Epidemiologic Studies on Vitamin A and Cancer

Abstract

Many observations have led epidemiologists to suggest that environmental factors, in the sense of those elements exogenous to the individual, are responsible for as much as 90% of cancer incidence. Recently, Doll and Peto (1981) attempted to define what proportion of cancer was “avoidable” because it is caused by exposures which might be controlled. The many causes of cancer they identified included tobacco, alcohol, food additives, reproductive and sexual behavior, occupation, pollution, medicines and medical procedures, and viral infections. They also specified a number of currently promising hypotheses regarding the ways that diet might affect the incidence of cancer: diet might contain carcinogens or their precursors; aspects of the diet might affect the formation, transport, activation or deactivation of carcinogens in the body; the diet might contain substances which promote or inhibit the. carcinogenic processes. Among this host of possible sources of environmental cancer risk and possible mechanisms whereby dietary factors may influence carcinogenesis, one aspect of diet, the intake of vitamin A, has been of considerable interest to laboratory scientists, epidemiologists, and nutritionists. We will review some of the bases for this interest and, in particular, examine the epidemiologic evidence which might link vitamin A to cancer in a protective role.