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On the whole, Paleo is fasting-positive. Intermittent fasting is a powerful tool for improving health, jump-starting weight loss, or beating cravings, and there’s plenty of medical literature to back up its usefulness. But what about longer fasts (more than a day or two)? If intermittent fasting is good, will a longer fast be even better?

Long-term fasting can take several different forms. The most extreme is a “dry fast,” consuming nothing at all (food or water). This is definitely not advisable, as it’s very dangerous to go for more than a day or so without drinking. Water fasting means drinking only water, but consuming no calories during the fast. Another technique is juice fasting, or consuming only fruit and vegetable juices. Some people also fast on broth, or extremely low-calorie protein mixes.

Juice fasting and bone broth fasting aren’t truly fasts, since they do involve some intake of calories and nutrients. And protein fasts can be downright dangerous, since humans just weren’t built to live on protein with no accompanying fat. This article focuses on water fasting: the benefits, the drawbacks, and precautions to take if you do decide to start a longer fast.

Long Fasts and Weight Loss

The most obvious and best-researched benefits of longer fasts are for weight loss: if you’re not eating anything, weight will drop off your body fairly quickly. During the first 24 hours or so, you go through all the glycogen in your liver. After that, your body needs to run on what it has stored, either protein or fat.

For the first few days, weight loss averages around 1-2 pounds a day, both because you’re shedding water weight and because you’re using more protein. But consuming protein for fuel is risky, because it means breaking down muscles – not only your biceps, but also more essential muscles like your heart. This makes fat by far a preferential energy source, so after a few days, your body turns to its stored fat reserves for energy (ketosis). Since fat is more energy-dense per pound than protein, weight loss during this phase slows down to a more reasonable but still rapid pace of a little over 1 pound every 2 days.

Essentially, then, a long fast is a way of staying in ketosis for an extended period of time, but forcing your body to rely entirely on its own fat stores instead of dietary fat intake. Being in ketosis makes for fairly easy weight loss, since it suppresses hunger (once you get through the first few days, which are always rough). Fasting also allows you to completely get your mind off food, instead of always thinking about what you’re going to eat next and worrying about whether or not you’re eating too much.

Long fasting is an effective way to lose a lot of weight quickly, but with a significant caveat: many people proceed to gain it all back again, because they just go back to their old obesogenic diet right afterwards. Like any other “crash diet,” fasting will help you lose weight, but won’t help you keep it off unless you also make a long-term change in your diet after the fast is over. One study in obese patients who lost weight during 2 weeks of fasting found that 50% of their 46 patients either did not respond to the hospital’s attempts to contact them (which the researchers took as a sign they’d regained the weight) or responded saying they’d regained all the lost weight after 2 years.

Long Fasts: Other Benefits

In addition to weight loss, fasting also promotes autophagy, which is like a “spring cleaning” for your cells. Since your body is essentially eating itself, it has a chance to get rid of any junk or waste material that may have built up, and repair the damage of oxidative stress. This is one of the biggest benefits of fasting even for people who are already at a healthy weight, since it has powerful anti-aging and muscle-building properties.

One study also found that an extended fast (10 days on average) was beneficial to patients with hypertension, also noting that even though the patients didn’t embark on the fast to lose weight, all of them did – average weight loss was around 15 pounds. Longer term fasting (up to 5 days) may also have some benefits for chemotherapy patients.

Another benefit of extended fasting is purely mental: for many fasters, it’s a way to “re-set” their relationship with food, break free from patterns of emotional eating, or start fresh at the end of the fast. Fasting is part of many religious and spiritual practices because of its value for meditation and mindfulness. Bear in mind that this doesn’t happen automatically: it requires a high level of self-awareness and effort on the part of the faster. It’s a very useful tool, but it’s not a miracle cure.

Long Fasts: Dangers and Drawbacks

The ultimate risk of fasting, of course, is death by starvation; this doesn’t usually happen to people fasting for medical reasons, but taking anything to extremes is perilous. In Ireland in 1981, for example, 10 political prisoners starved themselves to death in a hunger strike against the British government, fasting between 46 and 73 days before they died.

Even fasts of a few weeks or less can have dangerous consequences. Fasting puts two different types of stress on your heart. First, it cannibalizes cardiac muscle for fuel. The human body does everything it can to conserve muscle during a fast, but inevitably some muscle will be sacrificed at the beginning of the fast. After a few days, the body switches over to using fat, but researchers have discovered that protein (muscle) utilization actually increases again later on, even though fat stores are still available. This protein includes the muscle in your heart: weaken this too much, and heart failure will result.

Strict water fasting is also a risk for heart failure because during a fast, the body’s intracellular stores of minerals vital for cardiac function, like magnesium and potassium, are depleted, even though serum levels remain normal. The results of this cardiac muscle loss and mineral deprivation can be tragic. During the 1950s and 60s, fasting was used as an experimental treatment for obesity, and several patients died (many from heart failure). Other reports of people dying during long fasts include more cases of heart failure. More recently, in 2010, a woman in Florida died after 21 days of fasting.

Other fasters die of infectious diseases that they simply don’t have the energy to fight off without adequate nutrition. In 1978, for example, a man named William Carlton died of pneumonia at a fasting center after fasting for 29 days in an attempt to cure his ulcerative colitis. He was 49 years old, and in normal health other than the colitis. Worldwide, infectious diseases are actually the most common cause of death among starving people, because an immune system weakened by malnutrition tends to give in before heart problems start to show. This is particularly common among children who go on (or are forced to go on) long fasts.

Of course, many people also fast safely, but it’s worth noting that fasting isn’t a risk-free experiment. Less serious drawbacks also include intense mood swings, low energy, and irritability. Fasting lowers blood pressure, so you may feel weak, dizzy, or nauseous during the fast. It raises levels of the stress hormones norepinephrine and cortisol, probably an adaptation to give you more energy for finding food, but not beneficial for optimum health.

Another potential downside of long-term fasting is the rate of detox. Fat is your body’s storage organ for everything, including any toxins that may have accumulated over the years. When you lose weight, all these toxins have to be removed through your bloodstream, which can be extremely uncomfortable. During fasting, these symptoms are even more pronounced, since the rate of fat burning is so rapid – many people feel nauseous, sick, or otherwise unwell.

Detox is sometimes a necessary evil, but when you’re thinking about the potential dangers of long-term fasting, make sure not to get taken in by fanatical advocates who claim that everything is just another detox symptom. Sometimes it’s actually a symptom of a bigger problem, not just detox, and even rapid detox can be unhealthy in itself.

There’s also a darker side to the mental health benefits of fasting. For eating disordered people, fasting can quickly turn into another form of abuse (punishment for eating too much or anything “wrong.”) Because fasting is often accompanied by a strange kind of energy, it’s possible to get addicted to it, and ignore physical danger in pursuit of that “fasting high.” This is just as dangerous as any other form of chronic malnutrition and starvation.

Long Fasts: Case Reports

In humans, most of the studies on fasting have dealt with religious fasts, which are more like intermittent fasting (for example, during the month of Ramadan, observant Muslims may not eat or drink while the sun is up, but observers commonly do eat dinner after sundown, and sometimes a small “breakfast” before sunrise as well. These studies tell us a lot about the benefits of intermittent fasting, but not much about long-term water fasts. There haven’t been a lot of high-quality studies on long-term water fasting, since mainstream medicine regards it as very dangerous. There are a few case studies which are interesting, but difficult to draw conclusions from:

One study tracked an anonymous monk who undertook a 40-day fast, with no caloric intake at all except from taking communion, which the researchers estimated at 60 calories per day. After thoroughly measuring every baseline of health before the man started fasting, the researchers took daily and weekly measurements until day 36. During this time, the man:

Lost around 34.5 pounds

Developed symptoms of severe hypotension (blood pressure that’s lower than normal), to the point where he needed almost half an hour just to stand up in the morning.

Showed no major changes in serum potassium, magnesium, calcium, or phosphorous (although this isn’t a sign of mineral sufficiency; see the section on breaking the fast and refeeding syndrome below)

Showed a slight rise in serum zinc

Showed increased levels of uric acid for the first two weeks. Uric acid stabilized in the third week, and then normalized after that.

The man had originally planned to fast for 40 days, but stopped on day 36, when “profound weakness and symptoms of postural hypotension interfered with his daily activities in the monastery.”

One reporter from Harper’s Magazine lost 30 pounds on a 17-day fast, and suggested that fasting is so denigrated as a cure for chronic disease because it’s just not profitable for drug companies. His article also detailed the case of a suicidal doctor after the Civil War who tried to starve himself to death, only to find that the longer he fasted, the better he felt.

An extremely obese man in Scotland lost 276 pounds on a 382-day fast, while taking supplemental vitamins and minerals. A rarity among extreme weight loss patients, he actually kept the weight off once the fast was over.

One follower of the Perfect Health Diet fasted for 30 days, and reported that his migraines were cured at the end of the fast. However, he later discovered that just eating a ketogenic diet would have the same effect.

Perhaps most usefully to anyone considering a long fast, a blogger named Celestine Chua fasted for 21 days and kept an astonishingly detailed record of her physical and emotional reactions. Although she experienced severe mental and physical “detox,” she reported being glad that she’d fasted, and having a better relationship with food afterwards.

All these reports are biased, of course, because people who try a fast and fail are much less likely to write publicly about it. But they are interesting resources, and we’re not likely to get a randomized, controlled clinical study any time soon, so they’re the best we have for now.

Long Fasts vs. Shorter Fasts

With any dietary intervention, you want to get the most gain for the least amount of pain. Especially considering that the benefits of intermittent fasting (as opposed to longer fasts) are so well attested, it’s worth a look to see if shorter fasts aren’t a better idea overall.

So are long or short fasts superior? The answer is that it depends on why you’re fasting. You can get the physical benefits of longer fasts just as easily from intermittent fasting, or even from not fasting at all. Ketosis, for example, doesn’t require any calorie restriction, only carbohydrate restriction. Since a ketogenic diet includes all the vitamins and nutrients your body needs to keep functioning, it’s much less dangerous and easier to just eat ketogenic meals than to enter ketosis by fasting.

Autophagy is also achievable through intermittent fasting just as easily as longer fasts. Autophagy begins when liver glycogen is depleted, around 12-16 hours into a fast. The rate of autophagy peaks there, and then drops after about 2 days. If your goal is a “spring cleaning” for your cells, intermittent fasting may be even more effective, since you spend more time in the “early fasting” period when autophagy is at its peak.

Long fasts also lose points for the social aspect – it’s easy to plan family meals around an 8-hour feeding window, but much more difficult to stay engaged in a healthy social life if you’re avoiding food altogether.

Intermittent fasting also reduces many of the drawbacks of long fast. There’s typically no loss of lean tissue (muscle), since people who IF eat plenty of protein, just at different times. Intermittent fasting allows you to keep working out, while exercising during long fasts will just cause more muscle loss. There’s much less risk of malnutrition, heart failure, and infectious complications with intermittent fasting – from a physical perspective, it’s hard to see how anyone would prefer an extended water fast.

On the other hand, there are some emotional and psychological benefits of longer fasts that intermittent fasting just doesn’t provide. A 2 or 3 week fast can be a springboard for a radical change in dietary habits – fasters report that abstaining from food completely gave them a valuable chance to re-evaluate their eating habits. For example, even emotional eating of ketogenic or otherwise healthy foods doesn’t break you out of the cycle of eating to deal with uncomfortable feelings. A total fast forces you to find other ways of handling these emotions. This can make a fast very difficult, but also very rewarding for people who can stick it out.

On the whole, intermittent fasting is superior for physical health (the same gain with less risk), but longer fasting may be superior for emotional, psychological, or spiritual reasons.

Long Fasts: Precautions

If you do decide to embark on a long fast, some common-sense precautions can prevent prevent a disappointing failure – or worse, a trip to the emergency room. Some people simply shouldn’t practice extended fasts, period:

Young children are still growing rapidly and need adequate nutrition at every stage to make sure their bodies develop properly

Very elderly people often don’t have the physical resources to fast safely.

People who are seriously ill, or people with chronic heart or kidney conditions, shouldn’t fast since their bodies may not be able to withstand the stress of fasting.

Women who are pregnant, or trying to get pregnant, should eat plenty of nutrient-dense food, because a well-fed state is essential for healthy reproduction.

If you’re not in any of these groups, make sure to do your own research and have a plan beforehand. Read up on other people’s experiences with long fasts, so you understand what you’re getting into. Think about your physical and emotional relationship with food – what do you think will be the difficulties of fasting for you, and how will you overcome them? Are there any specific issues that you want to meditate on or work through while you’re fasting?

A useful fasting plan should also include a plan for dealing with other people during your fast. Unless you live alone, chances are good that someone will notice and be concerned. How will you reassure them that you’re not anorexic or starving yourself? What if you have to participate in a business lunch or another work meeting involving food?

Also, make a backup plan while you’re well-fed and healthy for what you’ll do if the fast isn’t going well. During a long fast, it’s normal to experience crazy emotional highs and lows; this can prevent you from making a rational decision about whether or not you want to continue. Write down your criteria for what will make you stop the fast (“I will stop fasting if my blood pressure drops below _____________” or “I will stop fasting if I lose __________ pounds” or whatever it might be for you) and stick to them during your fast.

An even better option is to find someone to supervise your fast. If at all possible, it’s wise to fast under the care of a doctor, but this isn’t always an option, since most mainstream doctors have a knee-jerk negative reaction towards fasting. Even a close friend with no medical training can help provide some valuable perspective on how the fast is going. Fasting centers are another option, but these also aren’t without their problems: some of the fasting experts who run them have been implicated in very sketchy practices, and it’s crucial to do a lot of research before entrusting yourself to them.

During your fast, make sure to drink plenty of water to avoid dehydration, and take supplemental electrolytes: sodium, potassium, calcium, phosphate, and magnesium. Your body needs these minerals to balance fluid levels, and supplementing also prevents one of the chief dangers of fasting: refeeding syndrome (see the section on breaking your fast below). Normally, you get those minerals from your food; since you’re continuing to drink water without eating anything, you need them from an alternate source during your fast. You can buy “electrolyte water” (make sure you don’t get sports drinks full of artificial flavorings, though), make your own with water, lemon, and a pinch of salt, or take supplements.

While you’re in the midst of a fast, don’t try to work out at all – this will just cause your body to lose more muscle than necessary. The closer you can get to bed rest, the better. Read, journal, meditate, sleep, listen to music, or talk to people you love. Many people take time off work to concentrate on their fast. Keeping a slow-paced and thoughtful environment will help you really get the most of the psychological and spiritual benefits of fasting.

Breaking a Fast

Breaking the fast correctly is one of the most important ways to prevent any negative consequences. Introducing too much food too rapidly can lead to a fatal condition called refeeding syndrome. A sudden shift from ketosis (fat-adapted metabolism) to carbohydrate-based foods causes your body to release a flood of insulin to digest all those carbs, a process that requires large amounts of phosphate, potassium, magnesium, and several vitamins, especially thiamine (Vitamin B1). If you’ve been fasting for any length of time, though, these nutrients are severely depleted, so suddenly needing a lot of them leads to serious acute deficiencies, causing heart failure, hypotension, and sudden death.

A set of guidelines from Scarborough Hospital in the UK for avoiding refeeding syndrome include:

Start slow: 10 calories/kilogram of body weight per day (or 5 calories/kilogram if you’ve been fasting longer than 15 days), with a daily increase of 5 calories/kilogram.

Supplement with thiamine and other B vitamins 30 minutes before breaking the fast (the guidelines are meant for hospitals and suggest an IV; another article gives an appropriate dose as 50-250mg).

Supplement with phosphorous, potassium, magnesium, and sodium before breaking the fast.

Additional guidelines include:

Supplement with electrolytes during the fast, as discussed above.

Break your fast with bone broth or diluted fruit juice, both rich in electrolytes. Squeeze your own juice, or if you buy some at the store, make sure it’s pure juice, with no sugar or other nasty additives involved.

The longer you fast, the longer you’ll need to spend breaking the fast; for anything longer than a week, plan on spending several days slowly reintroducing your body to solid foods.

Eat small portions, more often, and eat slowly. As your body starts to get more used to food again, pureed soups (like sweet potato lime soup, cream of tomato soup, or butternut squash soup) are a good transition food. Avocados are soft and easy to digest, and a good source of healthy fats. A bone broth soup with egg yolks and some well-cooked vegetables is loaded with nutrients, fat, and protein. Fruit is often recommended as a transition food, but this isn’t a great idea because the fructose can be hard on your gut.

As you slowly work your way up to eating normally, make sure to re-introduce foods slowly and carefully. If you’ve never done an elimination diet before, this is a great opportunity to do so: try testing nightshades, nuts, or FODMAPs separately to see whether or not you have any reaction to them.

The Upshot

Long-term fasting is definitely an intriguing idea. Humans certainly have the ability to endure long periods of famine, but that doesn’t mean it’s healthy. So far, there’s some substantial evidence showing benefits for weight loss, but whether this weight loss is actually sustainable in the long term remains to be proven. And in most cases, you can get all the physical benefits of long-term fasting from intermittent fasting or even just a ketogenic diet, without all the attendant risks of completely abstaining from food for several weeks at a time. So for purely physical health reasons, longer fasts aren’t great because they have increased dangers without any increased benefits.

For the vast majority of people, it’s a much better idea to use intermittent fasting or alternate-day fasting (24 hours of feeding followed by 24 hours of fasting) and perhaps a ketogenic diet to reap the physical benefits of food restriction, without the very real dangers of longer fasts. For mental health and spiritual practice, on the other hand, longer fasts do have some advantages over intermittent fasting. Whether the benefits outweigh the risks is a decision everyone has to make individually – don’t take the dangers lightly, and make sure to do your own research before you make your choice.