What the Heck is a Calorie, Anyway?

For all the time and energy we spend talking and thinking about calories, a very simple thing can be overlooked: Just what the heck is a calorie, anyway?

A calorie is a unit of energy. The energy we consume in food is stored in the body—in the liver and muscles for short-term use, in fat cells for longer-term use—after the food has been digested. Strictly speaking, a calorie is the amount of heat energy it takes to raise the temperature of one gram of water by one degree Celsius. But when we’re talking about food and diets, we’re talking about kilocalories, or kcals; that’s the amount of heat energy it takes to raise the temperature of a kilogram (2.2 pounds) of water by one degree Celsius.

That can be pretty wonky, so it may be more helpful to think in terms of which foods contain more calories than others. Here’s a list of nutrients, and how many calories are contained in a gram of each:

• Fat: 9 calories/gram
• Protein: 4 calories/gram
• Carbohydrate: 4 calories/gram
• Alcohol: 7 calories/gram

As you can see, foods high in fat have more calories than foods that are high in protein, and alcohol has more calories than carbohydrate and protein, but also has fewer vital nutrients. Of course, very few foods or drinks contain only one of the four basic nutrients; most are a combination. Lean chicken, for example, while high in protein, still has some fat and carbohydrate, while rice, which is high in carbs, still has some fat. That’s why the best bet for anybody—those on a weight loss program or not—to eat a variety of foods and a balanced diet (sorry, alcohol isn’t considered vital).