Nutrition is the science that interprets the relationship of food to the functioning of the living organism. It is concerned with the intake of food, digestive processes, the liberation of energy, and the elimination of wastes, as well as with all the syntheses that are essential for maintenance, growth, and reproduction. These fundamental activities are characteristic of all living organisms--from the simplest to the most complex plants and animals.
Nutrients are substances, either naturally occurring or synthesized, that are necessary for maintenance of the normal function of organisms. These include carbohydrates, lipids, proteins, vitamins and minerals, water, and some unknown substances.
The nutritionist, a scientist working in the field of nutrition, differs from the dietitian, who translates the science of nutrition into the skill of furnishing optimal nourishment to people.
Dietetics is a profession concerned with the science and art of human nutrition care, an essential component of the health sciences. The treatment of disease by modification of the diet lies within the province of the physician and the dietitian. Such modification can be, and often is, effected without the use of special foods, simply by changing the methods of food preparation or by restricting the diet.
Special foods for particular dietary uses differ from ordinary foods by their specific composition or by physical, chemical, or other modifications resulting from processing. If there is an inability to metabolize one of the normal constituents of a diet, then this must be removed from the diet.
Therapeutic diets fall into a number of categories. The foods consumed by humans must contain, in adequate amounts, about 45 to 50 highly important substances. Water and oxygen are equally essential. Starting only with these essential nutrients obtained from food, the body makes literally thousands of substances necessary for life and physical fitness. Most of these substances are far more complicated in structure than the original nutrients.
Energy metabolism and requirements are customarily expressed in terms of the calorie, a heat unit. Adoption of the calorie by nutritionists followed quite naturally from the original methods of measuring energy metabolism. The magnitude of human energy metabolism, however, made it awkward to record the calorie measured, so the convention of the large calorie, or kilocalorie (kcal), was accepted.
Atwater factors, also called physiologic fuel factors, are based on the corrections for losses of unabsorbed nutrients in thefeces and for the calorie equivalent of the nitrogenous products in the urine. These factors are as follows: 1 g of pure protein will yield 4 calories, 1 g of pure fat will yield 9 calories, and 1 g of pure carbohydrate will yield 4 calories.
Proteins are widely distributed in nature, and no life forms are known without them. They are made up of relatively simple organic compounds, the amino acids, which contain nitrogen and sometimes sulfur. Humans and animals build the protein they need for growth and repair of tissues by breaking down the proteins obtained in food into their component parts, the amino acids, and then building up these components into proteins of the type needed. The protein-rich foods from animal sources contain complete proteins, which supply all the amino acids in the proper proportions necessary in the human diet. Although it was formerly believed that plant proteins had to be combined at each meal, research shows that a balanced diet will provide the proper combinations.
Most foods contain several vitamins and minerals. Vitamins are organic food substances, needed only in minute quantities but essential for the normal metabolism of other nutrients. Many vitamins and minerals act as catalysts or help form catalysts in the body. Minerals--such as calcium, iodine, and iron--are an essential part of all cells and body fluids and enter into many functions.
Fats, which are widely distributed in nature, are a concentrated food source of energy. Fats are glyceryl esters of fatty acids and yield glycerol and many different fatty acids when broken down by hydrolysis.
Carbohydrates are the most abundant food sources of energy. Important dietary carbohydrates are divided into two groups--starches and sugars. The starches, which may be converted into utilizable sugars in plants or in the human body, are supplied in the grains, the pulses, the tubers, and some rhizomes and roots. The sugars occur in many plants and fruits, the most important being sucrose, which is obtained from sugarcane or the sugar beet.
Dietary fiber, also known as bulk and roughage, is also an essential element in the diet even though it provides no nutrients. It consists of plant cellulose and other undigestible materials in foods, along with pectin and gum. The chewing it requires stimulates saliva flow, and the bulk it adds in the stomach and intestines during digestion provides more time for absorption of nutrients.
Diets with sufficient fiber produce softer, bulkier stools and help to promote bowel regularity and avoid constipation and other disorders, such as diverticulosis. Fruits, vegetables, whole-grain breads, and products made from nuts and legumes are all sources of dietary fiber. A diet overly abundant in dietary fiber, however, can cut down on the absorption of important trace minerals during digestion.
The bread-cereal group includes all breads and cereals that are whole-grain, enriched, or restored. Protein content is not high in cereals, but the large intake of cereals in some diets makes these products a significant source of protein. All cereals are very high in starch and hence are good, generally inexpensive sources of energy. The fat content of cereal products generally is very low unless the germ is included in the food. The whole-grain products contribute significant quantities of fiber and such trace vitamins and minerals as pantothenic acid, vitamin E, zinc, copper, manganese, and molybdenum.
Most vegetables are an important source of minerals, vitamins, and cellulose. Certain vegetables, such as potatoes, contribute appreciable quantities of starch. Large amounts of the minerals calcium and iron are in vegetables, particularly beans, peas, and broccoli. Vegetables also help meet the body's need for sodium, chloride, cobalt, copper, magnesium, manganese, phosphorus, and potassium. Carotenes (the precursor of vitamin A) and ascorbic acid (vitamin C) are abundant in many vegetables. Vegetables are useful as sources of roughage.
The nutritional value of fruits varies. Some fruits are composed largely of water, but contain valuable vitamins. The citrus fruits are a valuable source of vitamin C, and yellow-colored fruits, such as peaches, contain carotene. Dried fruits contain an ample amount of iron, and figs and oranges are an excellent source of calcium. Like vegetables, fruits have a high cellulose content.
The milk group includes milk, cheese, and ice cream. Milk is a complete protein food containing several protein complexes. It also contains important amounts of most nutrients, but it is very low in iron and ascorbic acid and low in niacin. Calcium and phosphorus levels in milk are very high. Vitamin A levels are high in whole milk, but in the production of skim milk, this fat-soluble vitamin is removed. Riboflavin is present in significant quantities in milk unless the milk has been exposed to light.
The meat and meat substitutes group includes beef; veal; lamb; pork; variety meats such as liver, heart, and kidney; poultry and eggs; fish and shellfish; and dried peas, beans, and nuts. The meat group contains many valuable nutrients. One of its main nutrients is protein, but meat protein also contains cholesterol, which is believed to contribute to coronary artery disease. The minerals copper, iron, and phosphorus occur in meats in significant amounts. The quantity of iron and copper found in liver is of particular interest. Different meats vary in their vitamin content. Liver usually contains a useful amount of vitamin A. Thiamine, riboflavin, and niacin, all B vitamins, occur in significant amounts in all meats.
Butter, margarine, other fats, oils, sugars, or unenriched refined-grain products are included in the diet to round out meals and satisfy the appetite. Fats, oils, and sugars are added to other foods during preparation of the meal or at the table. These foods supply calories and can add to total nutrients in meals.
The dietary standards used in the United States are the Recommended Dietary Allowances (RDA), which were developed by the Food and Nutrition Board of the National Research Council, National Academy of Sciences. The allowances are described as the levels of intake of essential nutrients considered--in the judgment of the Food and Nutrition Board on the basis of available scientific knowledge--to be adequate to meet normal nutritional needs.
Energy allowances are determined according to age and average heights and weights. Individual activity level is also used to determine the level of ideal calorie consumption. For example, a decrease in recommended energy allowance with increasing age is consistent with the known decrease in basal metabolic rate that occurs with aging and with a possible decrease in physical activity. The recommended daily allowance for protein for adults is no more than 1.6 g/kg desirable weight. This assumes 70 percent efficiency in utilization of dietary protein.
For many years the United States Department of Agriculture (USDA) issued dietary guidelines based on four basic food groups--meat and meat substitutes, fruits and vegetables, milk and dairy products, and grains, including bread and cereals--and a balanced diet would include at least one food from each group in each meal every day. In 1980 the U.S. Department of Health and Human Services outlined guidelines recommending that people eat a variety of foods daily, including fruits; vegetables; whole and enriched grain products; dairy products; meats, poultry, fish, and eggs; and dried peas and beans.
While recognizing that certain people (for example, pregnant women, the elderly, and infants) have special nutritional needs, the report stressed that for most people the greater the variety of foods eaten, the less likely is a deficiency or excess of any single nutrient to develop. The report emphasized that people should increase their consumption of complex carbohydrates--fruits, vegetables, and other unrefined foods--and naturally occurring sugars. It also recommended reducing the consumption of refined and processed sugars. It encouraged a reduction in fat consumption by decreasing the amount of fatty meats and replacing foods that have saturated fats with those having unsaturated fats. A reduction in the sodium intake by decreasing the amount of salt added to food was also recommended.
The dietary goals suggested reducing cholesterol intake by decreasing the amount of meat, butterfat, eggs, and other sources in the diet. Controversy has arisen concerning cholesterol intake, however. Although there is abundant evidence that dietary fat and cholesterol are factors that increase the risk of atherosclerosis and coronary heart disease, the Food and Nutrition Board has concluded that the evidence does not warrant specific recommendations about dietary cholesterol for the healthy person, who does not need to be concerned about fat intake.
The proportion of the population that would benefit from reducing cholesterol intake is unknown. In 1984, however, the U.S. National Heart, Lung, and Blood Institute reported the results of a study showing that persons with high cholesterol levels are clearly under greater risk of heart attacks and heart disease than are persons with normal cholesterol levels.
To reflect the recent research findings on nutrition, in 1992 the USDA and the Department of Health and Human Services changed the daily diet recommendations from the square of the four food groups to a food pyramid, with foods that should be eaten more often at the base, and those used less frequently at the top. The emphasis is on consuming less of the groups meat and meat substitutes, dairy products, and oils and fats, and more of the breads and cereals, and fruits and vegetables. Some scientists feel that these recommendations do not go far enough and are pressing for the near elimination of meats and fats from the American diet.
Nutritional labeling is part of the solution to the problem of consumer ignorance of nutrients in foods. For a meaningful labeling program, serving size, the nutritional constituency in terms of the percentage of the RDA for each nutrient, as well as calories, proteins, carbohydrates, fats, and a system of freshness dating must be included on each label. Since 1973 the U.S. Food and Drug Administration has required complete nutrition labeling if a nutrient is added to a food or a nutrient claim is made by a manufacturer.
A 1990 law made the formerly voluntary full nutritional labeling necessary for all products, including meats, fruits, and vegetables, as well as setting standards for common terms such as "low fat." Nutritionists, physicians, and dietitians are faced with the problems of acquainting people with nutritious foods available to them, supplying them with the knowledge to make good nutritional decisions, and educating them to the dangers of poor food habits. Merely introducing people to wholesome food is not enough. People must also come to enjoy and actually prefer nutritious food if nutritional health is to become a reality.