Harrison's Principle of Internal Medicine 17th Edition. 2008
INTRODUCTION
Nutrients are substances that must be supplied by the diet because they are not synthesized in the body in sufficient amounts. Nutrient requirements for groups of healthy persons have been determined experimentally. For good health we require energy-providing nutrients (protein, fat, and carbohydrate), vitamins, minerals, and water. Specific nutrient requirements include 9 essential amino acids, several fatty acids, 4 fat-soluble vitamins, 10 water-soluble vitamins, and choline. Several inorganic substances, including 4 minerals, 7 trace minerals, 3 electrolytes, and the ultratrace elements, also must be supplied in the diet.
The required amounts of the essential nutrients differ by age and physiologic state. Conditionally essential nutrients are not required in the diet but must be supplied to individuals who do not synthesize them in adequate amounts, such as those with genetic defects, those having pathologic states with nutritional implications, and developmentally immature infants. Many organic phytochemicals and zoochemicals present in foods have health effects. For example, dietary fiber has beneficial effects on gastrointestinal function. Other bioactive food constituents or contaminants such as lead may have negative health effects.ESSENTIAL NUTRIENT REQUIREMENTS
For weight to remain stable, energy intake must match energy output. The major components of energy output are resting energy expenditure (REE) and physical activity; minor sources include the energy cost of metabolizing food (thermic effect of food or specific dynamic action) and shivering thermogenesis (e.g., cold-induced thermogenesis). The average energy intake is about 2800 kcal/d for American men and about 1800 kcal/d for American women, although these estimates vary with body size and activity level. Formulas for estimating REE are useful for assessing the energy needs of an individual whose weight is stable. Thus, for males, REE = 900 + 10w, and for females, REE = 700 + 7w, where w is weight in kilograms. The calculated REE is then adjusted for physical activity level by multiplying by 1.2 for sedentary, 1.4 for moderately active, or 1.8 for very active individuals. The final figure provides a rough estimate of total caloric needs in a state of energy balance. Formulas to provide more precise estimates of energy requirements are provided by the Food and Nutrition Board,
Dietary protein consists of both essential and other amino acids that are required for protein synthesis. The nine essential amino acids are histidine, isoleucine, leucine, lysine, methionine/cystine, phenylalanine/tyrosine, threonine, tryptophan, and valine. All amino acids can be used for energy, and certain amino acids (e.g., alanine) can also be used for gluconeogenesis. When energy intake is inadequate, protein intake must be increased, since ingested amino acids are diverted into pathways of glucose synthesis and oxidation. In extreme energy deprivation, protein-calorie malnutrition may ensue.
For adults, the recommended dietary allowance (RDA) for protein is about 0.6 g/kg desirable body weight per day, assuming that energy needs are met and that the protein is of relatively high biologic value. Current recommendations for a healthy diet call for at least 10–14% of calories from protein. Biologic value tends to be highest for animal proteins, followed by proteins from legumes (beans), cereals (rice, wheat, corn), and roots. Combinations of plant proteins that complement one another in biologic value or combinations of animal and plant proteins can increase biologic value and lower total protein requirements.
Protein needs increase during growth, pregnancy, lactation, and rehabilitation after malnutrition. Tolerance to normal amounts of dietary protein is decreased in renal insufficiency and liver failure, precipitating encephalopathy in patients with cirrhosis of the liver.
Fats are a concentrated source of energy and constitute on average 34% of calories in
For adults, 1.0–1.5 mL water per kcal of energy expenditure is sufficient under usual conditions to allow for normal variations in physical activity, sweating, and solute load of the diet. Water losses include 50–100 mL/d in the feces, 500–1000 mL/d by evaporation or exhalation, and, depending on the renal solute load, >1000 mL/d in the urine. If external losses increase, intakes must increase accordingly to avoid underhydration. Fever increases water losses by approximately 200 mL/d per °C; diarrheal losses vary but may be as great as 5 L/d with severe diarrhea. Heavy sweating and vomiting also increase water losses. When renal function is normal and solute intakes are adequate, the kidneys can adjust to increased water intake by excreting up to 18 L/d of excess water. However, obligatory urine outputs can compromise hydration status when there is inadequate intake or when losses increase in disease or kidney damage.
Infants have high requirements for water because of their large ratio of surface area to volume, the limited capacity of the immature kidney to handle high renal solute loads, and their inability to communicate their thirst. During pregnancy, 30 mL/d additional water is needed. During lactation, milk production increases water requirements by approximately 1000 mL/d, or 1 mL for each mL of milk produced. Special attention must be paid to the water needs of the elderly, who have reduced total body water and blunted thirst sensation, and may be taking diuretics.
Other NutrientsSee Other chapter for a detailed description of vitamins and trace minerals.
DIETARY REFERENCE INTAKES AND RECOMMENDED DIETARY ALLOWANCES
Fortunately, human life and well-being can be maintained within a fairly wide range for most nutrients. However, the capacity for adaptation is not infinite—too much of a nutrient, as well as too little, may have adverse effects on health. Therefore, quantitative benchmark recommendations on nutrient intakes have been developed to guide clinical practice. These estimates are collectively referred to as the dietary reference intakes (DRIs). The DRIs supplant but include the recommended dietary allowances (RDAs), the single reference values used in the
Dietary Reference Intakes: Recommended Intakes for Individuals—Vitamins.
Click to enlarge the table
Note: This table presents recommended dietary allowances (RDAs) in bold type and adequate intakes (AIs) in ordinary type. RDAs and AIs may both be used as goals for individual intake. RDAs are set to meet the needs of almost all individuals (97 to 98%) in a group. For healthy breastfed infants, the AI is the mean intake. The AI for other life stage and gender groups is believed to cover needs of all individuals in the group, but lack of data or uncertainty in the data prevent being able to specify with confidence the percentage of individuals covered by this intake.
aAs retinol activity equivalents (RAEs). 1 RAE = 1 µg retinol, 12 µg β-carotene, 24 µg α-carotene, or 24 µg β-cryptoxanthin. To calculate RAEs from retinol equivalents (REs) of provitamin A carotenoids in foods, divide the REs by 2. For preformed vitamin A in foods or supplements and for provitamin A carotenoids in supplements, 1 RE = 1 RAE.
bAs calciferol. 1 µg calciferol = 40 IU vitamin D.
cIn the absence of adequate exposure to sunlight.
dAs α-tocopherol. α-Tocopherol includes RRR-α-tocopherol, the only form of α-tocopherol that occurs naturally in foods, and the 2R-stereoisomeric forms of α-tocopherol (RRR-, RSR-, RRS-, and RSS-α-tocopherol) that occur in fortified foods and supplements. It does not include the 2S-stereoisomeric forms of α-tocopherol (SRR-, SSR-, SRS-, and SSS-α-tocopherol), also found in fortified foods and supplements.
eAs niacin equivalents (NE). 1 mg of niacin = 60 mg of tryptophan; 0–6 months = preformed niacin (not NE).
fAs dietary folate equivalents (DFEs). 1 DFE = 1 µg food folate = 0.6 µg of folic acid from fortified food or as a supplement consumed with food = 0.5 µg of a supplement taken on an empty stomach.
gAlthough AIs have been set for choline, there are few data to assess whether a dietary supply of choline is needed at all stages of the life cycle, and it may be that the choline requirement can be met by endogenous synthesis at some of these stages.
hBecause 10 to 30% of older people may malabsorb food-bound B12, it is advisable for those >50 years to meet their RDA mainly by consuming foods fortified with B12 or a supplement containing B12.
iIn view of evidence linking inadequate folate intake with neural tube defects in the fetus, it is recommended that all women capable of becoming pregnant consume 400 µg from supplements or fortified foods in addition to intake of food folate from a varied diet.
jIt is assumed that women will continue consuming 400 µg from supplements or fortified food until their pregnancy is confirmed and they enter prenatal care, which ordinarily occurs after the end of the periconceptional period—the critical time for formation of the neural tube.
Table 2. Dietary Reference Intakes: Recommended Intakes for Individuals—Elements.
Note: This table presents recommended dietary allowances (RDAs) in bold type and adequate intakes (AIs) in ordinary type. RDAs and AIs may both be used as goals for individual intake. RDAs are set to meet the needs of almost all individuals (97 to 98%) in a group. For healthy breastfed infants, the AI is the mean intake. The AI for other life stage and gender groups is believed to cover needs of all individuals in the group, but lack of data or uncertainty in the data prevent being able to specify with confidence the percentage of individuals covered by this intake.
Source: Food and Nutrition Board, Institute of Medicine—National Academy of Sciences Dietary Reference Intakes, 2000, 2002, reprinted with permission. Courtesy of the National Academy Press, Washington, DC. http://www.nap.eduEstimated Average Requirement
When florid manifestations of the classic dietary deficiency diseases such as rickets, scurvy, xerophthalmia, and protein-calorie malnutrition were common, nutrient adequacy was inferred from the absence of their clinical signs. Later, it was determined that biochemical and other changes were evident long before the clinical deficiency became apparent. Consequently, criteria of nutrient adequacy are now based on biologic markers when they are available. Priority is given to sensitive biochemical, physiologic, or behavioral tests that reflect early changes in regulatory processes or maintenance of body stores of nutrients. Current definitions focus on the amount of a nutrient that minimizes the risk of chronic degenerative diseases.
The EAR is the amount of a nutrient estimated to be adequate for half of the healthy individuals of a specific age and sex. The types of evidence and criteria used to establish nutrient requirements vary by nutrient, age, and physiologic group. The EAR is not useful clinically for estimating nutrient adequacy in individuals because it is a median requirement for a group; 50% of individuals in a group fall below the requirement and 50% fall above it. Thus, a person with a usual intake at the EAR has a 50% risk of an inadequate intake. For these reasons, other standards, described below, are more useful for clinical purposes.
Recommended Dietary Allowances
The RDA is the nutrient-intake goal for planning diets of individuals; it is used in the MyPyramid food guide of the U.S. Department of Agriculture (USDA), therapeutic diets, and descriptions of the nutritional content of processed foods and dietary supplements. The nutrient content in a food is stated by weight or as a percentage of the daily value (DV), a variant of the RDA that, for an adult, represents the highest RDA for an adult consuming 2000 kcal/d.
The RDA is the average daily dietary intake level that meets the nutrient requirements of nearly all healthy persons of a specific sex, age, life stage, or physiologic condition (such as pregnancy or lactation).
The RDA is defined statistically as 2 standard deviations (SD) above the EAR to ensure that the needs of most individuals are met.
The risk of dietary inadequacy increases as intake falls further below the RDA. However, the RDA is an overly generous criterion for evaluating nutrient adequacy. For example, by definition the RDA exceeds the actual requirements of all but about 2 to 3% of the population. Therefore, many people whose intake falls below the RDA may still be getting enough of the nutrient.
It is not possible to set an RDA for some nutrients that do not have an established EAR. In this circumstance, the AI is based on observed, or experimentally determined, approximations of nutrient intakes in healthy people. In the DRIs established to date, AIs rather than RDAs are proposed for infants up to age 1 year, as well as for calcium, chromium, vitamin D, fluoride, manganese, pantothenic acid, biotin, choline, sodium, chloride, potassium, and water for persons of all ages.
Tolerable Upper Levels of Nutrient Intake
Healthy individuals derive no established benefit from consuming nutrient levels above the RDA or AI. Excessive nutrient intake can disturb body functions and cause acute, progressive, or permanent disabilities. The tolerable UL is the highest level of chronic nutrient intake (usually daily) that is unlikely to pose a risk of adverse health effects for most of the population. Data on the adverse effects of large amounts of many nutrients are unavailable or too limited to establish a UL. Therefore, the lack of a UL does not mean that the risk of adverse effects from high intake is nonexistent. Individual nutrients in foods that most people eat rarely reach levels that exceed the UL. However, nutritional supplements provide more concentrated amounts of nutrients per dose and, as a result, pose a greater potential risk of toxicity. Nutrient supplements are labeled with "Supplement Facts" that express the amount of nutrient in absolute units or as the percent of the DV provided per recommended serving size. Total nutrient consumption, including both food and supplements, should not exceed RDA levels.FACTORS ALTERING NUTRIENT NEEDS
The DRIs are affected by age, sex, rate of growth, pregnancy, lactation, physical activity, composition of diet, coexisting diseases, and drugs. When only slight differences exist between the requirements for nutrient sufficiency and excess, dietary planning becomes more difficult.
Growth, strenuous physical activity, pregnancy, and lactation increase needs for energy and several essential nutrients, including water. Energy needs rise during pregnancy, due to the demands of fetal growth, and during lactation, because of the increased energy required for milk production. Energy needs decrease with loss of lean body mass, the major determinant of REE. Because both health and physical activity tend to decline with age, energy needs in older persons, especially those over 70, tend to be less than those of younger persons.
Dietary composition affects the biologic availability and utilization of nutrients. For example, the absorption of iron may be impaired by high amounts of calcium or lead; non-heme iron uptake may be impaired by the lack of ascorbic acid and amino acids in the meal. Protein utilization by the body may be decreased when essential amino acids are not present in sufficient amounts. Animal foods, such as milk, eggs, and meat, have high biologic values with most of the needed amino acids present in adequate amounts. Plant proteins in corn (maize), soy, and wheat have lower biologic values and must be combined with other plant or animal proteins to achieve optimal utilization by the body.
The RDAs apply only to oral intakes. When nutrients are administered parenterally, similar values can sometimes be used for amino acids, carbohydrates, fats, sodium, chloride, potassium, and most of the vitamins, since their intestinal absorption is nearly 100%. However, the oral bioavailability of most mineral elements may be only half that obtained by parenteral administration. For some nutrients that are not readily stored in the body, or cannot be stored in large amounts, timing of administration may also be important. For example, amino acids cannot be used for protein synthesis if they are not supplied together; instead they will be used for energy production.
Specific dietary deficiency diseases include protein-calorie malnutrition; iron, iodine, and vitamin A deficiency; megaloblastic anemia due to vitamin B12 or folic acid deficiency; vitamin D–deficiency rickets; scurvy due to lack of ascorbic acid; beriberi due to lack of thiamine; and pellagra due to lack of niacin and protein. Each deficiency disease is characterized by imbalances at the cellular level between the supply of nutrients or energy and the body's nutritional needs for growth, maintenance, and other functions. Imbalances in nutrient intakes are recognized as risk factors for certain chronic degenerative diseases, such as saturated and trans-fat and cholesterol in coronary artery disease; sodium in hypertension; obesity in hormone-dependent endometrial and breast cancers; and ethanol in alcoholism. However, the etiology and pathogenesis of these disorders are multifactorial, and diet is only one of many risk factors. Osteoporosis, for example, is associated with calcium deficiency as well as risk factors related to environment (e.g., smoking, sedentary lifestyle), physiology (e.g., estrogen deficiency), genetic determinants (e.g., defects in collagen metabolism), and drug use (chronic steroids) .
DIETARY ASSESSMENT
In clinical situations, nutritional assessment is an iterative process that involves (1) screening for malnutrition; (2) assessing food and dietary supplement intake, and establishing the absence or presence of malnutrition and its possible causes; and (3) planning for the most appropriate nutritional therapy. Some disease states affect the bioavailability, requirements, utilization, or excretion of specific nutrients. In these circumstances, specific measurements of various nutrients may be required to ensure adequate replacement.
Most health care facilities have a nutrition screening process in place for identifying possible malnutrition after hospital admission. Nutritional screening is required by the Joint Commission on Accreditation of Healthcare Organizations (JCAHO), but there are no universally recognized or validated standards. The factors that are usually assessed include abnormal weight for height or body mass index (e.g., BMI <18.5>25); reported weight change (involuntary loss or gain of >5 kg in the past 6 months); diagnoses with known nutritional implications (metabolic disease, any disease affecting the gastrointestinal tract, alcoholism, and others); present therapeutic dietary prescription; chronic poor appetite; presence of chewing and swallowing problems or major food intolerances; need for assistance with preparing or shopping for food, eating, or other aspects of self care; and social isolation. Reassessment of nutrition status should occur periodically in hospitalized patients—at least once every week.
A more complete dietary assessment is indicated for patients who exhibit a high risk of malnutrition based on nutrition screening. The type of assessment varies with the clinical setting, severity of the patient's illness, and stability of his or her condition.
Acute care settings, anorexia, various diseases, test procedures, and medications can compromise dietary intake. Under such circumstances, the goal is to identify and avoid inadequate intake and ensure appropriate alimentation. Dietary assessment focuses on what patients are currently eating, whether they are able and willing to eat, and whether they experience any problems with eating. Dietary intake assessment is based on information from observed intakes; medical record; history; clinical examination; and anthropometric, biochemical, and functional status. The objective is to gather enough information to establish the likelihood of malnutrition due to poor dietary intake or other causes and to assess whether nutritional therapy is indicated.
Simple observations may suffice to suggest inadequate oral intake. These include dietitians' and nurses' notes, the amount of food eaten on trays, frequent tests and procedures that are likely to cause meals to be skipped, nutritionally inadequate diet orders such as clear liquids or full liquids for more than a few days, fever, gastrointestinal distress, vomiting, diarrhea, a comatose state, and diseases or treatments that involve any part of the alimentary tract. Acutely ill patients with diet-related diseases such as diabetes require assessment because an inappropriate diet may exacerbate these conditions and adversely affect other therapies. Abnormal biochemical values [serum albumin levels <35>
Most therapeutic diets offered in hospitals are calculated to meet individual nutrient requirements and the RDA. However, there are exceptions including clear liquids, some full liquid diets, and test diets, which are inadequate for several nutrients and should not be used, if possible, for more than 24 h. As much as half of the food served to hospitalized patients is not eaten, so it cannot be assumed that the intakes of hospitalized patients are adequate. Dietary assessment should compare how much and what food the patient has consumed with the diet that has been provided. Major deviations in intakes of energy, protein, fluids, or other nutrients of special concern for the patient's illness should be noted and corrected.
Nutritional monitoring is especially important for patients who are very ill and who have extended lengths of stay. Patients who are fed by special enteral and parenteral routes also require special nutritional assessment and monitoring by physicians with training in nutrition support and/or dietitians with certification in nutrition support.
The aim of dietary assessment in the outpatient setting is to determine whether the patient's usual diet is a health risk in itself or if it contributes to existing chronic disease-related problems. Dietary assessment also provides the basis for planning a diet that fulfills therapeutic goals while ensuring patient adherence. The outpatient dietary assessment should review the adequacy of present and usual food intakes, including vitamin and mineral supplements, medications, and alcohol, as all of these may affect the patient's nutritional status. The assessment should focus on the dietary constituents that are most likely to be involved or compromised by a specific diagnosis, as well as any comorbidities that are present. More than one day's intake should be reviewed to provide a better representation of the usual diet.
There are many ways to assess the adequacy of the patient's habitual diet. These include a food guide, a food exchange list, a diet history, or a food frequency questionnaire. A commonly used food guide for healthy persons is the USDA's food pyramid, which is useful as a basis for identifying inadequate intakes of essential nutrients, as well as likely excesses in fat, saturated fat, sodium, sugar, and alcohol (Table 3). The guide is available online (www.MyPyramid.gov) and can be tailored to the needs of persons of different ages and life stages by varying the number of servings. The process of reviewing the guide with patients helps to identify food groups eaten in excess of recommendations or in insufficient quantities and helps them to transition to healthier dietary patterns. For those prescribed therapeutic diets, assessment against prescriptions stated as food exchange lists may be useful. These include, for example, the American Diabetes Association food exchange lists for diabetes, or the American Dietetic Association food exchange lists for renal disease.
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| Abbreviation: oz eq, ounce equivalent. Source: Data from United States Department of Agriculture. http://www.MyPyramid.com |
Nutritional Status Assessment
Full nutritional status assessment is reserved for seriously ill patients and those at very high nutritional risk when the cause of malnutrition is still uncertain after initial clinical evaluation and dietary assessment. It involves multiple dimensions, including documentation of dietary intake, anthropometric measurements, biochemical measurements of blood and urine, clinical examination, health history, and functional status.GLOBAL CONSIDERATION
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