Soy protein reduces the arterial low-density lipoprotein (LDL) concentration and delivery of LDL cholesterol to the arteries of diabetic and nondiabetic male cynomolgns monkeys

We have previously shown that soy protein consumption improves lipoprotein concentrations and reduces the progression of atherosclerosis in cynomolgus monkeys. The mechanism for these beneficial effects is unclear. The purpos e of this study was to determine potential mechanisms for the atheroprotect ive effects of soy and to determine if these effects extend to diabetic mon keys. We designed an experiment with a 2 x 2 factorial design in which adul t male monkeys (N = 23) were fed an atherogenic diet with a protein source of either soy isolate or casein and lactalbumin, and the monkeys were eithe r control or streptozotocin-induced diabetic. Diabetics had significantly i ncreased fasting glucose and glycated hemoglobin (GHb) levels; this relatio nship was not affected by the type of dietary protein. Diabetics also had i ncreased total (TC) and low-density lipoprotein cholesterol (LDLC) concentr ations. However, soy consumption significantly reduced TC and LDLC concentr ations in both control and diabetic monkeys. Plasma and arterial LDL metabo lism was determined by injecting I-125.LDL at 48 hours and I-131-tyramine c ellobiose LDL at 1 hour prior to necropsy. This allowed a determination of the arterial LDL concentration, permeability, and arterial LDL delivery. An increase in the whole-body plasma LDL fractional catabolic rate (FCR) was found with soy Soy significantly reduced the arterial LDL concentration acr oss all arterial sites by an average of 50%. Soy also significantly reduced the delivery of LDLC to all arterial sites by an average of 40%. While thi s was primarily due to the lower plasma LDLC concentration, LDL permeabilit y in the carotid bifurcation and internal carotid arteries was also reduced . There was no additional effect of diabetes. These beneficial effects on p lasma and arterial LDL metabolism would be expected to reduce atheroscleros is and were found in both control and diabetic monkeys. Copyright (C) 2000 by W.B. Saunders Company.