HUMAN BIOLOGY

212 Chapter 11

the Body’s nutritional requirements

n Diet has a major effect on body functions because

it supplies major nutrients as well as vitamins

and minerals.

n Links to Carbohydrates, lipids, and proteins 2.9–2.11

The nutrients we absorb are burned as fuel to provide

energy and used as building blocks to build and replace

tissues. In this section we focus on the three main classes

of nutrients—carbohydrates, lipids, and proteins—and we

take a look at guidelines for what makes up a healthy diet.

Complex carbohydrates are best

There are many views on the definition of a “proper”

diet, but just about all nutritionists can agree on this

point: The healthiest carbohydrates are “complex” ones

such as starch—the type of carbohydrate in fleshy fruits,

cereal grains, and legumes, including peas and beans

(see Figure 11.21).

Human digestive enzymes easily break down complex

carbohydrates to glucose, the body’s chief energy source.

Foods rich in complex carbohydrates also usually are high

in fiber, including the insoluble fiber that adds needed bulk

to feces and helps prevent constipation (see Section 11.9).

By contrast, simple sugars such as those in sweets don’t

have much fiber, and they lack the vitamins and minerals

of whole foods.

A person who eats lots of packaged food may consume

up to 2 pounds of refined sugars per week. Ingredient

labels may list these sugars as corn syrup, corn sweeten-

ers, and dextrose. They represent “empty calories” because

they add calories but meet no other nutritional needs.

Highly refined carbohydrates also have a high glycemic

index (GI). This index ranks foods by their effect on blood

glucose during the first 2 hours after a meal. For example,

white rice and breads or crackers made with refined white

flour have a high GI. They are digested quickly and cause a

surge in the blood levels of sugar and insulin.

Circulating insulin makes cells take up glucose quickly,

and it also prevents cells from using stored fat as fuel. At

the same time, glucose that is not needed as fuel for cells

is stored as fat. When blood sugar levels later fall, you feel

hungry. So you may eat more, secrete more insulin, and

keep storing fat, mainly in the form of triglycerides. Over

time, high triglyceride levels increase the risk of heart dis-

ease and type 2 diabetes.

some fats are healthier than others

The body can’t survive without fats and other lipids. The

phospholipid lecithin and the sterol cholesterol both are

building blocks of cell membranes. Fat stored in adipose

tissue serves as an energy reserve, cushions organs such as

the eyes and kidneys, and provides insulation beneath the

skin. A young child’s brain won’t develop properly without

a supply of cholesterol and saturated fat. The body also

stores fat-soluble vitamins in adipose tissues.

The liver can manufacture most fats the body needs,

including cholesterol, from protein and carbohydrates. The

ones it cannot make are essential fatty acids, but whole

foods and vegetable oils provide plenty of them. Linoleic

acid is an example. You can get enough of it by consuming

just 1 teaspoon a day of corn oil, olive oil, or some other

polyunsaturated fat.

Animal fats—the fat in butter, cheese, and fatty meat—

are rich in saturated fats and cholesterol. Eating too

much of these kinds of foods increases the risk for heart

disease and stroke, as well as for certain cancers. Trans

fatty acids, or “trans fats” (Chapter 2), are also bad for the

Figure 11.21 Fruits, vegetables, and whole grains are good

sources of both soluble and insoluble fiber. (Left: © Brian Chase/

Shutterstock.com; Right: © Antonio Gravante/Shutterstock.com)

11.12

Polyunsaturated Fatty acids:

Liquid at room temperature; essential for health.

Omega-3 fatty acids

Alpha-linolenic acid and its derivatives

Sources: Nut oils, vegetable oils, oily fish

Omega-6 fatty acids

Linoleic acid and its derivatives

Sources: Nut oils, vegetable oils, meat

Monounsaturated Fatty acids:

Liquid at room temperature. Main dietary source is olive oil. Beneficial

in moderation.

Saturated Fatty acids:

Solid at room temperature. Main sources are meat and dairy

products, palm and coconut oils. Excessive intake may raise risk of

heart disease.

Trans Fatty acids (Hydrogenated Fats):

Solid at room temperature. Manufactured from vegetable oils and

used in many processed foods. Excessive intake may raise risk of

heart disease.

Table 11.3 Main Types of Dietary Lipids

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