The topic of fasting is incredibly popular these days. News headlines report on benefits associated with fasting and you may have heard of celebrities or influencers trying a new fast. Popular fasting plans include intermittent fasting or fasting for short periods.
Some of the most popular intermittent fasting patterns include 12/12 (fasting for 12 hours a day), 16/8 (fasting for 16 hours a day), and 20/4 (fasting for 20 hours a day). Some patterns allow for 24-hour fasts for 1-2 days per week. You can read more about intermittent fasting here.
You may be wondering if there are any benefits to fasting for long periods. Prolonged fasting is generally defined as fasting that lasts longer than 48 hours. (1)
While fasting has become quite popular, people often have questions regarding the safety and efficacy of fasting. Perhaps you’ve tried intermittent fasting and seen some benefits and now are wondering if fasting for longer could be beneficial. While there are potential benefits to prolonged fasting, there are some risks you should be aware of before trying it.
Fasting has been linked to health improvements for several centuries(2). Recent research has explored the health benefits surrounding short-term and intermittent fasting. (3)
While there are several studies on fasting up to 72 hours, relatively little research has been done on fasting for longer periods. Some websites may claim to provide prolonged fasting guides, but there is very little evidence to back up these claims. A small number of studies have found some improvements with long fasts, but the results are generally mixed.
One study of 13 participants who finished four days of a calorie-restricted diet and then completed a 10-day complete fast found improvements in weight loss and loss of fat mass, but noted no change in cholesterol levels. (4)
Other studies have determined that while you may see weight loss during a prolonged fasting period of 5-20 days, up to 2/3 of the weight that is lost is lean muscle mass. For people with normal blood sugar control, there was a decrease in fasting glucose, fasting insulin, and insulin resistance. However, these improvements were not seen in participants with diabetes.(5)
Some studies have found longer fasting periods to have the potential to improve some health markers. Another recent study of 48 overweight or obese participants did a water fast for an average of 17 days with medical supervision and noticed a reduction in blood pressure. (6)
Fasting may also promote autophagy, a process that recycles old parts of our cells. Autophagy has been proven to reduce the risk for certain cancers, as it may stop damaged cell parts from accumulating in the body. (7)
Water fasting may promote autophagy and help with longevity, but more studies are required to determine how longer fasting periods may impact this. (8)
Studies have found health improvements with fasting as little as 16 hours per day with intermittent fasting. However, you’ll need to practice intermittent fasting consistently for several weeks to see weight loss or other benefits.
One study noted several positive health impacts that occurred on day 3 of a fast, including weight loss and certain protein levels in the blood. (9) It may also improve insulin resistance. (10)
Some people see the benefits of a 24-hour fast that is done once a week. This can be referred to as a 6/1 intermittent fast. Another popular pattern is 5/2 intermittent fasting, in which you fast for 24 hours on 2 non-consecutive days per week (e.g. fast on Monday and Thursday). One study of 30 healthy adults who fasted with only water for 24 hours showed an improvement in triglycerides. (11)
People can generally fast safely with only water for up to a 24-hour period. If you don’t have diabetes, or blood pressure issues, or are pregnant or breastfeeding, you may attempt to fast for up to 48 hours without many serious side effects.
A few studies have looked at fasting for up to 120 days, but many of these allow for up to 500-600 calories per day, so they are not complete water fasts.
If you’re considering fasting for longer than 72 hours or 3 days, you may want to check with your healthcare provider first. You should also seek medical supervision when attempting total fasting for longer than 72 hours. (12, 13)
If you fast for a longer period, research has found that certain changes occur in the body. Everybody is different, so it’s difficult to determine the prolonged fasting benefits by the hour. Generally, within the first few hours, your body will use food for fuel. At 8 hours or so, the stomach is empty and blood sugar levels may drop, which decreases insulin production. After 12 hours, the body will start using fat as energy and growth hormone levels increase. At between 16 and 24 hours later, autophagy begins and the body will produce ketone bodies for fuel. (14)
One result of prolonged fasting is an increase in ketone production or going into ketosis. This is a process in which the body starts to break down fat storage for fuel as not enough blood glucose is available. Another result is lower blood pressure. While this may be beneficial to a point, it can drop too low, which can be dangerous. An increase in growth hormones and a decrease in thyroid levels have also been noted. Finally, other long-term fasting stages can include organ damage. (15, 16)
For more information on the timeline of fasting, read here.
While some people have reported completing a 40-day fast in relatively good health, there are several significant risks to attempting to fast for this length of time. Fasting for long periods has been linked to dizziness, feeling lightheaded, low blood sugar, low blood pressure, headaches, and fatigue. Fasting for too long can result in organ damage. (17)
If you’re considering a longer fast, you may want to avoid complete or water fasts and opt for including healthy foods daily to provide several hundred calories. For example, during Ramadan, observant Muslims eat at sundown and once again before sunrise. This allows them to consume several hundred calories so they can fast safely for one entire month.
The short answer is that it depends on your goals. For weight loss, studies have found improvement with intermittent fasting that calls for as little as 16 hours per day. The key to this is consistency. You need to continue the 16/8 intermittent fasting pattern for several weeks.
Studies have also found that fasting for 24 hours on two alternate days in a week, sometimes called 5/2 fasting, can lead to health improvements.
Other studies have found that certain health improvements can be seen at the 48-72 hour mark for fasting. (18)
The benefits of fasting for 48 hours can include slowing cell aging, decreasing insulin levels, and improving insulin sensitivity. (19, 20)
You can read more about 48-hour fasts here.
Recently, more people have started to try water fasts. Water fasting involves consuming nothing but water. There may be some water fasting benefits, but you don’t want to fast for too long. A popular length for a water fast is 3 days.
Trying a water fast for 3 days may be safe for some, but others should consult their doctor before attempting this. If you have issues with your blood pressure, are pregnant or breastfeeding, or are diabetic, only drinking water for 3 days can potentially be quite dangerous.
In addition, if you have gout, only drinking water for several days may increase the uric acid in your body, which can aggravate the condition. (21)
One risk of fasting is dehydration, which can cause orthostatic hypotension. This is a drop in blood pressure that occurs when you stand up too quickly. You may feel dizzy, lightheaded, or at risk of fainting. (22)
You get approximately 20-30% of your daily water needs from food, so you’ll need to drink plenty of water to avoid dehydration.
You may lose weight from water, eating less carbohydrates, and muscle mass, but this kind of weight loss can easily be regained once you start eating again. (23)
There may be some 3-day or 72-hour fast benefits such as autophagy and improved levels of certain protein markers. (24) In addition, a 72-hour fast may help with insulin resistance. (25)
You don’t need to fast to cleanse your body. Your liver and kidneys cleanse and filter your body constantly. Avoid alcohol and processed foods, eat plenty of fruits and vegetables, and drink plenty of water as this will help your liver and kidneys do their job.
However, if you want to try a fast to see how your body feels, you may want to try a 24-hour fast.
You don’t have to do a 3-day fast, but if you attempt it, you should give your body plenty of time to recover after completing it. You should never attempt a 2- or 3-day fast more than once per month.
While certain health benefits and weight loss have been linked to fasting for shorter periods, many people can try fasting for 48-72 hours quite safely. However, there isn’t much research on the safety and efficacy of prolonged fasting. If you want to attempt to fast for longer than 72 hours, you should first contact your healthcare provider. Women who are pregnant or breastfeeding and people with blood pressure problems or diabetes should not attempt prolonged fasting.
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