I got in to an interesting discussion about calories at the gym. People seem to be misunderstanding what calories are, and how they work with food.
First and very foremost, a calorie is a scientific unit of ENERGY. Put in to scientific terms, one calorie is equal to 4.184 joules. This is very important to keep in mind. When you are talking about calories, you are talking about energy. More specifically we are talking about both Thermodynamic Energy. One calorie is the amount of energy it takes to raise one gram of water one degree centigrade. However, in one of those unknown twists of the American weights and measures system, the calorie that you see on food packages is actually a kilocalorie, or 1000 units of the calorie described above (just like 1000 grams is equal to 1 kilogram, get it?). Because it really isn't important to the conversation at this point, we are going to use the American system ('MERICA!!) and call the kilocalorie a calorie, ok?
So, we are talking about thermodynamic energy. Let's equate this to heating water, because, well... that is exactly what calories measure.
It takes a certain amount of energy to raise the temperature of a given amount of water at a given temperature to 50 degrees C (ignore pressure for the time being, or you can just assume that everything takes place at sea level). It stands to reason that if the water is frozen, it takes more energy to raise it to 50C than if the water were at room temperature. Given that the temperature of the water is the same, and volume of water remains the same, it will always take the same amount of energy to get it to 50C. It is simply a matter of adding the energy.
If I don't put enough energy in, the water does not reach 50C. If I put too much energy in, the water rises above 50C. Pretty elementary right?
The converse relationship is true in your body. You take in a certain amount of calories. Your body uses a certain amount of calories. If you take in fewer calories than your body uses, you loose weight. You have to. The body needs the energy, and if it is not in the food you have eaten, it has to go to its stores. If you run out of energy, you die. The body doesn't want to die, therefore it will do what it has to do to get the energy.
Now how you get the calories doesn't matter, from an ENERGY standpoint. If your body uses 2000 calories a day, and you take in 1000 calories in a day, no matter what it is, it could be all in bacon fat (mmmmmmmmmm bacon fat), you will loose weight. You have to. You have a deficiency of 1000 calories.
Now... Nutrition. Nutrition is how the body uses the FOOD it takes in. Not the calories. Calories are energy units. The body will take energy from any source that is available, fat, muscle, whatever.
Let's go back to the bacon fat example. If your only source of food is bacon fat, you may be skinny, BUT you will not be healthy. The body will process that bacon fat and will do some very bad things with it. This is how you can be thin, but very unhealthy.
SO, if you just want to lose weight, eat less. You will lose weight. IF you want to be healthy, you need to watch what you eat so you can be sure that what you take in will be used by your body with only good aftereffects.