The National Academy of Sciences collects data and publishes a series of tables known as Dietary Reference Intakes (DRIs), which provide quantitative estimates of nutrient intakes for planning and assessing diets for healthy people. The DRIs replace the Recommended Daily Allowance (RDA) last updated in 1989. The DRIs are actually a set of four reference values: Estimated Average Requirement, Recommended Dietary Allowances, Adequate Intake, and Tolerable Upper Intake Level. Links to the Dietary Reference Intakes updated in September, 2011, may be retrieved at www.iom.edu/Activities/Nutrition/SummaryDRIs/DRI-Tables.aspx.

The Estimated Average Requirement (EAR) is a nutrient intake value that is estimated to meet the requirement of half of the healthy individuals in a group. The most widely known component of the DRIs is the Recommended Dietary Allowances (RDAs) table. It lists the average daily dietary intake level that is sufficient to meet the nutrient requirements of almost all (97% to 98%) healthy individuals in a group. (Groups are based on gender, age and, if applicable, pregnancy or lactation.) Recommended dietary allowances are goals in meeting nutritional needs. RDAs do not meet the nutritional needs of ill patients and do not account for nutritional value that may be lost in cooking.

The RDA is based on the EAR plus twice the standard deviation: the RDA for a nutrient is a value to be used as a goal for dietary intake by healthy individuals. There is no established benefit for healthy persons if they consume nutrient intakes greater than the RDA or Adequate Intake.

Adequate Intake (AI) is a value based on observed or experimentally determined approximations of nutrient intake by a group of healthy people. The AI is used when the RDA cannot be determined.

Another category in the DRI tables is the Tolerable Upper Intake Level (UL). This level is defined as the highest level of daily nutrient intake that is likely to pose no risk of adverse health effects to almost all those in the general population. As intake increases above the UL, the risk of adverse effects increases. The UL is not intended to be a recommended level of intake. For many nutrients, there are insufficient data on which to develop a UL. This does not mean that there is no potential for adverse effects resulting from high intake. Over time, these data will help establish the value of megadoses of vitamins and nutrients and provide information about whether there are therapeutic and toxic effects to ingestion of large doses of these chemicals.

The Estimated Energy Requirement (EER) is defined as the average dietary energy intake that is predicted to maintain energy balance in a healthy adult of a defined age, gender, weight, height, and level of physical activity consistent with good health (Table 47-1).

The National Academy of Sciences published the Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein and Amino Acids in 2005. This report on macronutrients was sponsored for both the United States and Canada by several governmental agencies, including Health Canada, U.S. Department of Health and Human Services, U.S. Food and Drug Administration (FDA), National Institutes of Health, Centers for Disease Control and Prevention, USDA, and private sources. These guidelines are based on research of nutrients in foods and how the body uses energy from foods. The report recommends that to meet the body’s energy and nutritional needs while minimizing risk for chronic disease, adults should obtain 45% to 65% of their calories from carbohydrates, 20% to 35% from fat, and 10% to 35% from protein. (See Chapter 36 for the American Diabetes Association recommendations for intake of carbohydrates, fats, and proteins for a patient with diabetes mellitus.)

Carbohydrates, often referred to as “sugars” because many of them taste sweet, are the major source of energy for body activities and metabolism. They occur in nature as simple and complex molecules and are water-soluble. The simple carbohydrates are also known as monosaccharides and disaccharides. Monosaccharides such as glucose (also known as dextrose), fructose, and galactose are the only sugars that can be absorbed directly from the gastrointestinal (GI) tract into the blood. They are the most rapidly available sources of energy and are the only sugars capable of being used directly to produce energy for the body. Disaccharides such as sucrose (common table sugar), maltose, and lactose are the most common sugars in foods, but they must be metabolized to monosaccharides before being absorbed into the bloodstream. For example, a molecule of lactose is metabolized by the enzyme lactase into a molecule each of glucose and galactose, which are then absorbed through the gut wall into the blood. Complex carbohydrates such as starch, dextrin, and fiber are also known as polysaccharides. Complex carbohydrates must also be metabolized into simple sugars in the intestine before being absorbed. The carbohydrates provide about 4 kcal of energy per gram. Daily caloric needs from carbohydrates range from 3 to 5.5 g/kg/day, depending on energy requirements for daily living, stress, and wound healing. Fruits, grains, and vegetables are excellent sources of carbohydrates. Candies and carbonated beverages are commonly used sources of calories but contain no other nutrients. End products of carbohydrate metabolism are carbon dioxide (excreted primarily through the lungs) and water.

A by-product of some complex carbohydrate metabolism is fiber. Until recently, fiber was thought to be only a by-product of carbohydrate metabolism that needed to be eliminated from the body. It is recognized now as a macronutrient, a separate factor necessary for complete nutrition and wellness. Dietary fiber is derived from plant sources and consists of undigestible carbohydrates and lignin and digestible macronutrients (carbohydrates, proteins) such as cereal brans, sweet potatoes, and legumes that contribute to overall nutrition. Another category of fiber is functional fiber, which consists of undigestible carbohydrates that have a beneficial physiologic effect on humans. An excellent example of a functional fiber is psyllium, an undigestible fiber that adds bulk to fecal content, which helps timely passage of fecal contents in the GI tract, preventing constipation. Functional fiber content may delay gastric emptying, giving a sense of fullness, which may contribute to weight control. Delayed gastric emptying may also reduce postprandial blood glucose concentrations, potentially preventing excessive insulin secretion and insulin sensitivity. Fibers can reduce the absorption of dietary fat and cholesterol as well as enterohepatic recirculation of cholesterol and bile acids, which may also reduce blood cholesterol concentrations. Total fiber is the sum of dietary fiber and functional fiber.

Fats, also known as lipids, serve as the body’s major form of stored energy and are key components of membranes, prostaglandins, and many hormones. They are not water-soluble. Examples of lipids are cholesterol, fatty acids, triglycerides, and phospholipids. Excess dietary carbohydrates and proteins are converted to fat for storage. When used as an energy source, fats generate 9 kcal of energy per gram. Fat intake usually constitutes 25% to 40% of total caloric intake, although the DRIs recommend limits of 20% to 35% from fat. Healthy adults require 1 to 1.5 g/kg/day. Even when patients severely restrict intake for dieting purposes, 4% to 10% of total calories must be supplied in the form of fats to prevent essential fatty acid deficiency.

Essential fatty acids (EFAs) are not produced by the body and must be obtained from dietary sources. The most prominent EFAs are omega-3 and omega-6 fatty acids. They are polyunsaturated fatty acids also known as alpha-linolenic and linoleic acids, respectively. Both fatty acids are required for eicosanoid and prostaglandin production and cell membrane structure.

Dietary fats can be subdivided into four categories: monounsaturated, polyunsaturated, saturated, and trans fats (see Box 47-2 for sources of dietary fats). Monounsaturated fats decrease low-density lipoproteins (LDLs) and increase high-density lipoproteins (HDLs) and are considered to be cardioprotective. Polyunsaturated fats also lower LDL and raise HDL levels. Saturated fats raise both LDL and HDL and are thought to increase atherosclerotic plaque formation in the arteries. Only recently has it been recognized that trans fats may induce more heart disease than saturated fats because, in addition to raising LDL cholesterol, trans fats decrease HDL cholesterol and increase triglycerides as well as another undesirable blood fat, lipoprotein(a). Saturated fats and trans fats have no known beneficial nutritional effect and should be eliminated as much as possible from the diet. Note in Figure 47-2 how all plant oils used in food preparation contain saturated, monounsaturated, and polyunsaturated fatty acids. It is recommended that we minimize the use of those oils higher in saturated fatty acids. (See also Chapter 22, p. 344, for further discussion of lipoproteins and cholesterol.) End products of fat metabolism are water and carbon dioxide and insoluble substances excreted in sweat, bile, and feces.

Proteins are complex molecules composed of amino acid chains. Amino acids can be subclassified as essential and nonessential. The essential amino acids must be provided from external sources to sustain life; nonessential amino acids can be synthesized to meet metabolic requirements. Before absorption, proteins must be metabolized in the gut to the individual amino acids. Once absorbed, the amino acids are used to build new proteins, such as muscle and other vital tissues, or are used as an energy source if other energy sources are depleted. Amino acids generate 4 kcal of energy per gram, similar to carbohydrates. Sources of highest protein value are dairy products (e.g., milk, eggs, cheese), fish, and meat. Grains and beans have less protein value. End products of amino acid metabolism are nitrogenous products such as urea, uric acid and ammonium, carbon dioxide, and water. Protein requirements for healthy people are 0.5 to 1 g/kg/day. Depending on the amount of stress that the patient is experiencing and the amount of tissue building and wound healing required, protein requirements may range from 1.5 to 2.5 g/kg/day. Calories from proteins generally constitute 12% to 20% of total calorie intake, but the Academy of Sciences recommends 10% to 35% for healthy nutrition.

It is crucial that calorie intake be balanced among carbohydrates, proteins, and fats. If there is an inadequate amount of carbohydrates to provide energy to break down the proteins and carbohydrates, the body will actually metabolize body proteins and fats through a process called gluconeogenesis to provide glucose energy to use the incoming proteins and fats. Even though patients are receiving adequate total calories, they may develop a protein-wasting condition.

Vitamins, whose name derives from the term vital amines, are a specific set of chemical molecules that regulate human metabolism necessary to maintain health. To be classified as a vitamin, a chemical must be ingested, because the human body does not make sufficient quantities to maintain health, and the lack of a vitamin in the diet produces a specific vitamin deficiency disease (e.g., beriberi is a thiamine deficiency, and scurvy is an ascorbic acid deficiency). To date, 13 compounds have been identified as vitamins, 9 of which are classified as water-soluble and 4 as fat-soluble (Table 47-2). Vitamins were originally named according to letters of the alphabet, but as a result of the diversity of actions of different vitamins, they are commonly referred to by their generic names (e.g., phytonadione is vitamin K, and thiamine is vitamin B1).

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