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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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.
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