Nutrition begins with food.
The science of nutrition concerns everything that the body does with food in order to live, function, grow, and heal.
Foods that are eaten on a regular basis make up a diet.
Although geographic location and family traditions play major parts in forming dietary habits, food choices vary from person to person.
To a high degree, what people eat affects their health and their enjoyment of life.
In turn, healthy habits and a happy outlook can improve the ways in which the human body makes use of food.
With the help of the sun, plants make their food from chemicals in the earth, water, and air.
Animals human beings included cannot make their own food but must eat plants, other animals, or both.
The body’s most basic need is for energy.
To get energy it needs food as a fuel and oxygen to burn it.
The amount of energy foods can produce is measured in units called calories.
A food calorie, or kilocalorie, is the amount of heat required to raise the temperature of 1 kilogram (2.2 pounds) of water 1 degree Celsius (1.8 degrees Fahrenheit).
The body changes the calories in food into energy, which is necessary for every act from blinking an eye to running a race.
Energy is also used for the growing process, for rebuilding damaged cells, and for regulating body systems.
The number of calories needed each day depends upon how much energy an individual’s body uses.
An active child can need more calories than an adult who works at a desk.
The body needs more calories in cold weather to stay at an even temperature.
If a person takes in more food than required to meet the body’s needs, the excess calories are converted to fat a stored form of energy.
That causes weight gain.
Eating too little causes weight loss because the fat is used for energy. One pound (0.5 kilogram) of stored fat contains about 3,500 calories.
If weight loss is advisable, the best way to lose is by eating less of high-calorie foods and getting more exercise.
For most people a safe limit for losing weight is 2 pounds a week.
Strict dieting to lose weight should be attempted only under a physician’s care. So-called quick and easy weight-loss diets are fads.
Most are unbalanced, emphasizing one type of food and excluding others. The more unbalanced the diet, the more dangerous it is.
Knowledge of the body’s nutritional needs helps to guard against promoters of fad diets and special “health” foods that have no advantages and that may even be harmful.
To function, the human body must have nutrients.
Proteins are made of amino acids, small units necessary for growth and tissue repair.
Protein is the body’s most plentiful substance except for water and, possibly, fat.
Animal foods such as meat, fish, poultry, milk, and eggs are rich in protein. Good plant sources of protein are beans, peas, nuts, bread, and cereals.
Combining plant sources, such as peanut butter with whole-grain bread or rice with beans, provides excellent protein.
So does combining plant and animal sources such as cereal and milk or macaroni and cheese.
Starches and sugars are carbohydrates, the main source of the body’s energy.
Carbohydrates account for about half of the calorie intake for most Americans and up to four fifths of the calories in diets of African and Asian peoples.
Carbohydrate-rich foods are also the main sources of protein for most of the world. Rice, wheat, corn, and potatoes are common rich sources of carbohydrates.
Sugars are not essential foods. They provide energy (calories) but no nutrients. For that reason sugar is called an “empty calorie” food.
Occasional sweets are not harmful to a healthy, active person. But excessive sugar can lead to tooth decay when eaten between meals, especially in sticky snack foods that cling to the teeth.
Fats and Oils
Fats and oils (which are liquid fats) are a concentrated source of energy, Fats in the diet are necessary for good health.
They make certain vitamins available for use in the body, they cushion vital organs, they make up part of all body cells, and they help to maintain body temperature.
Fats also delay pangs of hunger because a food mixture containing fat remains longer in the stomach.
Nutritionists distinguish between different types of dietary fats, or fats in food.
Saturated fats usually are solid in form and of animal origin. In many typical diets, meat fat is the main source.
It is known that saturated fats can raise the level of cholesterol in the blood. Cholesterol is a natural waxy substance made by the body.
It helps to form digestive juices and does other important work. It is present in the body no matter what is eaten.
When the body cells cannot absorb any more cholesterol. Any excess begins to accumulate in the walls of the blood vessels and gradually narrows them.
This condition may lead to a heart attack or stroke.
Minerals are neither animal nor vegetable; they are inorganic. Almost all foods contribute to a varied intake of essential minerals.
Most minerals are easy to obtain in quantities required by the body.
A major exception is iron for children under age 4 and adolescent girls and women in the childbearing years. These groups need more iron than a normal diet may provide.
Iron helps to build red blood cells. It also helps the blood carry oxygen from the lungs to each body cell.
Rich sources of iron are meat, especially liver; egg yolks; and dark green vegetables.
Everyone at every age needs calcium.
This mineral builds bones and teeth, and it is necessary for blood clotting. The best sources are milk and hard cheese.
Others are leafy greens, nuts, and small fishes such as sardines with bones that can be eaten.
Phosphorus works with calcium to make strong bones and teeth. A diet that furnishes enough protein and calcium also provides enough phosphorus.
Other important minerals are sodium, potassium, iodine, magnesium, zinc, and copper.
The discovery of vitamins began early in the 20th century. It is likely that some still are undiscovered.
Eating a wide variety of foods ensures getting enough vitamins whether or not they are identified.
All living things need vitamins for growth and health.
The body either cannot manufacture them at all or cannot normally manufacture them in sufficient amounts, and so must absorb them from food.
Each vitamin has specific roles to play.
Many reactions in the body require several vitamins, and the lack or excess of any one can interfere with the function of another.
Four vitamins A, D, E, and K are known as the fat-soluble vitamins. They are digested and absorbed with the help of fats that are in the diet.
Vitamin A is needed for strong bones, good vision, and healthy skin. It is found both in dark green and yellow fruits and vegetables.
Vitamin D is essential for children because it helps calcium and phosphorus to form straight, strong bones and teeth.
With direct sunlight on the skin, the body can manufacture its own vitamin D. Infants and young children often need a vitamin D supplement.
Vitamin D is added routinely to most milk during processing.
Vitamin E helps to protect vitamin A and red blood cells. It is found in a wide variety of foods, and almost everyone gets enough.
Vitamin K is one vitamin that is made within the human body by bacteria that live in the intestinal tract.
Fat-soluble vitamins can be stored in the body for long periods. They are stored mostly in the fatty tissue and in the liver.
The vitamin B group of several vitamins helps to maintain healthy skin and a well-functioning nervous system. B vitamins also help to convert carbohydrates into energy.
Vitamin C, or ascorbic acid, is needed for building the connective tissue that holds body cells together.
Vitamin C is essential for healthy teeth, gums, and blood vessels. It also helps the body to absorb iron.
These water-soluble vitamins are not stored in the body for long. Good sources should be eaten every day.
In order to live, every cell in the body must be bathed in water.
Water takes an active part in many chemical reactions and is needed to carry other nutrients, to regulate body temperature, and to help eliminate wastes.
Water makes up about 60 percent of an adult’s body weight. Requirements for water are met in many ways.
Most fruits are more than 90 percent water.
FOOD GUIDE PYRAMID
In the early 1990s the long-standing, traditional basic four food groups, consisting of meat, dairy products, grains, and fruits and vegetables, were reworked into the more balanced and healthy food guide pyramid.
This pyramid has as its base the grain group; on the second level are the fruit and vegetable groups; on the third level are the meat and dairy groups; and at the top is the fats, oils, and sweets group.
At the base of the food guide pyramid are breads, cereals, rice, pasta, and other foods made from grain.
Human beings need more daily servings of these foods than any others because grain-based foods provide B vitamins, iron, carbohydrates, and some protein.
Six or more daily servings are recommended.
A serving, for example, is one slice of bread, one ounce (28 grams) of dry cereal, or one half cup of cooked pasta.
Fruit Group and Vegetable Group
The sources of most vitamins and minerals belong to these two groups.
They also provide fiber, which contains no nutrients but aids in moving food through the digestive system.
Three to five servings of vegetables and two to four servings of fruit should be eaten every day. One half cup of any fruit or vegetable counts as one serving.
Dairy Group and Meat Group
These groups are quite high in protein, calcium, iron, phosphorus, the B vitamins, and zinc. Two to three servings from each group are recommended daily.
Eight ounces (227 grams) of milk or 3 ounces (85 grams) of meat, for example, count as a serving.
Fats, Oils, and Sweets Group
The top of the pyramid includes foods that may add pleasure to eating but provide only calories and little nutritional benefit to one’s diet.
These foods include salad dressings, cream, butter, margarine, sugars, soft drinks, and candies. They should be eaten only sparingly.
The American Heart Association recommends that no more than 30 percent of one’s daily calories be derived from fat.
The word diet comes from the Greek diaita, meaning “manner of living.” Dietetics applies nutritional knowledge to the feeding of humans.
Foods from the food guide pyramid may be part of any meal.
A grilled cheese sandwich or a bowl of whole-grain cereal is just as nutritious in the morning as it is at noon.
A good breakfast consists of any foods that supply about one fourth of the necessary nutrients for the day.
The United States government publishes guidelines for appropriate nutrient intakes. These are known as the Recommended Dietary Allowances.
They are updated regularly on the basis of new evidence from the science of nutrition. Just as wise shoppers get the most for their money, so wise eaters select foods that give the most nutrition for the lowest number of calories.
For example, snacks can furnish about one fourth of the calorie requirements among teenagers.
Those snacks should also furnish that much of the day’s allowances for protein, minerals, and vitamins. Sandwiches, fruit, and milk make good snacks for active teenagers.
Fast Foods and Convenience Foods
Fast foods and convenience foods are major parts of many people’s diets. Convenience foods, such as TV dinners and cake mixes.
Are those prepared at home from foods already cooked or otherwise processed before reaching the retail store.
Fast foods are prepared in quick-order restaurants (see Fast Food).
A meal consisting of a cheeseburger with lettuce, tomato, and onion, french fries, and a milk shake, though high in fat and sodium, does include foods from each of the groups in the food guide pyramid.
So does a taco with its cereal shell, ground beef, shredded cheese, lettuce, olives, and tomato.
Frequent fast-food meals, though, require some thought about nutrients and calories.
High in protein, iron, and B vitamins, fast-food meals are sometimes low in calcium and vitamins A and C and often extremely high in sodium.
A milk shake provides calcium, but it can also be high in fat and contain more than 800 calories.
Depending on the items ordered, the calorie content of a typical fast-food meal ranges from 900 to 1,800.
A burger with all the trimmings has about 600 calories, but a plain cheeseburger has only about half as many, and a plain hamburger even less.
A serving of pizza can have from 300 to 600 or more calories.
Special diets for people with health problems should be prescribed by a physician. Many doctors refer patients to a dietitian, who draws up an individual diet plan.
The typical diet in the United States contains much more salt than the body needs. Persons with high blood pressure must avoid as much salt as possible.
Diabetes mellitus is another disease that requires careful meal planning. In diabetes mellitus the body does not make normal use of glucose, or blood sugar.
In addition to a special diet, diabetic children and some adults need medication.
If diabetes develops later in life, it often can be controlled with diet alone.
Diets high in fat and cholesterol have been linked with blockage of the arteries, a common factor in heart disease and stroke.
Studies show that this condition can begin in adolescence or earlier.
Some people are vegetarians; that is, they eat no meat and sometimes no eggs, cheese, or milk. Those who avoid all animal products are called vegans.
Such people must plan their diets carefully to get balanced nutrition.
Malnutrition is the imbalance between the body’s demand for nutrients and the available supply of nutrients.
This can result from an unsatisfactory diet or from a disorder that interferes with the body’s use of food.
Obesity, or the state of being excessively fat, is a form of malnutrition that contributes to many health problems.
It may be defined as body weight more than 20 percent above a person’s ideal weight.
Anorexia nervosa is a condition characterized by extreme weight loss. Nervosa means “of the nerves,” or neurotic.
This life-threatening disease usually occurs in young women. It requires professional treatment.
Protein-calorie malnutrition, known as kwashiorkor, is common among children in unindustrialized nations.
A lack of protein results in failure of the body to grow and often damages digestive organs. A severe calorie deficiency results from starvation.
Mineral and vitamin deficiencies are responsible for various disorders. Insufficient iron can cause iron-deficiency anemia.
Lack of iodine can cause goiter, an enlargement of the thyroid gland.
Vitamin A deficiency causes loss of vision in dim light.
Vitamin D deficiency leads to a faulty deposit of calcium in bones and teeth, resulting in rickets. A child with rickets may have bowed legs and a prominent sternum (breastbone).
Vitamin C deficiency is known as scurvy. It causes infected and bleeding gums and painful joints.
Vitamin B1 deficiency, or beriberi, damages the nerves, heart, and circulation. Whole-grain flour and enriched flour that is processed in the United States contain vitamin B1.
Vitamin B12 deficiency causes blood disorders and affects the nervous system.
It rarely results from an inappropriate diet but rather from a defect of absorption in the digestive tract.