Protection against cancer through nutritionally-induced increase of endogenous proteinase inhibitors--a hypothesis.

Population groups with a diet low in calories, fat and animal protein, but high in vegetables and fibre, have a low incidence of cancer of the colon, rectum, breast and prostate. It is proposed that such a diet protects against cancer by stimulating an increase of endogenous proteinase inhibitors (PI), in particular, alpha 2-macroglobulin (AM), above those of individuals on a diet rich in calories, fat and animal protein. Evidence in support of this hypothesis is discussed. In various models, PI has been shown to be anticarcinogenistic. Preschool and school children who although on a diet apparently below the requirements for protein and calories were still considered healthy, have been found to have increased AM levels. Raised PI levels have also been observed in rats low on protein and energy and restriction of food intake in an inbred colony of rats reduced the incidence of radiation-induced tumours.