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One usually does not relate the ingestion of a specific nutrient with concerns about the toxicity of that nutrient. However, intakes of essential dietary chemicals from zero to excessive produce responses, from lethal because of nutrient deficiency to an optimal health response and back to lethal because of intolerably high concentrations. With the exception of vitamin D, vitamin A, and some minerals, the intake of nutrients from natural food sources will not pose any significant health problems. However, one can argue that the health problems associated with high intakes of protein, fats, or energy are really manifestations of nutrient toxicity, i.e., cardiovascular diseases, cancers, and eye diseases such as macular degeneration and other chronic diseases. The other potential means whereby nutrient intakes can present health problems is the abuse of nutrient supplementation. A nonfood source of a nutrient can produce pharmacological actions at concentrations well above normal dietary amounts.
Over the last few years, dietary reference intakes (DRIs) have been developed by the Food and Nutrition Board of the National Academy of Sciences. The premise for developing DRIs is that such values reflect the current knowledge of nutrients, particularly with respect to their role in diet and chronic diseases. Similar to recommended dietary allowances (RDAs), DRIs are reference values for nutrient intakes to be used in assessing and planning diets for healthy people. A vital component involved in the development of DRIs is the value for tolerable upper level(UL). UL may be defined as the point beyond which a higher intake of a nutrient could be harmful. UL is the highest level of daily nutrient intake that is likely to pose no risk of adverse health effects in almost all individuals in a specified life stage group.
Often, ULs apply to nutrient intake from supplements because it would be extremely unusual to obtain such large quantities of a specific nutrient in food form. The objective of ULs is to indicate the need to exercise caution in consuming amounts greater than the recommended intakes. It does not mean that high intakes pose no risk of adverse effects.
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