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I really urge people to do a multi-day fast a few times a year. I wrote a little guide about it that I shared previously [1].

You can wonder about these longterm positive effects, but it's sort of a Pascals wager. You lose absolutely nothing and it could have significant upside. Worst case, it makes you confront your relationship to food and offers a great mental challenge.

One cool thing that you learn is that there is a maximum hungry you get. It doesn't ever go past that point (and you've felt it before without fasting). Even better, it ebbs and flows. You won't be in a state of "OMG I am starving!!" all day long. It hits at normal eating times, and then retreats to the background.

Anyway, just try it.

[1]https://www.nicholasjrobinson.com/blog/general/five-day-fast...

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I find that physically you are fine when you do this: you can walk around, do house work, etc... but mentally you are shot.

I would love to try this - but certainly dont have 5 consecutive days where I can be a mental vegetable.

The brain needs constant fuel. Anyone experience this as well?

I found my experience to be the opposite. I had clear mental focus, but felt physically drained of energy the entire time. I did a 3.5 day water fast, with the majority of my fluids from water/tea with salt for electrolytes.

My sleep was pretty terrible up until the last night, when I got a solid 8 hours without waking up once. I usually wake up 8-10 times a night, so this was a huge breakthrough. I also regained my energy on that last half day and felt incredible.

What type of fasting did you do, and what liquids did you drink?

I generally find myself sharper, more alert and with much more energy. This continues the longer your fast lasts, up until day 7ish in my case.

Your noradrenaline levels spike when fasting, and it can make it hard to sleep - too juiced haha.

If you're finding yourself tired, it could be a lack of electrolytes. Try a teaspoon of salt in a pint of water. I tend to drink coffee when water fasting, which has an outsize effect.

Same. It makes sense to me intuitively: when you're hungry, you go into an alert "hunting mode". My sense of smell is normally very weak, but when I am hungry it increases by a factor of 100.

(Consider using “I” instead of “you”.)

I have found that once I become fat adapted, I no longer suffer from mental clarity problems. It takes a couple of days without carbs to get there for me.

Naps help. Plan your day around one or even two naps.

Also watch your electrolytes. Salt, potassium and magnesium can greatly affect energy levels.

> The brain needs constant fuel. Anyone experience this as well?

That is true, however that fuel does not need to be glucose. As far as I understand it, people eating 3 meals a day and snacks (all containing carbohydrates), their body is not used to relying solely on fat/ketones for energy, and it takes time to adjust.

Also, it's very important when fasting to ensure you're replacing water and electrolytes since once you've burned through your stored glycogen you will have lost quite a bit of water. If you're feeling light headed or out of it, that is one of the causes, whether you're starting a ketogenic diet or fasting.

When I'm fasting I use an electrolyte mix which has sodium, magnesium, phosphorus, potassium and calcium. One of the main reasons for refeeding syndrome is that electrolytes are depleted, and when someone eats again, their cells take up what little electrolytes are left in the blood, which leads to dangerously low levels.

To be precise, some of the fuel does need to be glucose. There is no such phenomenon as solely relying on ketones for energy, and the brain is especially glucose greedy.

In a sufficiently fasted state, all of that glucose will be endogenously produced rather than coming directly from food, but it does need to be there.

Endogenously, from the fat/glucose pathway. Not sure that any glucose is required to be consumed directly. But we do love sweets, and fat cells love saving energy.

I did a weekly "don't eat anything on Sunday and don't eat until Monday lunch" for about a year. Sunday afternoon/evening I'd be kinda groggy and grumpy. I'd wake up on Monday morning completely locked in - it felt like a high dose of adderol without ANY of the side effects. I'd get 2 days worth of work done in a morning. I'd always be bummed Monday afternoon because as soon as I ate the focus went away.

In my experience, it seems the brain gets addicted to having a constant flow of glucose streaming into it (it's the most expensive organ to run, calorically) and it gets to think a lot more when it has its fuel. This isn't necessarily healthy, especially if left unchecked for a long period of time. Part of the benefit I believe I've gotten from fasting is re-training my brain to work well in a variety of glucose levels (including very low).

These days, I can feel my body switch modes when I enter a low-glucose state. It's a certain type of relaxation that feels like my brain just stops running certain programs. For example, it can't afford to hold on to dramatic, powerful emotions in that low-fuel state, and so it stops running its "replay enraging memories" program, so to speak.

You're correct that the brain needs constant fuel, but we have an unbelievable source of fuel in our bodies called fat. Increasing the metabolism's flexibility, such that it can gracefully tap into that massive fuel source, has had a noticeable impact on my mental and physical health.

IME, it's more like a hump you have to get over, for what that's worth.

It takes about a day for me to break through this.

I do prefer resting physically when fasting, but the mental clarity is unparallelled after 24 hours.

Wait. Why is it possible fasting has upsides, but it is not possible it has downsides?

You could ask the same question about eating 3-4 meals per day.

This is a relatively new phenomenon for humans - starting in the 1700s. Feast/famine is how we used to live. The Romans ate one meal a day and frowned upon breakfast, for instance - their default was what we would now consider OMAD fasting. [1]

[1] https://www.bbc.com/news/magazine-20243692

Just because you can ask the same question about eating 3-4 meals per day DOES NOT mean that we can automatically assume that fasting might be better but is definitely not worse.

It could be better, worse, or the same.

We don't know. And there's no reason to assume that it might be better while it is definitely not worse (and therefore an obvious choice).

There are reasons to assume it might be better. It dramatically raises HGH levels [1] which improves body composition (increases the proportion of muscle to fat), improves insulin sensitivity, lowers blood pressure and reduces general inflammation / oxidative stress - even in the absence of weight loss [2]. It also burns fat (obviously, since there's nothing better to burn when fasting) which for the vast majority of folks is strictly better than the alternative.

There's a lot of compelling studies being done at the moment, including the interaction of fasting with the immune system. [3]

> Last week, in a paper just published in the New England Journal of Medicine [4], Rafael de Cabo and Mark Mattson reviewed multiple strategies for fasting that have been tested in the years since Longo's study. The news continues to be very encouraging: intermittent fasting is good for you.

I'm not saying it's conclusive but to suggest there's no reason to think it might be better is disingenuous.

There's been some pretty interesting studies over the last few decades (few and far between, but interesting) - and the idea of fasting for heath is enjoying something of a renaissance in nutrition research these days.

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC329619/

[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5990470/

[3] https://www.forbes.com/sites/stevensalzberg/2020/01/06/can-i...

[4] https://www.nejm.org/doi/10.1056/NEJMra1905136

> I'm not saying it's conclusive but to suggest there's no reason to think it might be better is disingenuous.

I didn't say there's no reason to assume it might be better!

I said there's no reason to assume it might be better AND that it is definitely not worse.

I suppose that might be true, but I'm not sure how one would go about and prove a negative like that. Especially in the field of nutrition science which has famously alternated between calling coffee the Devil's Hot Bean Juice and the Second Coming. Current consensus appears to be, based on a 1.2 million person meta analysis that 3-5 cups per day reduces the risk of cardiovascular disease. [1] I suspect in reality there are just too many factors to do that.

[1] http://circ.ahajournals.org/content/129/6/643.long

From an evolutionary perspective, our bodies were designed for fasting. They were NOT designed for 7-8 decades of plenty.

Devil's advocate: From an evolutionary perspective, our bodies were designed as mere vessels to propagate our genes.

They were not designed to survive 7-8 decades regardless of diet, because by this time reproductive success (and therefore fitness and pressure) is zero for women and almost zero for many men.

Devil's advocate against this point: Grandparents might increase the reproductive success of their children's children, helping their genes propagate in accordance to Kin theory.

Devil's advocate against Devil's advocate of this point: Grandparents might also decrease the reproductive success of their offspring, since they must care for their own children as well as their parents. Hence, the near inescapable phenomena of dementia as age increases into the 100s, and the trait of Senicide (ex. Ättestupa) in many cultures.

My point being, maybe it is best to avoid categorical statements about "evolutionary perspectives."

edit: Correction, Ättestupa is mythological and not a good example of Senecide. The wiki for senecide has many other examples, however.

I'm afraid I have to disagree with your point.

From an evolutionary perspective, “vessels to propagate our genes” carries a lot of baggage with it, including the need to be healthy enough to take a baby to term, care for yourself/other family/group members, and raise it until it reaches sexual maturity, all of which require efficient energy gathering, storage, and usage strategies.

> My point being, maybe it is best to avoid categorical statements about "evolutionary perspectives."

Indeed, it's a wide, complex topic that I only know a little about, and I only had time for a brief comment.

BTW love your chain of devil's advocate statements. More people should think/counter that way.

Thank you! I am trying to argue with myself more. Partly because I can be a bit of an opinionated know-it-all which is unpleasant and very foolish!

Also I think about how young we are as a species that uses the scientific method to question our reality.

Maybe all of my knowledge today will not stand the test of time, and will be considered just as silly as someone from the 1600s seems to me today? (ex. A natural philosopher that believes in abiogenesis)

But a mutual respect and bond can exist if we both strive to prove ourselves wrong, adapt, and grow.

Of course, it has downsides—the length of the fast matters a LOT. Too long, and it’s starvation which has considerable downsides as I’m sure you’re already aware.

The biggest takeaway that I’ve gotten recently is that our bodies are literally designed to take fasting into account. Specifically, our vascular system is terrible at doing cleanup and nutrient delivery at the same time. So it needs ample time to do both.

> Specifically, our vascular system is terrible at doing cleanup and nutrient delivery at the same time.

I'd not heard this before, sounds cool. Do you have a reference handy? Thanks.

Here’s one. It’s not said directly, but that was my take away reading several articles along these same lines. https://www.sciencedirect.com/science/article/pii/S156816371... and one specifically about cancer https://acsjournals.onlinelibrary.wiley.com/doi/full/10.3322...

I like to fast for a couple days, simply because I find it helps a lot with mental clarity. However, I think you make a good point.

Fasting absolutely can have downsides. The link in the comment you are replying to, for example, recommends a five day fast which is a lot longer than I've ever seen recommended.

The first obvious downside is that people with health conditions like diabetes, underweight, immunocompromised, etc. need to consult a doctor first. They would probably call a 5 day fast with zero intake of food or electrolytes needlessly reckless with little evidence to support it over a shorter fast.

Medical studies also tend to be of poor quality and don't stand up to meta-analysis by organizations like Cochrane. For example:

https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD...

https://www.cochrane.org/CD013496/VASC_does-limiting-times-y...

The Mayo clinic also argues theres not enough high quality evidence even on intermittent fasting at this point to recommend for or against it in weight loss (where you think it would be of obvious benefit):

https://healthletter.mayoclinic.com/issues/july-2020/is-inte...

Speculating on in-vitro effects and microbiological mechanisms scaling to result in things like cancer prevention or life extension seems like a fools errand.

People saying "oh but it's natural" are also making a naturalistic fallacy. It was also "natural" for much of human history to be infested with parasites and ravaged by disease. Natural is not necessarily good.

> People saying "oh but it's natural" are also making a naturalistic fallacy. It was also "natural" for much of human history to be infested with parasites and ravaged by disease. Natural is not necessarily good.

I think you are committing a fallacy here too. The naturalistic appeal is not about how things were before, but in what conditions we evolved. For example, we could try to eliminate every single germ everywhere, but that would make our immune system weaker, because our immune system can adapt itself. Vaccines are a way of leveraging that knowledge about our immune systems. Same thing about light: we spent most of our evolution time following the Sun cycles, and now we see that people that work shift hours are in a worse condition than people that don't.

A big difference between the way we fast today and the way people may have faster before is that we don't have to hunt or gather for our food, we have certainty about the presence of food at the end of a fast. Though by writing that, I wonder if early humans felt constantly the need for food and the fear of not finding it. I don't even know if animals do.

The biggest downside is that I don't get to eat delicious cakes for hours or days.

> but it is not possible it has downsides

I remember reading somewhere that anorexia used to be more common in highly religious communities where ritual fasting was encouraged: the devout would fast so long that they would find themselves unable to eat again when the fast was over. If you go too long without eating, you can actually lose the ability to do so. I don't suppose that can happen after just a two-day fast, but it does seem like a "possible" downside to me.

The more studies pile up, the less it seems to matter the length of a given fast, but more the total time spent in a fasting state relative to a fed state.

So as a taster, you can do multiple 24-hour fasts and get the same experience from a health point of view (everything else is different, though).

That said, at some point, I’m interested in trying a multiple-day fast myself but haven’t taken the plunge yet.

> The more studies pile up, the less it seems to matter the length of a given fast, but more the total time spent in a fasting state relative to a fed state.

Yes, I've noticed this as well. Personally I fast for 17 hours every day and I've noticed I feel the benefits without negative consequences. Once in a while I'll go for a 3 day fast but aside from the personal challenge accomplishment I don't notice a major gain. My experience is all very non scientific. For the more rigorous science, I usually look to Peter Attia[1]

[1]: https://peterattiamd.com/

> you can do multiple 24-hour fasts and get the same experience from a health point of view

NO, that's incorrect. The length matters a lot. In fact, a major change occurs after the 2-3 day mark, when body glycogen stores are depleted and your body shifts to ketosis etc. Another major marker has to do with when a much coveted state called autophagy is activated.

I have not heard this. Can you share more? I’m familiar with ketosis but unfamiliar with it in conjunction with fasting. Or benefits for otherwise healthy people. Ketosis has lots of downsides for normally healthy people.

Ketosis is literally the start of starvation, so I'm dubious of widespread health benefits. I know it is extremely beneficial to people with a history of seizures, but that's it.

> Ketosis has lots of downsides for normally healthy people.

I've been considering trying ketosis but all I've heard are good things, what are the downsides?

The most significant downsides are psychological and kidney stones, plus its literally (by definition) your body’s signal of starvation.

https://en.wikipedia.org/wiki/Ketosis#Safety

Oh and this gem from the article on starvation.

> After two or three days of fasting, the liver begins to synthesize ketone bodies from precursors obtained from fatty acid breakdown. The brain uses these ketone bodies as fuel, thus cutting its requirement for glucose. After fasting for three days, the brain gets 30% of its energy from ketone bodies. After four days, this may increase to 70% or more.[11] Thus, the production of ketone bodies cuts the brain's glucose requirement from 80 g per day to 30 g per day, about 35% of normal, with 65% derived from ketone bodies. But of the brain's remaining 30 g requirement, 20 g per day can be produced by the liver from glycerol (itself a product of fat breakdown). This still leaves a deficit of about 10 g of glucose per day that must be supplied from another source; this other source will be the body's own proteins.

This is the biggest reason I've never done more than toy with ketosis is the body consuming muscle for protein to supply that remaining 10g of glucose per day. Esp. because 1 lb of muscle only 1,800 Cal vs 3,500 Cal / lb. Fat.

There's nothing in the article you linked to support your claim that it's "literally your body’s signal of starvation". It does happen during starvation, but that's it.

The first two phases of starvation are defined in terms of ketosis. It is by definition the start of starvation.

https://en.wikipedia.org/wiki/Starvation

Phase one: ... After that, the body begins to break down fat and protein. Fatty acids are used by the body as an energy source for muscles, but lower the amount of glucose that gets to the brain. Another chemical that comes from fatty acids is glycerol. It can be used like glucose for energy, but eventually runs out.

Phase two: Phase two can last for up to weeks at a time. In this phase, the body mainly uses stored fat for energy. The breakdown occurs in the liver and turns fat into ketones. After fasting has gone on for one week, the brain will use these ketones and any leftover glucose. Using ketones lowers the need for glucose, and the body slows the breakdown of proteins.

Ketosis being present when starvation occurs does not mean that starvation occurs every time ketosis is present. A in B doesn't mean B in A.

It seems we're arguing semantics here. My point is that it is biologically indistinguishable from starvation at this point again by definition. Without an interview a doctor could not tell the difference.

Your point seems be an objection to the use of the word starvation because it doesn't progress to the later stages due to maintenance of available fatty acids. And that it can be useful and therefore the word starvation is inappropriate. Does this sum it up?

No, we're not arguing semantics. The definition of starvation, according to wikipedia, is: "Starvation is a severe deficiency in caloric energy intake, below the level needed to maintain an organism's life.". However ketosis can occur at a normal caloric energy intake. Also from the article: "The symptoms of starvation show up in three stages. Phase one and two can show up in anyone that skips meals, diets, and goes through fasting.". But you can get into ketosis without fasting or skipping meals. I'm not sure what "diets" means here. If it means "following a non standard diet", that would cover low carb diet. However if it means "reducing your caloric intake for health/aesthetic reasons", it wouldn't cover it.

That’s interesting but I was planning to eat, which means that those 10 grams of protein would come from food. Unless I missed something important there?

ketosis increases risk of kidney stones, which is a pretty huge downside given that many people describe kidney stones as more painful than childbirth

That certainly doesn’t sound fun! Thanks, I obviously need to do more research.

This has been my personal experience. Things go strongly ketogenic around day 3. It’s an experience worth having at least once in your life, even if you’re not interested in a ketogenic diet. The longest I’ve fasted is 7 days, and things felt quite a bit different between day 2 and day 4.

I love fasting and typically I have one meal and fast for 22 hours a day, but please be careful with multi day fasting.

I drink a lot of water and on two occasions on 60 hour fasts I passed out cold, falling to the ground and chipping one of my front teeth in the process, but it could have ended much worse.

I eventually figured out I was not consuming enough electrolytes relative to my water consumption, I still recommend long term fasts but please do more research than I did and be careful.

I have kept that chip in my tooth to this day to remind me my naivety and how lucky I was.

I used to do 16/8 or 18/6 and it's pretty easy. I still usually don't eat breakfast because it makes it easy to get > 12 hours fasted.

A few months ago, though, I started doing a 24 hour fast every week. It takes some planning but it's surprisingly easy. I'm damn hungry by the end, but it's never been an issue. I'll even work out and lift heavy 22+ hours into the fast without a noticeable decline in strength. I have no plan to stop doing this.

I'm curious what planning you mean? I do it without planning and haven't had an issue.

Planning my social life around it. I'm not as fun at dinner if all I can have is water.

The study in the OP is specific to alternate day fasting though, which is generally much more tolerable than straight-up multi-day fasts, and can be done over a much longer term.

How do you overcome upset stomach? If I don't eat for half a day my stomach starts to hurt.

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