The effects of soy protein (40 g/d) containing moderate and higher concentrations of isoflavones on blood lipid profiles, mononuclear cell LDL receptor messenger RNA, and bone mineral density and content were investigated in 66 free-living, hypercholesterolemic, postmenopausal women during a 6-mo, parallel-group, double-blind trial with 3 interventions. After a control period of 14 d, during which subjects followed a National Cholesterol Education Program Step I low-fat, low-cholesterol diet, all subjects were randomly assigned to 1 of 3 dietary groups: Step I diet with 40 g protein/d obtained from casein and nonfat dry milk (CNFDM), Step I diet with 40 g protein/d from isolated soy protein containing 1.39 mg isoflavones/g protein (ISP56), or Step I diet with 40 g protein/d from isolated soy protein containing 2.25 mg isoflavones/g protein (ISP90). Total and regional bone mineral content and density were assessed. Non-HDL cholesterol for both ISP56 and ISP90 groups was reduced compared with the CNFDM group (P < 0.05). HDL cholesterol increased in both ISP56 and ISP90 groups (P < 0.05). Mononuclear cell LDL receptor mRNA was increased in subjects consuming ISP56 or ISP90 compared with those consuming CNFDM (P < 0.05). Significant increases occurred in both bone mineral content and density in the lumbar spine but not elsewhere for the ISP90 group compared with the control group (P < 0.05). Intake of soy protein at both isoflavone concentrations for 6 mo may decrease the risk factors associated with cardiovascular disease in postmenopausal women. However, only the higher isoflavone-containing product protected against spinal bone loss.

The long-term clinical effects of soy protein containing various amounts of isoflavones on lipoproteins, mononuclear cell LDL receptor messenger RNA concentrations, and other selected cardiovascular risk factors are not well known. Sixty-six hypercholesterolemic, free-living, postmenopausal women were investigated during a 6-mo parallel-group, double-blind trial with 3 interventions. After a control period of 14 d, all subjects were randomly assigned to 1 of 3 dietary groups (all with 40 g protein): a National Cholesterol Education Program (NCEP) Step 1 diet with protein from casein and nonfat dry milk (control), an NCEP Step 1 diet with protein from isolated soy protein containing moderate amounts of isoflavones (ISP56), or an NCEP Step 1 diet with protein from isolated soy protein containing high amounts of isoflavones (ISP90). Non-HDL cholesterol in both the ISP56 and ISP90 groups was reduced compared with the control group (P < 0.05), whereas total cholesterol was not changed. HDL cholesterol increased in both the ISP56 and ISP90 groups (P < 0.05), whereas the ratio of total to HDL cholesterol decreased significantly in both groups compared with the control (P < 0.05). Mononuclear cell LDL receptor messenger RNA concentrations increased in subjects consuming ISP56 or ISP90 compared with the control (P < 0.05). These results indicate that soy protein, with different amounts of isoflavones, may decrease the risk of cardiovascular disease via improved blood lipid profiles, and that the mechanism by which apolipoprotein B-containing lipoproteins were depressed may be via alterations in LDL receptor quantity or activity.

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Our findings show that consuming as little as 20 g soy protein/d instead of animal protein for 6 wk reduces concentrations of non-HDL cholesterol and apo B by approximately 2.6% and 2.2%, respectively. 2000;71:-84.

The effects of an ethanol-acetone extract from soy flour on serum lipids and thyroid hormones were studied in rats and hamsters. In Study 1, male Sprague-Dawley rats were fed diets containing protein from ethanol-acetone extracted isolated soy protein (ISP-), nonextracted ISP (ISP), casein (casein-), or casein to which 0.36 mg ethanol-acetone extract/g protein was added (casein+). Rats fed either ISP-containing diet had lower serum total cholesterol concentrations compared with those fed either casein diet (P < 0.05). Lower serum LDL-cholesterol concentrations were present in rats fed either ISP-containing diet and in those fed casein+ compared with those fed casein- (P < 0.05). Adding the extract to casein (casein+) produced higher serum thyroxine concentration and free thyroxine indices compared with all other groups (P < 0.05). In Study 2, male Golden Syrian hamsters were fed experimental diets containing protein from ISP, ISP with added ethanol-acetone extract (0.36 mg extract/g protein; ISP+), casein-, casein+ (0.36 mg extract/g protein), or casein with twice the level of extract (0.72 mg/g protein; casein++). Lower serum total cholesterol and LDL (LDL + VLDL + IDL) cholesterol concentrations were observed in hamsters fed ISP, ISP+ or casein+ compared with those fed casein- (P < 0.05). Addition of the extract at higher levels to casein (casein++) did not lower serum lipids relative to those fed casein-. Serum thyroxine concentration and the free thyroxine index were greater in both ISP groups as well as in hamsters fed casein++ compared with those fed casein- or casein+ (P < 0.05). In conclusion, both studies show that protein from soybeans decreases serum total and LDL cholesterol while the effects on thyroxine are different in rats and hamsters. It is also apparent that the ethanol-acetone extract of soy flour produces changes in serum cholesterol, particularly in the LDL fraction in both species.