To understand the problem with eating too
much saturated fat, we must examine its relationship to cholesterol. High levels
of cholesterol in the blood have been linked to the development of heart
disease, strokes, and other health problems. Despite its bad
reputation, our bodies need cholesterol, which is used to build cell membranes,
to protect nerve fibers, and to produce vitamin D and some
hormones, chemical messengers that help coordinate the body’s
functions. We just do not need cholesterol in our diet. The liver,
and to a lesser extent the small intestine, manufacture all the
cholesterol we require. When we eat cholesterol from foods that contain
saturated fatty acids, we increase the level of a cholesterol-carrying substance
in our blood that harms our health.
Human Nutrition, study of how food affects the health and survival of the human body. Human beings require food to grow, reproduce, and maintain good health. Without food, our bodies could not stay warm, build or repair tissue, or maintain a heartbeat. Eating the right foods can help us avoid certain diseases or recover faster when illness occurs. These and other important functions are fueled by chemical substances in our food called nutrients. Nutrients are classified as carbohydrates, proteins, fats, vitamins, minerals, and water. When we eat a meal, nutrients are released from food through digestion. Digestion begins in the mouth by the action of chewing and the chemical activity of saliva, a watery fluid that contains enzymes, certain proteins that help break down food. Further digestion occurs as food travels through the stomach and the small intestine, where digestive enzymes and acids liquefy food and muscle contractions push it along the digestive tract. Nutrients are absorbed from the inside of the small intestine into the bloodstream and carried to the sites in the body where they are needed. At these sites, several chemical reactions occur that ensure the growth and function of body tissues. The parts of foods that are not absorbed continue to move down the intestinal tract and are eliminated from the body as feces.
ESSENTIAL NUTRIENTS
Nutrients are classified as essential or
nonessential. Nonessential nutrients are manufactured in the body and do not
need to be obtained from food. Examples include cholesterol, a
fatlike substance present in all animal cells. Essential nutrients
must be obtained from food sources, because the body either does not produce
them or produces them in amounts too small to maintain growth and health.
Essential nutrients include water, carbohydrates, proteins, fats, vitamins, and
minerals.
An individual needs varying amounts of each
essential nutrient, depending upon such factors as gender and age. Specific
health conditions, such as pregnancy, breast-feeding,
illness, or drug use, make unusual demands on the body and increase its need for
nutrients. Dietary guidelines, which take many of these factors into account,
provide general guidance in meeting daily nutritional needs.
WATER
If the importance of a nutrient is judged
by how long we can do without it, water ranks as the most
important. A person can survive only eight to ten days without water, whereas it
takes weeks or even months to die from a lack of food. Water circulates through
our blood and lymphatic system, transporting
oxygen and nutrients to cells and removing wastes through
urine and sweat. Water also maintains the natural balance between
dissolved salts and water inside and outside of cells. Our
joints and soft tissues depend on the cushioning that water
provides for them. While water has no caloric value and therefore is not an
energy source, without it in our diets we could not digest or absorb the foods
we eat or eliminate the body’s digestive waste.
 |
| Water is Life! |
The human body is 65 percent water, and it
takes an average of eight to ten cups to replenish the water our bodies lose
each day. How much water a person needs depends largely on the volume of urine
and sweat lost daily, and water needs are increased if a person suffers from
diarrhea or vomiting or undergoes heavy physical
exercise. Water is replenished by drinking liquids, preferably those without
caffeine or alcohol, both of which increase the output
of urine and thus dehydrate the body. Many foods are also a good source of
water—fruits and vegetables, for instance, are 80 to
95 percent water; meats are made up of 50 percent water; and
grains, such as oats and rice, can have
as much as 35 percent water.
CARBOHYDRATES
Carbohydrates are the human
body’s key source of energy, providing 4 calories of energy per
gram. When carbohydrates are broken down by the body, the sugar
glucose is produced; glucose is critical to help maintain tissue
protein, metabolize fat, and fuel the central nervous system.
Glucose is absorbed into the bloodstream
through the intestinal wall. Some of this glucose goes straight to work in our
brain cells and red blood cells, while the rest makes its way to
the liver and muscles, where it is stored as
glycogen (animal starch), and to fat cells, where it is stored as fat.
Glycogen is the body’s auxiliary energy source, tapped and converted back into
glucose when we need more energy. Although stored fat can also serve as a backup
source of energy, it is never converted into glucose. Fructose and
galactose, other sugar products resulting from the breakdown of carbohydrates,
go straight to the liver, where they are converted into glucose.
| Dietary Fiber Sources |
Starches and sugars are the major
carbohydrates. Common starch foods include whole-grain breads and cereals,
pasta, corn, beans, peas, and potatoes. Naturally occurring sugars are found in
fruits and many vegetables; milk products; and honey, maple sugar, and sugar
cane. Foods that contain starches and naturally occurring sugars are referred to
as complex carbohydrates, because their molecular complexity requires our bodies
to break them down into a simpler form to obtain the much-needed fuel, glucose.
Our bodies digest and absorb complex carbohydrates at a rate that helps maintain
the healthful levels of glucose already in the blood.
| Wheat (flour), a staple diet |
In contrast, simple sugars, refined from
naturally occurring sugars and added to processed foods, require little
digestion and are quickly absorbed by the body, triggering an unhealthy chain of
events. The body’s rapid absorption of simple sugars elevates the levels of
glucose in the blood, which triggers the release of the hormone
insulin. Insulin reins in the body’s rising glucose levels, but at
a price: Glucose levels may fall so low within one to two hours after eating
foods high in simple sugars, such as candy, that the body responds by releasing
chemicals known as anti-insulin hormones. This surge in chemicals, the aftermath
of eating a candy bar, can leave a person feeling irritable and nervous.
Many processed foods not only contain high
levels of added simple sugars, they also tend to be high in fat and lacking in
the vitamins and minerals found naturally in complex carbohydrates.
Nutritionists often refer to such processed foods as junk foods and say that
they provide only empty calories, meaning they are loaded with calories from
sugars and fats but lack the essential nutrients our bodies need.
In addition to starches and sugars, complex
carbohydrates contain indigestible dietary fibers. Although such fibers provide
no energy or building materials, they play a vital role in our health. Found
only in plants, dietary fiber is classified as soluble or insoluble. Soluble
fiber, found in such foods as oats, barley, beans, peas, apples, strawberries,
and citrus fruits, mixes with food in the stomach and prevents or reduces the
absorption by the small intestine of potentially dangerous substances from food.
Soluble fiber also binds dietary cholesterol and carries it out of
the body, thus preventing it from entering the bloodstream where it can
accumulate in the inner walls of arteries and set the stage for
high blood pressure, heart disease, and strokes.
Insoluble fiber, found in vegetables, whole-grain products, and bran, provides
roughage that speeds the elimination of feces, which decreases the time that the
body is exposed to harmful substances, possibly reducing the risk of colon
cancer. Studies of populations with fiber-rich diets, such as Africans
and Asians, show that these populations have less risk of colon cancer compared
to those who eat low-fiber diets, such as Americans. In the United States, colon
cancer is the third most common cancer for both men and women, but experts
believe that, with a proper diet, it is one of the most preventable types of
cancer.
Nutritionists caution that most Americans
need to eat more complex carbohydrates. In the typical American diet, only 40 to
50 percent of total calories come from carbohydrates—a lower percentage than
found in most of the world. To make matters worse, half of the carbohydrate
calories consumed by the typical American come from processed foods filled with
simple sugars. Experts recommend that these foods make up no more that 10
percent of our diet, because these foods offer no nutritional value. Foods rich
in complex carbohydrates, which provide vitamins, minerals, some protein, and
dietary fiber and are an abundant energy source, should make up roughly 50
percent of our daily calories.
PROTEINS
  |
| Protein Molecule |
Dietary proteins are powerful
compounds that build and repair body tissues, from
hair and fingernails to muscles. In addition to
maintaining the body’s structure, proteins speed up chemical reactions in the
body, serve as chemical messengers, fight infection, and transport
oxygen from the lungs to the body’s tissues. Although
protein provides 4 calories of energy per gram, the body uses
protein for energy only if carbohydrate and fat intake is insufficient. When
tapped as an energy source, protein is diverted from the many critical functions
it performs for our bodies.
Proteins are made of smaller units called
amino acids. Of the more than 20 amino acids our bodies require,
eight (nine in some older adults and young children) cannot be made by the body
in sufficient quantities to maintain health. These amino acids are considered
essential and must be obtained from food. When we eat food high in proteins, the
digestive tract breaks this dietary protein into amino acids. Absorbed into the
bloodstream and sent to the cells that need them, amino acids then recombine
into the functional proteins our bodies need.
| Protein Sources |
Animal proteins, found in such food as eggs,
milk, meat, fish, and poultry, are considered complete proteins because they
contain all of the essential amino acids our bodies need. Plant proteins, found
in vegetables, grains, and beans, lack one or more of the essential
amino acids. However, plant proteins can be combined in the diet to provide all
of the essential amino acids. A good example is rice and beans. Each of these
foods lacks one or more essential amino acids, but the amino acids missing in
rice are found in the beans, and vice versa. So when eaten together, these foods
provide a complete source of protein. Thus, people who do not eat animal
products (see Vegetarianism) can meet their protein needs
with diets rich in grains, dried peas and beans, rice,
nuts, and tofu, a soybean product.
Experts recommend that protein intake make up
only 10 percent of our daily calorie intake. Some people, especially in the
United States and other developed countries, consume more protein than the body
needs. Because extra amino acids cannot be stored for later use, the body
destroys these amino acids and excretes their by-products. Alternatively,
deficiencies in protein consumption, seen in the diets of people in some
developing nations, may result in health problems. Marasmus and
kwashiorkor, both life-threatening conditions, are the two most
common forms of protein malnutrition.
Some health conditions, such as illness,
stress, and pregnancy and breast-feeding
in women, place an enormous demand on the body as it builds tissue
or fights infection, and these conditions require an increase in
protein consumption. For example, a healthy woman normally needs 45 grams of
protein each day. Experts recommend that a pregnant woman consume 55 grams of
protein per day, and that a breast-feeding mother consume 65 grams to maintain
health.
A man of average size should eat 57 grams of
protein daily. To support their rapid development, infants and young children
require relatively more protein than do adults. A three-month-old
infant requires about 13 grams of protein daily, and a
four-year-old child requires about 22 grams. Once in adolescence,
sex hormone differences cause boys to develop more
muscle and bone than girls; as a result, the protein needs of
adolescent boys are higher than those of girls.
FATS
| 3 Fatty Acids + 1 Glycerol = 1 Fat Molecule |
Fats, which provide 9 calories
of energy per gram, are the most concentrated of the energy-producing nutrients,
so our bodies need only very small amounts. Fats play an important role in
building the membranes that surround our cells and in
helping blood to clot. Once digested and absorbed, fats help the
body absorb certain vitamins. Fat stored in the body cushions vital
organs and protects us from extreme cold and heat.
 |
| Fat Sources |
Fat consists of fatty acids
attached to a substance called glycerol. Dietary fats are
classified as saturated, monounsaturated, and polyunsaturated according to the
structure of their fatty acids. Animal fats—from eggs, dairy products, and
meats—are high in saturated fats and cholesterol, a chemical
substance found in all animal fat. Vegetable fats—found, for example, in
avocados, olives, some nuts, and certain vegetable
oils—are rich in monounsaturated and polyunsaturated fat. As we will see, high
intake of saturated fats can be unhealthy.
Cholesterol, like fat, is a
lipid—an organic compound that is not soluble in water. In order to
travel through blood, cholesterol therefore must be transported through the body
in special carriers, called lipoproteins. High-density lipoproteins (HDLs)
remove cholesterol from the walls of arteries, return it to the
liver, and help the liver excrete it as bile, a liquid acid
essential to fat digestion. For this reason, HDL is called “good”
cholesterol.
Low-density lipoproteins (LDLs) and
very-low-density lipoproteins (VLDLs) are considered “bad” cholesterol. Both
LDLs and VLDLs transport cholesterol from the liver to the cells. As they work,
LDLs and VLDLs leave plaque-forming cholesterol in the walls of the arteries,
clogging the artery walls and setting the stage for heart disease. Almost 70
percent of the cholesterol in our bodies is carried by LDLs and VLDLs, and the
remainder is transported by HDLs. For this reason, we need to consume dietary
fats that increase our HDLs and decrease our LDL and VLDL levels.
 |
| Saturated Fat Molecule, Unsaturated Fat Molecule |
Saturated fatty acids—found in foods ranging
from beef to ice cream, to mozzarella cheese to
doughnuts—should make up no more than 10 percent of a person’s total
calorie intake each day. Saturated fats are considered harmful to
the heart and blood vessels because they are thought
to increase the level of LDLs and VLDLs and decrease the levels of HDLs.
Monounsaturated fats—found in olive,
canola, and peanut oils—appear to have the best effect
on blood cholesterol, decreasing the level of LDLs and VLDLs and increasing the
level of HDLs. Polyunsaturated fats—found in margarine and
sunflower, soybean, corn, and safflower
oils—are considered more healthful than saturated fats. However, if consumed in
excess (more than 10 percent of daily calories), they can decrease the blood
levels of HDLs.
Most Americans obtain 15 to 50 percent of
their daily calories from fats. Health experts consider diets with more than 30
percent of calories from fat to be unsafe, increasing the risk of heart disease.
High-fat diets also contribute to obesity, which is linked to high
blood pressure (see hypertension) and diabetes
mellitus. A diet high in both saturated and unsaturated fats has also
been associated with greater risk of developing cancers of the
colon, prostate, breast, and
uterus. Choosing a diet that is low in fat and cholesterol is
critical to maintaining health and reducing the risk of life-threatening
disease.
VITAMINS AND MINERALS
Both vitamins and minerals are needed by the body in very small
amounts to trigger the thousands of chemical reactions necessary to
maintain good health. Many of these chemical reactions are linked, with one
triggering another. If there is a missing or deficient vitamin or mineral—or
link—anywhere in this chain, this process may break down, with potentially
devastating health effects. Although similar in supporting critical functions in
the human body, vitamins and minerals have key differences.
Among their many functions, vitamins
enhance the body’s use of carbohydrates, proteins, and fats. They are critical
in the formation of blood cells, hormones, nervous system chemicals
known as neurotransmitters, and the genetic material
deoxyribonucleic acid (DNA). Vitamins are classified into two
groups: fat soluble and water soluble. Fat-soluble vitamins, which include
vitamins A, D, E, and K, are usually absorbed with the help of foods that
contain fat. Fat containing these vitamins is broken down by bile, a liquid
released by the liver, and the body then absorbs the breakdown
products and vitamins. Excess amounts of fat-soluble vitamins are stored in the
body’s fat, liver, and kidneys. Because these vitamins can be
stored in the body, they do not need to be consumed every day to meet the body’s
needs.
Water-soluble vitamins, which include
vitamins C (also known as ascorbic acid), B1
(thiamine), B2 (riboflavin), B3 (niacin), B6, B12, and
folic acid, cannot be stored and rapidly leave the body in urine if taken in
greater quantities than the body can use. Foods that contain water-soluble
vitamins need to be eaten daily to replenish the body’s needs.
In addition to the roles noted in the
vitamin and mineral chart accompanying this article, vitamins A (in the form of
beta-carotene), C, and E function as antioxidants, which are vital
in countering the potential harm of chemicals known as free
radicals. If these chemicals remain unchecked they can make
cells more vulnerable to cancer-causing substances. Free radicals
can also transform chemicals in the body into cancer-causing agents.
Environmental pollutants, such as cigarette smoke, are sources of free
radicals.
Minerals are minute amounts of metallic
elements that are vital for the healthy growth of teeth and
bones. They also help in such cellular activity as enzyme action,
muscle contraction, nerve reaction, and blood clotting. Mineral nutrients are
classified as major elements (calcium, chlorine, magnesium, phosphorus,
potassium, sodium, and sulfur) and trace elements (chromium, copper, fluoride,
iodine, iron, selenium, and zinc).
Vitamins and minerals not only help the
body perform its various functions, but also prevent the onset of many
disorders. For example, vitamin C is important in maintaining our
bones and teeth; scurvy, a disorder that attacks the gums,
skin, and muscles, occurs in its absence. Diets lacking vitamin B1,
which supports neuromuscular function, can result in beriberi, a
disease characterized by mental confusion, muscle weakness, and inflammation of
the heart. Adequate intake of folic acid by pregnant women is critical to avoid
nervous system defects in the developing fetus. The mineral
calcium plays a critical role in building and maintaining strong
bones; without it, children develop weak bones and adults experience the
progressive loss of bone mass known as osteoporosis, which
increases their risk of bone fractures.
Vitamins and minerals are found in a wide
variety of foods, but some foods are better sources of specific vitamins and
minerals than others. For example, oranges contain large amounts of
vitamin C and folic acid but very little of the other vitamins.
Milk contains large amounts of calcium but no vitamin C.
Sweet potatoes are rich in vitamin A, but white
potatoes contain almost none of this vitamin. Because of these
differences in vitamin and mineral content, it is wise to eat a wide variety of
foods.
Credential
Worthington, Roberts PhD
