However, recent analyses suggest that olive oil may have a protective effect against cancer at certain sites, particularly breast cancer. However, the role of n-6 PUFA becomes less clear when also evaluating animal studies. Investigations have been carried out on laboratory animals indicating that n-6 PUFA are more likely to increase cancer risk than other types of fatty acids. There is no evidence showing that there is an independent relationship between protein consumption and cancer risk. It is not clear which component of the total complex carbohydrates provides the protective effect which is seen.
Cereals appear highly protective against cancers of the colon, breast, endometrium and prostate.
A prospective study has confirmed the protective effect of high-fibre foods against colorectal cancer. Epidemiological evidence shows that a high intake of fruit and vegetables, particularly raw vegetables, protects against cancers at different sites, especially those of the digestive and respiratory tracts and the hormone related cancers. They have an anti-carcinogenic action at a wide range of sites and there is no positive correlation between fruit and vegetable intake and cancer. Only the actions of anti-oxidant vitamins and provitamins have been supported by human epidemiology, at present. However, it is likely that no single agent functions as a key protective factor in isolation, but that all of them have some protective role under some circumstances.