Ive always understood that you need “X” amount of calories for your body to do “Y” amount of activity.
So when someone who isn’t necessarily more active as you, and eats the same as you says “i just have a fast metabolism “, where is the energy output coming from? What’s your body burning even if you’re not doing anything. Is their body working twice as hard to do the same thing as someone else ?? Is that what a fast metabolism is and if so how??
I think about a kid i went to high school with. Roughly the same height and they were skin and bones. I played sports, they didnt, and yet they could eat whatever they want. They just always blamed their “fast metabolism.”
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Food calories are actually a different measure from regular calories, regular calories are just energy, food calories are energy the average human can extract.
Different peoples bodies are better or worse at processing energy they take in, some get rid of more of it, and some burn it faster. Different people have different workloads at different times and different storage methods for fat.
So when people say they have a fast metabolism they mean that their body doesn’t store stuff as fat as readily as some other peoples.
Also you do burn energy by just living, breathing, and being. That does take baseline energy to run.
Your thyroid regulates your metabolism to a large degree. People who have more active or less active thyroid while process food much more quickly than others. That’s why some people have a harder time losing weight than others. And then there’s simply the amount of activity you do in a day. If you’re constantly up and down in your fidgeting and you’re I’m probably going to burn more calories than someone else occasionally pops and Candy into
You can imagine your body as having a Furnace you shovel fuel into. (In real biology terms, this would be the Mitochondria in every cell)
A fast metabolism means you running your furnace at a higher temperature but only actually drawing off the energy you use, wasting fuel in other words.
The plus however is that you don’t store your fuel as fat. you just burn it as fast as you can and stay skinny.
A slower metabolism runs the furnace at a rate more appropriate to your physical activity, and stores excess fuel as fat-cells elsewhere in the body.
So if you eat more calories than you actually need day-to-day (as many many people do) it simply gets bunkered as fat until you need it.
It does however take more time to draw on fat, and your body will prioritise the more easily accessible energy in the food/nutrients that’s floating in your system.
I would bet your fast-metabolism friend has a higher body-temperature than you on average.
Mostly it doesn’t mean anything.
When Person A is gaining weight and Person B is eating lots of food but not gaining weight, the answer is never that Person B “has a fast metabolism”. Instead, the answer is always that either Person B is far more active than Person A, or that Person A is consuming far more calories than they realize or admit they are. Or both.
First off when people use the term, “fast metabolism” they just mean people who can seemingly never gain weight. That could be the result of a fast metabolism.. or lots of other things. The chances your friends who can’t gain weight ever had thyroid hormone tests and confirmed they actually have abnormal hormone levels is pretty low.
It means I have to get novocaine injected twice as often during dental procedures.
When people say someone has a “fast metabolism” what they really mean is that they don’t seem to gain weight when eating a given amount of food. And that generally means their bodies burn more calories than someone else’s body, even when they’re doing the exact same activities. It’s often BS but there are genuine reasons different people have different metabolisms.
There’s a minimum amount of calories your body has to burn each day to just keep you alive. This is called the basal metabolic rate (BMR), and it’s the amount of calories required to keep your heart beating, your lungs breathing, your brain functioning, your guts digesting food, keep your body temp at 98.6, and so on. Any activity you perform burns calories above and beyond that.
The most important reason different people have different metabolic rate has to do with how much muscle mass they have. Muscle burns more calories per hour than fat, even when you’re not doing anything. There are equations to estimate your BMR purely based on how much lean body mass you have, totally independent of your age, gender, height, or weight.
Other factors that affect your metabolism include if you’re diabetic, if you’re going through menopause, if you have an infection, thyroid health, if you live in a cold climate or if you live at a high elevation, etc.
Your body does a LOT of things when stationary. Your body has to be kept at 98.6F at all times which costs a lot of calories to keep your temperature in check. Your brain is also a huge calorie burner, it is always running, and do to the complexities of human brains it does a lot more subconsciously. Your heart uses a ton of calories pumping away at all times.
The variation in metabolism comes mainly from the variation in people’s body size and muscle/fat composition. Bigger bodies will need more calories than smaller bodies. You also have muscle and fat which both burn calories constantly but at different rates. Muscle burns more calories than fat.
So theoretically, someone with a “fast” metabolism is just someone who has a combination of larger body and a lot of muscle. Therefore they need to eat a lot more calories everyday compared to someone smaller and that has less muscle.
The biggest confusion we see with “fast” or “slow” metabolism comes from a TON of perception bias. Your friend that “eats a ton but doesnt gain weight” maybe skips breakfast or dinner a lot but you just see them eat a lot at lunchtime. If you were to track calories perfectly, you would immediately see huge differences in caloric intake. Your skinny friend most definitely eats less calories than your larger friend.