Consensus amongst both science and the general public is that a diet containing a wide variety of fruit and vegetables is beneficial to the health. Current recommendations are to eat more fruit and vegetables and reduce the intakes of fat and sugar. Although it seems self evident that plant based foods are beneficial, there is still much controversy as to the exact mechanism by which such a diet may be protective of health. It is known that fruits and vegetables are rich in phytonutrients that have biological effects in humans, the main groups of these being the polyphenols. A number of foods stand out as being particularly well know for their beneficial health effects, but the fact remains that even well researched food such as tea, garlic and red wine possess complex chemistries that are not fully understood.
The potential disease-preventative mechanisms of vegetables and fruit has been extensively studied and the main mechanisms are shown in table 1. The role of plant nutrients as antioxidants in vivo is well established, and the health benefits of red wine, tea and berries is thought to be due to their ability to quench free radicals. These foods contain a variety of polyphenols that are able to prevent oxidation of lipoproteins in plasma which has beneficial effects on the cardiovascular system. Linked to this antioxidant activity is the ability of these polyphenols to decrease blood pressure, which may come about as a result of an antioxidant effect in endothelial tissue, making the arterial linings less susceptible to oxidation. This may normalise endothelial function and thus improve blood flow and increase circulation.
Table 1. Potential disease-preventative mechanisms of fruits and vegetables
Some plant foods are known to stimulate the immune system in various ways. Garlic and the allium family of vegetables (chives, onion, shallots) are thought to possess strong anti-tumour effects in vitro, as well as possess the ability to modulate allergic reactions. This would suggest that these vegetables have a stimulatory effect on an under-active immune system and a modulatory effect on a hyper-active immune system. The allium family also possess anti-bacterial, antifungal and antiviral effects. Elderberry has been used traditionally as a cure for the common cold and has been shown to decrease the length of influenza infection by 3 to 4 days, probably by stimulation of inflammatory cytokine production. Alkylamides from Echinacea purpurea and Echinacea angustifolia have also been extensively researched in relation to their immune system enhancing effects.
A number of plant based foods appear to have a beneficial effect on the aggregation of platelets, including chocolate, red wine and grape juice. Research suggests this is on account of their high flavonoid content. Flax seed oil has shown a similar beneficial effect, probably because of its high content of alpha linolenic acid. Unlike other nuts, walnuts are low in monounsaturated fat and high in polyunsaturated fat and research shows that they are beneficial to the heart because they are able to decrease platelet aggregation and lower cholesterol levels. Garlic has also been researched in relation to its ability to alter cholesterol metabolism, although the mechanism of action is not fully understood. Studies indicate that tocotrienols from cereals such as oats are able to inhibit HMG-CoA reductase, the enzyme responsible for the synthesis of cholesterol in humans.
Vegetables of the genus brassica (the mustard family) have been shown to possess strong anti-cancer effects that is likely a result of their ability to modify the phase II detoxification enzymes systems. Broccoli, cauliflower, water cress, brussel sprouts and kale all possess this effect because they contain the same glucosinolate chemicals in their cells. A number of plant foods can modulate hormone levels in humans, including flax seeds and soy. Flax is able to alter hormone metabolism because it contains the fatty acid alpha linolenic acid, which is required for eicosanoid synthesis. Flax has been researched in relation to its ability to decrease inflammation. Soy has been extensively researched for its ability to regulate bone health as well as to modulate oestrogen levels and decrease plasma cholesterol. Soy contains a group of polyphenols called isoflavones which are likely responsible for these effects.
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