Oxidative stress is associated with the aetiology of many major Western lifestyle diseases including cardiovascular disease, cancer, diabetes and arthritis. Recently, evidence has been accumulating to show that obesity, and the related metabolic syndrome, are conditions characterised by inflammation and chronic oxidative stress. Many plant compounds act as reducing agents that can decreases levels of oxidative stress in vitro and in vivo, and some nutrients such as the flavonoid group have been shown to be protective of certain diseases in epidemiological studies. Many plant antioxidants are available in supplements, but the best method of increasing plasma levels appears to be through consumption of a variety of fruit and vegetables in the diet. This may relate to the higher quantity of antioxidants contained in plant foods compared to supplements, in combination with synergism between different chemical classes of reducing agents.
The optimal amount of vegetable and fruit content to the diet is not known, although most countries have government recommendations. Surprisingly, few trials have been conducted to ascertain the amount of plant food required to raise antioxidant status or to investigate physiological benefits of plant rich diets. One group of researchers1 conducted a 3-dose, crossover feeding trial to assess the effects of plasma carotenoid concentrations in 49 overweight postmenopausal women. The authors were also interested in determining any changes to oxidative stress levels. The doses of vegetables were 2 daily servings (150g), 5 daily servings (287g) or 10 daily servings (614g) and consisted of fresh greenhouse vegetables including baby lettuce, cruciferous leafy vegetables, red bell peppers and tomatoes. Treatment lasted for 3 weeks, and a 4 week washout period was present between each treatment.
There was a stepwise increase in plasma carotenoids with increasing serving sizes. Following analysis using pre- and post-paired t test, the plasma carotenoids increased from 1.63 to 2.07µmol/L, from 1.49 to 2.84 µmol/L and from 1.40 to 4.42 µmol/L, in the 3, 5 and 10 serving size groups respectively. However, despite the increase in plasma levels of carotenoids, the urinary excretion of the 8-isopostane F2α and hexanoyl lysine markers of oxidative stress did not decrease significantly, suggesting that there had been no increase in the reducing capacity of plasma despite increased carotenoid concentrations. To support this view, serum C-reactive protein did not change significantly. This may relate to the subject possessing adequate antioxidant status before the study. This is particularly relevant to fat soluble antioxidants such as vitamin E and carotenoids, which can stay at high concentrations in cell membranes for weeks following consumption.
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