Clinical and Investigative Medicine

 

Effects of substituting dietary soybean protein and oil for milk protein and fat in subjects with hypercholesterolemia

Elzbieta M. Kurowska, PhD
John Jordan, MD
J. David Spence, MD, PhD
Steven Wetmore, MD
Leonard A. Piché, PhD
Margaret Radzikowski
Paresh Dandona, MD
Kenneth K. Carroll, PhD

Clin Invest Med 1997;20(3):162-170.

[résumé]

Drs. Kurowska and Carroll are with the Department of Biochemistry and Dr. Jordan is with the Department of Family Medicine, University of Western Ontario, London, Ont.; Dr. Spence is with the Department of Clinical Neurological Sciences, University Hospital, London, Ont.; Dr. Piché is with the Food and Nutrition Program, Brescia College, London, Ont.; Ms. Radzikowski was with the University Hospital, London, Ont., and is now with Heritage Home, Utica, NY; and Dr. Dandona is with the Department of Medicine, University of Buffalo, Buffalo, NY.

Part of this work was presented at the Experimental Biology '96 meeting, Washington, Apr. 13­18, 1996 and at the Second International Symposium on the Role of Soy in Preventing and Treating Chronic Disease, Brussels, Belgium, Sept. 15­18, 1996.

(Original manuscript submitted Sept. 3, 1996; received in revised form Mar. 17, 1997; accepted Mar. 13, 1997)

Reprint requests to: Dr. Elzbieta M. Kurowska, Department of Biochemistry, University of Western Ontaro, London ON N6A 5C1

Contents

Abstract

Objectives: To determine whether, in individuals with hypercholesterolemia, substituting dietary soybean products for cows' milk products improves the plasma lipid profile and whether any change in the profile is due partially to soy oil.

Design: Randomized 3-treatment crossover trial.

Setting: Family practice clinics and an outpatient clinic in London, Ont.

Participants: Seventeen healthy men and 17 healthy women with elevated plasma levels of total and low-density-lipoprotein (LDL) cholesterol and with normal plasma levels of triglycerides.

Interventions: Participants incorporated into their normal diet either 2% cows' milk products, soybean products or a combination of skim milk products and soy oil, each over period of 4 weeks, with 2 2-week wash-out periods. Plasma lipid profile, blood pressure and body weight were assessed after each dietary and wash-out period.

Outcome measures: Plasma levels of total and lipoprotein cholesterol, plasma levels of triglycerides, apolipoprotein B and A1 levels, blood pressure and plasma lipid peroxidation.

Results: The change in diet had no effect on body mass index, levels of apolipoproteins B and A1 and most plasma lipids. During the soybean period, the subjects' mean level of high-density-lipoprotein (HDL) cholesterol increased 9% (p < 0.04) and their mean LDL/HDL cholesterol ratio decreased 14% (p < 0.007). These effects were less pronounced during the skim milk/soy oil period. In the 24 subjects with the highest initial LDL cholesterol level and LDL/HDL cholesterol ratio, the mean LDL cholesterol level decreased 11% after the soybean period. In all subjects, changes in the LDL/HDL cholesterol ratio induced by a soybean diet were negatively correlated with the initial LDL/HDL cholesterol ratio and positively correlated with the initial HDL cholesterol level.

Conclusions: In people with hypercholesterolemia, the plasma lipid profile improved after treatment with a soybean-product diet, and this improvement was partially due to soy oil. The degree of responsiveness was associated with initial risk factors for coronary artery disease.

Résumé

 Objectifs : Déterminer si, chez les personnes qui ont de l'hypercholestérolémie, le remplacement des produits à base de lait de vache par des produits diététiques au soja améliore le profil lipidique plasmatique et si tout changement du profil est attribuable en partie à l'huile de soja.

Conception : Étude randomisée croisée à 3 traitements.

Contexte : Cliniques de pratique familiale et clinique externe à London (Ontario).

Participants : Dix-sept hommes en bonne santé et 17 femmes en bonne santé qui avaient des taux plasmatiques élevés de cholestérol total et de lipoprotéines de basse densité (LDL) et des taux plasmatiques normaux de triglycérides.

Interventions : Les participants ont intégré à leur alimentation normale soit des produits à base de lait de vache à 2 %, soit des produits à base de soja, soit une combinaison de produits à base de lait écrémé et d'huile de soja, pendant 4 semaines dans chaque cas, suivies de 2 périodes d'élimination de 2 semaines. On a évalué le profil lipidique plasmatique, pris la tension artérielle et calculé la masse corporelle après chaque période d'alimentation et d'élimination.

Mesures des résultats : Taux plasmatiques de cholestérol total et de cholestérol à lipoprotéines, taux plasmatiques de triglycérides, taux d'apolipoprotéines B et A1, tension artérielle et peroxidation des lipides plasmatiques.

Résultats : Le changement d'alimentation n'a eu aucun effet sur l'indice de masse corporelle, les taux d'apolipoprotéines B et A1 et la plupart des lipides plasmatiques. Au cours de la période d'alimentation au soja, le taux moyen de cholestérol à lipoprotéines de haute densité (HDL) des sujets a augmenté de 9 % (p < 0,04) et le ratio moyen de cholestérol LDL/HDL a diminué de 14 % (p < 0,007). Ces effets ont été moins prononcés au cours de la période de consommation de lait écrémé et d'huile de soja. Chez les 24 sujets qui avaient les taux initiaux de cholestérol LDL et le ratio de cholestérol LDL/HDL les plus élevés, le taux moyen de cholestérol LDL a diminué de 11 % après la période de consommation de soja. Chez tous les sujets, on a établi un lien négatif entre les changements du ratio de cholestérol LDL/HDL causés par une alimentation au soja et le ratio initial de cholestérol LDL/HDL, et un lien positif avec le taux initial de cholestérol HDL.

Conclusion : Chez les personnes qui ont de l'hypercholestérolémie, le profil lipidique plasmatique s'est amélioré après traitement par une alimentation aux produits à base de soja et cette amélioration est attribuable en partie à l'huile de soja. On a établi un lien entre l'importance de la réaction et des facteurs de risques initiaux de coronaropathie.

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Introduction

 The antihypercholesterolemic effect of dietary soy protein, as compared with casein and other animal proteins, is well described in laboratory animals, and new data about this effect in humans continue to emerge.1­3 In clinical studies, both open and controlled, the substitution of soy protein for animal protein in the diet has been associated with various degrees of reduction of serum total and low-density-lipoprotein (LDL) cholesterol levels, especially in individuals with pre-existing hypercholesterolemia.3,4 Some reports have also demonstrated that treatment with soy protein results in moderate reduction in serum triglyceride levels5,6 and in small increases in high-density-lipoprotein (HDL) cholesterol levels.7,8 A recent meta-analysis of the effects of soy protein intake in 38 clinical trials has shown that a diet high in soy protein resulted in a significant reduction in hypercholesterolemia in individuals with elevated initial levels of serum total and LDL cholesterol and that a daily intake of more than 30 g of soy protein was necessary for the reductions.9 Initial hypercholesterolemia and a high soy protein intake may not be the only important variables in the soybean­plasma cholesterol equation. Also, the beneficial effects of dietary soy protein have been more pronounced in European than in comparable North American studies. It has recently been hypothesized that this difference may be due to interactions with other dietary components or to differences in the subjects studied or both.2,10

In most human trials to study the effects of soy protein intake, animal fat was added to the diets to match the fatty-acid composition of a standard, control diet. This manipulation could minimize any beneficial changes in blood lipids induced by soy protein, since saturated fats are known to have independent hypercholesterolemic properties.11 During the last few years, whole soybean products containing both soy protein and soybean oil have become more widely available for general consumption. These products could have greater hypocholesterolemic potential than low-fat soy protein products alone, as a result of the additional effect of unsaturated fatty acids present in soy oil.11 The objectives of this study involving people with hypercholesterolemia were therefore (1) to determine whether plasma levels of total and LDL cholesterol can be reduced by substituting dietary soybean products for cows' milk products containing comparable amounts of protein and fat, and (2) to assess the contribution of soy oil alone toward any effect on the lipoprotein profile attributed to soybean products.

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Materials and methods

Subjects

Thirty-four healthy volunteers with hypercholesterolemia (17 men and 17 women) were recruited from 2 family practice clinics and 1 outpatient clinic in London, Ont. Baseline characteristics of all the subjects are given in Table 1. Most of the participants had moderately elevated initial plasma total and LDL cholesterol levels (5.8 to 8.9 mmol/L and 3.5 to 5.4 mmol/L, respectively), with the exception of 1 subject who had a very high LDL cholesterol level (8.5 mmol/L) and another who had a normal LDL cholesterol level (2.9 mmol/L). To be included, participants also had to: (1) have initial fasting plasma triglyceride levels within the normal range (subject range 0.8 to 3.7 mmol/L); (2) be habitual milk drinkers (intake equivalent to 2 or more glasses per day); (3) be free of thyroid disorders, (4) kidney disease and (5) diabetes mellitus; and (6) have an alcohol intake that did not exceed 2 drinks per day. Most of the participants had not been prescribed cholesterol-lowering medication before the study, and those who were taking such medication were asked to discontinue it 2 to 3 weeks before starting the diets. The experimental protocol was approved by the Human Ethics Committee of the University of Western Ontario. The protocol was fully explained to the participants during group information sessions before the study. During these sessions, subjects were allowed to taste samples of the soybean products, in order to assess their ability to adhere to the protocol and to select flavours to their liking. Written and signed consent was obtained from all participants.

Experimental protocol

The study was a randomized 3-treatment crossover design, the details of which are included in Fig. 1. Participants consumed their normal diet during the full 16 weeks of the study except that they were asked to incorporate either cows' milk products, soybean products (Alpro Natural Soyfoods, Wevelgem, Belgium) or a combination of skim milk products and soybean oil into their diet over periods of 4 weeks at a time. Each dietary period was separated by a 2-week wash-out period during which subjects resumed their normal diet. Thus, each subject acted as his or her own control.

The food products incorporated into the subjects' diet during each period contained 31 g of protein (from cows' milk or soybean products) and a total of 18 g of fat (derived from cows' milk products, soybean products or a combination of pure soy oil [16 g] and skim milk fat [2 g], Fig. 2). To achieve these levels, participants were instructed to consume particular brands of low-fat milk desserts or skim milk desserts that had protein and fat contents similar to those in whole soybean desserts provided by Alpro Natural Soyfoods. Thus, participants incorporated 3 glasses of either milk (2% cows' milk, skim milk or soy milk, each containing 3.6% protein) and ½ cup of cows' milk dessert or soy milk dessert (each containing 3.0% protein) into their diet each day during any given dietary period. During the diet that included skim milk products, 1 tablespoon of soy oil was also added to the diet to keep the fat content of all 3 dietary regimens the same (Fig. 2).

At the outset of the study and at the end of each dietary period, fasting blood samples were taken (from the anticubital vein of the forearm) and body weight was recorded. Blood pressure readings, performed with a mercury manometer while the participants were seated, were also recorded, and means were calculated from 2 readings from the arm that gave the higher reading during initial measurement. Plasma lipoproteins (VLDL, LDL and HDL) were separated by discontinuous density gradient ultracentrifugation, as described by Redgrave, Roberts and West12 and modified by Terpstra, Woodward and Sanchez-Muniz.13 Cholesterol levels were measured with a commercially available enzymatic kit (Randox Laboratories Canada Ltd., Mississauga, Ont.). Plasma concentrations of apolipoprotein B and apolipoprotein A1 were determined by immunoprecipitation (Randox Laboratories Canada Ltd.) and plasma lipid peroxidation was assessed by thiobarbiturate oxidation (TBARS) as described by Ohkawa, Ohishi and Yagi.14 Three-day dietary food records were obtained at baseline and during each dietary period; each included 1 day on a weekend. Practical instructions on the preparation of food records and on planning menus were provided by 1 of the authors who is a registered dietitian (M.R.). The dietitian maintained contact with the participants at least weekly to ensure comprehension of and compliance with the dietary regimens.

Statistics

Statistical analysis of the effects of treatments was carried out using repeated-measures analysis of variance (ANOVA) of the change from the pretreatment baseline values. ANOVA was followed by pairwise comparison of the least significant difference. To reduce the number of comparisons, these tests were done only for parameters significantly different according to the ANOVA analysis.

For 2 participants who did not complete the last dietary period, single missing values were imputed by using the average values obtained from their remaining 2 dietary periods. Wash-out values were not included in the analysis, since it was assumed that they would be affected by any carry-over from previous dietary periods.

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Results

Analysis of dietary records for baseline and dietary periods is presented in Table 2. The intake of total energy, protein, carbohydrates, total fat and monounsaturated fatty acids was not significantly different in any of the treatment periods. In accordance with the design, the diets significantly affected the ratio of protein from animal versus plant sources (increasing it substantially during the cows' milk period and the milk/oil period, and reducing it during the soybean period). Also, the intake of saturated fatty acid (SAFA) and polyunsaturated fatty acid (PUFA) was significantly affected by the diets, with levels of SAFA being elevated during the milk period and levels of PUFA being elevated during both the milk/oil and soybean periods. In addition, the diets induced several significant effects on intake of many of the minor food components examined. Levels of calcium, phosphorus and vitamin D were elevated during the consumption of milk and skim milk products, whereas fibre intake was higher during consumption of fibre-rich soybean products. The diets had a significant effect on intake of cholesterol, increasing it during the 2% milk period compared with the skim milk/soy oil period but not with the soybean period. Also, a significant reduction in sodium intake was observed during the soybean period, but this could be due to lack of monitoring of discretionary salt use by the subjects.

Changes in the subjects' baseline characteristics in response to diet are presented in Table 3. Our results reveal that the diets had no effect on body weight and body mass index (not shown), on levels of apolipoproteins B and A1, or on most of the plasma levels of lipids. However, the diets significantly influenced the HDL cholesterol level and the LDL/HDL cholesterol ratio. HDL cholesterol levels increased a mean 9% and LDL/HDL cholesterol ratios decreased a mean 14% during the soybean period. The latter effect appeared to result from elevations in HDL cholesterol levels and nonsignificant reductions in LDL cholesterol levels during the soybean period. The levels of LDL and HDL cholesterol and the LDL/HDL ratio during the skim milk/soy oil period were intermediate when compared with values obtained during the milk and soybean periods.

In addition to influencing blood lipids, the diets had significant effects on diastolic blood pressure (DBP), systolic blood pressure (SBP) and on total plasma concentrations of peroxides (Table 3). During the milk period, there was a mean 6% reduction from baseline in both DBP and SBP, compared with a minimal change during the soybean period. The plasma concentrations of peroxides decreased by a mean 7% during the milk period and did not change significantly during the other 2 periods. The reductions in blood pressure and in plasma concentrations of peroxides induced by the milk diet were not significantly correlated with corresponding intakes of either calcium or iron, respectively. The order of the dietary periods and the interaction between the diet and the order of periods did not appear to significantly influence any of the responses. In addition, there was no consistent evidence of carry-over effects during the wash-out periods.

The dietary effects observed for all subjects generally disappeared when men and women were considered separately (data not shown). Thus, after a diet containing soybean products, a significant reduction in the LDL/HDL cholesterol ratio, DBP and SBP was observed in the men but not in the women, and increases in the HDL cholesterol level did not reach statistical significance for either sex. The effect of diet on plasma peroxide concentration remained significant for both men and women, with p values at the level comparable to that obtained for the entire group.

In 24 of the 34 subjects who participated in the study, LDL cholesterol levels were reduced (by a mean 11%) as a result of a diet of soybean products. In this group, which we will refer to as "responders," intake of soybean products also increased HDL cholesterol levels by a mean 9% and decreased the LDL/HDL cholesterol ratio by a mean 19%, but did not alter plasma triglyceride levels, plasma lipid peroxide concentrations or blood pressure. Intake of skim milk/soy oil products caused similar but less pronounced changes in the plasma lipid profile of responders, whereas intake of milk products had little effect on LDL and HDL cholesterol levels in this group. The responders had a mean 15% higher baseline LDL cholesterol level, a 13% higher LDL/HDL ratio, a 8% lower HDL cholesterol level, a 11% lower DBP and a 9% lower SBP than the remaining nonresponders.

To determine which baseline parameters were important in altering the LDL/HDL cholesterol ratio in all subjects, regression analysis was carried out for associations between these parameters and changes in the LDL/HDL cholesterol ratio induced by diet. The results revealed that changes in this ratio induced by the soybean diet were inversely related to the initial LDL/HDL cholesterol ratio (Fig. 3) and strongly positively correlated with the initial HDL cholesterol concentration (r = 0.52, p < 0.002). Analysis of changes in the LDL/HDL cholesterol ratio induced by the skim milk/soy oil diet showed an inverse relation to baseline plasma triglyceride concentrations (r = -0.45, p < 0.009) and a positive correlation with baseline HDL cholesterol levels (r = 0.43, p < 0.01). However, after this dietary period, alterations in the LDL/HDL cholesterol ratio were not significantly correlated with the baseline ratio of LDL/HDL cholesterol. Changes in the LDL/HDL cholesterol ratio induced by the milk diet were generally poorly associated with baseline parameters.

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Discussion

This study demonstrates that in a group of subjects consisting mainly of individuals with moderate hypercholesterolemia, substituting dietary whole soybean products for 2% cows' milk products containing similar amounts of protein and fat improves the plasma lipoprotein profile by significantly increasing HDL cholesterol levels and reducing the ratio of LDL/HDL cholesterol. These effects appear to be related mainly but not entirely to soy protein, since substituting a combination of soy oil and skim milk products for 2% cows' milk products results in only a modest reduction of the LDL/HDL cholesterol ratio and does not affect HDL cholesterol levels.

Our finding that a diet of soybean products containing 31 g protein and 16 g soy oil per day had a greater effect on the HDL cholesterol level and on the LDL/HDL cholesterol ratio than on the LDL cholesterol level is not entirely consistent with results of earlier trials.2,3,9 A rise in HDL cholesterol levels in response to soy protein intake has also been reported previously7,8 but, according to a meta-analysis by Anderson, Johnstone and Cook-Newell,9 the average change obtained in the studies examined was only 2.4% (range -3.1% to 5.4%) as compared with the 7% increase seen in our study. On the other hand, the reduction in the LDL cholesterol level (4%) induced by dietary soybean products in our study was less pronounced than the reduction induced by intake of soy protein for subjects in a similar range of initial hypercholesterolemia (6.8% to 9.8%), reported in the same meta-analysis.9 The fact that the dietary soybean products tested in our study had a relatively marked effect on the HDL cholesterol level, compared with the dietary soy protein examined in previous studies, could have been due in part to the soy oil content of the products we used. Indeed, we observed that a moderate elevation of the HDL cholesterol level was induced by a diet containing either soy oil in combination with skim milk or whole soybean products, but not by a diet containing 2% milk products. Similarly, a rise in the HDL cholesterol level was reported previously in a study in which the combined effects of dietary soy protein, cholesterol and a high proportion of PUFA were examined in normocholesterolemic subjects.15 However, other evidence suggests that the intake of PUFA is associated with a reduction in both LDL and HDL cholesterol levels.11,16 Another explanation for the moderately beneficial effect of dietary soybean products on the HDL cholesterol level is the relatively high content of flavonoids in these products (5 mg per 100 g soy milk). A recent study showed that a dietary soy protein preparation containing its flavonoid fraction substantially reduced combined VLDL and LDL cholesterol levels and, at the same time, moderately increased the HDL cholesterol level in peripubertal Rhesus monkeys, compared with a soy protein diet depleted of its flavonoids.17 Also, a recent study in postmenopausal women given a flavonoid-enriched soy protein diet found that the diet resulted in a moderately increased HDL cholesterol level (Dr. Susan M. Potter, Associate Professor, Department of Food Science and Human Nutrition, University of Illinois, Urbana, Ill.: personal communication, 1996).

Unlike the results reported above for the entire group of subjects, in the subgroup of responders, the effects of dietary soybean products on the LDL cholesterol level appeared to be similar to those observed earlier in individuals with comparable initial hypercholesterolemia given a soy protein diet.9 However, in the responders, the greater reduction in the LDL cholesterol level as a result of dietary soybean products was associated not only with relatively high initial LDL cholesterol levels, as it was in the previous trials of the effect of soy protein, but also with low initial HDL cholesterol levels and low initial blood pressure. The importance of this association should be investigated further in a larger group of people with baseline characteristics similar to those of the responders in our study.

Analysis of correlations between the changes in LDL/HDL ratio induced by dietary soybean products and the metabolic state of subjects at baseline showed that, in the heterogeneous group of people with hypercholesterolemia who participated in the study, some subjects appeared to be predisposed to the benefits of treatment with these products. For the entire group, a strong negative correlation was observed between the baseline ratio of LDL/HDL cholesterol and the reduction of this ratio caused by the intake of soybean products (Fig. 3). This suggests that, in general, individuals with the highest initial ratio of LDL/HDL cholesterol are the most likely to experience a reduction of this ratio when soybean products are consumed. The increased susceptibility of subjects with a high initial LDL/HDL cholesterol ratio to the reduction of this ratio by dietary soybean products appears to be due to their generally low initial HDL cholesterol levels and high initial LDL cholesterol levels. In all subjects given soybean products, a strong positive correlation was observed between the reduction in the LDL/HDL cholesterol ratio and the baseline HDL cholesterol level (r = 0.52, p < 0.002). No correlation was observed between the LDL/HDL cholesterol ratio and the baseline LDL cholesterol concentration; however, higher baseline LDL cholesterol concentrations were observed in responders than in nonresponders. Our results are, therefore, only partially consistent with previous data that showed an association between the high initial plasma level of cholesterol and its degree of reduction resulting from soy protein intake in individuals with hypercholesterolemia.3

Changes in the LDL/HDL cholesterol ratio induced by soy oil in combination with skim milk appear to be less strongly correlated with baseline levels of LDL and HDL cholesterol than changes in this ratio induced by the intake of soybean products. No correlation was observed between the baseline LDL/HDL cholesterol ratio and the reduction of this ratio resulting from dietary soy oil. This finding likely resulted from a weak influence of high initial HDL cholesterol levels on the drop in the LDL/HDL cholesterol ratio induced by this diet (r = 0.43, p < 0.01). However, the reduction in the LDL/HDL cholesterol ratio induced by dietary soy oil was highly significantly inversely related to the baseline plasma level of triglycerides (r = -0.45, p < 0.009). By comparison, a similar association was absent after a diet containing soybean products. A reason for the association between the baseline plasma triglyceride level and the decrease in the LDL/HDL cholesterol ratio after a diet containing soy oil remains unclear, since this diet did not cause any significant reduction in plasma levels of triglycerides. As well, PUFA, known to be abundant in soy oil, did not affect plasma triglyceride levels in previous clinical trials.16

Minor alterations in the LDL/HDL cholesterol ratio induced by the intake of 2% milk products did not seem to be related to any baseline parameters. This was consistent with the lack of significant improvement in the other plasma lipid variables examined after this diet. The significant reduction of blood pressure induced by the intake of 2% milk products is difficult to explain and should be further investigated. The reduction of plasma levels of lipid peroxides resulting from the diet containing 2% milk products but not from the diet containing soybean products could be related to the fact that the former diet contained mainly saturated fatty acids, which are not detectable by TBARS, whereas the latter diet was enriched with PUFA, including 18:3 (n-3), which has the ability to react with thiobarbituric acid.

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Conclusion

Our results demonstrate that dietary supplementation with whole soybean products moderately improved the plasma lipoprotein profile in all subjects and that the effect of a diet containing these products on the LDL/HDL cholesterol ratio was greater than that induced by soy oil in combination with skim milk. In addition, we showed that a susceptibility to treatment with soybean products and with soy oil is associated not only with a high initial level of LDL cholesterol but also with other metabolic risk factors for coronary artery disease: relatively low initial levels of HDL cholesterol and relatively high baseline plasma levels of triglycerides.

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Acknowledgements

 This study was supported by a grant from Alpro Natural Soyfoods, Wavelgem, Belgium. The authors wish to thank Ms. Trang Thu Tran and Ms. Rosine Francis for their expert technical assistance.

References
  1. Kritchevsky D. Dietary protein, cholesterol and atherosclerosis: a review of the early history. J Nutr 1995;125:589S-93S.
  2. Carroll KK. Review of clinical studies on cholesterol-lowering response to soy protein. J Am Diet Assoc 1991;91:820-7.
  3. Sirtori CR, Even R, Lovati MR. Soybean protein diet and plasma cholesterol: from therapy to molecular mechanisms. Ann N Y Acad Sci 1993;676:188-201.
  4. Carroll KK, Kurowska EM. Soy consumption and cholesterol reduction: review of animal and human studies. J Nutr 1995;125:594S-7S.
  5. Grundy SM, Abrams JJ. Comparison of actions of soy protein and casein on metabolism of plasma lipoproteins and cholesterol in humans. Am J Clin Nutr 1983;38:245-52.
  6. Verillo A, de Teresa A, Giarrusso PC, La Rocca S. Soybean protein diets in the management of of type II hyperlipoproteinaemia. Atherosclerosis 1985;54:321-31.
  7. Sirtori CR, Zucchi-Dentone C, Sirtori M, Gatti E, Descovich GC, Gaddi A, et al. Cholesterol-lowering and HDL-raising properties of lecitinated soy proteins in type II hyperlipidemic patients. Ann Nutr Metab 1985;29:348-57.
  8. Gaddi A, Descovich GC, Noseda G, Fragiocomo C, Nicolini A, Montanari G, et al. Hypercholesterolaemia treated by soybean protein diet. Arch Dis Child 1987;62:274-8.
  9. Anderson JW, Johnstone BM, Cook-Newell ME. Meta-analysis of the effects of soy protein intake on serum lipids. N Engl J Med 1995;333:276-82.
  10. Sirtori CR, Lovati MR, Manzoni C, Monetti M, Pazzucconi F, Gatti E. Soy cholesterol reduction: clinical experience. J Nutr 1995;125:598S-605S.
  11. Grundy SM, Denke MA. Dietary influences on serum lipids and lipoproteins. J Lipid Res 1990;31:1149-72.
  12. Redgrave TG, Roberts DCK, West CE. Separation of plasma lipoproteins by density gradient ultracentrifugation. Anal Biochem 1975;63:42-9.
  13. Terpstra AHM, Woodward CJH, Sanchez-Muniz FJ. Improved techniques for the separation of serum lipoproteins by density gradient ultracentrifugation: visualization by prestaining and rapid separation of serum lipoproteins from small volumes of serum. Anal Biochem 1981;11:149-57.
  14. Ohkawa H, Ohishi N, Yagi K. Assay for lipid peroxides in animal tissues by thiobarbituric acid reaction. Anal Biochem 1979;95:351-8.
  15. Meinertz H, Nilausen K, Faergeman O. Soy protein and casein in cholesterol-enriched diets: effects on plasma lipoproteins in normocholesterolemic subjects. Am J Clin Nutr 1989;50:786-93.
  16. Gordon DJ, Salz KM, Roggenkamp KJ, Franklin FA. Dietary determinants of plasma cholesterol change in the recruitment phase of the Lipid Research Clinics Coronary Prevention Trial. Arteriosclerosis 1982;2:537-48.
  17. Anthony MS, Clarkson TB, Hughes CB, Morgan TM, Burke GL. Soybean isoflavones improve cardiovascular risk factors without affecting the reproductive system of peripubertal Rhesus monkeys. J Nutr 1996;126:43-50.

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