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The Secret to Weight Loss? Calories.

The truth is, all diets boil down to a simple formula-eating fewer Calories than you burn. Break that rule and all the carb-cutting, fat-banning, low-glycemic-index-eating in the world won't make a bit of difference. This is why weight-loss experts are now advocating a back to-the-basics approach: Calorie counting.

Weren't paying attention in high school biology? No worries. This crash course on the science of Calories will catch you up fast. You'll learn to make smarter choices about the foods (and portion sizes) you put on your plate and how to maximize your daily Calorie budget. The savvier you are, the quicker you'll drop pounds.

Q: Exactly what is a
Calorie anyway?

A: In science equations, a calorie is a measure of
potential energy in
food. Technically, one
calorie is the amount
of energy it takes to
raise the temperature
of 1 gram of water by
1 degree Celsius. The Calorie we refer to generally in nutrition, which is found on food labels, is the kcal, or kilocalorie (which equals 1,000 calories). "Calories
are like gasoline. In the
same way that gas
makes your car go, calories
fuel your body," says
D. Milton Stokes, R.D.,
president of a nutrition
counseling firm in
Norwalk, Connecticut.

Q: How does my
body convert Calories?

A: Enzymes in your
digestive system break
down the chemical
bonds that hold food
molecules together,
explains Gary Miller,
Ph.D., an associate
professor of nutrition at
Wake Forest University.
Your digestive system
then releases the energy
contained in those
bonds, making
it available for use.

Q: What does my body
do with that energy?

A: It fuels everything
from basic activities like
breathing, thinking, and
growing hair to bigger
tasks, like carrying a
pregnancy or running
a marathon. However,
when you don't use
the Calories you've
consumed (maybe you
decide to skip the gym
today), those Calories
get shuttled to your
liver to refill your glycogen
stores. Glycogen
is your body's quick,
easy-access energy
reserve. Your ability to
store it means you
don't have to eat continuously
to keep your
body revved up. Still, it
gets depleted every
three to four hours.
When the liver is holding
as much glycogen as
it can, some of it is
passed on to muscles for
short-term storage (to
be used as needed to
move your body and get
you through a workout).

Between your liver
and muscles, you have a
ready supply of Calories
(roughly 300 to 400,
depending on your
weight and metabolism)
that you can access as
necessary throughout the
day. When you eat more
than you can save in these
temporary "accounts,"
the Calories get converted
to fat and distributed
throughout your body.

Q: How many Calories
do you have to burn to
lose a pound?

A: About 3,500. That
means you can eat 100
fewer Calories a day for
35 days, or 500 fewer
calories for seven days,
or walk an hour a day for
22 days, or do a combination
of the two by
eating less and moving
more. Remember, even if
you're exercising more
than usual, the Calories-in,
Calories-out rule
still applies: If you take
in more than you burn,
you'll gain weight.
Q: How do scientists
determine the number
of Calories in a food?

A: Typically, they use
different laboratory
methods to separate the
caloric components of
food-the macronutrients
protein, fat, and carbs
from the noncaloric
substances like water and
minerals. They figure out
the weight of each macronutrient
in grams and
then multiply by the following
to get the Calorie
count: fat, 9 Calories per
gram; protein and carbs,
4 Calories per gram.

Q: Can I have a food
analyzed to see if the
Calorie count listed on
the label is correct?

A: Yes, but it's expensive-
about $90 an
item. Contact Bodycote
FPL at for
more information.

Q: How do scientists
figure out how many
Calories a person
burns during exercise?

A: They place a plastic
tube over the exerciser's
mouth and measure the
amount of oxygen she
inhales and uses while
engaging in a specific
activity (e.g., walking,
rowing, biking). Every
liter of oxygen consumed
is equivalent to 4.8
Calories burned. As your
fitness level increases,
you're able to take in and
use more oxygen, which
lets you work out harder,
longer, and burn more
Calories (and fat).

Q: How much do I really
need to eat every day
to maintain my weight?

A: It's a complex formula
that depends on your
weight and height (the
heavier you are, the more
Calories you'll burn), age
(the older you are, the
fewer you need), and
activity level (computer solitaire
fans utilize far less than marathoners).
For example, a 25-year-old
woman who is 5'9",
150 pounds, and very
active (i.e., exercising six
days a week) requires
roughly 2,570 Calories a
day to maintain her
weight, while a 40-year-old
woman who is 5'4",
130 pounds, and only
moderately active
(working out three days
a week) needs 640 fewer
Calories (1,930). For an
estimate of your Calorie
needs, go to

Q: Can I bank Calories
so I can eat more
on a special occasion?

A: In theory, yes. If you
know you're going to a
party on Saturday night
and want the freedom
to indulge a little, you can
shave, say, 100 Calories
from what you normally
eat Sunday through
Friday and have an extra
600 Calories to play with
by the time the party
rolls around. At the
end of the week, your
Calorie intake-and your
weight-stays the same.
In practice, though, this
strategy may backfire.
It's tricky to cut enough
calories to matter, but
not so many that you
go hungry or affect your
metabolism. "Really,
what you're trying to do
is lose a little weight
so you can gain it back,"
says Michael B. Zemel, M.D.,
professor of nutrition
and medicine at the
University of Tennessee.

"But when you deprive
yourself, you typically
compensate by eating
way too much later-
and take in more Calories
than you banked."

Instead of saving up
Calories, on days when
you know you may want
a little more diet wiggle
room, eat foods that are
low-calorie but filling.
For instance, if you usually
have a turkey sandwich
for lunch, try a salad with
sliced turkey. You'll take
in fewer Calories by skipping
the bread but feel
just as satisfied because
of the fiber in the greens
and other veggies. Then
you won't arrive at your
event famished, and
you can afford to have a
little of your splurge
food. An even smarter
option: "Work out a little
more before and after
the party," says Zemel.
"Every extra mile you
walk burns 100 Calories."


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