A calorie is a unit of energy or heat and it is defined as the energy (or
heat) needed to raise the temperature of one gram of water from 14.5
degrees centigrade to 15.5 degrees centigrade. This is a so-called
"scientific calorie". When it comes to food labels and nutrition, what is
called a "calorie" is actually a "kilo-calorie" or 1000 scientific calories.
Therefore a food calorie is the amount of energy needed to raise 1000
grams (one liter) of water from 14.5 to 15.5 degrees centigrade.

A calorie is therefore not a physical thing but a specific amount of energy
much like a pound is not a thing but a specific amount of weight. At first it
may seem surprising to think about food containing energy, but it does in
the form of "chemical potential energy". The chemical bonds of the
molecules found in food can release energy when they are broken in the
same way that the molecules in gasoline can release energy and, just like
gasoline, the energy in food can be used to perform useful work (like
building proteins or fat needed in the body) or it can simply be lit on fire and
burned. In fact, the way nutritionists measure the calories in food is by
drying the food out, then placing it inside a machine called a "bomb
calorimeter" and literally incinerating the food and measuring the heat that
comes off. Anyone who has ever witnessed a grease fire can probably
attest to the heat energy released by burning food.

Unlike a fire, the human body normally does everything it can not to make
heat from the calories in food but instead to use that energy to make other
molecules. Inevitably, a certain amount of heat is still produced as a
by-product of metabolism, but the body is still very efficient at capturing
every calorie it can.

This is because the body can basically never get "enough" calories. Any
food energy that it cannot use at the moment will not be lost, but will
instead be used to make body fat. Body fat is the "reserve tank" of the
body. It is tissue that is saved until it is needed, and that occurs when we
can't get enough calories from the food we eat to satisfy our body's needs.
At that time, body fat is utilized to make up the difference. This is what
protects us against starvation during times of famine. Since our ancestors
often faced nearly constant famine and sadly, since half the earth's
population still does today, our species has been well served by having the
ability to "grab" extra calories and store them in the chemical bonds of fat.

While this caloric "thriftiness" serves us well when times are lean, it has
backfired in the modern western world where large numbers of people, for
the first time in human history, have not just enough to eat, but more than
enough. Essentially our bodies have never "learned" to cope with excess.
Instead, we go right on storing more and more fat for as long as we
consume more calories than we burn. In the simplest and most basic sense,
this is the cause of obesity. As we shall see on later pages herein, in truth
there is nothing simple at all about obesity but it's ultimate cause is
undoubtedly related to amazing ability we have inherited to resist
starvation.

Conclusion:

Calories are energy. The body uses energy to stay alive and makes fat out
of extra energy because it is not adapted to the modern lifestyle.
What is a "Calorie"?
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