DIETARY ANTIOXIDANTS AND LDL OXIDATION
The role of dietary factors in protecting against the change from native to oxidized LDL; received considerable attention. The antioxidant vitamins are derived from fresh fruits and vegetables, and from vegetable oil and polyunsaturated margarine to which vitamin E is .mally added as an antioxidant; they cannot be synthesized from simple precursors. Thus, ::.:zt.ary intake, absorption, metabolism, and storage determine concentrations of vitamins in 'lasma and body tissues. An overview of epidemiological research suggests that individuals °th the highest intake of antioxidant vitamins, whether through diet or supplements, tend experience 20 - 40% lower risks of CHD than those with the lowest intake or blood levels.
Vitamin E is the major lipid-soluble antioxidant present in LDL, preventing the formation of lipid hydroperoxides from polyunsaturated fatty acids. Vitamin C can scavenge free radicals in the aqueous phase and may also regenerate vitamin E. p-Carotene, a vitamin A precursor, does not have a confirmed antioxidant mechanism although it and other carotenoids are contained within LDL. There is some evidence that carotenoids may protect LDL against oxidation more efficiently at low pO2 levels, which could have relevance to the levels of -rotection provided in the arterial wall in vivo. When LDL is exposed to oxidative stress in ',tro, lipid peroxidation can only proceed after the sequential loss of its antioxidants in the der ubiquinol-10, a-tocopherol, y-tocopherol, lycopene, and p-carotene.
Accordingly, LDL supplemented with vitamin E in vitro is much harder to oxidize in vitro.
Similarly, the a-tocopherol content of the LDL has been shown to be the most important ieterminant of susceptibility to oxidation in an in vitro model. In addition to antioxidant ~ontent of LDL, several other factors can influence the susceptibility to oxidation. Polyunsaturated fatty acids appear to be the most vulnerable moiety following the application
f oxidative stress. The fatty acid composition of the diet is, therefore, an important factor etermining the susceptibility of LDL to oxidation, with monounsaturated fatty acids protecting :.DL against oxidation. Small dense LDL particles are also easier to oxidize than more buoyant particles. The binding of LDL particles to proteoglycans, glycation, and the presence of preformed lipid peroxides are also important factors.
