Nutrition

NUTRITION. Food is comprised of nutrients that are classified by their role in the body: the energy-yielding macronutrients (carbohydrates, protein, and fat), the essential micronutrients (vitamins, minerals, and water), and numerous other components. Although micronutrients do not supply energy to fuel the body, they are indispensable for the proper functioning of the metabolic and regulatory activities in the body. Other nonessential nutrients, such as flavonoids, phytoestrogens, carotenoids, and probiotics, also may have important health-promoting properties, and investigations are ongoing. The daily intake of a variety of foods provides energy and nutrients that are essential to the health and well-being of an individual. The relationships among food intake, nutrition, and health define the field of nutrition. More fully, nutrition is the study of food, its nutrients and chemical components, and how these constituents act and interact within the body to affect health and disease.

The scope of the field has grown in recent years and the boundaries between the science of nutrition and many other biological sciences have blurred. For example, the science of nutrition includes chemistry to study how food ingredients interact with each other; physiology to investigate how nutrients within food are assimilated into body tissues; engineering to design new fortified foods; anthropology to explore why we chose to eat certain foods in centuries past; and psychology to determine what attitudes and behaviors influence our dietary patterns today. Nutritionists often have either a college or advanced degree in nutrition or a related field, whereas clinical (human) nutrition specialists will have graduate degrees, which may include medicine, and have completed an examination for certification. Registered dietitians are nutrition professionals who are often responsible for applying nutritional science to clinical practice to promote health and treat disease. Dietitians frequently work in hospitals but also may be employed in universities, public health departments, restaurants, the food industry, and exercise facilities. Similarly, given the broad scope of the field, other nutrition professionals include but are not limited to physicians, biochemists, anthropologists, epidemiologists, geneticists, food scientists, and engineers.

For this review, the field of nutrition is divided into three major categories: (1) nutrition in research, (2) nutrition in clinical practice, and (3) nutrition in policy and education. An overview of nutritional research is presented, from how nutrients interact within the body and among themselves (nutritional biochemistry), to the investigation of the relationships between specific foods or food groups and the health status of populations (nutritional epidemiology). Research findings in the field provide the information needed to guide nutrition practice for the care of individuals as well as large groups of people. The development of nutrition policy comes from both research and clinical practice advances. Concise descriptions of each are given and a brief history of the field and projected directions of the future of the field are offered.

Nutrition: A Historical Perspective

Numerous advances in the field of nutrition have occurred within the last century. The major focus of nutrition research and practice shifted from concern over which foods are required to avoid nutritional deficiencies and overt illness, to what foods and supplements may be consumed to promote optimal health. Functional foods are a part of the vocabulary, and energy bars, herbal remedies, and nutritional supplement products are now widely available.

In biblical times certain foods were understood to have special healing properties; however, the concept of nutrients as essential for health is relatively new. Recent discoveries in the field have been dependent on the development of scientific methods to analyze nutrient content and interactions. Therefore, though some vitamins were understood to be essential in the early part of the twentieth century, trace elements such as zinc and selenium were not considered essential for humans until the 1970s.

As the field of nutrition has developed, it has also expanded. In 1950 the history of nutrition science during the two previous centuries was summarized by Dr. Elmer McCollum in just under five hundred pages. It would likely take ten volumes of such texts to encapsulate the nutrition-related findings and proceedings from the latter half of the twentieth century. Accomplishments in the field of nutrition over the last century are highlighted in five major eras: (1) food as energy, (2) micronutrient deficiency diseases, (3) nutrition in public policy, (4) nutrition and chronic disease, and (5) nutrition for optimal health.

Food as energy (1880–1920). By the end of the nineteenth century the major, energy-yielding components of food—protein, fat, and carbohydrate—had been identified, and nutrition research, especially concerning the metabolism of proteins and the energy composition of foods, was flourishing. Much of this work had been conducted in animals; therefore, the human nutrition experiments performed by Dr. W. O. Atwater (1844–1907) and colleagues were particularly novel. From their studies, the energy yield of carbohydrate, protein, and fat was derived (4, 4, and 9 kcal per gram, respectively), values that are still used today. Dr. Atwater also developed the first human calorimeter in the United States to measure energy expenditure. However, it was a pair of medical doctors, James Harris and Francis Gano Benedict, who perfected this methodology to establish standards for the energy needs of healthy individuals. Energy expenditure was measured in approximately 250 healthy men and women at the Carnegie Institute Laboratory in Washington, D.C., and equations were derived from the data. The Harris-Benedict energy expenditure prediction equations for men and women, published in 1919, remain some of the most useful tools in clinical nutrition assessment today.

Micronutrient deficiency diseases (1920–1940). The period between 1920 and 1940 brought about a paradigm shift in the understanding of the etiology of some common diseases. Until this time it was thought that all disease resulted from poor sanitation and hygiene; therefore, bacteria, mold, and toxins were identified as the likely cause of disease. As Alfred Harper has suggested, “the concept that a disease might be caused by a deficit of a substance that was nutritionally essential was beyond the grasp even of most nineteenth-century physicians and scientists” (p. 217). In order to combat disease as well as increase shelf life, food was sterilized, milled, and polished to reduce the danger of ingesting bacteria, mold, and toxins. Despite these efforts, pellagra, beriberi, and infantile scurvy actually increased in prevalence. In a number of studies conducted by Dr. Joseph Goldberger from 1914 to the 1920s, where the diets of individuals suffering from pellagra were compared to those of healthy individuals, foods that decreased the presence of diarrhea and dementia in pellagrous individuals were identified. From his work it was later determined that pellagra was due to a diet poor in the vitamin niacin and not infection. At approximately the same time, Dr. Christiaan Eijkman (1858–1930) won a Nobel Prize in medicine (1929) for the discovery of the “antineuritic” vitamin thought to be responsible for curing beriberi. Through his experiments, in which chickens were fed human hospital diets, combined with studies of beriberi in prisoners who survived on polished rice, he hypothesized that the hull of the rice grain contained an antidote to the neurological disorder. Although not completely correct, his observations led to the discovery of the essential vitamin thiamin.

As Kenneth J. Carpenter summarized, “new technologies of food processing that have obvious advantages may also have a downside” (p. 227). While technology decreased infectious disease and increased the shelf life of food products, it inadvertently led to nutritional deficiencies. The heat-sterilization of cow’s milk, which destroyed vitamin C, was related to the outbreak of infantile scurvy in well-to-do families. The practices of polishing rice and degerming corn to increase grain stability also led to increased prevalence of beriberi (thiamin deficiency) and pellagra (niacin deficiency), respectively.

Nutrition in public policy (1920–1964). One of the most fruitful periods in the history of public health nutrition followed on the coattails of World War I. It became possible to manufacture the micronutrients that had been identified by chemists as essential for health cheaply and efficiently. In 1922 the first of a series of public health efforts at eradicating nutrient deficiency in the United States was initiated by the voluntary addition of iodine to salt (see Table 1). The fortification of other foods was used to address rampant public health problems such as rickets (vitamin D), beriberi (thiamin), pellagra (niacin), and dental caries (fluoride). Since the initiation of fortification policies in the United States, clinically evident nutritional deficiencies have been virtually eliminated.

The first attempt at defining nutritional requirements was directed toward the prevention of nutrient deficiencies in military personnel during World War II. In the early 1940s the Food and Nutrition Board of the National Academy of Sciences reviewed the scientific evidence and developed the Recommended Dietary Allowances for energy, protein, and eight essential vitamins and minerals. The first national food supplementation program was initiated in 1946 (National School Lunch Act) to improve the dietary intake of children from economically disadvantaged families. Other national food assistance programs were added over the next fifty years.

Nutrition and chronic disease (1960–1990). The last forty years of the twentieth century saw continued discovery in the field of nutritional biochemistry and a new research emphasis on the role of nutrition in the cause of and treatment for chronic disease. Disease patterns shifted from infectious and nutrient deficiency diseases to increasing rates of cardiovascular disease, diabetes, cancer, and osteoporosis. Nutrient deficiencies, when present, were often secondary to restrictive dietary habits, economic deprivation, or the presence of another disease that altered nutrient metabolism. The more pressing problem now was the change in the American lifestyle and a dietary shift from too little to too much. Modern household technologies increased productivity in housework but decreased physical activity, and the home-cooked family meal became a thing of the past. Varied diets consisting of whole grains, fruits, and vegetables gave way to convenience foods resulting in a much higher consumption of fat and sugar. Results from the Framingham Heart Study were perhaps the first glimpse into the relationship between fat intake and cardiovascular disease and the realization that each type of fat plays a specific role in health and disease. During this era, links among fat intake, serum cholesterol, and cardiovascular disease were studied thoroughly, and the reasons for the increasing prevalence of obesity in the United States were explored. In 1985 Michael Brown and Joseph Goldstein were awarded the Nobel Prize in medicine for their work on the regulation of cholesterol metabolism and its influence on arteriosclerosis.

The essentiality of macrominerals (e.g., calcium, phosphorus, sodium) was understood in the 1850s. However, it was not until technological advances triggered an explosion of new research that trace and ultra-trace elements were identified as essential for humans. Working together, nutritionists, biochemists, biologists, immunologists, geneticists, and epidemiologists uncovered the mysteries behind minerals such as zinc, selenium,

copper, molybdenum, and chromium. Scientists first recognized human zinc deficiency in the mid-1960s. Severely growth-retarded, young Middle Eastern men were anemic, extremely lethargic, and hypogonadal. Their diet consisted mainly of wheat bread with little animal protein. When their diets were supplemented with zinc, their lethargy, growth, and genital development improved.

Nutrition for optimal health (1990–present). In the understanding of nutrition, the American public experienced yet another paradigm shift in the 1990s. They wondered if all nutrients that provided a health benefit needed to fit the traditional definition of “essential nutrient.” As a result of this question, herbal and botanical extracts, phytochemicals, and other alternative nutritional therapies to promote optimum health were explored. In 1999 the U.S. market for functional foods alone was estimated to be $6 billion (Hasler, p. 504) and it continues to grow by approximately 12 percent each year. The explosion of this market is likely due to the increase in social acceptance, changes in regulations, the booming economy of the 1990s, and the targeting of products to particular populations. The scientific validation of some therapies also is of increasing interest.

Pharmacological uses (larger amounts than required to prevent deficiencies) of essential nutrients are being explored. Although much of the current interest in megavitamin supplementation began in the 1990s, the work of Dr. Linus Pauling in the 1970s initiated the movement. Pauling was the only individual to be awarded two unshared Nobel prizes for his work in chemistry (1954) and peace (1962). In the field of nutrition, however, he is noted most for his unproven theories regarding the potential protective role of vitamin C on the common cold, cancer, and heart disease. Pauling himself reportedly took up to six hundred times the recommended daily amount of vitamin C. Given that many individuals also practice a “more must be better” approach, the national recommendations for nutrient intake now include guidelines for safe upper limits for individual nutrient intakes.

Nutrition in Research

Experimental nutrition research is one aspect of the science of nutrition. Nutrition research is conducted to answer questions raised both in clinical practice and policy. Research in nutrition can focus on individual cells, whole animals or humans, or entire populations, and often overlaps with research in genetics, biochemistry, molecular biology, toxicology, immunology, physiology, and pharmacology.

Nutritional biochemistry. Nutritional biochemistry is the backbone to the understanding of the structure and function of nutrients within food and the body. Nutrients serve as cofactors for enzymes, components of hormones, and participants in oxidation/reduction reactions through metabolic processes. Though required in small amounts, nutrients are essential for body growth, sexual development and reproduction, psychological well-being, energy level, and the normal functioning of most organ systems in the body. Nutritional biochemists study the functional roles of vitamins and minerals in the body, metabolic blocks that occur from deficiencies, the effects of hormones on nutrient metabolism, and interactions among nutrients within the body. In the 1990s a whole new area of research emerged that focuses on relationships between nutrition and genetics. An example of this type of study includes the identification of a genetic defect in folate metabolism (C677T), which increases a woman’s risk of delivering a baby with a neural tube defect.

Food science. Food science is the study of the composition of food materials and the reaction of food to processing, cooking, packaging, and storage. Food science integrates knowledge of the chemical composition of food materials; their physical, biological, and biochemical behavior; the interaction of food components with each other and their environment; pharmacology and toxicology of food materials, additives, and contaminants; and the effects of manufacturing operations, processes, and storage conditions.

The potential beneficial role of functional foods in the American diet has gained attention and recent food science research focuses on the development of such foods. Functional foods are generally defined as those that provide health benefits beyond basic nutrition, and include fortified, enriched, or enhanced foods, and whole foods, which have high levels of protective nutrient components. Examples of these foods include orange juice with added calcium or echinacea, or snack foods with antioxidants, fruit-flavored candy with vitamin C, various soy products, and margarine with added plant sterols. Factors that drive the market for such foods include a growing general public interest in nutrition and its impact on health, an aging population that is more concerned with health, research findings receiving media attention, and an increasingly unregulated consumer food market.

Human nutrition. Human nutrition, or clinical nutrition, research is that which focuses on the study of nutrients within the living human body. Although biochemical studies are extremely informative, until the nutrient is added to or depleted from the diet, the effects on individuals can only be hypothesized. Human nutrition research includes the study of individual nutrient requirements (e.g., nutrient intake assessment, energy expenditure assessment, nutrient turnover balance studies, and nutrient bioavailability), the effects of nutrients on body growth (e.g., body composition techniques, anthropometry, pubertal assessment), and the dietary, physiological, or disease factors that influence nutrient requirements. In the 1990s one important human nutrition study found that increasing folic acid intake in young women reduces the incidence of neural tube defects (spina bifida) in their babies.

Nutritional epidemiology. Nutritional epidemiology is the science of systematically studying the relationships between food choices and health status. Epidemiological studies are particularly valuable in understanding complex relationships between food intake (dietary exposure) and determinants of diseases with multiple etiologies and long latent periods. Examples of such studies include the relationships between low folic acid intake and increased incidence of spina bifida, and elevated saturated fat intake and elevated risk of arteriosclerosis. There are, however, limitations to these studies in that they describe relationships rather than prove cause and effect. Frequently, clinical trials and intervention studies are used as follow-up studies to evaluate more fully the questions raised by epidemiological evidence.

Nutrition in Clinical Practice

Scientific evidence continues to mount regarding the key roles that nutrients and their metabolism play in the prevention of the most common chronic diseases. Half of the leading causes of death in the United States (heart disease, cancer, stroke, and diabetes) are associated strongly with unhealthy eating habits. Clinical nutrition is the practice of applying research evidence to aid in the care of individuals with or at risk for diet-related diseases. These principles are used to develop individualized nutrition care plans. Generally, diseases may affect nutritional status by (a) decreasing the intake of nutrients, (b) altering the metabolism of nutrients (or unusual losses), or (c) altering energy expenditure. Alternatively, as mentioned briefly above, poor nutritional status can lead to disease. For example, zinc deficiency can decrease the function of the immune system that in turn leads to increased risk for diarrhea and infectious diseases.

Assessment of nutritional status is essential for identifying undernourished and overnourished states (obesity is now a major health problem) and estimating the optimum intake to promote normal growth and well-being. Nutritional assessment has several components, including the evaluation of dietary intake, growth status, body composition, energy expenditure, and biochemical measures of nutritional status in the context of a medical history, diagnoses, and current therapy. These data are used to develop individualized nutritional care plans, which may include recommendations for total energy intake, adjustments in the diet to increase or decrease the consumption of certain foods, and possibly the inclusion of nutrient supplements. For patients who cannot be fed orally, more technology-based nutritional support is used to maintain or improve nutrient intakes and nutritional status. This involves either feeding the patient through a tube directly into the stomach or intestine (enteral) or through an intravenous line directly into the bloodstream (parenteral). Because malnutrition will add to complications of illness and prolong the illnesses and hospitalization, appropriate assessment of the patient is extremely important. In the complex and rapidly changing context of critical illness, individualized nutrition assessments are crucial and require the sequential monitoring of all patients to maintain appropriate nutritional care plans.

It is unlikely that individuals who have not been seriously ill have had the opportunity to seek the counsel of a trained nutritional professional for developing an individualized diet plan. The average American displays a keen interest in how nutrition affects his or her health, and is disappointed with the information physicians are able to provide because traditional medical training has limited nutrition content. Therefore, greater numbers of individuals are seeking nutrition information for themselves, and using the information to self-diagnose and self-prescribe. The advances in communications technology, particularly the explosion of information on the World Wide Web, allow the ready accessibility of sound nutritional advice, and substantial amounts of quackery. Without training and a significant amount of time dedicated to the task, it is difficult to decipher truth from fraud. Future directions in nutritional education likely will include tools to aid Americans in deciphering information, particularly from the Internet, in order to make educated choices to optimize their diets and live healthier lives (see Table 2).

Nutrition in Public Policy: Monitoring and Education

Nutrition in public health or nutrition policy generally is regarded as the combined efforts taken toward improving

nutrition and health status of populations. With increasing emphasis on health promotion and disease prevention, there is a proliferation of nutrition-related disease prevention, screening, and education programs targeted at increasing fiber, fruit, and vegetable intake, and reducing saturated fat intake. Additionally, a number of food assistance programs and mandated food fortification programs have been instituted, all promoting a healthy diet and lifestyle.

Nutrition research, public policy programs, and nutrition surveillance systems work synergistically like spokes on a wheel. Evidence obtained from scientific research is used to set nutritional recommendations such as the Dietary Reference Intakes and the Dietary Guidelines for Americans. These standards are used to judge the adequacy of the American diet, provide the basis for nutrition labeling of foods, formulate special diets, and guide the development of food fortification and nutrition policy developed to assist those who are at nutritional risk. Specific food assistance programs (such as, food stamps, Special Supplementary Nutrition Program for Women, Infants, and Children) are targeted at specific economically disadvantaged and nutritionally at-risk populations. Fortification programs generally are less specific, but some target at-risk populations through specific foods, for example, vitamin D–fortified milk to prevent rickets in young children. Finally, the wheel is completed by nutrition monitoring programs that are used to evaluate the effectiveness of instituted policies. The National Health and Nutrition Examination Survey (NHANES) and the Continuing Survey of Food Intake of Individuals (CSFII) are ongoing monitoring tools used to assess the population’s nutrient intakes, nutrition and health status, and knowledge and attitudes about health.

Perhaps most important, public health nutrition includes the dissemination of scientific findings, the explanation of dietary recommendations, and outreach of federal assistance programs. The responsibility of communicating experimental findings in an understandable form falls on nutrition scientists, journalists, educators, and the public. The scientists are responsible for interpreting the research findings into a form that is understandable to the general public. Journalists are responsible for communicating the scientific message in an objective way, and the public is responsible for pursuing an accurate understanding of the issues. Various government agencies have the responsibility to organize and administrate the myriad of nutritional policies and programs, and to communicate information regarding these programs to the public.

The Future of Nutrition and Food Science

In the twentieth century nutrition research, practice, and public policy shifted from a focus on the quantitative aspects—to ensure food security and eradicate nutritional deficiencies—to a greater attention on the qualitative aspects—to achieve optimal, balanced, dietary intakes. In the twenty-first century nutrition research, practice, and policy will likely explore the following areas:
relationships between human genetics and nutrition, the role of genetically modified foods in human health,
the relationship of nonfood substances in the promotion of health and the bioengineering of functional foods,
the promotion of economic growth and food security in developing nations to prevent or delay the undesirable health effects of malnutrition, and
the prevention and treatment of the obesity epidemic in children and adults.

Relationships between food intake and human health will continue to be of great public interest, and nutrition and food scientists will face new challenges in a fasterchanging environment.

See also Assessment of Nutritional Status; Dietary Assessment; Dietary Guidelines; Dietary Systems: A Historical Perspective; Dietetics; Enteral and Parenteral Nutrition; Food Stamps; Functional Foods; Malnutrition; Nutrients; Nutritionists; Nutrition Transition: Worldwide Diet Change; Obesity;Physical Activity and Nutrition; Vitamins; WIC (Women, Infants, and Children’s) Program.

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Ellen B. Fung Virginia A. Stallings

Professional Nutrition Credentials in the United States

Definition of Terms

Nutrition: the study of foods, their nutrients, and other chemical components; their actions and interactions in the body; and their influence on health and disease.

Nutritional Science: the body of scientific knowledge that relates to the processes involved in nutrition.

Health: a state of optimal well-being—physical, mental, and social; relative freedom from disease.

Functional Foods: foods that provide a health benefit beyond basic nutrition.

Essential Nutrient: a substance that must be obtained from the diet because the body either cannot make it or cannot make adequate amounts.

Enteral Nutrition: nutrient solutions delivered into the gastrointestinal tract (e.g., stomach, small intestine) through a tube inserted through the nose or directly into the stomach.

Parenteral Nutrition: nutrient solutions delivered directly into the bloodstream through an intravenous catheter.

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