DRAFT Australian Dietary Guidelines - Eat For Health

2.2.5.2 Cancer mechanisms
There is no dominant mechanism to explain the protective effect of vegetables, legumes/beans and
fruit for some cancers, and this is complicated by the range of site-specific cancer mechanisms.
Some risk factors for cancer, such as oxidising radiation, can operate primarily from childhood or
early adult life and antioxidants or other protective constituents of vegetables, including
legumes/beans, and fruit may need to be consumed regularly from early life to be effective [211].
Phytochemicals and several vitamins and minerals found in vegetables and fruit are thought to
protect against some cancers by a range of mechanisms. Vegtables in the green leafy and Brassica
subgroup are particularly high in folate [10] and inadequate amounts of folate are thought to
increase the risk of cancer by leading to a rise in homocysteine and megaloblastic changes in bone
marrow and other rapidly dividing tissues [9]. Poor folate status is thought to affect the induction
of DNA hypomethylation, increasing chromosomal fragility or diminishing DNA repair, as well as
increasing secondary choline deficiency, reducing killer cell surveillance, and increasing risk of
faulty DNA synthesis and metabolism of cancer-causing viruses [9]. Some studies have suggested
that folate, primarily from fruit and vegetables, may be more effective in reducing cancer risk
among those with habitual high alcohol intake [212, 213].
Several studies have indicated that vitamin C (found in most fruit and vegetables but particularly in
citrus fruit, capsicum and tomatoes) is protective against cancer, whereas others have not found
convincing evidence of this [9]. Lycopenes found in tomatoes are thought to be protective of
prostate cancer, particularly when tomatoes are cooked in olive oil [42]. Carotenes found
predominantly in orange, red and yellow fruit and vegetables are also thought to be associated
with maintenance of immune function [9]. Dithiolthiones and isothiocyanates (found in Brassica or
cruciferous vegetables) and allyl sulphides (found in Allium vegetables) have been shown to
stimulate detoxification processes [37]. The anti-inflammatory action of other phytochemicals,
such as flavonoids, is also thought to be important in reducing cancer risk [214]. Potatoes are not
as rich in phytochemicals as other types of vegetables, and this may help explain the weaker
association with decreased cancer risk and consumption of starchy vegetables compared with
other vegetables [42]. There is increasing evidence that whole foods are more effective in reducing
risk of cancer than specific vitamin and mineral supplements, and that some supplements may
actually increase risk of cancer [215, 216].
Localised contact with phytochemicals may be an important factor in the aetiology of epithelial
cancers of the alimentary system (oral, oesophagus, stomach and bowel). Proposed protective
mechanisms include:
the reduced formation of cancer-promoting substances in the gastrointestinal tract due to
antioxidant activity
the part played by phytochemicals and micronutrients in the detoxification of carcinogenic
substances
functions relating to the containment and destruction of existing cancer cells by means of a
variety of physiological processes and improved immunological activity against cancer cells
[9, 217].
DRAFT Australian Dietary Guidelines- December 2011 41