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An important aspect of nutrition is the daily intake of nutrients. Nutrients consist of various chemical substances in the food that makes up each person's diet. Many nutrients are essential for life, and an adequate amount of nutrients in the diet is necessary for providing energy, building and maintaining
THE THREE FUNCTIONS OF NUTRIENTS
body organs, and for various metabolic processes. People depend on nutrients in their diet because the human body is not able to produce many of these nutrients—or it cannot produce them in adequate amounts.
Nutrients are essential to the human diet if they meet two characteristics. First, omitting the nutrient from the diet leads to a nutritional deficiency and a decline in some aspect of health. Second, if the omitted nutrient is put back into the diet, the symptoms of nutritional deficiency will decline and the individual will return to normal, barring any permanent damage caused by its absence.
There are six major classes of nutrients found in food: carbohydrates, proteins, lipids (fats and oils), vitamins (both fat-soluble and water-soluble), minerals, and water. These six nutrients can be further categorized into three basic functional groups.
Carbohydrates are the major source of energy for the body. They are composed mostly of the elements carbon (C), hydrogen (H), and oxygen (O). Through the bonding of these elements, carbohydrates provide energy for the body in the form of kilocalories (kcal), with an average of 4 kcal per gram (kcal/g) of carbohydrates (a kcal is equivalent to a calorie on a nutritional label of a packaged food).
Carbohydrates come in a variety of sizes. The smallest carbohydrates are the simple sugars, also known as monosaccharides and disaccharides, meaning that they are made up of one or two sugar molecules. The best known simple sugar is table sugar, which is also known as sucrose, a disaccharide. Other simple sugars include the monosaccharides glucose and fructose, which are found in fruits, and the disaccharides, which include sucrose, lactose (found in milk), and maltose (in beer and malt liquors). The larger carbohydrates are made up of these smaller simple sugars and are known as polysaccharides (many sugar molecules) or complex carbohydrates. These are usually made up of many linked glucose molecules, though, unlike simple sugars, they do not have a sweet taste. Examples of foods high in complex carbohydrates include potatoes, beans, and vegetables. Another type of complex carbohydrate is dietary fiber. However, although fiber is a complex carbohydrate made up of linked sugar molecules, the body cannot break apart the sugar linkages and, unlike other complex carbohydrates, it passes through the body with minimal changes.
Although carbohydrates are not considered to be an essential nutrient, the body depends on them as its primary energy source. The body utilizes most carbohydrates to generate glucose, which serves as the basic functional molecule of energy within the cells of the human body (glucose is broken down to ultimately produce adenosine triphosphate, or ATP, the fundamental unit of energy). When the supply of carbohydrates is too low to adequately supply all the energy needs of the body, amino acids from proteins are converted to glucose. However, the typical American individual consumes more than adequate amounts of carbohydrates to prevent this utilization of protein.
Proteins are composed of the elements carbon (C), oxygen (O), hydrogen (H), and nitrogen (n). They have a variety of uses in the body, including serving as a source of energy, as substrates (starter materials) for tissue growth and maintenance, and for certain biological functions, such as making structural proteins, transfer proteins, enzyme molecules, and hormone receptors. Proteins are also the major component in bone, muscle, and other tissues and fluids. When used for energy, protein supplies an average of 4 kcal/g.
Proteins are formed by the linking of different combinations of the twenty common amino acids found in food. Of these, ten are essential for the human in the synthesis of body proteins (eight are essential throughout a human's life, whereas two become essential during periods of rapid growth, such as during infancy).
Protein may be found in a variety of food sources. Proteins from animal sources (meat, poultry, milk, fish) are considered to be of high biological value because they contain all of the essential amino acids. Proteins from plant sources (wheat, corn, rice, and beans) are considered to be of low biological value because an individual plant source does not contain all of the essential amino acids. Therefore, combinations of plant sources must be used to provide these nutrients.
Protein deficiency is not common in the American diet because most Americans consume 1.5 to 2 times more protein than is required for the body to maintain adequate health. This excess intake of protein is not considered to be harmful for the average healthy individual. However, when protein intake is inadequate, but total caloric intake is sufficient, a condition known as kwashiorkor may occur. Symptoms of kwashiorkor include an enlarged stomach, loss of hair and hair color, and an enlarged liver. Conversely, if protein and caloric intake are both inadequate, a condition known as marasmus occurs. Marasmus presents with a stoppage of growth, extreme muscle loss, and weakness.
Lipids, which consist of fats and oils, are high-energy yielding molecules composed mostly of carbon (C), hydrogen (H), and oxygen (O) (though lipids have a smaller number of oxygen molecules than carbohydrates have). This small number of oxygen molecules makes lipids insoluble in water, but soluble in certain organic solvents. The basic structure of lipids is a glycerol molecule consisting of three carbons, each attached to a fatty-acid chain. Collectively, this structure is known as a triglyceride, or sometimes it is called a triacylglycerol. Triglycerides are the major form of energy storage in the body (whereas carbohydrates are the body's major energy source), and are also the major form of fat in foods. The energy contained in a gram of lipids is more than twice the amount in carbohydrates and protein, with an average of 9 kcal/g.
Lipids can be broken down into two types, saturated and unsaturated, based on the chemical structure of their longest, and therefore dominant, fatty acid. Whether a lipid is solid or liquid at room temperature largely depends on its property of being saturated or unsaturated. Lipids from plant sources are largely unsaturated, and therefore liquid at room temperature. Lipids that are derived from animals contain a higher amount of saturated fats, and they are therefore solid at room temperature. An exception to this rule is fish, which, for the most part, contain unsaturated fat. The important difference between saturated and unsaturated fatty acids is that saturated fatty acids are the most important factor that can increase a person's cholesterol level. An increased cholesterol level may eventually result in the clogging of blood arteries and, ultimately, heart disease.
Not all fatty acids are considered harmful. In fact, certain unsaturated fatty acids are considered essential nutrients. Like the essential amino acids, these fatty acids are essential to a person's diet because the body cannot produce them. The essential fatty acids serve many important functions in the body, including regulating blood pressure and helping to synthesize and repair vital cell parts. It is estimated that the American diet contains about three times the amount of essential fatty acids needed daily. Lipids are also required for the absorption of fat-soluble vitamins, and they are generally thought to increase the taste and flavor of foods and to give an individual a feeling of fullness.
Vitamins are chemical compounds that are required for normal growth and metabolism. Some vitamins are essential for a number of metabolic reactions that result in the release of energy from carbohydrates, fats, and proteins. There are thirteen vitamins, which may be divided into two groups: the four fat-soluble vitamins (vitamins A, D, E, and K) and the nine water-soluble vitamins (the B vitamins and vitamin C). These two groups are dissimilar in many ways. First of all, cooking or heating destroys the water-soluble vitamins much more readily than the fat-soluble vitamins. On the other hand, fat-soluble vitamins are much less readily excreted from the body, compared to water-soluble vitamins, and can therefore accumulate to excessive, and possibly toxic, levels. This means, of course, that levels of water-soluble vitamins in the body can become depleted more quickly, leading to a vitamin deficiency if those nutrients are not replaced regularly. Deficiencies of vitamins may result from inadequate intake, as well as from factors unrelated to supply. For instance, vitamin K and biotin are both produced by bacteria that live within the intestines, and a person can become deficient if these bacteria are removed by antibiotics. Other factors that may result in a vitamin deficiency include disease, pregnancy, drug interactions, and newborn development (newborns lack the intestinal bacteria that create certain vitamins, such as biotin and vitamin K).
Minerals are different from the other nutrients discussed thus far, in that they are inorganic compounds (carbohydrates, proteins, lipids, and vitamins are all organic compounds). The fundamental structure of minerals is usually nothing more than a molecule, or molecules, of an element. The functions of minerals do not include participation in the yielding of energy. But they do play vital roles in several physiological functions, including critical involvement in nervous system functioning, in cellular reactions, in water balance in the body, and in structural systems, such as the skeletal system.
Because minerals have a very simple structure of usually one or more molecules of an element, they are not readily destroyed in the heating or cooking process of food preparation. However, they can leak out of the food substance that contains them and seep into the water or liquid the food is being cooked in. This may result in a decreased level of minerals being consumed if the liquid is discarded.
There are many minerals found within the human body, but of the sixteen (or possibly more) essential minerals, the amount required on a daily basis varies enormously. This is why minerals are subdivided into two classes: macrominerals and microminerals. Macrominerals include those that are needed in high quantities, ranging from milligrams to grams. Calcium, phosphorous, and magnesium are macrominerals. Microminerals are those necessary in smaller quantities, generally between a microgram and a milligram. Examples of microminerals include copper, chromium, and selenium. Dietary requirements for some minerals have yet to be established.
Water makes up the last class of nutrients, though the fact that it is considered a nutrient is surprising to many people. Water, however, has many necessary functions in the human body. Some of its actions include its use as a solvent (a substance that other substances dissolve in), as a lubricant, as a conduction system for transportation of vital nutrients and unnecessary waste, and as a mode of temperature regulation.
There are many available sources of water other than tap water and bottled water. Some foods have a high water content, including many fruits and vegetables. In addition, the body can make small amounts of water from various metabolic prcesses that result in molecules of water as a by-product. This, however, is by no means sufficient for the body's needs of water. It is generally recommended that people drink eight cups (or nearly 2 liters) of water a day to maintain an adequate supply.
Susan S. Kim Jeffrey Radecki
Harper, A. (1999). "Defining the Essentiality of Nutrients." In Modern Nutrition in Health and Disease, 9th edition, ed. M. E. Shills, et al. Baltimore, MD: Williams and Wilkins.
Morrison, Gail, and Hark, Lisa (1999). Medical Nutrition and Disease, 2nd edition. Cambridge, MA: Blackwell Science.
Subar, A. F., et al. (1998). "Dietary Sources of Nutrients in the U.S. Diet, 1989 to 1991." Journal of the American Dietetic Association 98:537.
Wardlaw, Gordon M., and Kessel, Margaret (2002). Perspectives in Nutrition, 5th edition. Boston: McGraw-Hill.