Recommended dietary intake levels for phytochemicals: Feasible or fanciful?

Phytochemicals are biologically active compounds, found in plants in small amounts, which are not established nutrients but which nevertheless seem to contribute significantly to protection against degenerative disease. At present, most interest in phytochemicals is focused on the polyphenolic flavonoids and on the carotenoids, although allium compounds, glucosinolates, indoles and coumarins have also received attention, especially with respect to cancer. Mechanistically, phytochemicals are thought to act in many ways, which include their activity as anti-oxidants, antibacterial/viral agents, phytoestrogens and as inducers or inhibitors of a variety of key enzymes. Recommended dietary intakes (RDI) are the levels of intakes of essential nutrients considered adequate to meet the known nutritional needs of practically all healthy persons. To be regarded as an essential nutrient, a dietary component must be a single identified compound or a close derivative. It should have a demonstrated key biological role and characteristic deficiency syndrome, both of which should respond to nutritional manipulation and are used as a basis for setting an RDI. In these terms, allocating RDI to phytochemicals is problematic, due in part to the large number of chemically different phytochemicals and the lack of a distinctive deficiency syndrome or inherent physiological role in almost all cases. Accordingly, allocation of a single RDI to a general class of phytochemicals would be impracticable, although for an individual phytochemical it may be feasible if acceptable justification for an RDI is extended to include optimum health and evidence is forthcoming of a key inherent role for that compound in maintaining optimum physiological function. However, a distinction will need to be drawn between phytochemicals that participate as integral components of an essential biological system and become recognized as nutrients and those that act as valuable non-nutrient health-promoting agents. Both classes of phytochemicals occur in foods and both could be incorporated into functional foods. Both could be addressed in recommendations such as dietary guidelines, but at present only established nutrient phytochemicals would be eligible for an RDI.