Why is digestion important?
Digestion is important for breaking down
food into nutrients, which the body uses
for energy, growth, and cell repair. Food
and drink must be changed into smaller
molecules of nutrients before the blood
absorbs them and carries them to cells
throughout the body. The body breaks
down nutrients from food and drink into
carbohydrates, protein, fats, and vitamins.
Carbohydrates. Carbohydrates are the
sugars, starches, and fiber found in many
foods. Carbohydrates are called simple
or complex, depending on their chemical
structure. Simple carbohydrates include
sugars found naturally in foods such as fruits,
vegetables, milk, and milk products, as well
as sugars added during food processing.
Complex carbohydrates are starches and
fiber found in whole-grain breads and
cereals, starchy vegetables, and legumes.
The Dietary Guidelines for Americans, 2010,
recommends that 45 to 65 percent of total
daily calories come from carbohydrates.
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1
U.S. Department of Agriculture and U.S. Department
of Health and Human Services. Dietary Guidelines
for Americans, 2010. 7th ed. Washington, D.C.: U.S.
Government Printing Office; 2010.
Protein. Foods such as meat, eggs, and
beans consist of large molecules of protein
that the body digests into smaller molecules
called amino acids. The body absorbs
amino acids through the small intestine
into the blood, which then carries them
throughout the body. The Dietary Guidelines
for Americans, 2010, recommends that 10 to
35 percent of total daily calories come from
protein.
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Fats. Fat molecules are a rich source of
energy for the body and help the body
absorb vitamins. Oils, such as corn, canola,
olive, safflower, soybean, and sunflower, are
examples of healthy fats. Butter, shortening,
and snack foods are examples of less healthy
fats. During digestion, the body breaks down
fat molecules into fatty acids and glycerol.
The Dietary Guidelines for Americans, 2010,
recommends that 20 to 35 percent of total
daily calories come from fat.
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Vitamins. Scientists classify vitamins by
the fluid in which they dissolve. Water-
soluble vitamins include all the B vitamins
and vitamin C. Fat-soluble vitamins include
vitamins A, D, E, and K. Each vitamin has
a different role in the body’s growth and
health. The body stores fat-soluble vitamins
in the liver and fatty tissues, whereas the
body does not easily store water-soluble
vitamins and flushes out the extra in the
urine. Read more about vitamins on the
Office of Dietary Supplements website at
www.ods.od.nih.gov.
2 The Digestive System and How It Works