How Long Can You Fast For?

Guide to prolonged fasting - Episode 07

Did you know the longest fast ever on record was 382 days? That's absolutely insane. Yet this person survived, and didn't even require that much medical supervision! In today's episode I discuss prolonged fasting: What is it? How to do it? Is it helpful? Is it dangerous? How to avoid problems What about hunger? Won't your muscles disappear? What about your brain function?


What is it?

We are discussing prolonged fasting: that is fasting for more than 24 hours.

Now right out of the gates I'm going to tell you: is prolonged fasting dangerous?

It can be, so you should speak to your doctor or a doctor experience in prolonged therapeutic fasting before you start.

There's also some good books and other literature out there by medical doctors who do this kind of work like Dr Jason Fung that you could read. That being said why would you want to do prolonged fasting if it's dangerous?

Well it's not inherently dangerous as I said in my previous episode on intermittent fasting the longest fast ever recorded was actually 382 Days by a Scottish man called Angus Barbieri. He lost 276 pounds in that time and he was under medical supervision and was consuming vitamins and other supplements to help him.

Now there's no doubt this is an extreme fast and I believe he was fairly closely monitored by doctors during this time although interestingly he wasn't in the hospital, he was actually living at home and just went in every few days. When i was researching for this episode I couldn't find much documentation on exactly what happened and whether he suffered any ill effects or side effects, but my understanding the is that if he did it was pretty small and it was it was temporary.

382 day fasting?

No we're not talking about doing 382 day fasts. More like maybe one to three days. Now some people go further and do maybe five seven or even 14 day fasts -I know some people that have gone a longer. I saw someone the other day saying 42 day fast you know regularly that's that's extreme and we're not talking about that.

Myself personally I've done plenty of one to three day fast I've done a few seven day fasts I would also done one 14 day fast.

I've had a few patients do seven day fast as well and I've overseen them through this and monitored their electrolytes and blood tests at key intervals.  I also monitored myself through blood tests during my 14 day fast.


I guess the first  question is why are blood tests even required? Well I mentioned that prolonged therapeutic fasting can be dangerous and that's mainly due to electrolyte disturbances.

If you don't know what electrolytes are- they are substances, chemicals if you will, within the body that help to regulate and control bodily functions, cellular processes and generally many aspects of our health. If they're out of balance you can quite quickly have a problem. Symptoms of electrolyte disturbance can be minor or pretty profound and can even result in coma or death, so these things are not to be messed with or ignored.

Thankfully though the body is generally extremely good at tightly regulating these electrolytes even under stressful conditions like fasting, but things can go wrong. The risks increase almost exponentially with the longer you fast for.

There's many electrolytes and chemicals within our body that are important for all sorts of functions but the five most important perhaps are potassium, glucose, sodium, magnesium and phosphate. These are all usually tightly controlled within the body because even minor disturbances can have pretty serious consequences - however that's under normal conditions and prolonged fasts - the body can start to struggle with these key substances.

The most obvious one that will be affected is glucose.

Generally your blood glucose level Glucose will fall when fasting but it's usually maintained by the body by processes such as glycogenolysis and gluconeogenesis. Glycogenolysis refers to breaking down of muscle glycogen and liver glycogen into glucose and glucose-one-phosphate, and the latter - gluconeogenesis refers to the creation of glucose from non-glucose sources, for example protein amino acids or even lactate.

These processes are regulated quite carefully by your hormones: mainly cortisol, glucagon and insulin, but if your hormones are disordered then your homeostasis as we call it - your regulation - can be disordered as well as a result.

Glucose is usually kept between around four to six mmol/L or around 70 to 100 ml/dL in American units. Some variations will exist but this is the case for most people. Locally here in Australia normal reference ranges given are around 3.5 to 6 mmol/L for most people.

For the most part other than extreme readings a high glucose reading is actually not that dangerous in the short term but can be very very damaging in the long, term whereas a low glucose is the opposite - it's more likely to be dangerous in the short term and less of a concern in the long term. \

The level at which danger can occur does vary between individuals and to my knowledge that mechanism isn't terribly well understood, but likely it's due to your hormonal balance and how well your body can control and handle a low blood sugar.

I've seen patients with type 2 diabetes have severe hypo events around 3.5 mmol/L, and I've seen medically metabolically well patients with fasting glucose around 2.5mmol/L walking around like there's nothing happening and not even feeling faint at all.

Likely it's because the other processes such as the Ketone body and fatty acid metabolism is functioning much more optimally in these patients so the brain isn't as concerned with low glucose as it's still getting plenty of energy from those Ketone bodies.

Typically I'm recommending patients try and stay above 3.5 mmol/L  and you definitely must eat if you're feeling unwell.

How you then eat depends on how long you've been fasting for and we'll come back to this later, but there's a very dangerous condition called refeeding syndrome that you do need to be aware of.


Symptoms of low sugar low blood glucose, otherwise known as hypoglycemia include:

  • sweating
  • lethargy
  • tiredness
  • fatigue
  • drowsiness
  • dizziness
  • lightheadedness
  • feeling shaky or trembling
  • shaky or tingling lips
  • tingling extremities
  • heart palpitations
  • mood disturbance and
  • confusion


Potassium is another key electrolyte at risk in prolonged fasting. It's usually very tightly controlled within the body and both high and low potassium is very dangerous. Low potassium is mainly the concern with prolonged fasting and symptoms include :

  • weakness
  • cramps
  • feeling tired
  • confusion
  • constipation
  • diarrhea
  • drowsiness
  • tingling or numbness
  • chest pains
  • irregular heartbeats or more forceful heartbeats

As with all electrolyte disturbances very low potassium is very, very serious and extremely dangerous and can very quickly become a medical emergency. Generally most doctors recommending or overseeing prolonged fasting will be recommending some sort of way of maintaining your potassium levels such as consuming electrolyte tablets, drinking bone broths or coconut water or even maybe pickle juice.

Now theoretically most of these will actually break a fast, but realistically they're mostly unlikely to impact a great deal, yet they could stop the electrolyte disturbances you get and be very very useful.

As with glucose again if you feel any of these symptoms you should break the fast: but take care how you do it because again refeeding syndrome is a big concern.


Again as with the other electrolytes magnesium will deplete over time and the longer you fast, the more it will go down. Magnesium is very important but is generally less of a concern than other electrolytes - the body has very good mechanisms for controlling magnesium. Whilst it's estimated that around 75 percent of the world's population is actually magnesium deficient, the deficiency is minor in most individuals.

We don't know exactly why the majority of the world is magnesium deficient but it's postulated that it's mainly due to the low nutritional content of food. Most of the food that we consume has very poor levels of magenesium, as well as other minerals. Fluoride in drinking water may be a contributing factor but that's not been proven.

Symptoms of low magnesium include:

  • Muscle cramps
  • fatigue
  • tiredness
  • dizziness
  • nausea and vomiting
  • shaking
  • muscle spasms
  • fasciculations (that is minor movements of the muscles at rest)
  • loss of appetite
  • and sleepiness

Magnesium is harder to keep up than during a prolonged fast than the others, but the body is quite good at regulating its magnesium levels and a prolonged fast is unlikely to cause significant magnesium deficiency if the levels were good to start with: but it is possible. In particular if you're exercising a lot, particularly weight training it's going to put a lot more pressure on your magnesium levels and could reduce them significantly. It's therefore sensible to consider taking a magnesium supplement during a prolonged fast of more than a few days.

Some take powders, some take tablets. Bearing in mind both of these will probably break the fast so not ideal. You probably won't get any magnesium in bone broths so I typically recommend topical magnesium either in Epsom salts or flakes in the bath creams, or maybe spray oils. Please bear in mind though spray oils don't give a lot of magnesium.


Is probably simultaneously the least and the most concerning electrolyte. I know that sounds odd so let me explain.

Phosphate levels for most people are generally quite good because they're present in a lot of food - both meat vegetables, bread Etc. Phosphate levels in most people are generally good although some people can have low levels to start with so it's sensible to get your levels checked prior to doing a prolonged fast to sure to make sure that you are adequately replete (that means you've got enough before starting the fast).

Phosphate levels will drop with fasting and the longer you fast for the more it will drop.

Symptoms of low phosphate include:

  • muscle pain
  • bone pain
  • muscular weakness
  • altered conscious state
  • confusion
  • drowsiness
  • dizziness
  • numbness
  • reduced reflexes
  • and in extreme cases seizures coma and death

Thankfully this is rare and phosphate is actually quite easy to replenish during a fast by using bone broth hence why it's actually not that much of a concern. Most commercially available brain broths will have significant levels of phosphate in so consuming these during prolonged fast is likely to keep your phosphate levels very high. In fact when I did my 14 day fast my phosphate levels actually went up over the course of the fast because I was drinking the bone broths. Now I'm sure homemade bone broths are fine as well.

The main concern for phosphate levels is Refeeding Syndrome.

Refeeding Syndrome

This can affect all electrolytes as I said earlier. Refeeding syndrome is a condition in which the body shifts fluid and your electrolytes into cells from the bloodstream. Most electrolyte disturbances occur when levels in the blood drop not levels in the body as a whole and in refeeding syndrome the levels will drop far more significantly and low phosphate is usually the biggest problem and is usually the hallmark of refeeding syndrome.

Thankfully it's very rare in the general population and is really only seen in people who are very very malnourished, but it does increase with prolonged fasts and the longer you fast for the greater the risk generally.

Fasts of up to three days are very low risk for refeeding syndrome.

Fasting three to five days slightly higher risk.

Five to seven the risk is starting to go up.

Seven days or more the risk can be significantly increased.

This is mainly due to the action of insulin the longer you fast for the lower your electrolytes will go but also the lower your insulin will go and the lower your insulin resistance will go now this is generally the idea of prolonged fasting, but in refeeding syndrome this is very very dangerous.

This is because when your insulin resistance goes down insulin works more effectively (which is kind of the point). When your insulin resistance drops your insulin sensitivity also increases. This means your insulin will work more effectively which again is kind of the point, but we've got now almost really a double effect and a big spike of insulin, say from a high carbohydrate meal can have an enormous insulin effect on the body. With this double whammy effect this can force the shift of key electrolytes like phosphate, potassium, magnesium and of course glucose out of the blood and into tissues in the body, thereby significantly reducing the concentration in the blood.

If your levels are already low because you've been fasting for say seven days or 14 days and then they're suddenly dramatically lowered even more, it can put you at very real risk of refeeding syndrome which is very very dangerous. In fact, it's a medical emergency and it can result in severe disability, harm and death.

Sudden changes in glucose status could also shift sodium and other key electrolytes so this is the reason why we really want to optimize our electrolytes before the fast, and why you should be mindful of what you eat when you break the fast.

Ending the fast

I think I mentioned in the previous episode on intermittent fasting the breakfast isn't the most important meal of the day but break fast is the most important meal of the day. This is because traditionally we weren't eating three or four meals a day, no, we ate maybe once every few days.

With refeeding syndrome you really should eat a small, very low carbohydrate meal ideally even a zero carb meal with just some small amount of protein when you're breaking a long fast. You really don't want a big spike of insulin and eating a high carb meal will Spike insulin enormously.

Eating a small protein meal will cause a very small insulin spike.

Once your insulin has been triggered, the sensitivity reduces significantly and you're then at much lower risk of refeeding syndrome and can eat a slightly larger meal a few hours later, a few hours later and so on and so forth.

It's still recommended to eat smaller low-carb meals until you electrolytes are back within normal ranges.

It's hard to give a universal guide to prolonged fasting as it very much individual, what medications you take (like  SGLT2 inhibitors), it depends on how long you want to fast for, what your medical conditions are  your previous experiences with fasting and your electrolyte levels before starting the fast.

As a general guide I would recommend you discuss it first with your doctor as you may require some medical supervision, particularly depending on the medications I mentioned before in the last episode about SGLT2 Inhibitors. With intermittent fasting you really need to be very very careful if you have type 2 diabetes doing a prolonged fasting on these kinds of medications - there is a significant and very serious risk of a condition called euglycemic ketoacidosis or EKA.

It's also recommended that you should get your electrolyte levels checked before starting to make sure that they're optimised. Once optimised generally I would repeat the blood test every two to three days to make sure they aren't dropping too quickly at the end and then two to three days after the end of the fast depending on the levels then there may need to be subsequent testing.

If the levels drop too much I will advise my patients to stop the fast sooner and safely re-feed and give them guidance on what to do and then do follow-up testing.

Now this might sound unnecessary but remember those electrolyte disturbances are potentially deadly and I really can't overstate this now some people out there are going to say oh well yeah I do it all the time and it's fine. Great bully for you, but you're not every single person and there's others listening or watching this podcast who might actually have serious issues with this, so it's better to be safe than sorry.

Others out there might think oh he just wants the money for consults for these things well actually no I don't because when I see patients and set them up I just give them four or five blood tests to go and get them every few days - I don't charge them for that.

After a fast

Great fantastic - now what? Prolonged fasting isn't a magical solution it's not going to give you a Teflon shield to protect you from further metabolic damage if you go back to old habits - you'll get old issues coming straight back.

You need to make long-term changes for your metabolic health. There's no quick fixes.

A question I often get asked is how often can I do prolonged fasting?

Whilst intermittent fasting is generally safe to do forever I wouldn't recommend regular prolonged fasting as such, and most doctors doing this kind of work wouldn't recommend more frequently than maybe every few months to allow the body to recover electrolytes and hormones the balance and normalize. More frequent than this could potentially put you at greater risk.

It's also just unlikely to be necessary, unlikely to be beneficial and it is definitely going to give you diminishing returns on your fasting.

Actually that's a good point - diminishing returns. What that means is the longer you do it for the less the benefit is

Diminishing returns? there's definitely diminishing returns when it comes to prolonged therapeutic fasting and this is probably due to the hormonal changes.

If all you're interested in is the number on the scales, then yes, you're going to see linear benefits with the duration of the fast, because well you're not eating.

However if you want to be metabolically healthy then you're likely to see less benefits the longer you fast. Beyond a certain point you get minimal benefit. Experts in this field disagree at what point the maximum benefit really is but it's likely to be somewhere at or below seven days. Now I've personally done 14 day fast before and honestly I saw absolutely no benefits beyond seven days. This is probably because by around three days your insulin level is likely to be pretty much next to nothing. In fact after day three mine dropped to one which is barely detectable I mean it's almost like I'm not even alive, though I was clearly. The longer your insulin is low for the more the insulin resistance will drop but you don't need it to be 1 forever as long as you don't return to a high carb diet the insulin resistance levels will continue to fall after the fast.

Growth Hormone

Growth hormone changes can continue increasing and we do know at what point they're going to be at their maximum, but there is data to suggest that it's at maybe 40 days. However the number at 40 days is probably not much higher than seven days if you look at the data and a 40-day fast - well that's significantly risky - very dangerous for refeeding syndrome. At day three growth hormone's already 300% higher than baseline typically, and 40 days it's around 1250 percent. However this isn't going to maintain forever - growth hormone will drop pretty much immediately when you start eating, particularly if it's carbohydrates.

Protein and fat don't affect growth hormone that much and in fact protein can increase growth hormone, but it's not going to keep the levels at 1250 percent. It will drop when you eat so for those reasons I typically don't recommend more than seven day fast and I advise my patients that beyond three days really the benefits are actually quite low and the risks do go up.

I believe that most people in the field believe when you're doing a seven day fast or longer that you also require medical supervision with blood tests and so forth but realistically doing a three-day fast is relatively low risk other than if you have really serious health conditions or certain medications so for people doing three day fast generally for myself if I know them, I know the patient well I know their medical history Etc I often don't even need to monitor them at all they just they just go and do it.

Therefore I believe the benefits of a long fast are really outweighed by the downsides and I would probably stick to three days.

What's a fasting actually like?

So we've discussed the benefits of a prolonged fast and the dangers and how to do it but what's a prolonged fast actually like? What's the experience like? What's it like to actually do a long fast? Are you just going to be insanely hungry the whole time?

Well actually, no you won't. Not likely anyway, but this depends on your metabolic health to start with. Most people are not actually particularly hungry beyond around 24 hours. The hardest part I find personally is around maybe 18 hours then it gets easier. This is actually science and it's because of our hormonal changes, as you fast your body will produce ghrelin - the hunger hormone - this is to stimulate you to eat. However this isn't a constant production - it comes in pulses and generally by around 36 hours is actually pretty much next to nothing. If you can kind of push through your hunger will actually go down as ghrelin levels fall.

At around 36 hours very little to no ghrelin is actually produced - that's one and a half days. The other effect is as the insulin levels are falling a hormone called leptin can start to become unblocked. Now I mentioned this before in my previous episode on intermittent fasting but leptin rather sorry is effectively your your fuel gauge - you know on your car your fat stores are your gas tank, so leptin's basically saying it's okay calm down I've got enough storage you don't need to eat right - now don't panic you're not dying.

Insulin blocks the activity of leptin so as insulin levels fall, lectin activity effectively increases. So on one hand we have less hunger signals being produced, and on the other hand we have increasing signals saying we already have enough energy storage on board so we need to eat less and this is why you actually don't feel that hungry on prolonged fasts assuming, you are actually fasting and not just not eating.

Fasting will generally lower cortisol as well which can also further reduce your drive to eat and increased growth hormone further reduces ghrelin.

Beyond three days you'll actually feel little to no hunger.

Another question I often get asked is will I feel dizzy or lightheaded? Again surprisingly no it's unlikely to happen because whilst your blood sugar may be a bit on the low side it's actually likely to be much much more stable. Your leptin levels will be higher and your insulin a lot lower allowing your body access to your fat stores for fat metabolism to provide Ketone bodies for fuel.

This is a slightly simplistic view of what happens but somewhat accurate - the insulin levels will fall allowing more ketosis as we said. Now you may at times feel slightly lightheaded yes but it's temporary and short term however if you do feel unwell you should break the fast as we've discussed previously.

Muscles on a fast

Now one of the more common questions I think I get asked is well will my muscles just waste away? Well no this is actually a common myth that your muscles will just waste away if you don't eat - it's actually based on very little to no evidence and there's actually plenty of evidence to the contrary. We already discussed the rising growth hormone around 300 at three days and at seven days will be much higher now if you've ever done a prolonged fast you'll actually be able to feel this effect just ask anyone who's done one before they'll tell you almost certainly they actually got stronger during the fast.

If your muscles were simply wasting away this wouldn't be the case.

When I last did my last seven day fast my strength dramatically increased I could lift my wife up like she was just a piece of paper. Now she's not particularly heavy, but I'm not super strong. I could also carry one of my kids on each arm at the same time like they were just nothing didn't phase me at all my weight lifting went up all my weight went up but I certainly did not feel or looked like my muscles were wasting away.

I had a DEXA body scan afterwards which confirmed my muscles hadn't gone down at all in fact they'd gone up.

Actually just a quick note on DEXA body scans - you should compare like for like so kind of true fasting versus true fasting or eating versus eating or the results will be skewed muscle glycogen will deplete during a fast and will make your muscles look less dense, and actually fool with a scanner. They'll say they're fat when they actually aren't.

Anyway back to fasting

Brain on a fast

Ask anyone that's ever done a fast chances are they felt amazing. True mental clarity.

Most people report their reflexes, their cognition, their brain function is just kind of just on fire - in in a good way - they're just on point. Quick to respond, cognitively agile, able to recall memories just in intricate detail that they just never really could before.

I remember when I did my first fast my first seven day fast I was absolutely blown away by this - my memory was kind of like I'm in an 8K maybe even 16k screen in front of my face - just you know full clarity. I could recall words from my medical textbooks that I hadn't read in over a decade like I could just see the pages in front of me, it was really bizarre but great.

All of my senses became heightened. My vision improved, I got less sensitive to sunlight, my hearing was just pin drop sharp, my cognition and responses were just were just reflex without any effort. In fact my wife was actually really stunned when I actually, well, answered a question that she asked me um and not just the fact I actually heard her and answered but I actually answered the question correctly.

Now my memory isn't fantastic for small details especially dates, I kind of get a lot of things wrong and she always corrects me so actually it was a bit unusual that I would just fire off this answer without even thinking, getting it correct. Especially the fact that she was about 10 meters away and I was boiling the kettle and I could barely hear her - yet I just answered without even a thought.

Let's think about this from a nature or an evolutionary perspective and it might help to explain maybe why this is the case.

So predators in the wild, and of course humans we are predators by design, we should be no exception now predators don't eat 24/7. In nature they didn't go to McDonald's you know the lion doesn't go to KFC when they're hungry so they go hunt they run they expand energy they have to generally catch their prey. Scavengers are different of course, but most big predators will eat every few days maybe, sometimes they might even go weeks between meals if they can't catch anything.

Now if they got weaker and frailer the longer they didn't eat, basically by the time they try and catch something, then they're not going to be able t.

They don't get weak and frail, their muscles don't fall to pieces, their brain doesn't turn to mush. If they did they'd never catch anything, they'd die. In fact often it's actually the opposite. If you watch any any nature program you know their energy levels actually seem to go up, their brain and reflexes get more acute and they become better hunters. After all, at some point, I mean if you can't catch something, you're gonna die, so it makes no biological logical sense for your brain and body to just disintegrate and turn to mush if you don't eat for a few hours, even a few days. If anything it makes sense that it would improve to make sure you can actually catch your prey - this would be an evolutionary advantage.

Surely then repeated fasting should be a really good thing then? Well there's not really any evidence to actually support that. Doesn't mean it wouldn't be but I think you probably get very limited benefit from frequent fasts again you know lions tigers Etc they're not fasting for months on end every few months.

As a random thing yeah maybe every few months it could be a good idea to fast, maybe reset our metabolic Health Etc, but that's mostly just unproven and pretty much just conjecture on my part.


So should you do prolonged fasts and what's the ideal time frame? Well there's no real evidence to say the people that fast have better health overall, but there's lots of evidence that fasting and fasting mimicking diets (the FMD) can be beneficial for health. I personally think that most people should do prolonged fast occasionally and whilst there's little evidence on what the best time or duration for a fast is I usually recommend about three days it's sufficient to see the maximum return on investment with minimal risk.

After three days there's definitely diminishing returns and increased risks.

Who shouldn't do prolonged fasts? Well like with anything generally - pregnant or lactating women - breastfeeding women shouldn't do prolonged fast, children should not do prolonged fasts, babies definitely do not fast.

Again there's no evidence to say it's harmful but there's also no evidence to say it's helpful in these individuals and they would be a higher risk of harm people with certain medical conditions as I said and these people should also be wary about prolonged fasting particularly those with diabetes especially type 1 definitely type 2 if you're taking SGLT2 inhibitors or DPP4 medications, or you have glucose issues those with kidney problems, liver problems or a history of gout because fasting can trigger gout and history of kidney stones.

Anybody wanting to do a fast longer than 24 hours I believe should seek some medical guidance even though it's generally safe it's good to have these discussions with people just to make sure that they are being safe.

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