Micronutrients, dietary questionnaires and cancer.

There is increasing evidence that many chemicals, although present in the diet at only low levels, play an important role in protection against cancer. Micronutrients are defined as nutrients present in the body in amounts less than 0.005% of body weight. Some micronutrients suggested to play a protective role in cancer are beta-carotene, vitamin E and vitamin C. In addition to those chemicals with an established role in nutrition, there is also a less well-defined group of chemicals, often referred to as phytochemicals, which may prove even more important. Examples here are a group of sulphur-containing chemicals present in brassicaceous vegetables, such as broccoli and cabbage, that appear to be very effective anticarcinogens. Epidemiology will be essential in accurately defining the role of phytochemicals and micronutrients in cancer. However, the large prospective studies that would be most desirable increasingly utilize food frequency questionnaires containing a limited number of questions. Such an approach has been well validated for macronutrients. However, there is often less accurate information available on micronutrients and phytochemicals, and the food combinations necessary (and acceptable) for macronutrients may be inappropriate for these other factors. It would be most desirable that fruits and vegetables are individually itemized or grouped according to plant families rather than macronutrient status. This may be the most appropriate way of identifying potentially novel protective factors in the diets of countries such as Morocco or New Zealand.