In 1991, Sanchez tried to explain these documented hypocholesterolemic effects of soy protein by suggesting that soy protein contains higher amounts of arginine and glycine than casein (animal protein) and that these amino acids induce a low postprandial glucagon ratio in both hypercholesterolemic and normocholesterolemic subjects. (Casein is known to produce a high postprandial insulin/glucagon ratio, an effect that augments atherogenesis.) Their data suggest that control of cholesterol and glucagon may be regulated by these dietary plasma amino acids. Later that year, Carroll reviewed a small number of clinical trials and numerous laboratory studies on soy and its cholesterol-lowering properties. Brief clinical observations indicated that soy protein (either alone or when added to the diets) lowered both total and LDL by approximately 20% and that hypertriglyceridemia subjects experienced a decrease in serum triglycerides tablet.

Concomitant clinical observations spanning 20 years involving over 1,000 Italians with raised serum cholesterol indicates that textured vegetable protein (soy-based) induces a significant hypocholesterolemia, especially evident in Fredrickson type IIA. Anderson reported a meta-analysis of 38 controlled clinical trials to examine the relationship between soy protein consumption and serum lipid concentrations in humans. In most studies, the intake of energy, fat, saturated fat, and was similar in subjects that ingested control and soy-containing diets. Soy protein intake averaged 47 g per day. Soy protein produced the following net changes in serum lipid concentration when compared to control diets:
1) Soy protein decreased total cholesterol 23.2 mg/dl (0.6mmol/l) = 9.3%
2) Soy protein decreased LDL cholesterol 21.7 mg/dl (0.5 mmol/l)=12.9%
3) Soy protein decreased 13.3 mg/dl (0.15 mmol/l)= 10.5%
4) Soy protein ingestion was accompanied by a nonsignificant increase in HDL cholesterol of 2.4%
5) Changes in serum cholesterol and LDL concentrations were directly related to initial serum cholesterol (pO.OOl)

summarizes studies (University of Illinois, 1993-1998) using soy foods in humans with elevated serum lipids.

A small (French) study involving male smokers (n=12) with normal lipid profiles was entered into an eight-week trial of soybean oil (mixed with butter) and normal butter. The phytosterols were purified from soybean oil unsaponificable matter and mixed in butter during processing to final concentrations of 1.07 g/100 g butter. Subjects were then randomized to four weeks of either soybean butter or normal butter in traditional French diets then crossed over to the other diet. Their results indicated the soy butter decreased both the total and LDL cholesterol (pO.OOl) even in normal males.

Kurowska, in a family practice setting in London, Ontario, studied 17 men and 17 women who had increased total and LDL (but were healthy otherwise). A three-treatment crossover trial determined the effects of substituting soybean products for cows’ milk. Subjects were randomized to one of the following three diets for four weeks each and washed out for two weeks between each. The treatments were: 1) normal diet with cows’ milk products, 2) normal diet with soybean products, and 3) combination of skim milk/soy oil. Each diet contained 31 g protein and 18 g fat in each diet group. Their results suggest that the diets had no effect on body weight, body mass index, or levels of apolipoproteins В and A. However, HDL cholesterol was increased (mean 9%), and the LDL/HDL cholesterol ratios decreased (mean 14%) during soybean diets. Significant decreases in total cholesterol results primarily from a decrease in LDL cholesterol. In 24 subjects with the highest baseline values, the soybean diet reduced LDL cholesterol (mean 11%), decreased LDL/HDL ratio (mean 19%), increased HDL cholesterol (mean 9%), but did not significantly alter triglycerides canadian. Investigators found that soybean foods reduced both the DBPs and SBPs in men but not in women.

Soy Supplements: Studies in Normal and Perimenopausal Women

Crouse studied 156 healthy subjects (males and females) to compare the effects of casein and soy protein containing various amounts of isoflavones on plasma concentrations of lipids and lipoproteins. This nine-week randomized trial compared casein 25 g per day to 25 g/day of isolated soy protein (ISP). The ISP contained isoflavones at four different levels (27 mg; 37 mg; 62 mg; or ethanol-extract-ed soy proteins, which contained only 3 mg). All isoflavones quantities were expressed as their agly-cone equivalent. The study needed 30 participants per group to have 95% power to detect a 6% relative change in LDL cholesterol levels among groups. Therefore, there were 5 groups with 31 subjects per group. Investigators stated that baseline entry LDL levels ranged from 3.62 mmol/L (140mg/dL) to 5.17 mmol/L (200 mg/dL) while on a National Cholesterol Education Program Step-1 diet for one month prior to randomization. Diets were equivalent with respect to percentage of daily energy: 15% protein; 56% carbohydrates; and 32% fat, which contained 9% saturated, 12% monounsaturated, and 12% polyunsaturated fats. The results indicated that:

1) There was a 3-mmHg decrease in DBP in women receiving ISP-62 mg isoflavones (p<0.04), but there was no change in blood pressure in men.
2) ISP-62 mg isoflavone decreased total cholesterol by 4% (241.2 to 231.7 mg/dL).
3) ISP-62 mg isoflavone decreased LDL cholesterol by 6% (164.7 to 154.5 mg/dL) in both men and women.
4) There were no significant effects on either triglycerides generic or HDL cholesterol.

Subjects were thereafter subdivided in two groups depending on entry LDL levels. There were 70 subjects with high-LDL cholesterol; dose-dependent changes of ISP-62 and ISP-37 isoflavone were: ISP-62 mg isoflavones had the following effects: 1) total cholesterol decreased by 9% (261-237 mg/dL), 2) LDL cholesterol decreased by 10% (260-240 mg/dL), and 3) no effect on HDL cholesterol.

The ISP-37 mg isoflavone had similar effects: 1) total cholesterol decreased by 8% (260-240 mg/dL), 2) LDL cholesterol decreased by 8% (182-165 mg/dL), and 3) no effect on HDL cholesterol.

There was a small decrease in LDL even from the 3-mg isoflavones.

When a gender subset of postmenopausal women was considered, those that received 62-mg isoflavones had reductions of total cholesterol (7%) and LDL cholesterol (8%) when compared to casein but no change in HDL cholesterol or triglycerides.

Washburn entered 51 perimenopausal women as outpatients (ages 45-55 years) into a randomized, double-blind crossover trial for 18 weeks. Subjects were randomized to one of three treatment groups, then crossed over to the other therapies (six weeks per treatment). Soy protein supplements (not food) were supplied in powder form as follows: group 1— 20 g complex carbohydrates containing no phytoestrogens, group 2—20 g soy protein supplement containing 34 mg phytoestrogens once daily, and group 3—20 g soy protein supplement containing 34 mg phytoestrogen in split doses.

All the diets were isocaloric, and women were given the supplemental powder, which was to be mixed with their food. The results in this short-term study indicated that:
1) Significantly lower levels of serum total cholesterol (decrease of 6%) were observed in both soy diets when compared to the carbohydrate diet.
2) LDL cholesterol was decreased by 7.5% in both soy diets, but there was no change in HDL cholesterol.
3) The DBP was significantly lower (decrease of 4.9 mmHg) in the split soy diet, when compared to the carbohydrate diet. Night sweats and hot flash severity (not frequency) were decreased in the split dosage group.

A major primate study helps to identify possible mechanisms of action. Young, male cynomolgus macaques monkeys were fed moderately atherogenic diets for three months—thereafter, randomly assigned to one of three groups. The source of protein in each dietary treatment was: 1) casein/lactalbumin, n=27; 2) Supro 670-HG (soy + therapy which contained genistein 1.10 mg and daidzein 0.37 mg per gram of isolate, n=27; and 3) Supro-670-IF (containing genistein 0.12 mg and daidzein 0.05 mg per gram of isolate, n=28. Amounts of calcium and phosphorus were equal in all the diets. After the three-month baseline period, the diets were fed for 14 months during whichblood samples were collected at assigned intervals. At the end of 14 months, the youngest 11 monkeys were necropsied (according to Federal Government Guidelines), and atherosclerosis evaluations and testicular weights were analyzed. The results indicated that: 1) The soy+ group had significantly lower total and LDL/VLDL cholesterol when compared to the other two groups (pO.OOl); 2) The soy+ group had increased HDL cholesterol when compared to the other two groups (pO.OOl); 3) Morphometric and angio-chemical studies were done to quantify atherosclerosis. Coronary artery atherosclerotic lesions were smallest in the soy+ group (90% less than the casein group and 50% less than the soy- group; 4) Data on intimal area (lesion size) for coronary arteries, abdominal aorta and left carotid bifurcation, left common carotid, and left common iliac arteries indicated that the average intimal area (atheromatous lesion size) was smallest in the soy+ group; 5) Testicular weights were unaffected by the phytoestrogens.

An Australian study (placebo-controlled, crossover trial) in healthy women (perimenopausal and menopausal) employed isoflavone tablets (80 mg isoflavone, of which 45 mg was genistein) and placebo to measured systemic arterial compliance within the main conduit arteries. Investigators estimated arterial elasticity by frequent automated arterial pressure measurements at five- and 10-week intervals. Isoflavones (80-mg tablets) improved systemic arterial compliance, and the mean difference (placebo = 0.81 ± 0.4 and isoflavone = 0.99 ± 0.54) was highly significant (paired t-test analysis, p=0.011). Placebo had no effect on systemic arterial compliance, while the isoflavone group experienced a 26% improvement in arterial compliance. The authors speculated that the smooth muscle layer within the vessel wall may be under influence of endothelial cell events, and that soybean isoflavones stimulate endothelial-related arteriolar relaxation.

Cassidy, in studying 15 healthy (inpatient) non-vegetarian premenopausal women, provided evience that soybean protein may have both agonist (stimulation) or antagonist (inhibitory) effects on estradiol 17 Beta receptors. Subjects were assigned to one of three groups receiving: 1) 60 g textured soybean protein (TSP) containing 45 mg conjugated isoflavones, n=6); 2) 28 g textured soybean protein containing 23 mg conjugated isoflavones, n=6); and 3) 50 g miso (fermented soybean paste) which contains 25 mg unconjugated isoflavones, n=3). Methodological limitations exist in these small numbers, as no diet significantly affected transit timeover complete menstrual cycle. The menstrual transit time during the follicular phase was longer in the miso group (miso= 45.8 ± 5.3 hours; control = 35.1 ± 4.7 hours). These normal premenopausal females exhibited a decrease in total cholesterol in the group receiving with TSP-60 g with 45 mg isoflavones. In addition, the group receiving the higher (TSP-60 g with 45 mg isoflavones) exhibited increased follicular phase length and peak progesterone concentrations delayed (pO.Ol). Mid-cycle peaks in luteinizing hormone (LH) and follicle stimulating hormone (FSH) were suppressed with 45 mg isoflavonoids. (p<0.05). The menstrual cycle effects of soybean protein with intact isoflavones must be evaluated.
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