This isn't obvious to me. I don't run seriously, but I don't find that fasting has any adverse effect on my performance. If you are talking about running a marathon or something like that, then sure, but hardly anyone is doing exercise that intense multiple times a week, so you could still have fast days.

16 hours really isn't a very long time to go without food. Your body can cope with it fine, even if you're doing quite intense exercise.

I always do my runs in the morning, right out of bed, fasted. Your body is very good at storing energy and using it later, I've had no issues (if anything, trying to eat before a run just causes problems for me).

Honestly the hardest part was from other people, the fasting part was fairly easy and made me feel great.

I grew up doing sunrise to sunset fasts for Ramadan, with no water, and even that was mostly mental.

I can manage about seven hours sleep + a run first thing but I have to start eating as soon as I'm back in the house, and just keep grazing until 22:00

That's not because your current system has any problems--if it works for you, great! But that discomfort you describe is both abnormal and frequently linked to health issues that could cause serious problems over time.

Source: dietician relative on the couch next to me, FWIW.

Not obvious at all. My best marathon times followed training cycles where I didn't eat breakfast before long runs (i.e., 13+ miles).

Fasting doesn't give you the ability to break rules of biology.

Besides that the standard: this is done on 8 people over a very small period of time, so it's at best a pilot study anyway.

IF may be a great way for some people to comply to their dietary goals better, but there is still not a single study that shows it actually does anything besides that.

I think Keto is the same way. There isn't strong evidence of a clear metabolic advantage (I'm not saying there is not one), but its one of the few ways I have eat a caloric deficit and not hate my life.

I think if IF and Keto take people off a insulin roller-coaster and let them comply with their targets, that is still a huge win, in and of itself. We know their are other benefits too such as depletion of stored liver fat or better acute insulin responsiveness, but these may be gravy, the real win is in compliance IMHO.

> We demonstrate for the first time in humans that eTRF improves some aspects of cardiometabolic health and that IF's effects are not solely due to weight loss.

It fails to do so because it does not measure food ingested, only food served, and so we have no idea if the diet was actually isocaloric (hint: it wasn't).

Keto and IF proponents both want to see a metabolic advantage that simply is not there. Compliance is huge, but there is no existing study showing a metabolic advantage that doesn't also have a caloric deficit.

A caloric deficit eventually forces the metabolism to slow down, hence the yo-to effect afterwards, whereas fasting doesn’t, not when implemented correctly.

In other words, yes you have the caloric deficit with fasting, but without some of the downsides of the body going into starvation mode.

If you are to believe the hyperinsulinemia theory of obesity, it actually makes perfect sense — it has been shown that insulin inhibits the body from accessing its own fat deposits. It has also been shown that you can make anybody fat by injections with insulin, no matter how thin they are. And there are several drugs known to make people fat by stimulating insulin. These are facts.

So by eating multiple meals per day, you’re stimulating your pancreas to trigger insulin in the bloodstream multiple times per day.

Thus the body will not have access to its fat store while that happens. As long as your insulin is high, no matter how much fat you have, you’re starving without eating food.

N.b. there has been plenty of evidence of this for the last several decades. What is new is the discovery of leptin and glucagon, two other hormones that are the opposite of insulin and that do a lot to fill in the blanks and explain why insulin is having the effects we’re seeing.

Also the metabolism is slowing down on caloric deficit, with the result that people eventually regain that weight back and then some, this has been shown in many studies, including that huge one from the Women’s Health Initiative.

When you’re doing keto or fasting, the insulin goes down. Again, this is a fact.

This allows the body to consume energy from its own fat deposits. This means that the metabolism shouldn’t slow down, when implemented correctly. Having fat isn’t for good looks, but an evolutionary advantage, we are supposed to use our fat store when in need, otherwise we would have evolved to excrete the excess.

Now of course, here we are talking of healthy people. It’s debatable if the metabolism slows down or not when we are speaking of diabetics or the obese. And actually I talked with people that claimed they measure their metabolic rate and it did slow down with fasting. So it works better for some people than for others.

But on the metabolic advantage, that advantage is undeniable for diabetics at least. In the next couple of years you will hear more and more of keto being promoted in the treatment of diabetes, because it works and there’s also a large study that’s still in progress, but that promises great results.

And btw, the reason for why keto hasn’t taken off in the medical community is due to perverse incentives. Nobody wants to invest hundreds of millions of dollars in proving that a diet works, because a diet doesn’t generate profit. The cold, harsh truth is that it’s more profitable to sell statins and synthetic insulin.

Funny enough, they did invest hundreds of millions (billions?) in trying to show that saturated fats and salt are bad, anything and everything to blame, except for sugar. All such studies failed of course, but that didn’t stop the dogma, anything to keep us consuming sugar, which is where the “_a calorie is a calorie_” bullshit comes from.

The "starvation mode" idea is thrown around a lot but is categorically not something that overweight people ever go into. The premise came from a study done on a tiny sample size in the 1950s of severely malnutritioned soldiers. It is an extreme, near death state.

> A caloric deficit eventually forces the metabolism to slow down, hence the yo-to effect afterwards, whereas fasting doesn’t, not when implemented correctly.

Citation needed, specifically on the second part. Isocaloric diets with different compositions have not been shown to have any difference in weight loss amounts.

> So by eating multiple meals per day, you’re stimulating your pancreas to trigger insulin in the bloodstream multiple times per day. Thus the body will not have access to its fat store while that happens. As long as your insulin is high, no matter how much fat you have, you’re starving without eating food.

No, this is a misunderstanding of the metabolic system. The body is very good at storing and using fat, and there is zero evidence that an isocaloric diet prevents your body from accessing fat stores. If you eat the same amount of calories, you will supplement (or not) the same amount of energy from your fat stores regardless of when or what those calories are made up of. There may be small statistically significant differences here, but there absolutely has not been a study showing clinically significant differences. Every study that has shown improvement in metabolic markers has not compared isocaloric diets, as I mentioned in the comment you are replying to. I'd be happy to look at any study you think does, but this would be a massive shift in the current understanding of metabolism and so it is unlikely such a study exists.

> When you’re doing keto or fasting, the insulin goes down. Again, this is a fact.

I'm not sure what you mean by "the insulin goes down" but basically this is not a fact because it has not been showing in isocaloric diets.

> This allows the body to consume energy from its own fat deposits. This means that the metabolism shouldn’t slow down, when implemented correctly.

Again, this is a misunderstanding of the metabolic system.

> And btw, the reason for why keto hasn’t taken off in the medical community is due to perverse incentives. Nobody wants to invest hundreds of millions of dollars in proving that a diet works, because a diet doesn’t generate profit. The cold, harsh truth is that it’s more profitable to sell statins and synthetic insulin.

The medical community is extremely messed up, but the part of it that focuses on treating obesity regards keto as a way to build compliance, but has largely read the studies and come to the same conclusions anyone else who understands the basic biology has: the premise that keto or IF produce metabolic changes outside of the ones that come from caloric deficits brought on by increased compliance is extraordinary and requires extraordinary evidence, which has not been produced.

This one, mentioned in this context:

> This was a randomized crossover, isocaloric and eucaloric study. That is, all patients did both arms of the study eating the same foods and the same calories and then compared against themselves. The two arms of the study were eating between 8am and 8pm, and the eTRF strategy of eating between 8am and 2pm, but remember, both groups ate three meals per day of the same foods. Some would start with the conventional diet, then cross over to eTRF, and others did the opposite, separated by a seven-week washout period. Subjects were men with prediabetes.

> The benefits were huge. Mean insulin levels dropped significantly, and insulin resistance dropped as well. Insulin is a driver of obesity, so merely changing the meal timing and restricting the number of hours you ate, and also by moving to an earlier eating schedule, produced huge benefits even in the same person eating the same meals. That’s astounding. Even more remarkable was that even after the washout period of seven weeks, the eTRF group maintained lower insulin levels at baseline. The benefits were maintained even after stopping the time restriction. Blood pressure dropped as well.

Specifically this sentence:

> so merely changing the meal timing and restricting the number of hours you ate, and also by moving to an earlier eating schedule, produced huge benefits even in the same person eating the same meals

is incorrect, and is basically the lynchpin of the entire IF idea.

Maybe you meant to question the study itself, but I don’t understand your objection to that either. The study tests whether this form of IF improves certain markers of metabolic health, and it shows it does — within the design of the study, of course, but that’s true of every study and doesn’t make it meaningless. You seem to be implying it doesn’t work in the way IF proponents think it does. That may be true, but isn’t relevant to this study since it doesn’t attempt to answer.

The study, like all similar IF studies, showed that people who do IF sometimes are able to use it to consume fewer calories. That is important from a compliance standpoint: it means you can use this diet to treat obesity for some portion of the population for whom it is going to be effective because it will get them to comply with not eating too much. Not eating as much is associated with weight loss which is associated with all kinds of good outcomes. That's awesome, but it doesn't mean that metabolically there is something significantly different going on outside of eating less.

This is an important discussion in nutrition in general, and shows up in IF as well as keto discussion frequently, with proponents of each arguing that beyond the compliance aspect, there is also something special going on with your metabolism (ie: insulin sensitivity and other metabolic markers). The problem is that all of these are things that improve when you are in a caloric defecit, and the studies never disambiguate between the effects of a caloric deficit and the meal timing itself.

Fung makes the same mistake the authors of the paper do in interpreting their own results too optimistically. The dietary controls are set up in a way to make it seem convincing in an abstract if you are familiar with this area and what the current core issues are, but it is not at all if you dig in.

Exactly. I've had pretty bad experiences with a no-breakfast IF. After two or three weeks on the diet, I get insomnia, and I definitely feel more stress. On the latter, I've read that IF raises cortisol levels, so that might be it. That's actually pretty good when you're at low baseline stress levels (it puts you in a state of "positive anxiety"), but when you're stressed from work it can take you over the limit.

Ultimately I stopped it because I'm already at around ~12% BF - I couldn't even say why I was doing it anymore. Three healthy meals a day and regular exercise does it for me.

If this works for people without negative consequences, then all the more power to them. I really like how it goes against the "shove things into your mouth at every waking hour" philosophy, which has become a ridiculous dogma in modern society.

If you fast long enough, say 48 hours I think, your body enters a mode of "autophagy", which is highly beneficial. So there's that.

Does anything like that happen to you? Do you know if it would feel better during autophagy?

I think this is a pretty common experience. You don't start feeling the "good feels" until you get past the 4-5 day mark.

Anyway, this was about 7-8 years ago and I haven't tried again since, but I often think about it.

For what it's worth, I do IF as a lifestyle (18:6). Basically (for me) it's a very easy way to help restrict calories. Hopefully there are some health benefits, too.

The first time I did a longer fast (72 hours) I felt low-energy, as you describe. Then I ate a single large meal and did another long fast; that felt quite good for the duration of that fast... so I did that for a month, more or less, and lost a lot of body fay (I'm 6'4" and I went from 225 to 200).

And then I was being pretty healthy for 8 months before going back to occasional drinking, but I still don't do sweets other than fruit.

I mostly eat a single meal in the middle of the day (though I make a lot of compromises due to being around other humans) and I don't usually find myself hungry in a way that is intrusive on my thoughts. However, I did start rock climbing in a gym and I upped my caloric intake and would eat a small meal afterwards.

I did another series of fasts this spring to bring my weight down to 175, and it's stayed there. And then I did a short series of longer fasts this fall, and didn't feel low-energy during them.

I can say that after all this, I was climbing hard 5.10s and easy 5.11s, and I'm 40, and I haven't been able to climb stuff like that since I was about 22.

https://www.amazon.com/Complete-Guide-Fasting-Intermittent-A... - another book by Jason Fung that covers water fasting more in depth.

As for fat reserves, even fairly lean individuals can probably go a week or two without food without breaking down much muscle. 14 days at 2500 calories is merely 10 pounds of fat. Unless you are morbidly skinny, you should have 10-15 pounds of fat ready.

I did a one week Buchinger fasting and did several long distance runs (as I always do) after 2 days fasting. (after switching to cetogen burning) I noticed a small decrease in power, but in no way this was a no go. It was actually fun.

Fasting is followed by animals too. When they fall sick they fast. Deliberate fasting in humans is a necessity because humans eat unnatural food.

Let's begin with the fact that we're not lions. We're not even carnivores. And there's some research evidence that in omnivores, short-term fasting leads to increased acidification in the stomach, while it doesn't for carnivores - that's a rather substantial difference.

Then, we come to the magic of "unnatural" food. We'll leave out the fact that such a thing doesn't exist and assume you mean processed food. What exactly is the proposed mechanism here where fasting is beneficial? And why can't that same mechanism be achieved via less processed food? And which processing are we even talking about?

Also:

> Why do all wild lions look the same?

Have you actually seen wild lions? They look roughly the same in the sense that "all white people look the same". I.e. not at all.

It is evident we are not lions. It is also evident that we are not carnivores. Are you sure we are omnivores?

Fasting has been recommended in Eastern cultures for thousands of years. If they were bad, we would have known.

> Then, we come to the magic of "unnatural" food. We'll leave out the fact that such a thing doesn't exist and assume you mean processed food.

Processed food is unnatural. We were not designed to eat such crap. The diseases we have to endure are the evidence. Try feeding your dog or cat the same crap and they get sick too. What more proof is needed?

> Have you actually seen wild lions? They look roughly the same in the sense that "all white people look the same". I.e. not at all.

What I meant is that they look lean and mean. Not one obese lion will you find in the wild.

Yup. We have evolved to consume meat, as well has plant matter. You can argue this point down the rabbithole, but it's irrelevant to the broader point. If you want to say we're herbivores, fine. Lets not get lost in that debate.

> Fasting has been recommended in Eastern cultures for thousands of years. If they were bad, we would have known.

So has circumcision. I say that as someone not categorically opposed to it - but to say that just because something has been done a lot, doesn't mean it's not harmful.

> Processed food is unnatural....

Processed food is not harmful on account of being processed. It's can be harmful for what it contains. Most food is processed.

I assume you're talking about weird nasty foods. I don't have a proper label to define them by. Things with a ton of Hydrogenated Oils, or whatever is inside "Cheez Wiz"

> Try feeding your dog or cat the same crap and they get sick too.

For your very broad statement of processed food, this isn't true. I mean, dog food itself is processed. For food that the animal should not be eating, yes. That'll make them sick. Dogs can't eat all human food. Humans can't eat cow food. A dog will get sick if it eats chocolate. A human will get sick if they eat nothing but grass.

> What I meant is that they look lean and mean. Not one obese lion will you find in the wild.

That's caused by external factors, not because a wild antelope isn't "processed" or "unnatural". You can feed a lion antelope meat until it is obese. But it has evolved to be in a healthy balance when it exercises through hunting, and its intake is limited by the available prey.

OK. Let us not argue this point.

> So has circumcision. I say that as someone not categorically opposed to it - but to say that just because something has been done a lot, doesn't mean it's not harmful.

This assumes that ancient cultures were naive and we are too evolved to even bother about our ancestors.

> Processed food is not harmful on account of being processed.

I would recommend you to read the WHO report that puts all processed meat under category 1 carcinogen. Processed wheat, rice, oil... everything has been proven to be harmful.

> I mean, dog food itself is processed

That is because we are involved. It does not have to be. Raw meat is available and is natural for them. Dogs and cats raised in urban world are obese and have health complications like humans. Diabetes, heart diseases are common.

> You can feed a lion antelope meat until it is obese.

Sure. Good food and no exercise is a bad combination. Bad food and good exercise is also not a good combination.

And raw red meat is a category 2a carcinogen.

That makes it safe to consume?

Yes, you can look at our digestive system and teeth to see clear omnivore adaptations. Both are closer to say bears than either cows or lions.

What's really interesting is lions for example can't grind their teeth.

'dry-aged beef' is still rather sought after. "The process of dry-aging usually also promotes growth of certain fungal (mold) species on the external surface of the meat. This does not cause spoilage, but rather forms an external "crust" on the meat's surface, which is trimmed off when the meat is prepared for cooking." https://en.wikipedia.org/wiki/Beef_aging

https://honest-food.net/on-hanging-pheasants-2/

I could go on, but people can digest uncooked or well aged meat. It does have a higher risks, but that's more about how meat is raised and prepared not necessarily simply being old and uncooked.

> 'dry-aged beef' is still rather sought after. "The process of dry-aging

"The process of dry-aging"... Processed. Cannot count this too.

We do not have protein receptors in our tongue. We cannot taste flesh the way carnivores and omnivores can. They do not need to cook, add spices, process flesh. Raw meat can kill us.

such as the dreaded raw egg. The only thing making this easier to digest is the grinding part.

Unprocessed entire raw and often living fish are regularly consumed as part of fraternity initiations and bets. It does not cause problems.

Cooking increases food safety, but it's really not required for digestion.

PS: To use a slightly older tradition. People used to do something similar in Europe. Hang birds up by the neck, when it fell down it was 'ready'. Calling that processed is ridiculous.

But that's not even germane to a discussion. Omnivores are not necessarily scavengers, nor does it mean that they eat literally everything. It means they eat meats, and plants. (Look e.g. at the maned wolf for another example of an omnivore that pretty much avoids rotten meat. They get 50% of their diet from - fresh - prey, and 50% from plants. That makes them an omnivore)

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

And I’m not sure what you thought I said, but I’m not arguing that milk consumption is great. Well, not in the post you responded to; I do think milk is pretty amazing. All I was saying is that it is an example of humans evolving adaptations to their diets.

But on the topic of milk consumption: there may be drawbacks, but the gene for lactase persistence is one of the fastest spreading genes in recent history. Consuming milk was an utter (there’s a pun to be made here, but I will exercise restraint) game changer for Northern Europeans. Even today, milk is an amazing source of well-balanced nutrition. It’s cheap, easy to consume, and pretty tasty at room temperature.

If it doesn’t cause explosive things to happen to your intestines, that is.

Same with milk, meat, sugar...