A calorie, as defined by the Oxford University Press dictionary:

calorie >noun (pl. calories)  1 (also large calorie) a unit of energy, often used in specifying the energy value of foods, equal to the energy needed to raise the temperature of 1 kilogram of water through 1 °C (4.1868 kilojoules).  2  (also small calorie) a unit of energy equal to one-thousandth of a large calorie.
-ORIGIN from Latin calor ‘heat’

Excuse me? What does this have to do with why my pants won’t fit?

A calorie is also the unit of measurement of energy produced by food when it is oxidized, or used, in the body. Calories are like fuel for our bodies. We need them for our bodies to run.

If more calories are eaten than are needed for organ function or muscle work, the body efficiently (too efficiently in my book) stores up the excess as fat. Because excesses were rare in nature, our ancestors became very efficient at storing fat. 

Today, the muscular work of day-to-day living has steadily declined and the energy needs of the average person have, in turn, declined. Combined with easy access to tasty food and drink, most Americans consume more calories than they need for survival.

You need to maintain a balance between the amount of calories you take in (food) and the amount of calories you use (exercise). You “burn” your body’s fuel – calories – when you perform physical activities, and you also burn calories just by sitting still, breathing and living each day.

Each person is different when it comes to the amount of calories he or she needs each day. The amount of calories a person needs for fuel varies according to age, height, gender, amount of physical activity and other factors.

Carbohydrates contain 4 calories/gram of food; protein also contains 4 calories/gram; but fat contains 9 calories/gram. Therefore, it is easy to see how quickly one can exceed his or her daily needs when fat content is high. Fat is also very tasty and dense (non-bulky), so it is difficult to “tell” when to stop eating. Most people stop eating only when the discomfort of a tense stomach is worse than the reward of the food.

Fat is used for energy if a deficiency in carbohydrate occurs (e.g., when we diet). It can be used straight from the diet or stored in fat cells for future use. Fat is broken down from fat cells, which are taken to the liver and converted to carbohydrates before the fat is used for energy. Fats can be found in plants or animal foods. Nuts, olives, corn, and avocados are examples of high-fat vegetables. These are unsaturated fats, which have the same calories as saturated fat. Unsaturated fat may not be as harmful as saturated fat to the arteries, unless it is taken in excess.

Alcohol, by the way, goes directly to fat. It does not pass “Go” nor does it collect $100.

Protein can also be converted to fat if too much of it is found in the diet for basic needs.

Individual differences in the ability to feel full and to stop eating are known as satiety, and vary greatly from person to person. Protein seems to be the most satiety-inducing food, followed by fat, and then by carbohydrates. Most protein is found in meats, which are also high in fat (with the exception of seafood).

Most starches are made from vegetables and, therefore, are carbohydrates—such as potatoes, rice, and pasta. Simple sugar, like that found in soft drinks and snacks, is more concentrated than that found in complex forms, such as in vegetables, where it is bound up with fiber (cellulose). Therefore, one can consume many more calories from starches and sugar without any bulk and never feel “full.”
I am always amazed at what little awareness people have of the calories in soft drinks. Many people who say they do not eat anything—but are overweight—drink their calories in soft drinks, juices, and milk—all dense calorie items.

The reason dietitians urge people to eat more complex, unrefined carbohydrates—such as whole fruits and vegetables, lean meats, and very little fat—is simple: Bulky, low-fat foods tend to fill the stomach faster with fewer calories. Items that say “no fat” still have plenty of carbohydrates in a very dense package. Items that say “no fat, no sugar” really mean no simple sugar, and are loaded with complex carbohydrates, at 4 calories/gram.

No matter how well you understand calories, it never hurts to read labels, note the texture and weight of food (fat is heavy, floats, and is greasy to the touch), and learn how to balance the taste reward of food with the desire to live healthier and longer. The challenge is how to keep track of your daily caloric intake without going crazy!