Much human experience suggests that the potential effects of dietary carcinogens are more likely to be realized when the specific foods in which they occur form too large a part of the diet. The varied and balanced diet needed for good nutrition also provides significant protection from natural toxicants. Increasing dietary fruit and vegetable intake may actually protect against cancer. The NRC report Diet and Health concluded that macronutrients and excess calories are likely the greatest contributors to dietary cancer risk in the United States.

Second, the committee concluded that natural components of the diet may prove to be of greater concern than synthetic components with respect to cancer risk, although additional evidence is required before definitive conclusions can be drawn. Existing concentration and exposure data and current cancer risk assessment methods are insufficient to definitively address the aggregate roles of naturally occurring and/or synthetic dietary chemicals in human cancer causation and prevention. Much of the information on the carcinogenic potential of these substances derives from animal bioassays conducted at high doses (up to the maximum tolerated dose, or MTD), which is difficult to translate directly to humans because these tests do not mimic human exposure conditions, i.e., we are exposed to an enormous complex of chemicals, many at exceedingly low quantities, in our diet. Furthermore, the committee concluded upon analyzing existing dietary exposure databases, that exposure data are either inadequate due to analytical or collection deficiencies, or simply nonexistent. In addition, through regulation, synthetic chemicals identified as carcinogens have largely been removed from or prevented from entering the human diet.

Third, the committee concluded that it is difficult to assess human cancer risk from individual natural or synthetic compounds in our diet because the diet is a complex mixture, and interactions between the components are largely unknown.

The committee's major conclusions are presented in detail below. They address the complexity and variability of the human

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