Have you ever wondered why, for so many people (and especially for anyone older than 30 years old), weight gain seems to be a fact of life? It's because the human body is way too efficient! It just does not take that much energy to maintain the human body at rest; and when exercising, the human body is amazingly frugal when it comes to turning food into motion.

At rest (for example, while sitting and watching television), the human body burns only about 12 calories per pound of body weight per day (26 calories per kilogram). That means that if you weigh 150 pounds (68 kg), your body uses only about:

150 X 12 = 1,800 calories per day

Twelve calories per pound per day is a rough estimate -- see How Calories Work for details.

Those 1,800 calories are used to do everything you need to stay alive:

• They keep your heart beating and lungs breathing.
• They keep your internal organs operating properly.
• They keep your brain running.
• They keep your body warm.

In motion, the human body also uses energy very efficiently. For example, a person running a marathon (26 miles or 42 km) burns only about 2,600 calories. In other words, you burn only about 100 calories per mile (about 62 calories per km) when you are running.

You can see just how efficient the human body is if you compare your body to a car. A typical car in the United States gets between 15 and 30 miles per gallon of gasoline (6 to 12 km/L). A gallon of gas contains about 31,000 calories. That means that if a human being could drink gasoline instead of eating hamburgers to take in calories, a human being could run 26 miles on about one-twelfth of a gallon of gas (0.3 L). In other words, a human being gets more than 300 miles per gallon (120 km/L)! If you put a human being on a bicycle to increase the efficiency, a human being can get well over 1,000 miles per gallon (more than 500 km/L)!

That level of efficiency is the main reason why it is so easy to gain weight, as we will see in the next section.