hours, to give their body more time to reap the benefits.

A fast longer than 48 hours is referred to as a prolonged fast, also called an extended fast or water fast. Most people who do prolonged fasts go 3-7 days without food. It is recommended that such a fast be done under medical supervision.

This article will focus on the 24 to 48-hour intermittent fasting long fast.

Why should you try fasting longer than 24 hours ?

Clock showing 12 o'clock.
  • After eating, it takes the body time to process the meal before stored fat burning begins at around the 12th-18th hour into the fast, depending on what you’ve eaten or drunk in the hours prior.  When you fast for more extended amounts of time, your body transitions from burning sugar for energy to burning stored fat. You will not only burn more fat the longer you fast, but you will also reduce the number of calories you consume, contributing to weight loss success.
  • Fasts of 24-48 hours may be more beneficial for blood sugar control than shorter fasts, and they increase insulin sensitivity so that your body works more efficiently (1).
  • The longer the fast, the more time the body has for cellular cleaning and repair through the process of autophagy. A person’s metabolism will determine how long it takes for this process to start during fasting. Although some people report signs of autophagy around 12 hours, studies done on animals show autophagy starting around 24 hours and peaking at 48 hours (2). Autophagy removes the damaged parts of the cells, reducing inflammation, providing more energy, and giving a more youthful appearance while preventing disease.
  • The human growth hormone (HgH) is triggered by fasting around the 12 hour mark. By 48 hours, HgH production increases by 500% (3). This hormone boosts metabolism, burns fat, preserves lean muscle mass, and provides anti-aging effects. 

Although it isn’t required to do longer fasts to enjoy the benefits of intermittent fasting, some experts recommend a 24-48 hour fast a few times a month to boost weight loss results. You could also try 24h fast once a week.

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Building up to a long fast

If you’re new to intermittent fasting, first sit back and get comfortable with the daily 16-ish hour fasts for at least a month. Let your body and mind adjust. 

If you’re a more seasoned faster who has hit a weight plateau, wants to boost results, or gain the additional health benefits, a periodic long fast of up to 48 hours might be a great fit. Start with a goal of 24-36 hours the first time and build to a 36-48 hour fast.

The most important thing is to honor your body. There’s a difference between low energy because you haven’t eaten in a day and a half and actually feeling ill. Pay attention to what your body is experiencing and adjust as needed. Don’t push yourself to a dangerous place for the sake of a few additional hours of fat burning.

Long fast tips

A woman explaining something to another woman.
  • Stay hydrated! Twenty percent of our water intake usually comes from our food, and since you won’t be eating, it is essential to make sure you are drinking plenty of allowed liquids. During the long fast, you can have water, black coffee, and unsweetened tea.
  • Be careful of low electrolyte levels if fasting over 36 hours. Electrolytes keep you hydrated (4). Ingest a pinch of salt, take electrolyte supplements, or drink bone broth. 
  • Ease back into eating with a small snack, like a handful of nuts or a protein shake.
  • An hour or two after your snack, have your first meal, focusing on healthy fats like avocado and coconut oil.
  • Eat iodine-rich foods (like seaweed, shrimp, tuna, or eggs) following a fast to support the production of your thyroid hormones. This is especially important for women (5).

Downsides of long fasts

While the benefits of periodic long fasting are many, there are some drawbacks to fasting for more than a day, including hunger, dizziness, sluggishness, and exhaustion. Long fasts can also make social situations uncomfortable.

Who should avoid long fasts 

The following people should either avoid or be extra cautious if fasting over 24 hours:

  • those with type 1 diabetes
  • those with low blood pressure
  • those who are underweight or have a history of eating disorders
  • women who are pregnant, breastfeeding or trying to conceive

Mix up your intermittent fasting routine with a 24-48 hour fast periodically to boost your long-term results.

If you need help with intermittent fasting, join our intermittent fasting community for women only. There you get all the support you need!


1. Patterson RE, Sears DD. Metabolic Effects of Intermittent Fasting. Annu Rev Nutr. Aug 21 2017;37:371-393. doi:10.1146/annurev-nutr-071816-064634

2. Alirezaei M, Kemball CC, Flynn CT, Wood MR, Whitton JL, Kiosses WB. Short-term fasting induces profound neuronal autophagy. Autophagy. Aug 2010;6(6):702-10. doi:10.4161/auto.6.6.12376

3. Hartman ML, Veldhuis JD, Johnson ML, et al. Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men. J Clin Endocrinol Metab. Apr 1992;74(4):757-65. doi:10.1210/jcem.74.4.1548337

4. James LJ, Shirreffs SM. Effect of electrolyte addition to rehydration drinks consumed after severe fluid and energy restriction. J Strength Cond Res. Feb 2015;29(2):521-7. doi:10.1519/JSC.0000000000000657

5. Chung HR. Iodine and thyroid function. Ann Pediatr Endocrinol Metab. Mar 2014;19(1):8-12. doi:10.6065/apem.2014.19.1.8

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