NUTRIENTS AND NUTRITION

CONCEPT

In the modern world people are accustomed to hearing a great deal about nutrients and nutrition. Words such as protein, carbohydrate, vitamins, minerals, and fats are a regular part of daily life, yet few people who talk about these nutrients really know what they are. In fact, these are the basic building blocks of nutrition, whereby animal life is sustained. Whereas plants can get their energy directly from the Sun and the atmosphere, animals (including humans) depend on other organisms to provide them with nutrition. These other organisms include plants, which generate carbohydrates as a result of photosynthesis, as well as other animals that eat plants and thereby build proteins and fats. Plants also may contain proteins and fats, and both plants and animals contain vitamins and minerals. These nutrients, consumed in the proper forms and proportions, sustain life and prevent the miseries of malnutrition—a condition that can involve either under nourishment or over nourishment.

HOW IT WORKS

Nutrients and Nutrition

In order to live, animals must consume nutrients, of which there are five major classes: carbohydrates, proteins, lipids or fats, vitamins, and minerals. In addition to these constituents, of course, animal life requires other materials for its sustenance-water, oxygen, and fiber, which aids in the digestive processing of foods-but these components usually are not regarded as nutrients.

Nutrition itself is the series of processes by which an organism takes in nutrients and makes use of them for its survival, growth, and development. The term nutrition also can refer to the study of nutrients, their consumption, and their processing in the bodies of organisms. Here the general term organism has been used, but for the most part the present essay is concerned with animal nutrition, or at least the nutrition of primary consumers (animals that eat plants) and secondary consumers (animals that eat other animals).

AUTOTROPHS AND THEIR NUTRIENTS.

By contrast, plants and a few other types of organism are autotrophs, or primary producers in the food web. Autotroph means "self-feeder," and these organisms are distinguished by the fact that they do not depend on other organisms as a source of energy. Instead, plants obtain energy from the Sun and carbon dioxide from the atmosphere, and from these materials they build the large organic molecules that they need to survive.

Though plants are the most obvious example of an autotroph, they are not the only ones. In the deep oceans, far from any plant life, primary consumers depend on phytoplankton, which are microscopic organisms that encompass a range of bacteria and algae. Nonplant autotrophs may use means different from those employed by plants in generating their own food. For example, there are certain nonplant autotrophic organisms that live in the deep oceans near hydrothermal vents, which are cracks in the ocean floor caused by volcanic activity. These organisms, unlike most autotrophs, do not need sunlight to survive. Instead, they build their own nutrients in a sunless world, using sulfur compounds found near the vents.

Chemical Elements and Nutrition

An element is a chemical substance made of only one kind of atom, whereas in a compound, atoms of more than one element are chemically bonded to one another. Unlike compounds, elements cannot be broken chemically into other substances. There are approximately 90 elements that occur in nature, and many of these elements—but not nearly all—are important to nutrition.

ELEMENTS IN THE HUMAN BODY AND BIOGEOCHEMICAL CYCLES.

Even when we rule out obviously harmful elements, such as lead or uranium, there are still numerous chemical elements that play a part in the nutrition of living things. This can be illustrated by a glance at the abundance of various chemical elements in the human body, which include oxygen, carbon, and hydrogen. Oxygen alone accounts for a whopping 65% of the human body's mass, and carbon (18%), hydrogen (10%), and oxygen together make up 93% of the mass in the human body.

A great deal of oxygen and hydrogen, of course, is found in that most useful of all chemical compounds, water. In this vein, it should be noted that all the elements that take part in biogeochemical cycles, which are essential to the functioning of Earth, appear in relatively large proportions within the human body. These elements are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur. (For more about biogeo-chemical cycles and the elements involved in them, including their proportion within the human body's mass, see The Biosphere.)

Carbon is present in all living things, and its presence in certain forms is key to distinguishing organic from inorganic substances. Contrary to popular belief, organic substances are not just living things, their parts, and their products. Something that has never been living still can be considered organic, provided that it contains compounds that include carbon. (The only exceptions would be carbonates and carbon oxides, two groups of carbon-based compounds that are excluded from the ranks of organic substances.) As we shall see, carbon, along with oxygen and hydrogen, plays a key role in nutrition.

Most of the remaining 7% of the body's mass is composed of ten other elements. Among these elements are the other three involved in biogeochemical cycles, whose names are italicized: nitrogen (3%), calcium (1.4%), phosphorus (1.0%), magnesium (0.50%), potassium (0.34%), sulfur (0.26%), sodium (0.14%), chlorine (0.14%), iron (0.004%), and zinc (0.003%). Note that many of these elements are found in vitamin and mineral supplements that people might take on a daily basis to augment the essential nutrients in their bodies. There are exceptions, however, such as sodium, of which most people already ingest too much in the form of salt.

TRACE ELEMENTS.

Generally speaking, it is safe to assume that any element that appears naturally in the human body is healthful as a nutritional component. This rule of thumb goes only so far, however: chlorine, for instance, is poisonous in large quantities, whereas in the very small proportions found in the human body, it can be essential to health and well-being. It is certainly possible to ingest some elements in unhealthy quantities, a fact that is particularly true of trace elements.

Copper is an example of a trace element, so named because only traces of them are present in the human body. In tiny quantities, copper is beneficial to human health, but if that small amount is exceeded, the effects can run the gamut from sneezing to diarrhea. In the proper proportions, however, trace elements are essential: without enough iodine, for instance, goiter, a large swelling of the thyroid gland in the neck area, can develop. Chromium helps the body metabolize sugars, which is why people concerned with losing weight or toning their bodies through exercise may take a chromium supplement. Even arsenic, which is lethal in large quantities, is a trace element in the human body, and medicines for treating such illnesses as "sleeping sickness" contain tiny amounts of arsenic. Other trace elements include cobalt, fluorine, manganese, molybdenum, nickel, selenium, silicon, and vanadium.

REAL-LIFE APPLICATIONS

Nutrients

If you glance at the side of a cereal box, or virtually any other food product manufactured in the United States, chances are that you will see a table listing proportions of nutrients per serving. This is the "Nutrition Facts" label, which replaced the old "Nutrition Information Per Serving" label in the early 1990s. Both forms of information label were administered by the United States Food and Drug Administration (FDA), with the newer labeling format being far more extensive in terms of the information it provides.

Usually these tables show the amount of nutrients both in terms of mass (usually rendered in metric components, such as grams or milligrams) and as a proportion of recommended daily value according to the FDA. These listings must include information about some components such as calories, fat, sodium, sugars, certain vitamins, and so on. In addition to these mandatory listings, labels may contain information that the manufacturer chooses to provide concerning other food components, such as potassium or insoluble fiber.

Today, even fast-food restaurants such as McDonald's, which is probably not the first name that comes to mind when one thinks of healthy eating, provide extensive nutritional information to customers. Additionally, makers of fast food or supermarket "junk food" such as potato chips have introduced offerings that represent a nod to nutritional concerns. These include products that are fat-or sodium-free, or are otherwise geared toward greater health consciousness.

Clearly, diet is a significant concern to Americans, the most well-fed group of people that has ever existed, and terms from the Nutrition Facts label—proteins, carbohydrates, fats, minerals, and vitamins—are household words, known to almost everyone but understood by only a few. In much of the remainder of this essay, we explore these concepts, discussing what they mean in very basic scientific terms, as well as in terms of their significance in the diets of humans and other animals.

PROTEINS.

Proteins are large molecules built from long chains of amino acids, which are organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations. Proteins serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body's immune system, and making enzymes. (An enzyme is a protein material that speeds up chemical reactions in the bodies of plants and animals.)

Proteins in the human body contain about 20 different amino acids, of which the body is able to manufacture 12 from the foods we eat. The other eight, which the body requires for protein production but is unable to manufacture on its own, are known as the essential amino acids. When a protein contains all of the essential amino acids, it is known as a complete protein. Among the best forms of complete protein are fish, red meat, and poultry as well as eggs, milk, cheese, and other dairy products. Fittingly, a protein that lacks at least one of the essential amino acids is known as an incomplete protein. Examples include peas, beans, lentils, nuts, and cereal grains. These can, however, be combined in such a way as to make a complete protein, beans and rice being a good example.

CARBOHYDRATES.

Carbohydrates are natural compounds that consist of carbon, hydrogen, and oxygen and whose primary function in the body is to supply energy. When a person ingests more carbohydrates than his or her body needs at the moment, the body converts the excess into a compound known as glycogen. It then stores the glycogen in the liver and muscle tissues, where it remains, a potential source of energy for the body to use in the future.

Sugars, starches, cellulose, and various other chemically related substances are part of the carbohydrate group. Most carbohydrates are produced by green plants in the process of undergoing photosynthesis. Nutritionally, the carbohydrates include sugar in its various forms as well as another class of food that people do not always think of as carbohydrates: fruits. Additionally, such starchy foods as potatoes, rice, and wheat products (bread, pasta, and so on) rank as important carbohydrates, while cereal grains and corn are examples of foods that contain both starchy carbohydrates and proteins.

LIPIDS.

All fats and oils are lipids; these substances are distinguished by the fact that they are soluble only in compounds made of nonpolar molecules. Water is an example of a polar molecule, because the oxygen and hydrogen atoms tend to occupy opposite "ends" of the molecule, with one end exerting a negative electric charge and the other end a positive one. Therefore, water molecules tend to stick closely together. On the other hand, oil molecules, which consist of carbon and hydrogen, are nonpolar, because the atoms of the two elements do not tend to drift to opposite ends of the molecule. As a result, oil has the slippery texture for which it is known.

With their affinity for nonpolar molecules, lipids are soluble, or capable of dissolving, in oily or fatty substances but not in water. In the body, lipids, like carbohydrates, supply energy, only in different ways and on a different timetable. When burned, a gram of lipid actually produces about three times as much energy as a gram of carbohydrate, but this energy release takes place much more slowly. Among the other functions performed by lipids in the body are protection of organs from shock and damage and the provision of insulation for the body, for instance, from toxins. (It is for this reason that toxins often are stored in fat cells. See Food Webs for more about DDT bioaccumulation in the fat cells of animals.)

VITAMINS AND MINERALS.

Vitamins are organic substances that, in extremely small quantities, are essential to the nutrition of most animals and some plants. In particular, they work with enzymes in regulating metabolic processes, but they do not in themselves provide energy; thus, vitamins alone do not qualify as a form of nutrition. Much the same is true of minerals, except that they are inorganic substances. And whereas vitamins are usually chemically complex (the formula for vitamin A, for instance, is C₂₀H₂₉OH), minerals may be as simple as a single element—for instance, iron or calcium.

Though the body can produce some vitamins, in general, vitamins and minerals are substances that the body is incapable of making for itself. Therefore, for optimal health, it is necessary to include them in the diet on a regular, if not daily, basis. They also have in common the fact that the body needs them only in very small quantities, for which reason they are sometimes known as micronutrients.

Vitamin A, for example, is a substance necessary to the functioning of the eye's retina in adjusting to light, and thus proper vitamin A levels are essential for night vision. Without vitamin A, a person can be afflicted with a condition known as night blindness, as well as with dryness of the skin. Vitamin A is also essential to bone growth. This vitamin occurs naturally in such foods as green and yellow vegetables, eggs, fruits, and liver and particularly in fish liver oils, such as cod liver oil.

Calcium, a mineral, helps build strong bones and teeth. It also has a role in the normal functioning of nerve and muscle activity. Ninety-nine percent of the body's calcium is stored in the skeleton and teeth, while the remainder circulates in the bloodstream, where it helps make possible muscle contractions. Bones are 70% calcium by weight, which gives them their strength and rigidity. Calcium, which is even more prevalent than iron, is the most abundant metallic element in the human body. Good dietary sources of calcium include milk and milk products, eggs, such leafy green vegetables as spinach, and sardines.

The U.S. Department of Agriculture Food Pyramid

The U.S. Department of Agriculture (USDA) has developed a diagram, called the food pyramid, to illustrate the components needed in a healthy diet. The bottom and widest level of the pyramid contains the cereal foods, such as breads, pastas, and rice. Primarily carbohydrates, these foods are a major source of energy, and therefore the USDA recommends 6-11 servings of 1-2 oz (30-60 g) from this food group. As to the exact number of servings, this is a function of such variables as age, gender, weight, and degree of regular physical activity.

The second level of the food pyramid, which is smaller than the first, consists of fruits and vegetables. These foods, which are also primarily carbohydrates, are especially important in supplying vitamins and minerals. A secondary function is the delivery of indigestible fiber, which improves the functioning and health of the large intestine, or colon. From this group, the USDA recommends 5-9 servings a day.

At the third level of the pyramid are proteins, including meats, eggs, beans, nuts, and milk products. According to the USDA, the percentage of these foods in one's diet should be much smaller than the percentage of carbohydrates. Smaller still is the quantity of servings at the top level, which contains the lipids. The small space allotted to this food emphasizes the fact that fats and oils should be consumed in small quantities for optimum health.

Fat and the American Diet

What we have just described is the orthodox view of nutrition in the United States as the nation entered the twenty-first century. By that time, however, physicians, nutritionists, dieticians, and other specialists had begun to question the emphasis on carbohydrates in the USDA food pyramid and other mainstream diets. For a young person, whose body is still growing, the food pyramid is a good dietary plan. But for a person past the early twenties, particularly those who are overweight or suffering from a condition such as diabetes, other approaches may be needed.

The average American adult is considerably overweight for his or her height and age group, a fact for which a number of practices can be blamed. Among these practices are inactivity. Things have changed a great deal since our ancestors spent their whole lives in a flurry of physical activity, hunting animals for food and remaining ever on the move. Our bodies themselves—a product of natural selection that took place over countless generations (see Evolution)—know nothing about this change. They are still programmed to perform as they did 10,000 years ago, storing fat for use in lean times.

Thus, inactivity breeds obesity, a condition that cannot be addressed successfully by diets aimed simply at reducing consumption. In such a situation, the body simply holds on to its fat more fiercely, and this is one reason why a starvation diet is less than useless as a means of bringing about healthy weight loss. Starving oneself also reduces lean muscle mass, which further slows the metabolism and makes it still harder to burn fat. In fact, one of the best ways to lose weight is by combining resistance exercise (i.e., weight lifting) with proper eating.

Then, of course, there are the things Americans eat: junk food and fried foods, for instance. Eating junk food, pumped full of chemicals and white sugar, is like dumping garbage into a gas tank. As for fried foods, an American seldom realizes how much is in his or her diet until making a visit overseas. In Germany, for instance, virtually nothing is fried, and though people eat hearty meals with plenty of sausage, potatoes, bread, and beer, obesity is far less of a problem in Germany than in America. (Furthermore, the American traveler is likely to have far better bowel movements on the high-fiber German diet than on the greasy, fatty, salty American diet.)

The Case for Proteins

As healthy as a diet based on the food pyramid is, an overweight adult who stuck religiously to it most likely would remain overweight, even if he or she combined it with a regular program of aerobic exercise (e.g., jogging). Thus, by the late twentieth century, numerous experts across a wide spectrum of health professions began to challenge the old orthodoxy that held up carbohydrates as the central component of a healthy diet. Instead, a growing body of opinion favored diets that rely more on proteins and even healthy fats, such as those in broiled or grilled salmon or olive oil.

Whereas carbohydrate consumption can help an athlete gain a burst of energy, for most people carbohydrates are simply the raw materials for fat, which the body will store when it discovers that it does not need the carbohydrates themselves as an immediate source of energy. Furthermore, the brain has a mechanism for signaling the body that it has consumed enough protein, whereas there is no such mechanism where carbohydrates are concerned. To test this, try eating a meal of just protein: chances are that you will feel you have had enough fairly quickly. On the other hand, try eating a meal of just starches; you will find that you can eat and eat and eat, piling on calories as you do.

Malnutrition

As we noted earlier, corn has both protein and carbohydrate components, but this does not mean that a diet heavy in corn and corn products is a healthy one. Such a diet was not uncommon among poor people in the American South during the late nineteenth and the early twentieth centuries and among even poorer people in Mexico and other parts of Latin America to the present day. With Southern foods, such as grits, hominy, and cornbread, and Latin American foods, such as corn tortillas and polenta, it is quite possible to eat cheaply from a diet that relies primarily on corn. Yet someone who does so is at risk of serious health problems, because corn is lacking in two essential amino acids, lysine and tryptophan.

This is just one example of malnutrition, a condition that develops when the body does not obtain the right amount of the vitamins, minerals, and other nutrients it needs to maintain healthy tissues and organ function. Malnutrition occurs in people who are either undernourished or overnourished. Undernourishment is a consequence of consuming too few essential nutrients or using or excreting them more rapidly than they can be replaced. This brings about the alltoo-familiar scenarios most of us associate with malnutrition: scenes of starving children in the third world, their bellies distended from kwashiorkor (which we discuss next) and thus abnormally large, far out of proportion to their bony arms and legs.

Such scenes of horror, and the virtually unimaginable misery of which they provide a hint, are almost beyond the comprehension of the average American. In America even the poorest of people are reasonably well fed, certainly compared with the poor of Africa, Asia, or Latin America. But that does not mean that malnutrition is not a problem in the affluent Western world. In the United States under nourishment may not be nearly as much of a problem, but dietary imbalances and excesses are, and they have become associated with many of the leading causes of death and disability. Overnutrition results from eating too much, eating too many of the wrong things (including junk foods, especially those containing white sugar), not exercising enough, or taking too many vitamins or other dietary replacements. As with many things, where vitamins are concerned, it is not the case that if a little is good, a lot is better. Vitamins can only be absorbed by the body if ingested on a full stomach, and an excess of fat-soluble vitamins such as A or D could actually be toxic. (See Vitamins for more on this subject.)

KWASHIORKOR.

For any nutrient that the body requires, there is a corresponding disease, ailment, or condition (or many of them) that will develop if a person's body is deprived of that nutrient. For instance, people whose diets lack protein may become susceptible to a condition known as kwashiorkor, characterized by apathy, exhaustion, fatigue, wasting of muscle tissue, and edema (swelling, a result of water collecting in the body.)

Kwashiorkor, in fact, is the condition that causes swelling in the stomachs of children suffering from malnutrition. When people eat a diet consisting mainly of starchy vegetables, as is common in parts of Africa, Latin America, and southern Asia, they consume an appropriate amount of calories, but they do not get certain essential amino acids that are important for growth. This is particularly serious for children. Nursing babies usually do not suffer from kwashiorkor, because they receive adequate protein from their mother's milk. A child who is weaned, however, is at much greater risk.

In much of the third world, where human populations are high relative to the number of animals raised for meat, meat itself is a luxury. A child of a poor family is unlikely to have meat in his or her diet, and without the mother's milk the child is left virtually without sources of protein. This situation may have informed the choice of the word kwashiorkor to describe this condition. Derived from the Ga language of coastal Ghana in Africa, the word is based on kwásíokó, a Ga term for "the influence a child is said to be under when a second child comes." In regions where birth control is unavailable or simply is not used, it is easy to imagine how an older child would be forced to give up the mother's milk to make room for a second child.

The results are devastating for the child deprived of protein sources. The most shocking outward manifestation of the condition is, of course, the swollen, bloated abdomen, brought about by a decrease in the amounts of the protein albumin (also found in egg whites) in the bloodstream. Skin discoloration may occur, along with severe diarrhea, an enlarged liver, and atrophy, or withering, of muscles and glands. Kwashiorkor can bring about retarded mental and physical development as well, but at least there is a treatment: adding proteins to the diet. For this reason, aid organizations supply powdered milk, which, if administered to these undernourished children in time, can save them from further damage to their bodies and minds.

MALNUTRITION IN THE UNITED STATES.

Malnutrition exists even in the United States and not only in the form of over nourishment. Particularly among the nation's poor, under nourishment is serious, though it is not necessarily because food is unavailable to poor families. Rather, the right food may be unavailable, because the health foods described at the conclusion of this essay tend to be expensive. Furthermore, lack of education and information ensures that poor families maintain destructive dietary practices that leave their bodies deprived of essential nutrients. Low-cost, high-bulk items, such as sugar, white flour, and corn meal, may provide immediate satisfaction to the stomach, but they leave the body undernourished in the long run.

Nevertheless, the most prevalent form of malnutrition in America is still over nourishment. Implied in the term over nourishment, of course, is the idea that the person so afflicted is not eating the right kinds of things, because it is hard to be overnourished with a proper diet. Junk foods, such as one finds in the candy aisle of a convenience store or in a fast-food restaurant, contain carbohydrates and some proteins (not to mention plenty of fats); usually, these nutrients are presented in such a way as to maximize taste and minimize nutritional value. The result is "empty calories" that simply go to the waistline, buttocks, and abdomen of the consumer.

America is awash in food, but very little of it is the kind of food that sustains the health of the body or prolongs life. People have been conditioned by advertising to believe that white bread is the only kind of bread (even though the process of refining white flour leaches out most of the nutrients); that it is possible to have "zero fat" cookies sweetened with white sugar (which simply turns to fat as soon as the body digests it); and that a diet of burgers, fries, and soft drinks is "normal." The result is an overnourished, over-weight populace whose life spans are being cut short.

There is another way, however, as a visit to a well-stocked health-food grocery store (or even the organic/health food section of a mainstream grocery store) will illustrate. After trying genuine whole wheat or sprouted wheat instead of white bread, fructose or cane-juice sweeteners in place of refined sugar, free-range chicken and grass-fed beef rather than meats pumped full of chemicals, many Americans might be surprised to learn just how good food can taste—and how good they can feel.

WHERE TO LEARN MORE

Anderson, Jean, and Barbara B. Deskins. The Nutrition Bible: A Comprehensive, No-Nonsense Guide to Foods, Nutrients, Additives, Preservatives, Pollutants, and Everything Else We Eat and Drink. New York: William Morrow, 1995.

Food and Nutrition Information Center, National Agricultural Library, U.S. Department of Agriculture (Web site). <http://www.nal.usda.gov/fnic/>.

Hands, Elizabeth S. Nutrients in Food. Philadelphia: Lippincott, Williams and Wilkins, 2000.

Kiple, Kenneth F., and Kriemhild Coneé Ornelas. The Cambridge World History of Food. New York: Cambridge University Press, 2000.

Nutrition.gov (Web site). <http://www.nutrition.gov/>.

Nutrition and Healthy Eating Advice. About.com (Web site). <http://nutrition.about.com/>.

Patent, Dorothy Hinshaw. Nutrition: What's in the Food We Eat. Illus. William Muñoz. New York: Holiday House, 1992.

Renders, Eileen. Food Additives, Nutrients, and Supplements A-Z: A Shopper's Guide. Santa Fe, NM: Clear Light Publishers, 1999.

Schwarzbein, Diana, and Nancy Deville. The Schwarzbein Principle: The Truth About Losing Weight, Being Healthy, and Feeling Younger. Deerfield Beach, FL: Health Communications, 1999.

U.S. FDA Center for Food Safety and Applied Nutrition (Web site). <http://vm.cfsan.fda.gov/list.html>.

KEY TERMS

AMINO ACIDS:

Organic compounds composed of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations. Strings of amino acids make up proteins.

AUTOTROPHS:

Primary producers. Autotroph means "self-feeder," and these organisms are distinguished by the fact that they do not depend on other organisms as a source of energy. Instead, they need only sunlight, water, and a few simple chemical compounds, from which they build the large organic molecules that they need to survive.

BIOGEOCHEMICAL CYCLES:

The changes that particular elements undergo as they pass back and forth through the various earth systems (e.g., the biosphere) and particularly between living and nonliving matter. The elements involved in biogeochemical cycles are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur.

CARBOHYDRATES:

Naturally occurring compounds, consisting of carbon, hydrogen, and oxygen, whose primary function in the body is to supply energy. Included in the carbohydrate group are sugars, starches, cellulose, and various other substances. Most carbohydrates are produced by green plants in the process of undergoing photosynthesis.

COMPLETE PROTEIN:

A protein that includes all eight essential amino acids.

COMPOUND:

A substance in which atoms of more than one element are bonded chemically to one another.

ELEMENT:

A substance made up of only one kind of atom. Unlike compounds, elements cannot be broken down chemically into simpler substances.

ENZYME:

A protein material that speeds up chemical reactions in the bodies of plants and animals.

ESSENTIAL AMINO ACIDS:

Amino acids that cannot be manufactured by the body and which therefore must be obtained from the diet. Proteins that contain essential amino acids are known as complete proteins.

FOOD WEB:

A term describing the interaction of plants, herbivores, carnivores, omnivores, decomposers, and detritivores in an ecosystem. Each of the seorganisms consumes nutrients and passes them along to other organisms (or, in the case of the decomposer food web, to the soil and environment). The food web may be thought of as a bundle or network of food chains, but since the latter rarely exist separately, scientists prefer the concept of a food web to that of a food chain.

LIPIDS:

Fats and oils. With their affinity for nonpolar molecules, lipids dissolve in oily or fatty substances but not in water-based liquids. In the body, lipids supply energy in slow-release doses, protect organs from shock and damage, and provide insulation for the body, for instance, from toxins.

MALNUTRITION:

Any one of several conditions that develop when the body does not obtain the right amount of vitamins, minerals, and other nutrients it needs to maintain healthy tissues and organ function. Malnutrition may result from under nourishment or over nourishment.

MINERALS:

In organic substances that, in a nutritional context, serve a function similar to that of vitamins. Minerals may include chemical elements—particularly metallic ones, such as calcium or iron—as well as some compouands.

NUTRIENT:

Materials essential to the survival of organisms. They include proteins, carbohydrates, lipids (fats), vitamins, and minerals.

NUTRITION:

The series of processes by which an organism takes in nutrients and makes use of them for its survival, growth, and development. The term nutrition also can refer to the study of nutrients, their consumption, and their use in the organism's body.

ORGANIC:

At one time chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals), and oxides, such as carbon dioxide.

PHOTOSYNTHESIS:

The biological conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants. In this process carbondioxide and water are converted to sugars.

PRIMARY CONSUMERS:

Animalsthat eat green plants. Compare with sec on dary consumers.

PRIMARY PRODUCERS:

Green plants that depend on photosynthesis for their nourishment.

PROTEINS:

Large molecules built from long chains of amino acids. Proteins serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body's immune system, and making enzymes.

SECONDARY CONSUMERS:

Animals that eat other animals.

TRACE ELEMENT:

A chemical element that appears within an organism or other natural system in very small quantities, or traces. In the human body, for instance, such trace elements as copper or iodine are essential to health, though they comprise far less than 1% of the body's mass.

VITAMINS:

Organic substances that, in extremely small quantities, are essential to the nutrition of most animals and some plants. In particular, vitamins work with enzymes in regulating metabolic processes; however, they do not in themselves provide energy, and thus vitamins alone do not qualify as a form of nutrition.