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They Studied All Fasting Lengths, This Dropped the Most Fat (12hr, 16hr, 24hr, 36hr)

Thomas DeLauer·youtube.com·13 min read·Mar 28

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Daily 16-8 fasting feels productive until it suddenly stops working, and that's not really

willpower, it's biology. When you compare 12-hour fasts, 24-hour fasts, 36-hour fasts, 72-hour fasts,

you can actually see where 16-8 goes wrong and why it eventually slows down the very metabolism

that you're really trying to speed up. I actually had a moment years ago where I was doing 16-8

just about every single day. I felt really sharp. I actually felt really light. I mean,

all the feelings that you know of, like it feels really good. And then one morning I kind of woke

up and I just felt flat. And it was like the first day I realized, wait a minute, my energy was really

low. My strength had dipped. And I remember thinking like, how am I eating less and somehow burning less?

Like it feels like my engine's just not running hot. And that moment sent me down this scientific

rabbit hole of trying to understand the frequency of fasting, not just the duration. And once you

understand this and you see it, you can't really unsee it because fasting is a lot more effective

and honestly just way more reliable. It's more dependable, sustainable. You can use it as an

accurate tool. And that's what I do now. And it's so effective. So here's where we're going with all

of this today. And I want to keep it super clear and really not get too far in the weeds of the

science. But first we're going to look at what fasting actually does to metabolic rate because

there's a sweet spot where the metabolism increases, but then there's a point where it actually

nosedives. And then secondly, we're going to look at glucose tolerance because the goal isn't to burn

fat for one day. The goal is to become metabolically flexible long-term. So you're not having to fast

every day. Then third, we're going to put it all together. So you know how often you should fast

without really slowing your metabolic rate and also how to do it without wrecking glucose control,

because that can actually happen. So let's go ahead and let's just jump into section one,

which is going to be all about the metabolism piece with some cool studies there. And I want to talk

about one study in particular that was in the British Journal of Nutrition. And it might be one

of the most misunderstood fasting papers that's out there. And it's good that I'm covering that

because it was a big one. And they looked at 12-hour fasts. They looked at 36-hour fasts. And they looked

at 72-hour fasts. Now intuitively, you'd think that the longer you go, the more your metabolism ramps up

because fasting does increase epinephrine and norepinephrine and adrenaline. So things do actually

ramp up as you go into a fast. It's partially true, but only up to a point. Because here's what

they found. After 12 hours, the metabolic rate barely changed. It was really just kind of neutral.

I mean, there was more fat oxidation for sure, but metabolic rate wasn't changing. What they found is

that after about like 24 to 36 hours, metabolic rate was increasing. Your body really starts ramping

up norepinephrine. You get a major increase in circulating fatty acids in conjunction with the

adrenaline, which just puts you into like fat-burning mode. You really upshift into that fat-burning

gear. Then after about 72 hours, metabolic rate doesn't go any higher than the 36-hour mark. So what's

happening is that short-term fasting increases the catecholamines. We got adrenaline, norepinephrine,

and that keeps energy expenditure from dropping. It actually protects you. But once you extend the fast

beyond 36 hours, but this also ties in with fasting too frequently, the body recognizes a consistency

pattern and it starts to say, oh, oh, oh, we're starving. We should conserve energy. So what this is

telling us is that a well-timed long fast speeds your metabolism up. But doing it too often, even with

shorter fasts or stacking calorie restriction on top of it on the days you're not fasting, will slow it

down. So that brings me to the second point in this video, because short-term fasting is just one side

of the coin. We need to kind of talk about how this all plays together, right? Because the other side is

what happens when you're chronically under eating without realizing it. Been my problem for like a half

a decade, right? You get hooked on feeling good with the lower calories. And there's a really good study

that was in the American Journal Clinical Nutrition, and it looked at what happened when people overate for

one week, but then went into a 50% caloric deficit for three weeks, then refed for two. So overeat one

week, then calorie deficit for three weeks, and then refeed for two. Three weeks of what most people

would consider just dieting a little too hard, all right? So in that short window, their resting metabolic

rate tanked by 266 calories per day. In three weeks, their daily metabolic rate didn't down 266 calories per day.

Almost half of that was adaptive thermogenesis, meaning that the body voluntarily slowed itself down

to survive. They lost 5% of their muscle mass in three weeks, which further dropped their metabolism. So in other

words, if your fasting routine turns into chronic calorie restriction, especially with something like daily

16-8, you can unintentionally shut the system down that you're trying to speed up. And this is exactly why people

feel great for the first couple weeks of fasting and then they stall, or even a couple of months, or even it just

becomes a lifestyle and just nothing happens, they don't really change it, they just feel good, but their body doesn't

change. It's wild how fasting can happen, especially if you don't have a whole lot of fat on you, right? So within about two to three

weeks of continuous reduction in calories, your metabolism begins to downshift. It's not months, it's not years,

it's 14 to 21 days. Here's the kicker. That same slowdown does not happen with infrequent, strategically placed,

moderate length to longer fasts. Why? Well, because the body interprets those as acute temporary stressors,

not starvation. You also want your body to be able to recover and build off of those fasts.

What I mean by that is, you have all kinds of cellular changes and repair that happen during a fast. You have

reorganization, reallocation, autophagy, and then you need to actually rebuild. It takes protein, it takes calories to

rebuild. So all the mitophagy you're going through, where the mitochondria are going through their own respective

autophagy to consolidate and get stronger,

if you don't have periods of rebuilding, that doesn't do you any good. So fasting is a stressor

on the mitochondria. Calorie restriction is a stressor on the mitochondria. Then you need to be

able to repair from that stressor. The stressor is the signal and then you need the repair. So you don't

just consolidate your mitochondria or consolidate cells and then continue to fast and have that happily

continue. You consolidate and then you power them up, right? So it's like you want to take all the

junked vehicles in your junkyard, consolidate all the parts to make one supercar, but then you got to

put fuel in that supercar. Otherwise, it's still just sitting there, right? This is where that product

timeline can come into play. I think this is cool science with this because this is a compound,

urolithin A, that induces mitochondrial autophagy. Mitophagy, it's been published in a lot of papers,

been published in JAMA, all kinds of different research journals, because it induces autophagy or

of the mitochondria. It's called mitophagy. So you take that regularly, but you can take that even when

you're not fasting and it kind of continues that autophagy effect and it helps rebuild those

mitochondria. So this is one of the most foundational like longevity compounds that's out there,

but personally, I take it more so because I get better recovery and I feel like my mitochondria is

stronger from it. I popped the link down below. That's a special discount link. So you can use

that link down below. So you can use their capsules. They've got a powder. They also have gummies, which

taste pretty good too, and they don't have sugar in them. So that link is down below. These guys are

great. They're a Swiss company that really focuses on precision too. So this stuff is

legit. Link down below. So don't crash your metabolism. All right. We talked about that.

And now that we've covered that piece, let's jump into part two of this, which is the glucose

tolerance piece. Because fat loss is not just about what you're burning today. It's about what your body

is choosing to burn today and tomorrow. Okay. The rate limiting step here is how well your cells

respond to insulin. And you probably know this by now. Fasting helps with insulin dynamics and all this.

But if your glucose tolerance is dropping, your insulin goes up and your fat burning shuts off,

and that's not good, right? That could be a serious problem. And there's a really interesting

study that was in the Journal of Applied Physiology that kind of outlined this a little

bit more. They had subjects fast for either 13 hours or 72 hours before actually eating a test meal.

And after the 72-hour fast, fat oxidation was higher, as you'd expect. But glucose tolerance

got worse. Their blood sugar glucose spiked higher, insulin spiked higher, and glucose oxidation dropped.

And this is the body going, I've been running on fat for a few days. I'm not switching gears very

easily over here. Like the body likes to kind of find these grooves. So researchers think that this

might be due to elevated growth hormone levels, which is good, but it also could be elevated free

fatty acids that are impairing the insulin signaling. Again, it has its place, but may not always be a good

thing. So long fasts burn fat like crazy, but if you do them too often, you start losing your ability

to process carbs normally. And you might be thinking like, I'm not going to have carbs anymore, so who

cares? There will come a time when you have carbs. So let's be real here. I don't care. Even the

strongest carb war people still occasionally will have carbs. It just happens, right? And are you 99%

of the population or are you 1% of the population? Who's going to eat carbs, right? So even me, I don't eat a lot

of carbs, but I want my body to be adjusted to them. But here's the cool counterpoint, because there

was a study that was in endocrinology, and this was a mouse study, but they found that mice doing

three 24-hour fasts per week, like non-consecutive, they did this and the glucose tolerance improved

dramatically if they were doing like these 24-hour fasts non-consecutive. Glucose tolerance improved.

They got better. So again, frequency is everything. How often we do them, how long we do them.

Shorter fasts done intermittently with breaks improve insulin sensitivity because you have a

period of time in between to actually mess around with your new insulin sensitivity that you just

got from fasting. Ultra long fasts done too frequently actually impair it. I think this is

where like Peter Attio was like what he was getting at when he was talking about how he quit fasting.

Like I think he was just like an all-or-nothing guy and going like full bore into fasting and was

doing like long fasts all the time. Of course that's going to be problematic. Like no one's saying

to do that. Anyway, now that we've covered the metabolic rate and glucose tolerance piece,

I want to move into the part where we kind of tie it all together. How often you should actually fast,

like what you can do personally. And here's the pattern that the research paints when you kind

of overlay everything we've talked about. If you fast every day, especially with 16-8 and you

unintentionally under eat, you have about two to three weeks of a window before your metabolism begins

to slow down. And that's why people see amazing results for like two, three weeks and then the

wheels fall off. But when you back up and you kind of zoom out, the research says something pretty

empowering. You don't need to fast every day to get the benefits, okay? You might not get as strong

of fat loss benefits right in the beginning, but you're going to get them forever. In fact,

not fasting every day is what keeps your metabolism high. If you do one 36-hour fasts per week,

or two non-consecutive 24-hour fasts, or even three separate 16-hour fasts, you get the norepinephrine

bump, you increase fat oxidation, you improve glucose tolerance, you avoid this metabolic downshift.

Here's the part I really want people to hear. Consistency doesn't mean every day. We have it

programmed that we need to grind every day. Consistency just means it's repeatable. It means

it's sustainable. It means it's something your body can adapt to long-term without defending itself.

The Cell Reports Journal data is actually fascinating here. They found the longer that

you can sustain a form of caloric restriction across your life, fasting, etc. Not extreme

restriction, just mild restriction. The longer you can sustain it, the more you will see improvements

consistently and reliably. You're going to see improvements in cellular housekeeping, in autophagy,

in metabolic health markers, in fat loss. But you can only sustain it long-term if you're not

constantly shrinking your metabolic rate. Fasting works because it is a pulse. It is a stressor.

It is a strategic interruption. When it becomes daily deprivation, the system breaks down.

So here's what I would suggest, based on everything we've covered today. Use fasting like a sprint,

not a marathon. Intervals. Go hard for a short burst and take a break. If you're doing daily 16-8,

you've got to give yourself a reset every couple weeks. Okay, I understand it's a lifestyle thing

and it works great. You have to take a few days off every couple weeks then. If you prefer longer

fasts, use them sparingly. Do them once a week, maybe once every other week. Okay, it's enough to

get the metabolic pop, but not enough to really trigger the slowdown if you're keeping an eye on it.

And also keep the protein super high because you're breaking down. You need to rebuild. Don't be afraid

of mTOR. Don't be afraid of rebuilding. Okay, let me give you three simple takeaways you can implement

just right now. Okay, number one, if you are doing daily fasting, insert a break every 7-14 days

for like 2-3 days. Okay, let your metabolism breathe. Just let it breathe. Number two, if you

like longer fasts, try doing one or two 24-hour fasts weekly or at least like maybe one 36-hour

fast every 10 days or every two weeks. Do not do multiple days in a row. It is never a real reason

to do that unless you have a very specific thing. Number three, keep protein high during eating windows

to preserve muscle. Okay, this is super important because protein is your absolute like metabolic

insurance policy. It's making sure you have that muscle so that you can have the metabolism going

forward. And number four, pay attention to how you feel day to day and week to week. If your energy is

dropping, that means your calories are probably too low the following couple of days or the previous

couple of days. So if energy drops, if strength drops, or if sleep falls off, if it feels really weird,

you're not sleeping well, that is your really early warning sign that you're sliding into metabolic

conservation. At the end of the day, it's not about discipline. It's about sustainability,

rhythm, and consistency. When you hit the right rhythm, your metabolism doesn't fight you anymore.

It works with you because that's the way it's supposed to be. It's a beautiful system that we have

from our creator, right? Like we can work in harmony with our environment and that includes our food.

And that matters because fasting is often used for one specific reason, right? It's to drive autophagy and

cellular repair and all these other things like to clean up. And that's important because fasting is used

for all kinds of reasons. Like whether it's spiritual, whether it's cellular cleanup, or fat loss, or

just because it feels natural, right? I think there's some other components of fasting that people really

like that can activate some of these sort of longevity like cleanup attributes. In another video,

I broke down a compound that's derived from an actual leaf, believe it or not, that activates many

of those same repair pathways without requiring a lot of extended fasts or you could couple it with some

fasting to get an extra benefit. It just gives you another tool when it comes down to cellular repair

on the days maybe when fasting isn't the right tool or when you want to like accelerate it fast a bit

more. So that video is right here. I recommend you check it out. Please do subscribe and I will see you tomorrow.

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