The energy in foods is expressed as kilocalories. One kilocalorie represents the amount of heat it takes to raise the temperature of one liter of water 1°C. A person needs about 2000 kilocalories a day to meet his or her energy needs. In common usage, people refer to kilocalories as calories, which is a much smaller energy unit: 1 kilocalorie contains 1000 calories. This text uses the familiar word calorie to stand for the larger energy unit; you’ll also find calorie used on food labels.
Of the six classes of essential nutrients, three supply energy:
• Fat = 9 calories per gram
• Protein = 4 calories per gram
• Carbohydrate = 4 calories per gram
Alcohol, though not an essential nutrient, also supplies energy, providing 7 calories per gram. (One gram equals a little less than 0.04 ounce.) The high caloric content of fat
Calories Photo Gallery
A pound of body fat is equal to 3500 calories. If you eat 100 calories more than you expend every day (36,500 extra calories), you will gain more than 10 pounds in a year. is one reason experts often advise against high fat consumption; most of us do not need the extra calories to meet energy needs. Regardless of their source, calories consumed in excess of energy needs can be converted to fat and stored in the body.
Just meeting energy needs is not enough. Our bodies need enough of the essential nutrients to function properly. Practically all foods contain combinations of nutrients, although foods are commonly classified according to their predominant nutrients. For example, spaghetti is considered a carbohydrate food, although it contains small amounts of other nutrients. The following sections discuss the functions and sources of each class of nutrients.