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
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.
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.
|Mechanism of Action|
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
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
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, Side Effects, Adverse
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
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
|Training, Certification, and
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
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