The term nutrition can have many different meanings. A broad definition is the relationship of food to the well-being of the human body (Krause and Mahan 1984). In scientific terms, nutrition is the study of nutrients and how they are ingested, digested, absorbed, transported, metabolized, stored, and excreted (Whitney and Hamilton 1984). Nutrition also encompasses the study of environmental effects on the quality and safety of foods, and the impact of nutritional factors on health and disease.

The study of human nutrition dates back to the 18th century, when the French chemist Lavoisier discovered the relationship between the metabolism of food and the process of respiration. Lavoisier was later named the father of nutrition, and his discoveries inspired much research on the caloric value of foods during the 19th century. Subsequent investigations led to the discovery of food factors other than carbohydrate, protein, and fat that influenced health (Krause and Mahan 1984). By the beginning of the 20th century, four different diseases (scurvy, beri-beri, rickets, and pellagra) were known to be associated with certain types of diet, which were later discovered to be nutrient deficiencies. In 1912, the Polish chemist Casimir Funk isolated an anti-beri-beri factor (known today as vitamin B₁) from rice husks; Funk coined the term "vitamine" to designate such accessory food factors necessary for life (Combs 1992).

In the early 1940s, Recommended Dietary Allowances (RDAs) were established by the National Research Council. The RDAs define the minimal nutrient intakes necessary for the prevention of basic deficiency diseases (National Research Council 1989). Until recently, these guidelines were used to set nutritional adequacy standards for the general population.

Today, RDAs are being revised and replaced with Dietary Reference Intakes (DRIs). Unlike RDAs, DRIs are designed to maximize health and lower the risk of chronic disease. DRIs are a new approach to nutritional planning and provide quantitative estimates of nutritional thresholds (Anonymous 1997).

DRIs refer to four categories: an Estimated Average Requirement (EAR), a revised Recommended Dietary Allowance (RDA), an Adequate Intake Level (AI), and a Tolerable Upper Intake Level (UL). The EAR is the level at which 50% of healthy individuals in a given life-stage and gender group have their intake needs met. The RDA is the dietary intake level that is sufficient to meet the nutrient requirements of nearly all (97% to 98%) of those in a given group. (For a single individual, the RDA is meant to represent that person's nutritional goal.) When there is insufficient scientific evidence to calculate an EAR (and thus establish an RDA), an AI is used instead. The AI is a level estimated to meet the needs of all individuals in a group. The UL is the maximum level of daily nutrient intake that is considered safe (i.e., unlikely to pose a risk of adverse health effects in 97% to 98% of people) (Anonymous 1997; Yates et al. 1998).

DRI reports are created by the Food and Nutrition Board (Institute of Medicine, National Institute of Health) and by the Canadian national health service (Health Canada); these reports are released periodically with joint recommendations for Americans and Canadians. The demographic or "life stage" groups have been altered to include two groups for people over age 50. The new groups include those between the ages of 51 and 70 years old, and an over-70 group (Anonymous 1997).

Nutritional science is based on the principle that food intake and nutritional status significantly affect health. How significant this impact can be is a matter of controversy. For example, it is well known that adequate nutrition is necessary to prevent classical deficiency diseases such as beri-beri and pellagra (Combs 1992). Likewise, a significant amount of research supports the principle that nutrition can help prevent certain chronic diseases such as osteoporosis and heart disease (NIH Consensus conference 1994; Kendler 1999). It is now recognized that manipulation of food and nutrient intake can impact a wide variety of genetic and environmentally induced diseases. Diabetes and phenylketonuria are two classic examples of genetic diseases where nutritional intervention is crucial to the health of the individual.

Nutritional science principles become controversial when the concepts of "nutritional medicine" and "marginal nutrient deficiencies" are introduced. The concept of nutritional medicine is based on the assumption that food can have medicinal and therapeutic effects, especially when individual nutrients are given in pharmacologic doses (Ghen and Corso 2000). This concept is controversial because it advocates the use of higher levels of nutrients than are available in foods; such nutrients must therefore be provided in supplement form. The concept of marginal nutrient deficiencies is based on the hypothesis that subtle nutrient deficiencies occur before the onset of frank, classical deficiency. Such marginal deficiencies may ultimately contribute to the development of degenerative diseases (Health Media of America and Somer 1992).

According to experts in this field, the question is not whether certain diseases can be prevented or treated with pharmacological doses of vitamins or trace elements, but which diseases can be treated, with which supplements, at which doses, and with what consequences (Werbach 1999).

A wide variety of treatment protocols are used to improve nutritional status. Some of these fall under the definition of fads while others have scientific merit. Billions of dollars are spent each year on such diets as high protein, low carbohydrate, low fat, low protein, Paleolithic, blood typing, zone, starvation, food pyramid, juice, and many others. Unfortunately, these diets do not meet the needs of all individuals and some may be harmful. The optimal diet for improving nutritional status is one that is individualized to the unique needs of the patient (Ghen and Corso 2000). In the clinical setting, treatment is based on scientifically documented protocols that address specific health conditions, such as diabetes and heart disease.

Nutrients are involved in the creation of every molecule in the body. Therefore, their mechanisms of action are as varied as the molecules, cells, and tissues they help to create. The body requires more than 45 nutrients to maintain health (Health Media of America and Somer 1992). The macronutrients (carbohydrates, proteins, and fats) provide energy for bodily processes. Vitamins and minerals are not metabolized for energy; however, they are used as coenzymes and cofactors in the conversion of macronutrients to energy. Many vitamins act as coenzymes in promoting essential chemical reactions (Krause and Mahan 1984). They help regulate metabolism, assist in the formation and maintenance of bones and tissues, hormones, nervous system chemicals, and genetic material (Health Media of America and Somer 1992). Minerals maintain acid-base balance and osmotic pressure, facilitate transfer of essential compounds across membranes, maintain nerve conduction and muscle contraction, regulate the metabolism of many enzymes, and provide structure to bones (Krause and Mahan 1984).

A variety of methods is used to evaluate nutritional status. The following are some of the more common tools used in the clinical evaluation process. In the initial interview, a thorough medical history is taken that includes diet and digestion, diseases and disorders, medications, history of weight loss or gain, sleep and exercise patterns, and relaxation habits (Shils et al. 1994). Information about food allergies, family history, and personal lifestyle may also be taken. Anthropometric measurements of body weight and height, and if necessary skinfold thickness, are sometimes performed. Other factors such as age and life changes (e.g., menopause) are noted during the interview. Laboratory tests are used to determine iron or micronutrient deficiencies, malnutrition, immune status, plasma glucose, and renal and hepatic function (Shils et al. 1994). The next phase is assessing dietary intake and patterns, including diet history, scheduling of meals and snacks, lifestyle habits, meal planning, and dietary supplements. This information is evaluated by the health care professional and used to create a diet and lifestyle plan for the individual. Follow-up steps include setting nutrition goals, reviewing digestion and bowel health, patient self-monitoring, and assessment of patient progress (Begany 1999).

Clinical applications for nutrition intervention vary depending on the needs of the individual. In the hospital setting, the intent is to use nutrition to improve patient outcomes during illness and trauma. In this regard, nutrition is used in the treatment of a wide range of conditions such as AIDS, cancer, diabetes, osteoporosis, heart disease, pulmonary disease, obesity, surgery, burns, metabolic disorders, and kidney, liver, or pancreatic insufficiency. It is not possible to include a complete list of all conditions in this brief monograph or to exhaust the list of healthful dietary nutrients (Deckelbaum et al. 1999).

Eating habits play a major role in the high prevalence of certain chronic diseases such as atherosclerosis, obesity, cancer, and diabetes. Numerous studies suggest a significant benefit from nutritional intervention in the prevention of these conditions (Deckelbaum et al. 1999). Some examples of dietary interventions to achieve specific outcomes include lowering saturated fat and cholesterol intake, in order to lower blood cholesterol levels; reducing caloric intake to lose weight; and reducing intake of simple sugars to maintain normal blood sugar levels.

In a large prospective study of more than 20,000 male physicians, consumption of one fish meal a week correlated with a 52% reduction in risk of sudden death from heart attack (Albert et al. 1998). Results from the Nurses Health Study suggest that increased intake of whole grains may protect against coronary heart disease (Liu et al. 1999). A high intake of dietary fiber, particularly soluble fiber, has been shown to improve glycemic control, decrease hyperinsulinemia, and lower plasma lipid concentrations in patients with type 2 diabetes (Chandalia et al. 2000). A prospective study of 42,254 women found that women who consume a diet that parallels the current dietary guidelines (fruits, vegetables, whole grains, low-fat dairy, and lean meats) have a lower risk of mortality (Kant et al. 2000).

Another aspect of nutritional intervention is the use of individual nutrients and food components to prevent and treat disease. In one study, vitamin E supplements were shown to decrease the risk of angina in patients without previously diagnosed coronary artery disease (CAD), and to decrease nonfatal myocardial infarction and cardiovascular death in patients with established CAD. Conflicting data from other studies, however, imply no beneficial effect of vitamin E supplementation; therefore, a high intake of fruits and vegetables rich in vitamin E has been recommended over supplements (Spencer et al. 1999). A prospective study on 77,466 women found that increased consumption of foods rich in the carotenoids lutein and zeaxanthin was associated with a moderate decrease in risk of cataracts (Chasan-Taber et al. 1999). According to one study, lutein from dietary sources may also help reduce the risk of developing colon cancer (Slattery et al. 2000). A higher intake of foods rich in flavonoids may protect against certain forms of lung cancer (Le Marchand et al. 2000). The typical Western diet is high in omega-6 and low in omega-3 (n-3) fatty acids. The n-3 fatty acids are essential components of cell membranes. Docosahexaneoic acid, or DHA, is an important n-3 fatty acid required for the development of retina and brain, particularly in infants, and for optimal cardiovascular and mental health (Simopoulos 2000). Numerous studies have been conducted to assess the benefit of individual nutrients, such as copper, zinc, selenium, and essential fatty acids, in the treatment of rheumatoid arthritis. Although some studies have found promising results, larger, controlled studies are necessary to confirm any benefits (Gaby 1999).

Risks and side effects associated with nutrition, in otherwise healthy individuals, are most often due to food allergies and pharmacologic doses of dietary supplements. Common food allergens include, but are not limited to, wheat gluten, nuts, eggs, fish, soy, shrimp, bananas, and chicken (Shils et al. 1994). Peanuts, seafood, and milk, as well as a wide spectrum of other foods, can induce potentially lethal anaphylaxis (Wuthrich 2000). Excessive intakes of vitamins A, D, and C, pyridoxine, niacin, and folic acid have been associated with adverse effects (Sinatra and Sinatra 1999). Chemical pollutants in food and public water supplies may pose a potential safety hazard. Certain health conditions such as kidney, liver, and pancreatic insufficiency can increase the risk of adverse reactions from foods (Ghen and Corso 2000).

Nutritional contraindications, such as food allergies, are determined during the clinical evaluation process. Numerous medical conditions require exclusion or manipulation of specific foods and/or nutrients in the diet. For example, simple sugars are contraindicated in diabetic individuals (Ghen and Corso 2000).

Nutritional medicine is a rapidly growing field. Ongoing research is seeking to determine how nutrients affect the aging process, brain chemistry, and diseases such as osteoarthritis, rheumatoid arthritis, inflammatory bowel disease, cancer, and cataracts, among other conditions.

Nutritional science is in constant flux as new research emerges that contradicts older findings. One current controversy involves the relationship of protein intake to risk of ischemic heart disease. A recent study found that replacing carbohydrates with protein may be associated with a lower risk of ischemia (Hu et al. 1999). These findings oppose current practices for the treatment of heart disease that reduce fat intake by limiting protein ingestion and substituting carbohydrates. More research is needed before recommendations on protein intake can be made to the public.

Functional foods—foods that include added vitamins, minerals, herbs, amino acids, and other dietary substances—are now prevalent in the market, touting claims of beneficial health effects on specific functions in the body beyond adequate nutritional effects (USGAO 2000; Katan 1999). Examples of functional foods include calcium-fortified orange juice and soups that contain such ingredients as the herb St. John's wort. The FDA regulates the safety and claims made in the labeling of both functional foods and dietary supplements. However, there is concern that a lack of a clearly defined, consistent safety standard for new ingredients in dietary supplements may make some of these products unsafe for certain consumers. In addition, a lack of scientifically based information on the health benefits of functional foods may make it difficult for consumers to make informed dietary choices (USGAO 2000). The functional foods industry is likely to experience continuous growth as health-conscious consumers seek out new foods to enhance health and prevent disease. This controversial trend will inevitably have an impact on upcoming research and, in turn, on the practice of nutritionists and other healthcare professionals (USGAO 2000).

The level of education and training can vary greatly among nutrition experts. A nutritionist may have a degree from an accredited university or a certificate from a two-month training program. Registered dietitians and formally trained nutritionists have a minimum of four years of education from an accredited university. Nutritional advice is also provided by individuals who mainly practice other disciplines, but have undertaken further training in nutrition. These include naturopaths, nurses, chiropractors, osteopaths, pharmacists, and physicians. Credentialed individuals are the best sources for accurate nutrition advice. Training can be obtained from most organizations of healthcare professionals (Ghen and Corso 2000). Many accredited universities offer higher degrees in nutritional science.

For more information contact the American Dietetic Association in Chicago, Illinois at 800-877-1600 or visit them on the web at www.eatright.org/; the Price-Pottenger Nutrition Foundation at 800-366-3748 or visit the foundation on the web at www.price-pottenger.org/index.html; and the Clinical Nutrition Certification Board in Dallas, Texas at 972-250-2829 or visit the group on the web at www.cncb.org/. In addition, the Tufts University Nutrition Navigator at www.navigator.tufts.edu/ maintains a rating guide to nutrition websites.

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