Calories get a bad rap. We blame them for everything -- from making us feel guilty about enjoying a hot fudge sundae with extra nuts to the way our jeans fit (or don't fit, as the case may be).

Yet, demonizing calories is like bad-mouthing oxygen: It's impossible to survive very long without either one. Calories fuel the body. There's nothing bad or magical about calories, it's just that body weight comes down to a simple equation of calories in (from food) versus calories out (as physical activity).

What is a Calorie?
A calorie is a measurement or unit of energy; calories in the foods you eat are a measure of the number of energy units that food supplies. Those energy units are then used by the body to fuel physical activity as well as all metabolic processes, from maintaining your heartbeat and growing hair to healing a scraped knee and building muscle. However, when you don't use the calories you've consumed, those calories get shuttled to your liver to refill your glycogen stores.

What is Glycogen?
Glycogen is your body's quick, easy-access energy reserve. Your ability to store it means you don't have to eat continuously to keep your body revved up. Still, it gets depleted every three to four hours. When the liver is holding as much glycogen as it can, some of it is passed on to muscles for short-term storage (to be used as needed ).

What causes fat?
Between your liver and muscles, you have a ready supply of calories (roughly 300 to 400, depending on your weight and metabolism) that you can access as necessary throughout the day. When you eat more than you can save in these temporary "accounts," the calories get converted to fat and distributed throughout your body.

Which food sources have calories?
Only four components of food supply calories: protein and carbohydrates (4 calories per gram), alcohol (7 calories per gram) and fat (9 calories per gram). Vitamins, minerals, phytochemicals (natural chemicals found in plants), fiber and water do not supply calories.