It seems clear, based on current evidence, that soy protein as a part of a diet or as a supplement exerts beneficial effects on plasma lipids and lipoproteins, but the magnitude of the beneficial effects is uncertain. The uncertainty probably relates to a number of poorly understood variables that relate to differences in metabolism among human and nonhuman primates and lower animals, and the effects of intervening variables, such as stage of the menstrual cycle and perhaps plasma concentrations of nonovarian-derived estrogens. In this brief review we attempt to put these various issues in the context that is possible based on current knowledge. Since soy supplements are more widely used by postmenopausal females than by males, the majority of the studies have focused on females.
Our group conducted and reported on several studies comparing the effects of diets containing casein/lactalbumin (C/L) and isolated soy protein on the plasma lipids and lipoproteins of surgically postmenopausal monkeys. Compared to C/L, soy protein resulted in reductions in low density lipoprotein plus very low density lipoprotein (LDL + VLDL) cholesterol of 30 to 40% and increases in high density lipoprotein (HDL) cholesterol of 20 to 50%.8-10
Using the cynomolgus monkey model, our group also sought to determine how much of the plasma lipid lowering effect of soy protein relates to the presence of the isoflavones and how much relates to soy peptides.11-13 Monkeys fed intact soy (+) protein generally had higher HDL concentrations (~12%) than those fed soy (-) protein that had been alcohol washed to remove the isoflavones. The LDL + VLDL cholesterol concentrations of monkeys fed soy (+) were about 16% lower than those fed soy (-).
Studies of human patients consuming intact soy protein, however, showed only modest changes in plasma lipoprotein concentrations. Human subjects consuming intact soy protein had reductions in plasma LDL cholesterol of 2.6 to 6.5%, with no effect seen on HDLC concentrations.14-16 A comparison of soy's effects on plasma lipids of postmenopausal monkeys11 and women15 is depicted in Figure 13.1. In a recent and carefully controlled nutritional study,17 very small reductions in LDL cholesterol (2%) and increases in HDL (3%) were reported when soy/soy isoflavones were administered to postmenopausal women in amounts comparable to those used in monkey studies.
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