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Carbohydrates -
Carbohydrates form one of the three macro-nutrient
categories; proteins and fats are the other macro-nutrient
categories. Carbohydrates come in a variety of forms.
The most common and abundant are sugars, fibers,
and starches. The basic building block of a carbohydrate
is a sugar molecule, a simple union of carbon, hydrogen,
and oxygen. Starches and fibers are essentially
chains of sugar molecules. Some contain hundreds
of sugars. Some chains are straight, whereas others
branch wildly.
Carbohydrates were once grouped into two main categories:
simple and complex. Simple
carbohydrates included sugars such as fruit sugar
(fructose), corn or grape sugar (dextrose or glucose),
and table sugar (sucrose).
Complex carbohydrates
included everything made of three or more linked
sugars. Simple sugars were considered bad and complex
carbohydrates good. However, the picture is much
more complicated than that. The digestive system
handles all carbohydrates in much the same way -
it breaks them down (or tries to break them down)
into single sugar molecules, since only these are
small enough to cross into the bloodstream. It also
converts most digestible carbohydrates into glucose
(also known as blood sugar), because cells are designed
to use this as a universal energy source.
Fiber
is an exception. It is put together in such a way
that it can't be broken down into sugar molecules,
and so passes through the body undigested.
The Glycemic Index
A new system for classifying
carbohydrates calls into question many of the old
assumptions about how carbohydrates affect health.
This new system, known as the glycemic index, measures
how fast and how far blood sugar rises after you
eat a food that contains carbohydrates.
For example, white bread is converted almost immediately
to blood sugar, causing it to spike rapidly. It's
classified as having a high glycemic index. On the
other hand, brown rice is digested more slowly,
causing a lower and more gentle change in blood
sugar. It has a low glycemic index.
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