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.