vidence supports the theory that fruit and vegetable consumption is beneficial to health. In particular, plant foods may protect against chronic Western lifestyle diseases. The current recommendation is to consume around five portions of fruits or vegetables per day in order to benefit long-term health. However, despite a large body of research, it is unclear as to how fruit and vegetables can protect from disease. Denham Harman in the 1950’s was the first to suggest that free radical damage might be the cause of chronic disease, and since this time the free radical theory of disease has gained popularity. Free radicals are implicated in the aetiology of a number of diseases including cancer and cardiovascular disease. Because fruits and vegetables are rich in antioxidants it has been suggested that this might be the mechanisms of their health benefits. Further, research suggests that consumption of fruits and vegetables increases plasma levels of antioxidants.
Most studies investigating the protective effects of plant foods have been observational in nature, but some clinical trials have been performed. For example, researchers1 investigated the effects of fruit and vegetable consumption on the plasma antioxidant concentrations of 87 healthy individuals who habitually consumed low amounts of plant foods. Subjects were randomly assigned to receive advice to increase fruit and vegetable consumption to eight servings per day, or to maintain their present diet. Diet records were kept to monitor compliance and blood sample were used to monitor increases in plasma levels of antioxidants. Those in the treatment group had significant increases in plasma concentration of vitamin C, α-carotene and β-carotene when compared to the control group. In addition, increases in these antioxidants were associated with the increase in the intake of fruit and vegetables. No increases in plasma concentrations vitamin E were observed, possibly because most fruit and vegetables are poor sources of tocopherols.
If plant foods increase plasma concentrations of antioxidants, this might explain the epidemiological evidence that shows an inverse associated between fruits and vegetable consumption and cancer and cardiovascular disease. While the antioxidant theory of fruit and vegetables is compelling, and supported by many in vitro studies, other mechanisms could explain the benefits of high intakes of plant foods. For example, plant material also contains high levels of fibre, which is known to decrease the glycaemic effect of food and can prevent the absorption of carcinogens. Compounds in cruciferous vegetables can favourably modify detoxification enzyme systems and reduce circulating toxins. Flavonoids from plants as well as being good antioxidants are also able to regulation gene expression in humans. Because the mechanism by which plant food protect from chronic diseases is not known, diets should include a wide variety of fruits and vegetables, because possible synergistic effects might be expected.
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