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Anti-oxidant effect

Oxidation is an important risk factor for both carcinogenesis and atherogenesis. Oxidative damage of LDL particles in the subendothelium of blood vessels appears to contribute substantially to foam cell formation with subsequent plaque formation.

There is some convincing evidence that soy protein, naturally rich in isoflavones, can be effective antioxidants, acting like vitamin E, to reduce risk for CHD in humans. These effects can be achieved at levels of soy and isoflavone consumption that are achievable with modest dietary changes.

Tikkanen et al (1) studied the effects of soy protein containing 60 mg isoflavones per day on LDL oxidation in a group of six healthy volunteers. They measured copper-induced LDL oxidation during a baseline period, after 2 week of soy consumption and then after a 2-week washout period. They found that soy treatment significantly prolonged LDL oxidation lagtime by ~20 min.

Wiseman et al (2) evaluated both F2-isoprostanes and LDL resistance to copper-induced oxidation in 19 premenopausal women and 5 men who consumed 15 g soy protein that was either high in isoflavones (56 mg/day) or low (1.9 mg/day) for 17-day periods. They found that the high isoflavone soy resulted in significantly lower concentrations of 8-epi-prostaglandin F2 (= a biomarker of in vivo lipid peroxidation) and longer lag time for copper-induced LDL oxidation.

Jenkins (3) studied the effect of 50 g of soy protein per day during 1 month containing high (73 mg) and low amounts (10 mg) of isoflavones in 41 hyperlipidemic men and women on oxidized LDL assessed as conjugated dienes. Compared to the control group both soy diets resulted in significant reduction in oxidized LDL.

A trial of Vega-Lopez (4) observed very little effect on plasma antioxidant capacity and on biomarkers of oxidative stress. Although the plasma total antioxidant performance was 10% higher with soy protein (regardless of the isoflavones presence), soy protein did not significantly affect most individual markers of oxidative stress (LDL oxidizability, urinary F2-isoprostanes, malondialdehyde, protein carbonyls in native plasma).

On the contrary, Henneman et al (5) found that soy protein with and without isoflavones failed to substantially increase postprandial antioxidant capacity, thus on an acute basis, a significant increase in serum antioxidant capacity is not detectable following consumption of soy protein. Although some intervention studies showed an antioxidant effect, other studies did not show such an effect (6-8).

Anti- inflammatory effects

Inflammation of components of the subendothelial plaque is likely to contribute to the progression of the atherosclerotic process (9). Scientific evidence has shown that soy protein and it’s isoflavones have broad anti-inflammaroy effects that can dampen this vascular inflammation (10).  More recent evidence confirms these anti-inflammatory effects of soy protein intake on serum C-reactive protein levels (11).

Metabolic syndrome

When postmenopausal women with the metabolic syndrome replaced 1 portion of red meat in their diet by a portion of soynuts or soy protein this had a beneficial effect on oxidative stress as the total antioxiant capacity of plasma increased and a marker for oxidation (malondialdehyde) decreased (12). Further more endothelial function improved and some markers for inflammation were reduced when soy was consumed compared to red meat (13).

References :

  1. Tikkanen MJ, Wahala K, Ojala S, Vihma V& AdlercreutzH. Effect of soybean phytoestrogen intake on low density lipoprotein oxidation resistance.Proc Natl Acad Sci USA. 1998 Mar 17;95(6):3106-10. 

  2. Wiseman H, O'Reilly JD, Adlercreutz H, Mallet Al, Bowey EA, Rowland IR, Sanders TAB. Isoflavone phytoestrogens consumed in soy drecrease F2-isoprostane concentrations and increase resistance of low-density lipoprotein to oxidation in humans.Am J Clin Nutr 2000;72:395-400.

  3. Jenkins DJ, Kendall CW, Jackson CJ, Connelly PW, Parker T, Faulkner D, Vidgen E, Cunnane SC, Leiter LA, Josse RG.Effects of high- and low-isoflavone soyfoods on blood lipids, oxidized LDL, homocysteine, and blood pressure in hyperlipidemic men and women. Am J Clin Nutr. 2002 Aug;76(2):365-72.

  4. Vega-Lopez S, Yeum KJ, Lecker JL, Ausman LM, Johnson EJ, Devaraj S, Jialal I, Lichtenstein AH. Plasma antioxidant capacity in response to diets high in soy or animal protein with or without isoflavones. Am J Clin Nutr. 2005 Jan;81(1):43-9.

  5. Heneman KM, Chang HC, Prior RL, Steinberg FM.Soy protein with and without isoflavones fails to substantially increase postprandial antioxidant capacity. J Nutr Biochem. 2006 Apr 17 

  6. Nestel PJ, Yamashita T, Sasahara T, Pomeroy S, Dart A, Komesaroff P, Owen A, Abbey M. Soy isoflavones improve systemic arterial compliance but not plasma lipids in menopausal and perimenopausal women. Arterioscler Thromb Vasc Biol. 1997 Dec;17(12):3392-8.

  7. Samman S, Lyons Wall PM, Chan GS, Smith SJ, Petocz P. The effect of supplementation with isoflavones on plasma lipids and oxidisability of low density lipoprotein in premenopausal women. Atherosclerosis. 1999 Dec;147(2):277-83.

  8. Hodgson JM, Puddey IB, Croft KD, Mori TA, Rivera J, Beilin LJ. Isoflavonoids do not inhibit in vivo lipid peroxidation in subjects with high-normal blood pressure. Atherosclerosis. 1999 Jul;145(1):167-72

  9. Ross R. Atherosclerosis is an inflammatory disease. American Heart Journal 1999; 138: S419-S420 Ref Type: Journal (Full)

  10. Sadowska-Krowicka H, Mannick EE, Oliver PD, Sandoval M, Zhang XJ, Eloby-Childess S, Clark DA & Miller MJS. Genistein and gut inflammation: role of nitric oxide. Proceedings of the Society for Experimental Biology and Medicine 1998; 217: 351-357

  11. Jenkins DJA, Kendall CWC, Marchie A et al. Effects of a dietary portfolio of cholesterol-lowering foods versus lovastatin on serum lipids and C-reactive protein. J Amer Med Assoc 2003; 290: 502-10

  12. Azadbakht L, Kimiagar M, Mehrabi Y, Esmaillzadeh A, Hu FB, Willett WC. Dietary soya intake alters plasma antioxidant status and lipid peroxidation in postmenopausal women with the metabolic syndrome. Br J Nutr 2007;98:807-13.

  13. Azadbakht L, Kimiagar M, Mehrabi Y, Esmaillzadeh A, Hu FB, Willett WC. Soy Consumption, Markers of Inflammation, and Endothelial Function: A cross-over study in postmenopausal women with the metabolic syndrome. Diabetes Care 2007;30:967-73.