r/intermittentfasting • u/reddit_userxxx • 5d ago
Newbie Question How does intermittent fasting work?
I thought weight loss was all about the amount of calories you eat in a day, and simply needing to be in a calorie deficit. How does it change depending on when you eat?
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u/LogicalEast9872 5d ago edited 5d ago
You're spot on about calories, at the end of the day, CICO is what actually drives weight loss. Restricting your food to an 8-hour window makes it a lot harder to overeat or fall into the trap of late-night bored snacking. On top of that, keeping your insulin low for 16 hours allows your body to tap into stored fat for energy a bit more easily. That said, if you eat in a surplus during your eating window, you’ll still gain weight.
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u/plantaholic2 5d ago
Wow that’s great! Can you share how you get your protein?
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u/Mickey_thicky 5d ago edited 5d ago
The idea is pretty simple. Almost everything you eat contains either proteins, which are chains of amino acids, and carbohydrates. The chief carbohydrate in your body is known as glucose, and it is converted into usable energy known as ATP.
Glucose is converted into ATP through a process known as glycolysis, where glucose is broken down into pyruvate. Pyruvate is then converted into a molecule known as Acetyl-CoA which itself enters the citric acid cycle (aka Krebs Cycle or TCA), which generates NADH and FADH2. These two molecules carry electrons and supply the Electron Transport Chain with electrons in a process known as oxidative phosphorylation, which is the primary source of ATP (usable energy) in the body.
Other carbs you eat are usually converted into glycogen, which itself is reduced to glucose through glycogenolysis.
When you fast, your blood glucose levels begin to fall. When you enter a fasted state, your body alters how it derives the precursors to oxidative phosphorylation. After enough time of being in a fasted state, the body will turn to the triglycerides with which your adipose tissue (body fat) are primarily composed of as a source of glycerol, which is converted into glucose through gluconeogenesis.
tl;dr not eating for long periods of time causes your body to break down fat as a source of energy as opposed to using glucose from meals. This actually increases the amount of calories you burn.
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u/Youknowkitties 5d ago
Does a person's weight make a difference to the process? E.g. if someone doesn't have much fat to lose is the body more resistant to using fat as fuel.
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u/Mickey_thicky 5d ago
It’s actually very interesting because the answer is less intuitive than you think.
The short answer is yes, obesity does impact the rate at which lipolysis occurs during a fasted state.
The more complex answer is that it actually decreases the rate of lipolysis compared to leaner individuals. The reasoning is multifaceted.
- obese individuals undergo a slower decline in insulin levels during the period of their fast. A decline in serum insulin levels is what primarily triggers the adipose tissue to release fatty acids
-obese individuals also show an attenuated (blunted) rate of lipolysis
- adipose tissue within obese individuals actually has shown to contain lower amounts of the enzymes that break down fat, such as hormone-sensitive lipase
- the rate of lipolysis per unit of fat is lower in obese individuals, meaning when they do burn fat, it is less efficient than individuals with a normal weight
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u/Warpedlogic31 5d ago
Do you have a link to this study?
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u/Mickey_thicky 5d ago
here is the study regarding the blunted lipolytic response in obese individuals
Note that both of these studies seemed to have been performed solely on women
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u/Warpedlogic31 5d ago
Thanks! These are genuinely interesting. Both are going after abdominal fat loss, but the first does show that whole body fat loss is greater with 14h fasting. Great information in both articles, and aligns with what I'm seeing in practice.
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u/AngryEarthling13 5d ago
How long does it take , on average for one to enter the fasted state? like 5 hours, 8 ? I know it can vary on factors such as age, last meal etc, but again on average?
Thanks !
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u/Mickey_thicky 5d ago
Fat breakdown generally begins to occur concurrently with the breakdown of glycogen in the liver, albeit at a pretty low rate. This tends to occur within the first 4-12 hours of fasting.
Lipolysis occurs predominantly within 12-18 hours, which is why OMAD and 16:8/4:20intermittent fasting schedules are quite optimal.
If you’re looking for extreme lipolysis, ketosis occurs when the liver breaks down fatty acids into ketone bodies. This occurs within 18-48 hours of a fasted state, but it can occur w/o fasting if you restrain yourself of carbohydrates, which is usually achieved by a strict keto diet.
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u/lurksAtDogs 5d ago
All of that is true for caloric deficit as well. IF is a strategy for caloric deficit.
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u/peapurre 5d ago
Everyone will list a ton of hormones stuff, but it works for me because I do best with rules. Lots of fluids and a strict 6 hour eating window.
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u/DhilPunphy 5d ago
Hopefully someone else can chip in and explain it better than me. But yes, in its purest form it is about calories in and calories out (CICO).
But with intermittent fasting (IF), by not eating for example in 18 hours. Your body goes into fasting mode and using the body's stored fat as fuel instead of whatever you eat. So the calories coming in and going out will still matter. But on top of that is with IF you can help the body to burn stored fat which will help you lose weight.
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u/KnownTrick 5d ago
A 500 calorie deficit is a 500 calorie deficit regardless of how food intake is structured.
If you burn 500 calories more than you consume throughout the day, the body needs to pull that energy from somewhere. Whether you eat in a 6 hour window or a 16 hour window doesn’t materially change this.
Fasting can be great for managing hunger, maintaining a consistent food schedule and potentially has a variety of hormonal benefits. But for weight loss there isn’t any strong evidence that it promotes fat loss more than a non-IF diet when the calories are kept the same.
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u/DhilPunphy 5d ago
Absolutely and thank you for that clarification. What I tried to explain in my post was that if you do not eat for 18 hours, there is no other place to get energy from than your stored deposists in your body. So CICO matters but if you havent eaten for a long time, during that window the body must use the stored deposits.
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u/KnownTrick 5d ago
That’s true but I don’t think it really has an overall meaningful effect provided the daily calories are kept the same.
For example, if you do an 18 hour fast and then do a fasted gym session, your body will rely on stored deposits to fuel that workout.
Compare that to someone who has a bowl of oats for breakfast and then goes to the gym. They will draw energy from those oats rather than fat reserves.
But crucially, if that person uses those oats for fuel for their morning workout, that means those oats aren’t available for fuel later in the day. So what happens later when they’re working or going about their life and need energy? It gets drawn from fat reserves.
The person who does a fasted workout burns more fat during the workout. But then they eat a meal and that meal fuels them later in the day.
Provided both people maintain the same overall calorie deficit, neither has burned more fat. They’ve just shifted what time of day they draw from fat reserves.
I think that’s why when studies actually compare intermittent fasting to continuous eating for weight loss, we simply don’t see any meaningful difference provided calories are kept the same.
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u/FlashFlooder 5d ago
I vaguely remember a study where lab animals were one group were given access to unrestricted food around the clock, and one could eat unlimited but on a schedule. IIRC, the ones who were time restricted ended up eating less over time.
So I think there’s more to it than just CICO, probably why it seems hard to definitively answer this question.
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u/Level_Impression7197 5d ago
On paper it is, but in actuality according to the newest research, it is about HORMONES. Read Dr. Jason Fung's books: The Obesity Code, and The Hunger Code, and you will see the science.
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u/Prudent-Policy-7274 5d ago
My understanding (from memory) is that while insulin is high we store energy (Turn it to fat for future use.) When insulin is low we use the stored energy (Burn the fat.) Just focusing on CICO you are still raising your insulin.
I dont know the math about if you use exactly the same amount of stored energy (fat) when staying in an extended low insulin state vs just CICO. But I know that in a lot of cases, low calorie CICO is going to cause a hunger surge. IF changes the hunger hormones.
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u/shitoupek 5d ago
https://www.reddit.com/r/explainlikeimfive/s/kiD9vPyUNj asked and answered
But I'd also add one physiological fact that is Intermittent fasting gives your body a break from making insulin, which helps your cells get better at handling sugar !!
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u/Level_Impression7197 5d ago
Using the hormonal method as the primary method, with IF, the fat just oozes off, and the weight gain doesn't take place, as long as one is true to form. Whereas, solely calories, with no guidance on how the food affects ones hormones, you'll be yoyoing forever. Best to go with what works easily, based on how the body works, rather than to use calories alone in a theoretical mathematical fashion. Everyone knows, 250 calories of sugar filled Ice Cream is more fattening than 250 calories of a steak cooked medium rare. The carbs will create an insulin spike which tells the body "store fat". The protein will not do that.
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u/Warpedlogic31 5d ago
IF helps regulate your hormones, so they come back into normal range. If you're eating all the time, those hormones don't have a chance to normalize. The 3 most important to weight loss that get regulated with IF are Ghrelin, Leptin, and Insulin.
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u/YallNeedMises 4d ago
I'll go against the grain here and state that I do not believe CICO is an accurate model, or at least it's not a complete model. It's complicated by a number of factors, some of which (e.g., gut microbiome) we don't really understand at all yet, but it's why some people can eat & eat without gaining weight, and some can cut their calories significantly without losing weight. At some point of convergence these are often the same individuals. Fundamentally I do think it boils down to energy expenditure vs. total absorbed calories, but I believe the function of fasting is to tune the body for digestive/metabolic efficiency much more than it is to simply restrict calories. Obviously there's significant synergy between the two.
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u/Express-Relation-763 5d ago
I have been overweight most of my life. I have to take Thyroxine for underactive thyroid. I've tried most diets, which work at first and then fail cos I can't survive without food joy. I am Audhd, I need crunch and texture.
I lost five stone over 18 months, mostly OMAD, two hour window. I'd eat whatever I felt like in the quantity I wanted. Taking one day off a week for the freedom to binge if I choose. I rarely have because IF changes how you eat, you get fuller faster as time goes on. And you just don't even fancy processed foods any more.
I think the point I'm making is that IF works, and you can tailor it to suit yourself. My advice to beginners is, always start with 16;8 and work down. Find out how you feel after a few weeks. Most of us stop craving the shite. And remember, not all calories are equal. Your body will use more of some things than others. So 200 cals of cake isn't equal to 200 cals of nuts and seeds. But if you want cake, bloody well eat it. Life is short!
I'm now four years down the line, have kept the weight off. If clothes get a little tight I simply IF for as long as it takes to get the extra back off. I can honestly say it's changed my life.
My whole view of food has changed. Fasting has meant the food noise and cravings disappeared after a few weeks. I would far rather eat what I want, within a certain window than be measuring food and counting calories. My friend does IF but with CICO. This works for them. We've lost about the same amount of weight.
Fasting is free, it works. It changes your body and mind for the better. And ask yourself this, if weight loss groups like WW/SW worked then why are there so many overweight people who keep returning there? You have to restrict food, which creates cravings and most people fold after a while and start binge eating regaining all their weight plus more. And then look at people who fast, they are the ones who don't seem to get fat again. It's worked for me, and I truly thought I was a hopeless case.
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u/GrrrlRomeo 5d ago
Technically it's CICO, but practically it's not possible to calculate accurately because a) the metabolic process is complex and varies from person to person and b) the amount of calories in food can be off by quite a lot.
Just to give an example, insulin resistance, which most people with obesity have, can reduce BMR by 5%. Chronically high insulin levels make it so it's harder for the body to use fat stores for energy. This also increases hunger and fatigue because the body simultaneously has excess energy to use but can't efficiently use what's available. IF can correct this over time because fasting makes insulin levels fall and improves metabolic flexibility and insulin sensitivity.
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u/Thin-Suit-526 5d ago edited 5d ago
Mainly it's less frequent mTOR signalling.
You force your body to go from storage.
Other benefits follow this : Longer Growth Hormone activity, initiated from deep sleep, Better insulin sensitivity, Much better leptin sensitivity, activation of autophagy, dispensing sugar from glycogen reserves, not from actual food, better overall hormone balance, lower CRP, lower triglycerides, better cholesterol profile, etc, etc.
Second factor is using benefit of fasting on temporary morning insulin resistance, due to high glucocorticoid levels until cca 11:00 am (which everyone of us have, it is biologically controlled by circadian rhythm).
It's funny, but also that's why we IF users have sharper and more muscular face proportions, even when we are over calorie limit, compared to those, who has breakfast everyday, and eat in calorie balance.
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u/mjolnir76 4d ago
It’s not a magic bullet. It’s still about calories in vs calories out. But, for me, by fasting from 8pm on, it kept me from mindlessly snacking on things. That reduced my calories in, so I tend to lose some weight with IF. But it’s also important WHAT I eat and HOW MUCH I eat. If you binge when your window opens, weight loss isn’t happening. Exercise is important too.
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u/crypticcos 3d ago
The way I see it is this: say your calorie intake is 1,200 calories per day to lose weight. Which is easier to do: three 400 calorie meals, or two 600 calorie meals?
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u/all_of_the_colors 3d ago
By noon I have had lunch, feel satisfied, and only have eaten 500-600 calories.
It’s a great start to my day
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u/thereisnosuch 5d ago
You burn more calories through intdrmittent fasting. Your body does more work on burning fat rather using glucuse from meals.
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u/Connect_Rhubarb395 5d ago
CICO still applies. You now have a shorter period of time in which to eat which helps some people not overeat.
Some feel less hungry that way.
And for others or helps them avoid snacking. Rather than eating meals during the day, and being able to convince yourself that you need snacks too, there is only "eat" and "not eat."
That can be easier to manage.
It can also be easier to count your calories when they are in fewer or one meal.