There is energy from the food that we just ate, and then there is energy from the fat reserves. I was wondering, how much time would I have left to get rid of the calories of a meal I just ate, before it becomes fat. I’m guessing that this transition most probably isn’t as simple as that, but I believe I have made the idea clear. Also, how is this transformation done?
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Your food needs to be digested before it turns to fat. Once its digested the nutrients are sent into your bloodstream from your gut. What is not immediately used for energy and construction is sent to the liver to be converted to fat.
Regular fat in your food can be turned into stored fat in about 4 hours (https://www.sciencefocus.com/science/how-long-before-extra-calories-show-on-the-scales/)
Others such as proteins need to be processed for longer.
By burning off food people mean burning calories so that it’s s net positive. Say you eat 300 calories. You want to burn off those 300 but you are not burning the cheeseburger you just ate but perhaps the one you ate 2 days ago that is now stored fat.
Not gaining fat is actually very simple.
We gain energy from the food we eat during the day, measured in calories. (If you want to get picky about it, it’s kilocalories, technically written as Calories or kcal, but in everyday use we just say “calorie”.)
We use energy during that same day in order to stay alive, which can also be measured in calories.
Extra calories consumed and not used (calories eaten > calories used) get stored as fat as a survival mechanism, to protect against times when we don’t get enough food (calories eaten < calories used). Then our body breaks down fat to get energy to make up the difference needed to stay alive and keep doing what you need to do.
If they are equal (calories eaten = calories used) then you are not creating new fat or using up existing fat; you are in balance.
If you want to stay in balance, then you match your food intake to activity level to balance it out. If you like to eat more or higher calorie foods, then you would need to increase your physical activity to burn more calories off. Professional athletes eat a LOT of calories to fuel their intense workouts and competitions.
On the flip side, if you are a couch potato and dislike exercise (it’s good for you!) then you would balance by eating fewer calories — less food in general and lower calorie of what you do eat. Or, compromise so you can eat a bit more sweets and go for a brisk walk every day to burn it off.
You still burn quite a lot of calories just by existing — it takes energy to keep your heart beating, your brain thinking, your muscles moving (even if it’s just to keep you standing or sitting upright!) and even to digest what you eat; so you have to eat to stay alive.
Cutting your calories to zero starves your brain and muscles, making you irritable, irrational, and weak while your body cannibalizes itself for energy. Then eventually you would die once your body had nothing left to give (or you have other issues, like electrolyte imbalances that give you heart arrhythmia or a heart attack). *Reduce* calories (or increase exercise) so that you run at a caloric deficit, while still eating enough to support your basic functions without crashing.
Why it’s so difficult to actually do this is related to our survival instincts, habits, and culture. For millions of years we hunted and gathered, where our diet heavily depended on what we could find, kill, or scavenge.
The early humans who put on some fat in good times had more energy to stay warm and do more energy-intensive activities like hunting for food in lean times like winter or drought, so we developed a taste for high calorie foods with more sugar and fat.
Now, though, when most of humanity lives in an abundance of food, and companies selling us food put in more fat and sugar to make it more appealing (liking high calorie fat and sugar are survival traits for living off the land!) then we develop the opposite problem of gaining too much weight while being too sedentary, which our bodies are not designed to handle well.
And of course now our culture is much more sedentary: we largely sit to work in offices, or sit and watch TV, and sit to drive where we want to go, instead of walking or running for miles to find food. And our always-on work culture (at least in the US) tends to leave us stressed and short of time, so we eat to soothe our emotions — often fatty or sugary foods, to soothe our old survival instincts — or grab fast and convenience foods that have excess fats and sugars to entice us to buy them.
But the balance still is the same: to lose weight, eat less and exercise more. To stay the same weight, balance calories in and out. To gain weight, eat more and exercise less (though exercising for health or to gain muscle is a good thing; but then you need to eat even more to support that).
Of course this is also ignoring the question of nutrients, which are vital too. You can still lose weight by eating only doughnuts, just fewer of them — but that’s still not healthy. You need protein, fiber, some fats (if only to use fat-soluble vitamins, since you need fat to dissolve them in!), vitamins, minerals, and micronutrients. So paying attention to the balance of what you eat is just as important as the caloric level of your diet.
I think you are thinking about this a little bit wrong. A better ELI5 description would be like this:
Think of all of the building blocks in your body like investing. Carbs are your money market account that you can spend freely from with a debit card. Fats are like stocks, where you can put more money in and take money out, but it takes a little bit longer to do both. This though is the preferred savings plan of the body as it’s the best balance for availability and investment potential. Finally protein is like real estate where money in and out need to be planned a bit more as it takes time and energy to both buy and sell.
When your body needs money in the money market for spending it (when you are exercising for example) it doesn’t really care if that energy is coming from your most recent paycheck (what you just ate that is currently digesting) or from a paycheck two weeks ago, it just wants to make sure your money market account has money. So it will start selling off stock if it senses a spending spree coming up. If you start exercising regularly, your body will make sure you are keeping more in your money market and less in stocks, just to make sure it has that cash available. Additionally depending on the type of exercise, it may start converting stocks to real estate for more long term investments as well (building muscle from your exercising and burning fat to do so). If you starting slowing down your level of activity then your body will go back to investing in stocks more and stop selling those stocks as much.
The digestion process will also be allocating funds to the money market, stocks and real estate regardless if you are actively spending or not. There’s other processes that worry about reallocating as needed, so digestion is just gonna keep doing what it does, but maybe some tweaks to where the allocation is based on spending history and cash flow coming in. If you started fasting for an extended time, your body is going to be doing more selling of stock to make sure you have enough in your money market. It will also slowly start to sell real estate. The longer the fast goes on, the more stocks and real estate you will be selling. It will also be adjusting the algorithms for the next paycheck and how to divvy that up based on what it has been selling.
Tl;dr your body is always going to be converting carbs to fat and fat to carbs regardless of what you just ate and doesn’t really care if the energy is coming from the bag of chips you just ate or the bag of chips you ate a week ago that’s been stored as fat or the bag of chips you ate when you were 6 that it stored as fat back then.
Mitochondria in each cell are what eventually create usable energy in the form of ATP. For the most part, mitochondria will use glucose (from carbs) or fatty acids (from dietary or stored fat) to do this.
Your body will generally use up available glucose before tapping into ingested or stored fat.
When ingesting carbs, your body sends the glucose into your bloodstream, where it is transported all over and absorbed by all your body’s cells.
Your blood and cells can only store so much glucose. Excess glucose is converted into glycogen for short term storage (~12 hrs) in the muscles and liver. When glucose levels drop, this glycogen is once again broken down into glucose for energy.
Your body can only store so much glycogen. When glycogen stores are filled up, any additional glucose is converted into fat. And again, this fat will only be burned when glucose/glycogen stores are used up.
In terms of calories/energy, this fat converted from glucose is no different from fat ingested or fat already stored in adipose tissue.
To touch on the third macronutrient, protein can be broken down into glucose if need be, but typically is used for other more important things.
Keep in mind this is a simplified explanation and reality isn’t this clear cut.
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