Encyclopedia of Health and Medicine

nutrients 191
hydrogen. Nutritionists further classify carbohydrates
as monosaccharides (single molecule), disaccharides
(two molecules), and polysaccharides
(multiple molecules). Monosaccharides and disaccharides
are simple carbohydrates; polysaccharides
are complex carbohydrates. Nearly all foods contain
or deliver as a product of metabolism some
form of carbohydrate. Monosaccharides and disaccharides
convert to energy fairly quickly after consumption;
the sugars from fruits and fruit juices
and from candies and sodas (soft drinks) can enter
the blood circulation within 10 minutes. Polysaccharides
such as pastas take longer for the body to
digest and metabolize, up to several hours.
Polysaccharides are starches and fibers.
Starches are storage forms of glucose the LIVER
converts to glycogen. Fibers are structural components
of plants that the body cannot digest. Some
forms of fiber, such as pectin, are soluble (dissolve
in water). These fibers acquire a gel-like consistency
in the intestines that bind with lipids
(including cholesterol), BILE, and other substances.
The primary dietary sources of soluble fibers are
fruits, oats, and legumes. Nonsoluble fibers absorb
water but do not change consistency. These fibers
add bulk to digestive waste in the large intestine,
aiding the COLON in moving the waste through and
out of the body. Though not itself a nutrient, fiber
is essential for the healthy function of the gastrointestinal
tract.
Monosaccharides
CARBOHYDRATES
GLUCOSE fructose galactose
Disaccharides
lactose maltose sucrose
Polysaccharides
cellulose fiber glycogen
Enzymes carry out the chemical actions that
metabolize carbohydrates to the end form of glucose.
Carbohydrate digestion begins in the MOUTH
with the aid of amylase, an enzyme in the saliva.
Amylase breaks down dietary carbohydrates into
smaller polysaccharides and disaccharides.
Because the STOMACH does not contain any
enzymes that metabolize carbohydrates, the next
stage of carbohydrate digestion takes place in the
SMALL INTESTINE. The enzymes lactase, maltase, and
sucrase break down lactose, maltose, and sucrose,
respectively. Lactose and sucrose each produce
one molecule of glucose; maltose produces two.
From the small intestine the monosaccharides
enter the BLOOD circulation. Fructose and galactose
travel to the liver where chemical processes convert
them to glucose. Depending on the body’s
needs, the liver may further convert glucose to
glycogen for storage.
Fats (lipids) Dietary fats are chemical combinations
of carbon and hydrogen atoms that form
structures called fatty acids. The number of hydrogen
atoms in a fatty acid determines whether the
fat is saturated or unsaturated, which is one of the
most important features of the fat from a health
perspective. A fatty acid’s saturation determines
how the fat behaves in the body.
Saturated fats, which come primarily from animal-based
foods such as meats and dairy, contribute
to elevated CHOLESTEROL BLOOD LEVELS, a risk
factor for CARDIOVASCULAR DISEASE (CVD). Saturated
fats are the primary source material for the liver’s
production of cholesterol and the carriers that
transport them through the blood, lipoproteins.
Palm oil and coconut oil are also saturated fats.
Saturated fats are solid at room temperature.
TRANSFORMED THINKING ABOUT TRANS FATS
In the 1980s and 1990s researchers and doctors
believed trans fats, created through a manufacturing
process called hydrogenation that adds
hydrogen atoms to unsaturated fatty acid structures
to make them more stable in food products,
were less harmful for health than the saturated
fats they were marketed to replace. However,
further research demonstrated that trans fats are
instead considerably more harmful to health,
causing a rapid and significant rise in blood cholesterol
levels and thus dramatically raising the
risk for CARDIOVASCULAR DISEASE (CVD). Health
experts now recommend avoiding trans fats; and
in 2006, US regulations began requiring food
labels to list trans fat content.
Unsaturated fats come from plant-based foods.
Commonly called oils, unsaturated fats are liquid
at room temperature. They are monounsaturated
or polyunsaturated, depending on their chemical