Soy Protein Reduces Arterial LDL Concentration and LDL Delivery in Both Diabetic and Nondiabetic Cynomolgus Monkeys

We previously showed that soy protein consumption improves lipoprotein concentrations and reduces progression of atherosclerosis in cynomolgus monkeys. The mechanism for these beneficial effects is unclear. The purpose of this study was to determine potential mechanisms for the atheroprotective effects and to determine whether these effects extend to diabetic monkeys. As such, we designed an experiment with a 2 x 2 factorial design in which adult male monkeys (n = 23) were fed an atherogenic diet; the protein source was soy isolate (n = 11) or casein lactalbumin (n = 12), and monkeys were either control (n = 12) or streptozotocin-induced diabetic (n = 11). Diabetics had significantly increased fasting glucose and glycated hemoglobin levels; this relationship was not affected by protein consumption. Soy consumption significantly reduced total cholesterol and increased HDL cholesterol concentrations in both control and diabetic monkeys. To determine plasma and arterial LDL metabolism, radiolabeled LDL was injected before necropsy, 125I-LDL was injected 48 h and 131I-tyramine cellobiose-LDL was injected 1 h before necropsy to determine arterial LDL concentration and permeability, respectively. An increase in whole-body plasma LDL fractional catabolic rate was found with soy consumption. Soy reduced arterial LDL concentration across all arterial sites and LDL permeability in the carotid bifurcation and internal carotid arteries. Arterial delivery of LDL was calculated as LDL permeability X plasma LDL concentration and was also reduced across all arterial sites. There was no additional effect of diabetes. In conclusion, these beneficial effects of soy on both plasma and arterial LDL metabolism would be expected to reduce atherosclerosis progression and were found in both control and diabetic monkeys.