WHOLE FOODS AND LDL OXIDATION
Although intervention trials are important in evaluating possible beneficial effects of antioxidants against development or progression of CHD, they have limitations and should be considered as only one component in the totality of available research evidence. It may be that a lifetime of intake is required to show a protective effect, or that a mixture of natural antioxidants found in fruits and vegetables provide the necessary protective mixture. A number of studies of people who eat a diet rich in fruit and vegetables, and therefore rich in antioxidant nutrients, have tried to test the hypothesis that fruits and vegetables lower the risk of CHD:· In general, observational studies of vegetarians and those with diets rich in fruits and vegetables support the hypothesis that such a diet might lower the risk of CHD.
Vegetarians generally have high intakes of cereals, nuts, vegetable oils, vegetables, and fruit. However, vegetarians differ from the rest of the population in a number of important ways: they tend to smoke less, have a lower BMI and alcohol intake, and come predominantly from higher social classes, all of which are known to confer a health advantage. Few studies have looked at the effects of whole foods on biochemical end-points. A recent study asked subjects with normal lipid concentrations who ate three or fewer servings of fruit and vegetables daily to consume eight servings per day.
Plasma concentrations of vitamin C, retinol, a-tocopherol, a- and p-carotene, lipids, and lipoproteins were assessed before and after an 8-wk intervention period. The plasma vitamin C, a-carotene, and p-carotene concentrations increased, whereas concentrations of retinol, atocopherol, lipids and lipoproteins remained unchanged, despite some increase ,in dietary vitamin E and a small reduction in saturated fat intake. An interesting addition to the results would have been the inclusion of data on the susceptibility of LDL to oxidation. The authors concluded that more specific dietary advice to modify fat intake may be necessary to reduce the risk of CVD. By contrast, Singh and colleagues found over a 12-wk period that fruit ar::vegetable administration to subjects at high risk of CRD lowered total and LDL choleste and triglyceride levels, and increased RDL-cholesterol.
Another study by Wise and coworkers using dehydrated fruit and vegetable extracts 0, = a period of 28 d in 15 healthy adults aged 18 - 53 yr produced increases of 50 - 2000-fold plasma carotenoid and tocopherol levels. During the same interv..ention period, plasma li1= peroxides decreased fourfold, with much of this lowering taking place during the first w Other antioxidant micro nutrients may be important in increasing the resistance of LDL oxidation. Flavonoids are plant-derived compounds that inhibit in vitro copper-catalyzed LD_ oxidation (100 -103). The inhibition of oxidation of human LDL by consumption of red willi or tea has been attributed to the presence of antioxidants such as flavonoids and otb __ polyphenols in red wine and catechin in tea.
Isoflavonoids such as genistein also reportedly increase LDL resistance to oxidatiol: although this has not been confirmed, whereas aged garlic extract has recently been sho~ to have antioxidant properties. Ubiquinol-10 is another effective lipid-soluble antioxidant thainhibits LDL oxidation due to aqueous or lipid-phase peroxyl radicals. The effect of antioxidan' supplementation may be sex-specific with 17 p-estradiol at physiological levels increasing thr resistance of LDL to oxidation in some studies. Further studies, especially in humans, arE required to validate the role ofthese antioxidants in inhibiting LDL oxidation.
