Although the human diet contains both naturally occurring and synthetic carcinogens, these chemicals are generally present at very low concentrations, and are thus unlikely to pose an appreciable cancer risk. Application of pesticides to food crops is subject to stringent control. Thus, most foods contain no detectable levels of such residues and the residues that are detected are generally present at levels well below established tolerances. Similarly, with the exception of a few agents such as caffeic acid, naturally occurring carcinogens, including "natural pesticides," are also present at very low levels.

The human diet contains a number of nutrients, including fibre and micronutrients such as the antioxidant vitamins A, C and E, that appear to reduce cancer risk. In addition, several non-nutritive agents, including the isoflavonoids, phenolic acids, and isothio-cyanates, have been shown to demonstrate anticarcinogenic effects. Diets low in calories and fat are also associated with reduced cancer risk.

Epidemiologic studies have associated diets rich in fruits and vegetables with reduced rates of human cancer. These foods are sources of a number of constitutive chemicals with anticarcinogenic properties.

Fruits and vegetables contain both anticarcinogenic and carcinogenic constituents. Because such foods are associated with reduced cancer risk in humans, the anticarcinogenic properties would appear to be greater than their carcinogenic properties.