How fasting or changing the amount of food that you eat, tampers with your metabolism?


How fasting or changing the amount of food that you eat, tampers with your metabolism?

In: Biology


I think the main benefit from fasting is that it accustoms your body to be used to burning fuel reserves(that empty stomach growling feeling) so when that happens (pushing off eating for a little while longer) you learn to eat less/need less because the fasting is that hunger that you would otherwise try to quell. Best, method is start going six hours between small meals.

The body will respond to long spans of reduced food by basically reducing unimportant activity. A lot of this is sub-conscious reduction, (less fidgeting, leaning against a wall instead of standing freely, laying in bed a little longer, etc.). Over time, you might feel a little lazier too, where before you might be excited to do something, but now you’re maybe a little hesitant to do things. You may not even really notice.

There is a lot of advertising and pseudoscience jargon out there right now concerning intermittent fasting since this the latest fad diet craze.

The science behind how it changes your metabolism and its effect on weight loss is not firmly well established, and do not let anyone tell you otherwise.

However, I am not saying intermittent fasting is ineffective.

It is like any other diet plan. There are many of them and the effectiveness of every diet plan depends firmly on your own personal behaviors.

The most important thing about picking any diet, is how easily you will be able to stick to it.
Whatever biological, metabolic, body changes or whatever differences between each diet is minuscule compared to your own behavior pattern.

If you find a fasting diet allows you to more control over your appetite and craving and it becomes easy and effective for you to stick to, then by all means do it.
If find another diet is easier for your lifestyle or behaviors or cravings then do that one.

Be honest with yourself and use your own sense and knowledge about body and behavior when dieting rather then trying to pick a diet based on whatever metabolic tricks any particular diets provide.

It doesn’t. Starvation mode and metabolism slowing is bullshit either by marketing, false wives tales, or an excuse that fat people use to shirk responsibility. It can for the most part be a simple matter of tdee (total daily energy expenditure). Assuming you have no conditions that would actually affect your metabolism. Your body uses a certain amount of calories based on your age, activity level, body mass, and body composition. If you eat less than your tdee, you will lose weight. And since your weight contributes to your tdee, it goes down as well.

If you go to more extreme cases of eating less, you may reduce neat (non exercise activity thermogenesis) basically fidgeting around and moving. This may impact your tdee negatively but it’s not your metabolism slowing, its your body unconsciously adapting to reduced calorie intake.

In general or body has three fuel sources: sugar, glycogen (a complex carb stored inside many cells in our body), and fat. (there is a fourth: protein, but our bodies will only start burning protein for energy if something is wrong or you are starving for a very long time)

When we eat a meal our blood sugar starts to go up. When our blood sugar gets high enough our body starts storing the excess as glycogen. But each cell can only store so much of that glycogen, so any left over sugar gets turned into fat and stored in fat cells, which are made to do that one thing, and fat cells can store an endless amount of fat. They’ll just keep packing it in and each individual fat can can become huge.

Our body really likes to hold onto the fat, it’s a fail safe for when food is scarce. You can imagine our hunter gatherer ancestors needed to be good at storing energy in case they failed to hunt or gather food that day. Or you can think about bears who get super fat before the go into hibernation. It’s all about energy storage.

When we burn energy we burn it in the same way: first the sugar in our blood, then the body will break down glycogen, then finally fat. When fasting in order to maintain energy levels, blood glucose levels, it will first start to break down glycogen to make those sugars. When glycogen stores run low the body then starts pulling from its emergency reserve: fat.

This is the purpose of intermittent fasting as a dieting technique. You’re forcing your body to burn through its glycogen stores and maximizing the amount of time your body is burning fat for energy. This is called ketosis, and is probably what you’re referring to when you say ‘metabolism’. Our bodies change they way they are metabolizing different stores of energy to ensure it has enough to function.

Edit: people keep commenting that my comment about intermittent fasting is wrong. It’s only wrong if you think intermittent fasting is just skipping breakfast. Many people who do intermittent fasting do OMAD, one meal a day, or implement 48-72 hour fasts to their intermittent fasting regime.

And in general, even by just skipping breakfast, you will be burning more fat during that period. Very rarely does the body do only one thing at a time. When you start fasting for any length of time you are going to be burning glycogen, metabolizing fat, and inducing gluconeogenesis (making sugar from fat and other sources). Fasting is a great way to burn extra fat stored in the liver (fatty liver disease, a precursor to type two diabetes and metabolic syndrome) b/c gluconeogenesis happens pretty much exclusively in the liver.

If you want to get more technical low blood sugar, like when you are fasting, will increase the amount of glucagon in your blood. Glucagon acts on peripheral tissues to tap into glycogen stores, on the liver to induce gluconeogenesis as well as burn glycogen, and on adipose tissue to induce lipolysis (break down of triglycerides into few fatty acids so they can be turned into energy in the liver). Even short periods of fasting will induce these actions. Will you go into full ketosis after a short fast? No. Will your body start the process after a few hours? Yes, even if it’s just a little at first. Metabolizing fat from peripheral tissues takes time, so the longer the fast the better.


Your body uses the easiest energy source. Eating 3 – 5 meals a day, means your body has a constant supply of easily available energy. So available, that there is typically leftover energy from each meal, and our body turns that extra energy into fat and other proteins, to be used later when food is scarce… But eating 3 – 5 meals a day, means food never gets scarce, so fat continues to build/stay the same. Even if you cut calories with this method, you still are giving your body an easy energy source to pull from first.

However, when you go extended periods without eating (8, 10, 12, 16, …hours) your body goes “I want some energy” and pushes the “I’m hungry” button which you think actually means your hungry so you eat. By fasting, you’re telling your body “figure it out” to which your body (eventually) gives up on pushing the “I’m hungry” button, and gets to work breaking down the fat it has stored up.

Our body creates an eating cycle, just like we have a sleep cycle. By maintaining a constant routine for either of these, your body will adjust and it will become normal and won’t be such a struggle. The longer someone has struggled with overeating, the more difficult this mental change will be to overcome.

The tldr is that it doesn’t. Metabolism changes and such is largely pseudoscience designed to sell fad diets. Intermittent fasting, alongside something like keto for example, is a tool available to you if you would like to change how you diet.

Some people, like myself, binge regardless of how hungry we actually are, or how much we have eaten. I was binging 2-3 times a day despite having no want for food. Once I started IF, I lost weight simply because I was eating a third of what I had been. From there I was able to actually focus on a diet and work on what I needed to get healthy, not just not-fat. I’ve lost over 100 pounds and have kept it off for a few years now.

Things like keto are the same way. Avoiding carbs helps some people control portions and their body reacts in a way that they feel better or less hungry or more energetic or whatever. It doesn’t mean that eating steak for every meal is going to work for everyone.

Most “diets” are actually “dieting tools” and should be experimented with if you are interested in order to determine what works best for you. Remember, calories in < calories out means weight loss, however there are different ways of going about it that make it easier or harder depending on who you are.

Humans evolved to eat big meals less frequently.

Modern humans eat massive amounts of calories throughout the day and don’t exercise much.

Used to starving historically, hormones are released when you have low amounts of food that is great for metabolism, and extends your life span, for complicated reasons beyond an eli5.

Eating in an 8 or 10hr window works well for me. I never was into breakfast anyways. I can eat whatever I want and I’m a heathy thin weight with only minimal exercise.

Completely recommend trying intermittent fasting, its a better system of eating than the typical one.


Think of your fat stores like a garbage bag. You cannot take things out and put things inside simultaneously. If you keep putting things inside without taking stuff out, the garbage bag will keep growing until it’s too big and far.

Basically what is happening when you fast, you’re letting your insulin fall. Whenever you eat food, your body releases insulin into your bloodstream to signal cells to uptake the glucose in your blood, thus, lowering blood sugar. However, insulin also stimulates the pathway for filling fat stores. If you keep eating multiple.meals throughout the day, you won’t give your body a chance to let the insulin fall. When you fast, you allow insulin levels to decrease in your blood. Once insulin levels have decreased, that is when the signal to store fat stops, allowing your body to be able to start using energy from your fat stores again.

Proper ELI5 – consider your body is like a house with a wood burning stove, you put fuel in to release energy you need. Now, having 0 fuel on hand is dangerous so you have a modest stack of wood inside, somewhere nearby. This is normal for anyone since you don’t want the house to freeze, so everyones body has some fat stores to burn on hand.

But Being obese fat is like bringing more and more wood in than you can burn constantly. Eventually you are building addition after addition on to that house, but only in order to stock the more and more wood inside you keep bringing. Time passes until you could last month’s without getting more wood, stacked to the rafters in every room and corridor where it can go.

Intermittent fasting would be like focusing on burning the wood you have in the house first and only bringing more in at the end of the day. You still need to burn more wood than you bring in, but that deficit will slowly over time whittle away at your stockpiles. It’s made easier to have less calories since your restricting based on time, it’s harder to get them in.

Months pass, Your additions get smaller and smaller, and even if you want to bring more in you can’t fit it. This is because your stomach volume shrinks over the hours it remains empty, and it’s harder to eat thousands of calories in the little time you have with a smaller stomach.

In time, the additions get emptied little by little, and the amount of wood you bring in becomes more manageable and normal because you simply aren’t used to carrying in an over abundance anymore, both physically and mentally.

I lost 100lbs on intermittent fasting, starting from 400+ to my 305 now. At 6 ft 6 I have a dad bod at that weight but I’m lower than I weighed in highschool. I’m working on losing more, thinking of getting to the gym (hotter stove means more wood gets burned!). I made no major changes other than restricting to 6-10pm and not drinking calories before. I ate like crap still, handfuls of lucky charms, full pizzas, beer, munchies out on weekends and still got here because I can’t eat as much in one go anymore. Still have moments where I order or plate up food and chuckle when I can’t finish more than half of what I once did.

Funny, I just realized the stove metaphor is actually spot on since any weight loss is actually expelled as CO2 from breathing!

The last peer-reviewed review i read on the subject reported little difference in efficacy when IF was compared to daily caloric energy restriction (a classic diet). Can link paper when i get home if people are interested. Basically the authors’ interpretation of the available data was that IF is good if it helps ypu eat less, but does little other than that.