Our body uses food by breaking it down into its components and taking what it needs to maintain the many different types of cells in our bodies and give us energy for the things we do through our day.  Let's look at an example...

Angela is an average 30 year old woman weighing 160 pounds who needs approximately 1494 calories per day just to stay alive (i.e., keep the heart pumping, the lungs breathing, the brain alert to do tasks, and all of her organs functioning properly).  This is called her "Basal Metabolic Rate", or "BMR".  It is effected by a variety of factors including how and when she eats, as well as her activity level.  If Angela is very active, she will need more calories, or energy, to feed the muscles to do the activity.  It's much like putting gas in a vehicle.  If you want to go on a long trip, or drive faster, you will need more fuel.  Likewise, if you want your body to do more, you have to give it more fuel.  Alternatively, if you want to burn stored energy (fat), you would have to either put less food in your mouth, or increase your activity level to use more energy! The best plan for weight management is to do a little of both in a safe way. 

So, Angela has decided that she wants to lose weight because she is in the "overweight" range for her height of 5' 4" according to the Body Mass Index.  She has determined that her total daily calorie needs are 1494, which is the number of calories required to maintain her weight.  However, she has calculated through self-monitoring that she takes in on average 2000 calories per day and has noticed that she has gained 15 pounds over the past few years (550 calories per day in excess adds up over time!).  In order to lose weight safely, (e.g. one pound per week), Angela should consume 3,500 calories less per week, or 500 calories less per day (one pound of weight is equal to 3,500 calories).  Alternatively, she can consume 2,000 fewer calories per week and burn an extra 1,500 calories by exercising regularly. 

Another important factor to consider is that our body takes from our food only what it can use, the rest becomes waste! So, be careful what you put in your mouth.  You wouldn't put garbage in your vehicle and expect it to run, would you?