ENJOY A WIDE VARIETY OF NuTRITIOuS FOODS
41Guideline 2
Most fruit, vegetables and legumes have a low energy (kilojoule) density and high dietary fibre and water content,
providing a plausible mechanism for the association of consumption of these foods with reduced risk of weight
gain. Any effect is likely to be mediated through potentially increasing satiety, and also through taking longer to
chew, which leads to reducing total energy (kilojoule) intake.8,9 High dietary fibre intakes have been linked to lower
rates of obesity, type 2 diabetes and cardiovascular disease, mainly through an effect on plasma cholesterol.^8
The majority of vegetables are nutrient-dense and low in kilojoules – in particular, green, Brassica and other ‘salad’
vegetables are relatively low in energy (kilojoules).^9 However, starchy vegetables are less nutrient-dense and are
higher in kilojoules, but provide a rich source of complex carbohydrates.^9
Diets high in fibre and specific complex carbohydrates such as non-starch polysaccharides have been used
with modest success by people with type 2 diabetes attempting to lose weight. The small effects seen in
these experimental situations might relate to a satiating effect due to the prolongation of food absorption and a
smoothing of the blood glucose response after meals.^8
In the prevention and dietary control of type 2 diabetes, some vegetables are likely to be of particular value
because of their low energy density and relatively high content of dietary fibre, although longer term studies
may be required to demonstrate effects.
legumes are also believed to confer cardiovascular health benefits because they provide a valuable low saturated
fat source of protein as an alternative to meats, and because of their glycaemic properties and phytoestrogen
and isoflavone content.417,418 These reviews suggest the isoflavone in soy foods may have a role in cholesterol
reduction, improved vascular health, preservation of bone-mineral density^417 and anti-oestrogenic, anti-proliferative,
pro-apoptotic, anti-oxidative and anti-inflammatory processes.^418 However more research is needed to determine
the relative effect of legumes/beans and soy foods themselves as opposed to isoflavones specifically.
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.^419 Phytochemicals and several vitamins and minerals found in vegetables
and fruit are thought to protect against some cancers by a range of mechanisms. Vegetables in the green leafy
and Brassica subgroup are particularly high in folate^9 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.^8 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.^8 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.420,421
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.^8 l ycopenes found in tomatoes are thought to be protective of prostate cancer, particularly when tomatoes
are cooked in olive oil.^43 Carotenoids found predominantly in orange, red and yellow fruit and vegetables are also
thought to be associated with maintenance of immune function.^8 Dithiolthiones and isothiocyanates (found in
Brassica or cruciferous vegetables) and allyl sulphides (found in Allium vegetables) have been shown to stimulate
detoxification processes.^36 The anti-inflammatory action of other phytochemicals, such as flavonoids, is also
thought to be important in reducing cancer risk.^422 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.^43 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.423,424
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:8,425
• 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.