Calories are simply units of measure, not actual things. They are labels like an inch which really isn’t anything, but it measures the distance between two points.
So what do calories measure?
Your body creates energy from the food you eat, whether it's healthy food or not. It creates energy from fruits and vegetables using the same process that it uses to create energy from chocolate bars and candy.
While you know it's better for your body to get energy from fruit and vegetables, your body doesn't evaluate the food. It creates energy from whatever you feed it.
It sounds strange, 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 it must do something with that energy. If a carrot delivers 100 calories to the body, it has to accept those 100 calories. The same goes for 200 calories from chocolate bars and candy.
The body does one of two things to the 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.
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, the whole calorie/metabolism/weight gain thing is really just about excess energy. When there are too many calories in the body, they are transformed into fat.
Sometimes those extra calories are transformed into muscle. In fact, muscles require calories to maintain their mass, so 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. The more lean muscle you have, the more places excess calories can go before they’re turned into fat.
There’s a nasty rumor floating around that fat cells are permanent. Unfortunately, the rumor is true. Most experts agree that once fat cells have been created, they're permanent. But this doesn’t spell doom and gloom for those of us who could stand to drop a few pounds. Even though experts believe that fat cells are permanent, they also agree that fat cells can be shrunk. So even if the number of fat cells in your body remains the same, their size, appearance and percentage of your overall weight, can be reduced.