For years now, calories have been all the rage -- people
are counting them and cutting them, and you'd be hard-pressed
to find something at the supermarket that does not list its
calories per serving somewhere on the package. But have you
ever wondered what exactly a calorie is?
In this edition of HowStuffWorks,
we'll find out what calories are and why we need them, and
examine the relationship between calories and weight.
What is a Calorie?
A calorie is a unit of energy. We
tend to associate calories with food, but they apply to
anything containing energy. For example, a gallon (about 4
liters) of gasoline contains about 31,000,000 calories.
Specifically, a calorie is the amount of energy, or
heat, it takes to raise the temperature of 1 gram of
water 1 degree Celsius (1.8 degrees Fahrenheit). One calorie
is equal to 4.184 joules, a common unit of energy used in the
Most of us think of calories in relation to food, as in
"This can of soda has 200 calories." It turns out that the
calories on a food package are actually kilocalories
(1,000 calories = 1 kilocalorie). The word is sometimes
capitalized to show the difference, but usually not. A food
calorie contains 4,184 joules. A can of soda containing 200
food calories contains 200,000 regular calories, or 200
kilocalories. A gallon of gasoline contains 31,000
The same applies to exercise -- when a fitness chart says
you burn about 100 calories for every mile you jog, it means
100 kilocalories. For the duration of this article, when we
say "calorie," we mean "kilocalorie."
What Calories Do
Human beings need energy to
survive -- to breathe,
move, pump blood -- and
they acquire this energy from food.
of calories in a food is a measure of how much potential
energy that food possesses. A gram of carbohydrates
has 4 calories, a gram of protein has
4 calories, and a gram of fat has 9
calories. Foods are a compilation of these three building
blocks. So if you know how many carbohydrates, fats and
proteins are in any given food, you know how many calories, or
how much energy, that food contains.
1 g Carbohydrates: 4 calories
1 g Protein: 4 calories
1 g Fat: 9 calories
If we look at the nutritional label on the back of a packet
of maple-and-brown-sugar oatmeal, we find that it has 160
calories. This means that if we were to pour this oatmeal into
a dish, set the oatmeal on fire and get it to burn completely
(which is actually pretty tricky), the reaction would produce
160 kilocalories (remember: food calories are kilocalories) --
enough energy to raise the temperature of 160 kilograms of
water 1 degree Celsius. If we look closer at the nutritional
label, we see that our oatmeal has 2 grams of fat, 4 grams of
protein and 32 grams of carbohydrates, producing a total of
162 calories (apparently, food manufacturers like to round
down). Of these 162 calories, 18 come from fat (9 cal x 2 g),
16 come from protein (4 cal x 4 g) and 128 come from
carbohydrates (4 cal x 32 g).
"burn" the calories in the oatmeal through metabolic
processes, by which enzymes
break the carbohydrates into glucose and other sugars, the
fats into glycerol and fatty acids and the proteins into amino
acids (see How
Food Works for details). These molecules are then
transported through the bloodstream to the cells, where
they are either absorbed for immediate use or sent on to the
final stage of metabolism in which they are reacted with
oxygen to release their stored energy.
for a simplified diagram of these metabolic processes.
Your Caloric Needs
Just how many calories do
need to function well? The number is different for every
person. You may notice on the nutritional labels of the foods
you buy that the "percent daily values" are based on a 2,000
calorie diet -- 2,000 calories is a rough average of what a
person needs to eat in a day, but your body might need more or
less than 2,000 calories. Height, weight, gender, age and
activity level all affect your caloric needs. There are three
main factors involved in calculating how many calories your
body needs per day:
metabolic rate (BMR) is the amount of energy your body
needs to function at rest. This accounts for about 60 to 70
percent of calories burned in a day and includes the energy
required to keep the heart
beating, the lungs
breathing, the kidneys
functioning and the body temperature stabilized. In general,
men have a higher BMR than women. One of the most accurate
methods of estimating your basal metabolic rate is the
- Basal metabolic rate
- Physical activity
- Thermic effect of food
- Adult male: 66 + (6.3 x body weight in lbs.) +
(12.9 x height in inches) - (6.8 x age in years)
- Adult female: 655 + (4.3 x weight in lbs.) + (4.7
x height in inches) - (4.7 x age in years)
(Note: The first number in the equation for
females is, in fact, 655. Strange but true.)
The second factor in the equation, physical
activity, consumes the next highest number of calories.
Physical activity includes everything from making your bed to
jogging. Walking, lifting, bending, and just generally moving
around burns calories, but the number of calories you burn in
any given activity depends on your body weight. Click here
for a great table listing the calories expended in various
physical activities and for various weights.
The thermic effect of food is the final addition to
the number of calories your body burns. This is the amount of
energy your body uses to digest the food you eat -- it takes
energy to break food down to its basic elements in order to be
used by the body. To calculate the number of calories you
expend in this process, multiply the total number of calories
you eat in a day by 0.10, or 10 percent. If you need some help
determining how many calories you eat in a day, check out
number of calories a body needs in a day is the sum of these
three calculations. If you only want a rough estimate of your
daily caloric needs, you can skip the calculations and click
Calories, Fat and Exercise
So what happens
if you take in more or fewer calories than your body burns?
You either gain or lose fat,
respectively. An accumulation of 3,500 extra calories is
stored by your body as 1 pound of fat -- fat is the body's way
of saving energy for a rainy day. If, on the other hand, you
burn 3,500 more calories than you eat, whether by exercising
more or eating less, your body converts 1 pound of its stored
fat into energy to make up for the deficit.
One thing about exercise is that it raises your metabolic
rate not only while you're huffing and puffing on the
treadmill. Your metabolism takes a while to return to its
normal pace. It continues to function at a higher level; your
body burns an increased number of calories for about two hours
after you've stopped exercising.
Lots of people wonder if it matters where their calories
come from. At its most basic, if we eat exactly the number of
calories that we burn and if we're only talking about weight,
the answer is no -- a calorie is a calorie. A protein calorie
is no different from a fat calorie -- they are simply units of
energy. As long as you burn what you eat, you will maintain
your weight; and as long as you burn more than you eat, you'll
But if we're talking nutrition,
it definitely matters where those calories originate.
Carbohydrates and proteins are healthier sources of calories
than fats. Although our bodies do need a certain amount of fat
to function properly -- an adequate supply of fat allows your
body to absorb the vitamins you ingest -- an excess of fat can
have serious health consequences. The U.S.
Food and Drug Administration recommends that a maximum of
30 percent of our daily calories come from fat. So, if you eat
2,000 calories a day, that's a maximum of 600 calories from
fat, or 67 grams of fat, per day. However, many doctors and
nutritionists now set the maximum number of fat calories at 25
percent of our daily caloric intake. That's 56 grams of fat
per day for a 2,000 calorie diet.
Here are some calorie and fat contents that may surprise
For more information on calories, dieting, nutrition and
related topics, check out the links on the next page!
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