What is the difference between the dietary calorie and the physical calorie? Is there a relation between the two?

Asked by: Dee Hampton

Answer

They are both units of energy, but the dietary Calorie (note the use of upper case 'C') equals 1000 'physics' calories. The physics calorie represents an amount of energy needed to raise (or lower) one gram of liquid water by one centigrade degree. That amount of energy is too small to conveniently describe the energy content of food, so the physics 'kilocalorie' is used instead and is, confusingly, called simply a Calorie in dietary terminology. A food Calorie, then, is the amount of energy needed to change the temperature of one KILOGRAM of liquid water (a volume of one liter) one centigrade degree. The food Calorie is often spelled with an upper case 'C' to distinguish it from the smaller calorie.

By the way, this metric measure of energy has an English counterpart which you've probably seen used to describe air conditioners and furnaces. The amount of energy needed to change the temperature of one POUND of liquid water by one degree FAHRENHEIT is called a British Thermal Unit, or BTU. A food Calorie is the equivalent of about 4 BTU's.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor