HUMAN BIOLOGY

DiGestioN aND NUtritioN 213

essential amino acid

Amino acid that cannot be

made in the body and so

must be obtained from the

diet.

essential fatty acid Fatty

acid that cannot be made

in the body and so must be

obtained from the diet.

glycemic index Ranking

of foods by their effect on

blood glucose during the

two hours following a meal.

What is a “healthy diet”?

• A healthy diet must provide essential nutrients in the proper
  proportions and amounts.


• Complex carbohydrates provide nutrients and fiber without
  adding “empty” calories.


• Fats and other lipids are used for building cell membranes,
  energy stores, and other needs.


• Various foods, including vegetable oils, provide essential
  fatty acids.


• Proteins provide essential amino acids.

taKe-Home message

cardiovascular system. Food labels are now required to
show the amounts of trans fats, saturated fats, and choles-
terol per serving. Table 11.3 lists the main types of lipids
in food.

proteins are body-building nutrients

When the digestive system breaks down and absorbs pro-
teins, their amino acids become available for protein syn-
thesis in cells. Of the twenty common amino acids, eight
are essential amino acids. Our cells cannot make them,
so we must obtain them from food. The eight are isoleu-
cine, leucine, lysine, methionine, phenylalanine, threonine,
tryptophan, and valine (Figure 11.22).
Most animal proteins are complete, meaning their ratios
of amino acids match human nutritional needs. Nearly all
plant proteins are incomplete, meaning they lack one or
more of the essential amino acids. (The proteins of quinoa,
pronounced keen-wah, are an exception.) To get required
amino acids from a vegetarian diet, one must combine plant
foods so the amino acids missing from one are present in
others. Examples are combining beans with rice, cornbread
with chili, tofu with rice, and lentils with wheat bread.

guidelines for healthy eating include
limiting meat, refined grains, saturated
and trans fats, salt, and refined sugar

Scientists at the U.S. Department of Agriculture (USDA)
and other U.S. government agencies study diets that may
help prevent health problems such as heart disease, type 2
diabetes, and certain cancers. The USDA periodically
updates its nutritional guidelines. Figure 11.23 shows the
most recent USDA guidelines, presented in the form of
a plate with recommended portions of various types of
foods. The guidelines suggest eating less meat and more
dark green and orange vegetables, fruits, and milk prod-
ucts. They call for eating less of foods containing refined

grains (such as white flour and white rice), saturated and

trans fats, salt, and refined sugars.

Respected alternative diets include the Mediterranean

diet, which is associated with a lower risk of heart disease,

among other chronic ills. It emphasizes grains, fruits, and

vegetables. Its main fat is olive oil, an excellent antioxidant.

The diet also limits weekly intake of animal protein, eggs,

and refined sugars.

Also popular are “low-carb” diets: fewer carbohydrates

and more proteins and fats. People often lose weight rapidly

on such diets. However, high-fat,

high-protein diets make the kidneys

work harder, raising the risk of kid-

ney stones and other kidney prob-

lems. Studies show that following

a low-carb diet for six months does

not increase LDLs, the “bad” form

of cholesterol. Still, given all the

evidence that a diet high in satu-

rated fat increases the risk of heart

disease, low-carb dieters are advised

to obtain their protein from fish, lean

meat, or vegetable sources.

Rice, corn, or

other grains

Lentils, soybeans

(for example, tofu),

or other legumes

Eight essential amino acids

Methionine

Tryptophan

Leucine

Phenylalanine

Threonine

Valine

Isoleucine

Lysine

Figure 11.22 Numerous foods can supply the eight essential
amino acids. Goosefoot plant seeds, known as quinoa (lower
right), provide a balanced set of the amino acids humans require.
(© Cengage Learning; Scimat/Science Source)

Figure 11.23 The USDa formulates nutritional guidelines.

Recommended proportions add up to a daily 2,000 kilocalorie

intake for sedentary females aged 10 to 30. The recommended

intake and serving sizes are larger for males and highly active

females and less for older females.

ChooseMyPlate.gov/USDA

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