Nutrition is the science that deals with food and how the body uses it. People, like all living things, need food to live. Food supplies the energy for every action we perform, from reading a book to running a race. Food also provides substances that the body needs to build and repair its tissues and to regulate its organs and systems.
What we eat directly affects our health. A proper diet helps prevent certain illnesses and aids in recovery from others. An improper or inadequate diet increases the risk of various diseases. Eating a balanced diet is the best way to ensure that the body receives all the food substances it needs. Nutrition experts recommend that the daily diet include a certain number of servings from each of the five major food groups: (1) breads, cereals, rice, and pasta, (2) vegetables, (3) fruits, (4) milk, yogurt, and cheese, and (5) meat, poultry, fish, dried beans and peas, eggs, and nuts. Fats are also essential, but, like sweets, should be eaten in small quantities.
Workers in the field of nutrition oversee school food services, plan menus for hospitalized patients, and provide nutrition counseling for individuals. They administer international food programs and investigate the relationship between diet and health. They seek improved ways of processing, packaging, and distributing foods, and they create new foods.
How the body uses food
Food provides certain chemical substances needed for good health. The substances, called nutrients, perform one or more of three functions. (1) They provide materials for building, repairing, or maintaining body tissues. (2) They help regulate body processes. (3) They serve as fuel to provide energy. The body needs energy to maintain all its functions.
The body breaks food down into its nutrients through the process of digestion. Digestion begins in the mouth. As food is being chewed, saliva moistens the particles. The saliva begins to break down such starchy foods as bread and cereals. After the food is swallowed, it passes through the esophagus, a tube that leads into the stomach. In the stomach, digestive juices speed up the breakdown of such foods as meat, eggs, and milk.
The partly digested food, called chyme << kym >> , passes from the stomach into the small intestine. In the small intestine, other juices complete the process of digestion. They break down the food into molecules that pass through the walls of the intestine and into the blood.
Blood distributes nutrients throughout the body. In the body’s cells and tissues, the nutrients are broken down to produce energy, or are used to rebuild tissues or to regulate chemical processes. Some of the nutrients are stored in the body, and others are used over and over again. But most of the nutrients undergo chemical changes as they are used. These chemical changes produce waste products, which go into the bloodstream.
Some of the wastes are carried to the kidneys, which filter them from the blood. The body expels these wastes in the urine. The liver also filters out some wastes and concentrates them into a liquid called bile. In addition to concentrating certain wastes, bile helps digest fats.
Bile is stored in the gallbladder until it is needed to aid in digestion. Then the gallbladder empties bile into the small intestine. Bile then passes into the large intestine, along with parts of the food not digested in the small intestine. The large intestine absorbs water and small amounts of minerals from this waste material. This material, along with bacteria present in the large intestine, becomes the final waste product, the feces, and it is eliminated from the body.
Kinds of nutrients
The foods we eat contain thousands of different chemicals. However, only a few dozen of these chemicals are absolutely essential to keep us healthy. These few dozen are the nutrients—the substances we must obtain from the foods we consume.
Nutritionists classify nutrients into six main groups: (1) water, (2) carbohydrates, (3) fats, (4) proteins, (5) minerals, and (6) vitamins. The first four groups are called macronutrients, from a Greek prefix meaning large, because the body needs them in large amounts. The last two groups, which are required in small quantities, are known as micronutrients, from a Greek prefix meaning very small.
Water is needed in great amounts because the body consists largely of water. Usually, between 50 and 75 percent of a person’s body weight is made up of water.
The body requires large quantities of carbohydrates, fats, and proteins because these nutrients provide energy. The energy in food is measured in units called kilocalories. A kilocalorie is equal to 1,000 calories. A calorie is the amount of energy required to raise the temperature of one gram of water one Celsius degree. However, kilocalories are often referred to as simply “calories.” The “calories” mentioned in this article are actually kilocalories.
Although minerals and vitamins are needed in only small amounts, they are as vital to health as any other nutrients. Minerals and vitamins are needed for growth and to maintain tissues and regulate body functions.
Water
is, perhaps, the most critical nutrient. We can live without other nutrients for several weeks, but we can go without water for only about one week. The body needs water to carry out all of its life processes. Watery solutions help dissolve other nutrients and carry them to all the tissues. The chemical reactions that turn food into energy or tissue-building materials can take place only in a watery solution. The body also needs water to carry away waste products and to cool itself. Adults should consume about 2 1/2 quarts (2.4 liters) of water a day in the form of beverages or water in food.
Carbohydrates
include all sugars and starches. They serve as the main source of energy for living things. Each gram (0.035 ounce) of carbohydrate provides about 4 calories.
There are two kinds of carbohydrates—simple and complex. Simple carbohydrates, all of which are sugars, have a simple molecular structure. Complex carbohydrates, which include starches, have a larger and more complicated molecular structure that consists of many simple carbohydrates linked together.
Most foods contain carbohydrates. The main sugar in food is sucrose, ordinary white or brown sugar. Another important sugar, lactose, is found in milk. Fructose, an extremely sweet sugar, comes from most fruits and many vegetables. Foods containing starches include beans, breads, cereals, corn, pasta (macaroni, spaghetti, and similar foods made of flour), peas, and potatoes.
Fats
are a highly concentrated source of energy. Each gram of fat provides about 9 calories.
All fats are composed of an alcohol called glycerol and substances called fatty acids. A fatty acid consists of a long chain of carbon atoms, to which hydrogen atoms are attached. There are three types of fatty acids: saturated, monounsaturated, and polyunsaturated. A saturated fatty acid contains as many hydrogen atoms as its carbon chain can hold. A monounsaturated fatty acid is lacking a pair of hydrogen atoms. In a polyunsaturated fatty acid, the carbon chain contains at least four fewer hydrogen atoms than it could hold. A process called hydrogenation can attach additional hydrogen atoms to the carbon chains of unsaturated fatty acids. Hydrogenation may produce trans fats, in which newly added hydrogen atoms are connected in a slightly different position than the one in which they attach naturally.
Most saturated fatty acids occur in foods derived from animals, such as butter, lard, dairy products, and fatty red meats. But some come from vegetable sources, such as coconut oil or palm oil.
Polyunsaturated fatty acids are found in the oils of such plants as corn and soybeans and in such fish as salmon and mackerel. Certain polyunsaturated fatty acids called essential fatty acids must be included in the diet because the body cannot manufacture them. These essential fatty acids serve as building blocks for the membranes that surround every cell in the body. Common sources of monounsaturated fatty acids include olives and peanuts.
Proteins
provide energy—like carbohydrates, 4 calories per gram—but more importantly, proteins serve as one of the main building materials of the body. Muscle, skin, cartilage, and hair, for example, are made up largely of proteins. In addition, every cell contains proteins called enzymes, which speed up chemical reactions. Cells could not function without these enzymes. Proteins also serve as hormones (chemical messengers) and as antibodies (disease-fighting chemicals).
Proteins are large, complex molecules made up of smaller units called amino acids. The body must have a sufficient supply of 20 amino acids. It can manufacture enough of 11 of them. Nine others, called essential amino acids, either cannot be made by the body or cannot be manufactured in sufficient amounts. They must come from the diet.
The best sources of proteins are cheese, eggs, fish, lean meat, and milk. The proteins in these foods are called complete proteins because they contain adequate amounts of all the essential amino acids. Cereal grains, legumes (peanuts and other plants of the pea family), nuts, and vegetables also supply proteins. These proteins are called incomplete proteins because they lack adequate amounts of one or more of the essential amino acids. However, a combination of two incomplete proteins can provide a complete amino acid mixture. For example, beans and rice are both incomplete proteins, but eaten together they provide the correct balance of amino acids.
Minerals
are needed for the growth and maintenance of body structures. They are also needed to maintain the composition of the digestive juices and the fluids that are found in and around the cells. People need only small amounts of minerals each day.
Unlike vitamins, carbohydrates, fats, and proteins, minerals are inorganic compounds—that is, they are not created by living things. Plants obtain minerals from the water or soil, and animals get minerals by eating plants or plant-eating animals. Unlike other nutrients, minerals are not broken down by the body.
Required minerals include calcium, chlorine, magnesium, phosphorus, potassium, sodium, and sulfur. Calcium, magnesium, and phosphorus are essential parts of the bones and teeth. Calcium is also necessary for blood clotting. Milk and milk products are the richest sources of calcium. Cereals and meats provide phosphorus. Whole-grain cereals, nuts, legumes, and green leafy vegetables are good sources of magnesium.
Still other minerals are needed only in extremely tiny amounts. These minerals, called trace elements, include chromium, copper, fluorine, iodine, iron, manganese, molybdenum, selenium, and zinc. Iron is an important part of hemoglobin, the oxygen-carrying molecule in red blood cells. Copper helps the body make use of iron to build hemoglobin. Manganese and zinc are required for the normal action of various enzymes. Green leafy vegetables, whole-grain breads and cereals, seafood, and liver are good sources of trace elements.
Vitamins
are essential for good health because they regulate chemical reactions in which the body converts food into energy and tissues. There are 13 vitamins: vitamin A; the vitamin B complex, which is a group of 8 vitamins; and vitamins C, D, E, and K. Scientists divide vitamins into two general groups, fat-soluble vitamins and water-soluble vitamins. The fat-soluble vitamins—vitamins A, D, E, and K—dissolve in fats. The vitamins of the B complex and vitamin C dissolve in water. Small amounts of these compounds are needed daily.
Vitamin A
is necessary for healthy skin and development of strong bones. Sources of this vitamin include liver, green and yellow vegetables, and milk.
Vitamin B
also called thiamine, is necessary for changing starches and sugars into energy. It is found in meat and whole-grain cereals.
Vitamin B
also known as riboflavin, is essential for complicated chemical reactions that take place during the body’s use of food. Milk, cheese, fish, liver, and green vegetables supply vitamin B2.
Vitamin B
also called pyridoxine, and two other B vitamins known as pantothenic acid and biotin all play a role in chemical reactions essential for growth. Liver, yeast, and many other foods contain these vitamins.
Vitamin B
also called folate, are both needed for forming red blood cells and for a healthy nervous system. Vitamin B12 is found in animal products, especially liver. Folic acid is present in green leafy vegetables. Doctors recommend that all women who are capable of becoming pregnant consume small amounts of folic acid each day to reduce the risk of spina bifida, a serious birth defect.
Niacin
is also part of the B complex. Cells need niacin in order to release energy from carbohydrates. Liver, yeast, lean meat, fish, nuts, and legumes contain niacin.
Vitamin C,
also called ascorbic acid, is needed for the maintenance of the ligaments, tendons, and other supportive tissue. It is found in fruits and in potatoes.
Vitamin D
is necessary for the body’s use of calcium. It is present in fish-liver oil and vitamin D-fortified milk. Vitamin D also forms when the skin is exposed to sunlight.
Vitamin E,
also known as tocopherol, helps maintain cell membranes. Vegetable oils and whole-grain cereals are especially rich in this vitamin. It is also found in small amounts in most meats, fruits, and vegetables.
Vitamin K
is necessary for proper clotting of the blood. Green leafy vegetables contain vitamin K. It is also manufactured by bacteria in the intestine.
Nutrition guidelines
Eat a balanced diet.
Nutritionists have devised various systems to aid in planning a balanced diet. The Food Guide Pyramid is one such system, developed by the United States Department of Agriculture (USDA). The USDA introduced the pyramid in 1992 and revised it in 2005. The pyramid’s shape and colors called attention to the recommended amounts of each of the major food groups that make up a healthy diet. The department replaced the pyramid in 2011 with the MyPlate symbol. This symbol is combined with guidelines issued in 2015 to help encourage people to eat healthy and make better food choices. The guidelines recommend that people consume a variety of vegetables; fruits; grains (with at least half being whole grains); fat-free or low-fat dairy products; a variety of proteins, including seafood, lean meats and poultry; and healthy oils, including those from plants and nuts. The guidelines emphasize that a healthy diet should be low in saturated fats, trans fats, and salt. Less than 10 percent of daily calories should come from added sugars. They also recommend that children ages 6 to 17 years get at least 60 minutes of physical activity per day, including aerobic, muscle-strengthening, and bone-strengthening activities. Adults should get at least 150 minutes of moderate intensity physical activity each week and should perform muscle-strengthening exercises on two or more days each week.
In the United States, the Food and Nutrition Board of the National Academy of Sciences issues Recommended Dietary Allowances (RDA’s). RDA’s are estimated amounts of various nutrients needed daily to avoid deficiency and maintain good nutrition in healthy people. The board issues a standard called adequate intake (AI) in cases where the board’s experts feel there is insufficient evidence to establish an RDA. RDA’s and AI’s for particular nutrients may vary, depending on a person’s sex or age. This article includes a table of certain RDA’s.
RDA’s and AI’s are part of a larger category of standards called dietary reference intakes (DRI’s). DRI’s also include guidelines called tolerable upper intake levels (UL’s). UL’s define the maximum daily intake that is unlikely to harm health. DRI’s are being developed in a joint effort by the U.S. Food and Nutrition Board and Health Canada. The World Health Organization is creating a set of similar standards.
The Nutrition Labeling and Education Act of 1990 requires that all packaged and processed food sold in the United States carry labels with nutritional information. The labels help people compare their daily dietary needs with the nutritional content of the foods they eat. Nutrition labels list the amounts of fats, carbohydrates, cholesterol, and certain other nutrients in one serving of the labeled food. The nutrients listed are those considered most important to health, as determined by the U.S. Food and Drug Administration (FDA). The label also provides a Daily Value percentage for each nutrient to show how it fits into a healthful diet. The percentages are based on guidelines called Daily Reference Values (DRV’s), which the FDA established as the amounts of these nutrients that a healthy person should consume each day.
In addition, health claims, such as “reduces risk of heart disease,” and descriptive terms, such as “fresh” and “low fat,” can appear on food packages only in accordance with FDA regulations. A list of some of these descriptive terms of a standard food label accompany this article.
People vary in their needs for energy. A person who plays sports daily, for example, needs more calories than someone who does little physical work. Children need more calories than their size would indicate because they are growing. Pregnant women need extra calories to provide enough nutrients for a healthy baby.
Include fiber.
Dietary fiber consists of cellulose and other complex carbohydrates that cannot be absorbed by the body. Fiber passes out of the body as waste. Fiber moves food along through the stomach and intestines, thus helping to prevent constipation (difficulty in emptying the bowels). Many experts believe that it also helps reduce the risk of such rectal and intestinal disorders as hemorrhoids, diverticulitis, and, possibly, cancers of the colon and rectum. Good sources of fiber include whole-grain breads and cereals, beans and peas, vegetables, and fruit.
Limit intake of saturated fats, trans fats, and cholesterol.
Health experts recommend a diet that is low in saturated fats, trans fats, and cholesterol, a waxy substance found in many animal foods. Consumption of these substances raises the level of cholesterol in a person’s blood, which increases the risk of heart disease. Animal products are the source of all dietary cholesterol and most saturated fats except coconut oil and other tropical oils. To reduce intake of saturated fats and cholesterol, experts suggest choosing lean meats, fish, poultry with the skin removed, and low-fat dairy products. They also advise using fats and oils sparingly. Trans fats occur chiefly in solid margarines and certain processed foods. Nutrition labels often list the amount of trans fats. The words partially hydrogenated in a product’s ingredients often indicate that trans fats are present.
Limit intake of sodium and sugar.
A diet that includes a great deal of sodium may increase the risk of high blood pressure. Sodium is found in many foods, including canned vegetables, frozen dinners, pickles, processed cheese, table salt, and such snack foods as pretzels, chips, and nuts. One way to reduce sodium intake is to use herbs and other seasonings instead of salt in cooking and at the table. Another way is to select fresh foods rather than canned or frozen items.
Foods that contain much sugar are often high in calories and fat but low in minerals, proteins, and vitamins. Nutritionists sometimes call these foods “empty calorie” foods, because they may make a person feel full but provide few nutrients. In addition, sugar that remains in and around the teeth contributes to tooth decay. Foods that have a large amount of sugar include candies, pastries, many breakfast cereals, and sweetened canned fruits. In place of sugary foods, nutritionists advise people to snack on fresh fruits and vegetables.
Beware of alcohol.
Alcoholic beverages supply calories, but they provide almost no nutrients. In addition, alcohol is a powerful drug, and habitual drinking can lead to many health problems. Health experts recommend that if people choose to drink alcoholic beverages, they consume only small amounts. They suggest that certain people avoid alcohol altogether, including children and adolescents; pregnant women; people who are about to drive; anyone who is taking medicine; and individuals who are unable to limit their drinking.
Do not overeat.
When people consume more calories than they need, their bodies store most of the extra calories as fat. Accumulation of fat can result in obesity, a condition in which a person has too much body fat for good health. Obesity increases the risk of such diseases as adult-type diabetes, stroke, high blood pressure, gallbladder disease, heart disease, and certain cancers. Conditions such as osteoarthritis and lower back pain may be worsened by the pressure of excess weight.
A number of techniques can help people avoid obesity. Most health experts recommend that people exercise regularly to help control their weight. Another strategy is to avoid using food as a reward or as a way to overcome loneliness or boredom. Experts also advise against snacking on foods that are high in fat or sugar. Instead, they recommend such choices as fruits or vegetables, unsalted crackers, nonfat yogurt, and unsweetened fruit juice, fat-free milk, or sparkling water.
Store and cook foods properly
to retain their nutritional value. For example, many fresh foods should be kept in the refrigerator. They should be washed thoroughly and eaten as soon as possible. Frozen foods must be kept frozen in a freezer.
Vegetables should be cooked quickly in as little water as possible to minimize vitamin loss in the water. Avoid frying meats and other animal foods. Instead, cook them by broiling, stir-frying, braising, or poaching. Microwave cooking reheats food quickly and keeps nutrient values high.
Evaluate claims about foods and dietary supplements carefully.
People should use caution, common sense, and critical thinking to evaluate popular ideas or advertising claims about foods and dietary supplements. Some ideas about foods and supplements become popular even though they are wrong. For example, some people believe that they need not eat a balanced diet if they take a vitamin pill every day. But, in fact, people who rely on vitamin pills may not get all the calories, minerals, or proteins that they need.
Although labeling and advertising claims are regulated by federal agencies, people may develop mistaken ideas from claims that meet legal requirements. For example, some people think they can eat unlimited quantities of foods labeled “low fat.” In fact, careful comparison of labels shows that many low-fat foods contain almost as many calories as do versions not labeled low-fat.
People should carefully consider the source of nutritional claims. Reliable sources of information include state, provincial, and national health and nutrition agencies; public departments of agriculture; and universities with expert knowledge in health and nutrition.
Nutrition and disease
An improper or inadequate diet can lead to a number of diseases. On the other hand, good nutritional habits can help prevent certain diseases.
Heart disease
in its most common form is called coronary artery disease (CAD). CAD narrows the coronary arteries and so reduces the blood supply to the heart. It can lead to attacks of intense chest pain and, eventually, to life-threatening heart attacks. High blood pressure and high levels of blood cholesterol are two of the major risk factors for CAD. In many cases, people can lower these risk factors with good nutritional practices.
Many people can reduce mild high blood pressure by limiting their intake of salt and calories. Similarly, many people can lower their blood cholesterol level by reducing the amount of fat—particularly saturated fat—cholesterol, and calories in their diet. They can achieve these reductions by avoiding such foods as butter, cakes, cookies, egg yolks, fatty meats, tropical oils, and whole-fat dairy products.
Cancer.
Scientists have found that heredity, environment, and lifestyle all play a role in causing cancer. They have also learned that good nutrition can help prevent certain kinds of cancer in laboratory animals. Large doses of vitamins A and C have been proved to prevent some cancers in animals. Many scientists believe that certain foods contain substances that may help prevent some cancers in people. Such foods include broccoli, cabbage, carrots, cauliflower, fruits, spinach, whole-grain breads and cereals, cooked tomatoes, and some seafoods. Lessening intake of fats and increasing the intake of fiber may also help prevent some cancers.
Deficiency diseases
result from an insufficient amount of certain nutrients in the diet. Such diseases can usually be eliminated when adequate amounts of the nutrient are provided. Deficiency diseases are most widespread in less developed countries, where people may lack adequate food. These diseases are less common in most developed countries, where a variety of foods are available all year, and many foods are fortified with vitamins and minerals.
Protein-calorie malnutrition,
also called protein-energy malnutrition, occurs when the diet is low in both proteins and calories. If the diet is especially low in proteins, the condition is called kwashiorkor. Signs of kwashiorkor include changes in the color and texture of the hair and skin; swelling of the body; and damage to the intestines, liver, and pancreas. The disease, which is common in some less developed nations, usually attacks children who are suffering from an infectious disease. Kwashiorkor is fatal unless the patient is given protein along with food providing calories. If the diet is especially low in calories, the condition is called marasmus. Marasmus usually attacks infants and young children, and it causes extreme underweight and weakness.
Vitamin deficiencies.
The signs and symptoms of vitamin deficiencies vary according to the missing vitamin. Vitamin C deficiency, also called scurvy, causes sore and bleeding gums, slow repair of wounds, and painful joints. Vitamin D deficiency, also called rickets, causes an abnormal development of the bones. A deficiency of niacin and the amino acid tryptophan, found in protein, causes pellagra. The early symptoms of pellagra include weakness, lack of appetite, diarrhea, and indigestion.
Mineral deficiencies.
The most common mineral deficiency disease is iron-deficiency anemia. In this disease, a lack of iron prevents formation of enough oxygen-carrying red blood cells. As a result, the blood cannot supply the tissues with sufficient oxygen. Thus, the person feels weak or tired. Other symptoms include dizziness, headaches, rapid heartbeat, and shortness of breath. A lack of iodine can cause goiter, a disease in which the thyroid gland becomes enlarged.
Other diseases
may result from poor nutritional habits. For example, excessive intake of alcohol causes some forms of liver disease. Obesity increases the risk of gallbladder disease and of diabetes in adults. The risk of osteoporosis (loss of bone tissue) is higher for people—especially women—whose intake of calcium and level of physical activity are low. To prevent osteoporosis, physicians recommend a lifelong combination of regular exercise and a diet with adequate calcium.
Careers in nutrition
Nutrition offers a variety of careers in work settings that range from restaurants to research laboratories. Jobs in this field require at least a high school education. Some positions require a two-year associate’s degree or a bachelor’s degree. High school students interested in a career in the field of nutrition should take such science courses as biology and chemistry. College students preparing for a career in nutrition study physiology—the science that deals with how the body works—as well as biochemistry—the science of the chemical reactions that take place in living things. Other required courses include bacteriology and general chemistry. Students interested in careers in medical nutrition learn how diseases affect the body. Students pursuing careers in food service take such courses as food preparation, food purchasing, accounting, and personnel management.
Work as a dietetic technician requires an associate’s degree. Dietetic technicians work as members of a food service staff or a health care team, under the supervision of or in consultation with a registered dietitian. These technicians supervise support staff, develop nutrition care plans, and provide nutrition counseling for individuals or small groups.
A bachelor’s degree and a period of supervised practice are necessary to become a registered dietitian. Candidates must also pass a registration examination. Many registered dietitians manage food services in schools, nursing homes, and restaurants. Others work in hospitals or clinics, planning special diets for people who are ill. Some registered dietitians coordinate disease prevention programs for public health organizations or work with community food programs. Others work for corporations, helping to develop food products. A number of registered dietitians offer personal nutrition counseling directly to the public.
People with advanced degrees in nutrition and food science may teach; conduct research projects for government, business, industry, and health care institutions; or hold administrative positions in international food or agricultural programs. These experts may investigate the effect of diet on health or develop improved methods for processing and packaging food. Some create new foods, such as artificial sweeteners or fat substitutes.
