For several decades after the concept of nutritional essentiality was established in the early 1900s, foods were primarily considered to be sources of essential nutrients required for critical physiologic functions that, if impaired by dietary deficiencies, caused specific diseases. Except for the debilitating effects of malnutrition, little consideration was given during this time to the idea that the type of diet consumed might influence development of diseases other than those caused by inadequate intakes of essential nutrients. By the 1950s, dietary deficiency diseases were virtually eliminated in industrialized nations. Improvements in nutrition, sanitation, and control of infectious diseases had resulted in immense improvements in health; life expectancy had lengthened, and chronic and degenerative diseases had become the major causes of death. This aroused interest in the possibility that susceptibility to such diseases might be influenced by the type of diet consumed.
Associations observed subsequently between diet composition, intakes of various individual diet components, and the incidence of heart disease and cancer have implicated food constituents such as fatty acids, fiber, carotenoids, various nonnutrient substances in plants, and high intakes of some essential nutrients (especially vitamins E and C, which can function as antioxidants) as factors influencing the risk of developing these diseases ( 6) (see CMp.te.L76,, .Chapter— and CMp.t.e,r...8..1). This has led to proposals for modifying the criteria for essentiality or conditional essentiality to include dietary constituents reported to reduce the risk of chronic and degenerative diseases or to improve immune function, and for considering such effects of high intakes of essential nutrients as part of the basis for establishing RDIs (2, 3, 4, 5 and 6).
The definitions for essential and conditionally essential nutrients are clear from the criteria used to establish them. If the definitions were broadened to include substances that provide some desirable effect on health but do not fit these criteria, the specificity of the current definitions would be lost. Providing a health benefit, as for example is the case with fiber, is obviously not an adequate criterion for classifying a food constituent as essential or conditionally essential. Altering the criteria for establishing RDIs on the basis of effects of intakes of essential nutrients that greatly exceed physiologic needs or amounts obtainable from usual diets would have similar consequences—the specificity of the term RDI would be lost.
Food Constituents Desirable for Health. A straightforward way of avoiding these problems is to treat food constituents that exert desirable or beneficial effects on health, but do not fit the criteria established for essentiality or conditional essentiality, as a separate category of food constituents termed desirable (or beneficial) for health (1). Another more general term for such substances, which embraces both beneficial and adverse effects, is physiological modulators (31). A dietary guideline for including plenty of fresh vegetables and fruits in diets as sources of both known and unidentified substances that may have desirable effects on health or in preventing disease has been readily accepted. Individual food constituents that may confer health benefits different from those of physiologically required quantities of essential nutrients, whether they are nonnutrients, dispensable nutrients, or essential nutrients in quantities exceeding those obtainable from diets, are more appropriately included in guidelines for health than in the RDI. Some nutrients and other food constituents that have prophylactic actions are presently dealt with in essentially this manner. Fiber and fluoride are discussed in dietary guideline publications, and this has been suggested as the most appropriate way of dealing with the potential beneficial effects of high intakes of antioxidant nutrients ( 32).
Fluoride, in appropriate dose, reduces susceptibility to dental caries without exerting a toxic effect. Whether fluoride meets criteria for essentiality, whether it is essential for tooth and bone development, or even if it should be considered a nutrient is controversial. Nonetheless, in low doses it acts as a prophylactic agent in protecting teeth against the action of bacteria. It is discussed in RDI and dietary guidelines publications on this basis, and it is certainly classified appropriately as a dietary constituent that provides a desirable health benefit.
Fiber has been long recognized to be beneficial for gastrointestinal function, to prevent constipation, and to relieve signs of diverticulosis. There is no basis for classifying fiber as an essential nutrient, but some forms of fiber that are transformed in the gastrointestinal tract into products that can be oxidized to yield energy fit the definition of nutrients. Without question it is a food constituent that provides a desirable health benefit when ingested in moderate amounts ( 33). Fiber is discussed with carbohydrates in RDI publications and with plant foods in dietary guidelines. A recommendation for inclusion of fiber in diets is appropriate, but recommended intakes should not be considered as RDIs, which are reference values for intakes of essential nutrients.
To develop a separate category of food constituents of this type (substances with desirable effects on health that are different from effects attributable to the physiologic functions of essential nutrients), specific criteria must be established to identify those to be included. Establishing appropriate criteria for assessing the validity of health claims for a category of food constituents that will include a variety of unrelated substances with different types of effects, many of which apply to only segments of the population, will be more complex than establishing criteria for assessing the validity of claims for essentiality of food constituents. The latter criteria apply uniformly to all substances proposed for inclusion and can be measured objectively. Assessing the effects of food constituents on health or in preventing disease involves a greater element of judgment and is more subjective than evaluating the essentiality of nutrients. Thus, claims for such effects must be evaluated especially critically.
In establishing criteria for assessing claims for desirable health benefits, consideration must be given to the need for subcategories of substances having different effects. Susceptibility to chronic and degenerative diseases is highly variable and may be influenced by many factors, including genetic differences among individuals or between populations, lifestyle, and diet-genetic interactions that can influence expression of genetic traits. Among questions that require answers are, Does the effect result from alteration of a basic mechanism that prevents a disease from developing or is it due to modulation of the disease process? Does the benefit apply to the entire population or only to individuals at risk? This has been a source of controversy in relation to dietary recommendations for reducing the risk of developing heart disease (34). The effects of dietary constituents on immunocompetence should be analyzed in a similar manner: Are they of general significance or of consequence only if the immune system is impaired? When is stimulation of the immune system beneficial and when might it have adverse effects?
An immense number of plant constituents with anticarcinogenic actions are currently under investigation. These constituents differ in both their effects on cells and the stage of tumor development at which they act, and some have both adverse and beneficial effects (35). A number of subcategories would seem to be needed for which specific criteria will be required.
Pharmacologic Effects of Nutrients. Nutrients that function in large doses as drugs fall logically into a separate category of pharmacologic agents ( 36). Nicotinic acid in large doses is used to lower serum cholesterol. This represents use of a nutrient as a drug (see Chapter23). The effect is unrelated to its function as a vitamin required for oxidation of energy-yielding nutrients and can be achieved only by quantities that far exceed nutritional requirements or usual dietary amounts. Use of tryptophan as a sleep inducer (37) and of continuous intravenous infusions of magnesium in the treatment of preeclampsia or myocardial infarction fall into this category (38). Essential nutrients that fit this pattern are functioning as pharmacologic agents not as nutritional supplements, as are substances, such as aspirin or quinine, originally isolated from plants, that are used as medicines.
With the current state of knowledge, it is undoubtedly premature to try to resolve definitively the problems encountered in classifying food constituents that have desirable effects on health or have been implicated in disease prevention. Such actions are not related to the physiologic functions of essential nutrients. Nonetheless, even though solutions proposed at this stage must be considered tentative, an orderly resolution of questions relating to health effects of food constituents that do not fit current nutritional concepts must be started. The confusion that would be created by accommodating them through modifying the criteria for essentiality or conditional essentiality is to be avoided at all costs. They should be considered within the context of dietary guidelines for health, not as part of the scientifically based RDIs.
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