Calories are simply units of measure. They aren't actually things in and of themselves; they are labels for other things, just like how an inch really isn't anything, but it measures the distance between two points.
So what do calories measure?
Easy: they measure energy.
Yup, the evil calorie – the bane of the dieter's existence – is really just a 3-syllable label for energy.
And it's important to highlight this, because the body itself, despite its vast intelligence (much of which medical science cannot yet understand, only appreciate in awe) does not really do a very intelligent job of distinguishing good energy from bad.
Actually, to be blunt, the body doesn't care about where the energy comes from. Let's explore this a little more, because it's very important to the overall understanding of how to boost your metabolism, particularly when we look at food choices.
In our choice-laden grocery stores, with dozens of varieties of foods – hundreds, perhaps – there seems to be a fairly clear awareness of what's good food, and what's bad or junk food.
For example, we don't need a book to remind us that, all else being equal, a plum is a good food, whereas a tub of thick and creamy double-fudge ice cream is a bad food.
Not bad tasting, of course; but, really, you won't find many fit people eating a vat of ice cream a day, for obvious reasons. So what does this have to do with calories and energy?
It's this: while you and I can evaluate our food choices and say that something (like a plum) is a healthy source of energy, and something else (like a tub of ice cream) is an unhealthy source of energy, the body doesn't evaluate. Really.
It sounds strange and amazing, but the body really doesn't care. To the body, energy is energy. It takes whatever it gets, and doesn't really know that some foods are healthier than others. It's kind of like a garbage disposal: it takes what you put down it, whether it should go down or not.
So let's apply this to the body, and to weight gain. When the body receives a calorie – which, as we know, is merely a label for energy – it must do something with that energy.
In other words, putting all other nutrients and minerals aside, if a plum delivers 100 calories to the body, it has to accept those 100 calories. The same goes for 500 calories from a (small) tub of ice cream: those 500 calories have to be dealt with.
Now, the body does two things to that energy: it either metabolizes it via anabolism, or it metabolizes it via catabolism. That is, it will either convert the energy (calories) into cells/tissue, or it will use that energy (calories) to break down cells.
Now the link between calories/energy, metabolism, and weight loss becomes rather clear and direct.
When there is an excess of energy, and the body can't use this energy to deal with any needs at the time, it will be forced to create cells with that extra energy. It has to.
It doesn't necessarily want to, but after figuring out that the energy can't be used to do anything (such as help you exercise or digest some food), it has to turn it into cells through anabolism.
And those extra cells? Yup, you guessed it: added weight!
In a nutshell (and nuts have lots of calories by the way, so watch out and eat them in small portions…), the whole calorie/metabolism/weight gain thing is really just about excess energy.
When there are too many calories in the body – that is, when there's too much energy from food – then the body transforms those calories into stuff.
And that stuff, most of the time, is fat. Sometimes, of course, those extra calories are transformed into muscle; and this is usually a good thing for those watching their weight or trying to maintain an optimal body fat ratio.
In fact, because muscles require calories to maintain, people with strong muscle tone burn calories without actually doing anything; their metabolism burns it for them.
This is the primary reason why exercising and building lean muscle is part of an overall program to boost your metabolism; because the more lean muscle you have, the more places excess calories can go before they're turned into fat.