What are calories? Is it the amount if energy in a food? How does the body uses calories, burns calories, and how does excess calories becomes fat?
Why are food “low in calories” good for you? Why are food of the same size, have different amount if calories?
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Your body turns most of the food you eat into glucose (a type of sugar), then feeds that glucose to your cells as fuel. Some bits get used as building blocks, but most of it just gets burned for energy.
Your cells absorb the glucose, combine it with oxygen from the bloodstream (basically burning it, but more controlled), and use the energy this gives off to power all the being-alive stuff they do – muscles, metabolism, immune system, growth and repair, basically everything. Burning the glucose gives off CO2, and your blood returns that to the lungs so you can breathe it out, and grab fresh oxygen from the air.
If you have more glucose in your bloodstream than you need to power your cells right now, your fat cells wake up and start combining glucose molecules (along with some water / etc) together to build fat molecules, and secreting the fat into the surrounding tissues.
At a later time, if you don’t have enough glucose in your bloodstream to power everything, your body will start scavenging fat from your tissues, and your liver will break it down into glucose again, and use that for fuel.
(there’s actually a whole buffer system of stuff called glycogen that it keeps on hand before it goes for the fat, but let’s not complicate things)
The amount of usable energy that can be extracted from food (via the amount of glucose you can turn it into) is described in terms of calories – a measure of heat. One calorie is enough energy to warm 1ml of water one degree.
A small gotcha is that when talking about dietary stuff, it’s actually measured in *kilo*calories – enough heat to raise a whole litre of water one degree, but they call those ‘calories’ anyway just to be difficult and annoying.
Very roughly speaking, if you burned a tictac in a spoon and then stuck that spoon in a carton of milk, your milk would get about a degree warmer.
Obviously if you keep eating a bunch of foods with a whole lot of calories, more than you need, then your fat cells are going to keep busy turning all the leftover glucose into fat, and you will get podgy.
If you keep eating less food than you need to cover your energy requirements, then you’re going to keep scavenging fat from your tissues, and you’ll get thinner.
Different foods have different amounts of calories per gram, because a) not everything can be turned into glucose, and b) of the things that can, some can be converted into a whole lot more of it than others.
Fats are extremely calorie-dense, which is why our bodies use them as longterm storage. One gram of oil or bacon grease, etc will provide about 9 (kilo)calories worth of energy – as opposed to say a gram of protein, which only provides about 2.5. Starches are about 3, and sugars are about 4.
Things like water and cellulose (the cell walls of plants) can’t be turned into glucose at all, so something like a stick of celery, which is almost nothing *but* water and cellulose, has approximately fuckall calories in it.
Depending on how much of what kinds of components a food is made up of, it’ll have a different total number of calories.
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