In one sense, then, the metabolic rate could be defined as the speed at which our internal engines run.

Four factors contribute to your metabolic rate.

1. By far the largest contributor is your "basal" or resting metabolic rate. (They're not exactly the same thing, but for our purposes they're interchangeable.) This is basically the number of calories you burn at rest, just sitting in a chair.

2. The second biggest component of your metabolic rate is determined by your level of activity and exercise.

3. The third component is sometimes called the "thermic effect of food," and is defined as the amount of calories (or energy) it takes to actually digest and process the food you eat. ("Thermic," by the way, means "heat", and "thermogenic" means "heat producing." Remember that energy production is going on in the body at all times, and one of the byproducts of energy production is heat. Which food type has the highest thermic effect? Protein.)

4. The last and smallest contributor to the metabolic rate is something with the intimidating name of "facultative thermogenesis," which basically means the heat-producing effect of things like fidgeting, reaction to cold and stress, and other similar factors.

Now, when you and I say things like, "I have a slow (or sluggish) metabolism," what we mean is, "How do we get this whole process to move faster? How do we speed up the engines? How do we run the whole show at a higher temperature?"