The effects of natural mixed diets on lipid peroxidation were investig
ated in humans. In the first study, 59 subjects were fed a rapeseed oi
l-based diet rich in monounsaturated fatty acids (MU FA) and a sunflow
er oil-based diet rich in polyunsaturated fatty acids (PU FA) in a cro
ss-over manner for three and a half weeks. The lipid peroxidation prod
ucts in plasma were determined by measuring conjugated dienes and malo
ndialdehyde (MDA). In a second study, plasma thiobarbituric acid react
ive substances (TEARS), lipid hydroperoxides, and the susceptibility o
f very low density lipoprotein + low-density lipoprotein (LDL) to in v
itro oxidation were measured from subjects fed similar MUFA and PUFA d
iets for six week diets. No significant differences in plasma MDA or c
onjugated diene concentrations were found after the rapeseed oil diet
or the sunflower oil diet in Study 1. In the second study, a small but
significant decrease (P < 0.05) in both lipid hydroperoxides and TEAR
S was observed in the LDL fraction after the sunflower oil diet. The i
n vitro oxidation gave opposite results, showing increased oxidation a
fter the sunflower oil diet. Despite a high intake of alpha-tocopherol
during the oil periods, no increase in plasma alpha-tocopherol was no
ticed in either study. The results suggest that moderate changes in th
e fatty acid composition in the Western-type diet may be adequate to a
ffect lipoprotein susceptibility to oxidation in vitro, but there is c
onsiderable disparity with some indices of in vivo lipid peroxidation.