Soy Protein And Isoflavones: What's The Connection? (2025)

Soy protein and isoflavones have been the subject of much research, particularly in relation to their potential health benefits. Soy isoflavones are phytoestrogens, which means they have biological activities that are unrelated to their interactions with estrogen receptors. They have been found to have a positive effect on cardiovascular health, with studies showing that they can lower cholesterol. However, the FDA has stated that there is no evidence to support a significant role for soy isoflavones in the cholesterol-lowering effects of soy protein.

Soy protein and isoflavones may improve risk factors for cardiovascular disease

Soy protein does contain isoflavones. Soy protein and isoflavones have gained considerable attention for their potential role in improving risk factors for cardiovascular disease.

In 1999, the US Food and Drug Administration (FDA*)* approved labelling for foods containing soy protein as protective against coronary heart disease. This decision was based on clinical studies showing that at least 25g of soy protein per day lowered total and LDL cholesterol.

In the majority of 22 randomised trials, isolated soy protein with isoflavones, as compared with milk or other proteins, decreased LDL cholesterol concentrations. The average effect was approximately 3%. This reduction is very small relative to the large amount of soy protein tested in these studies, averaging 50g, about half the usual total daily protein intake. No significant effects on HDL cholesterol, triglycerides, lipoprotein(a), or blood pressure were evident.

Soy isoflavones and their metabolites also have biological activities that are unrelated to their interactions with estrogen receptors. By inhibiting the synthesis and activity of certain enzymes involved in estrogen metabolism, soy isoflavones may alter the biological activity of endogenous estrogens and androgens. Soy isoflavones have also been found to inhibit tyrosine kinases, enzymes that play critical roles in the signalling pathways that stimulate cell proliferation. Additionally, isoflavones can act as antioxidants in vitro, but the extent to which they contribute to the antioxidant status of humans is not yet clear.

The hypocholesterolemic effect of soy is well-documented and this has led to the regulatory approval of a health claim relating soy protein to a reduced risk of cardiovascular disease. However, soybeans contain additional components, such as isoflavones, lecithins, saponins and fibre that may improve cardiovascular health through independent mechanisms.

The FDA has stated that soy isoflavones do not have a significant role in cholesterol-lowering effects

Soy protein does contain isoflavones. Soy isoflavones and their metabolites have biological activities that are unrelated to their interactions with estrogen receptors. By inhibiting the synthesis and activity of certain enzymes involved in estrogen metabolism, soy isoflavones may alter the biological activity of endogenous estrogens and androgens. They have also been found to inhibit tyrosine kinases, enzymes that play critical roles in the signalling pathways that stimulate cell proliferation.

In the majority of 22 randomised trials, isolated soy protein with isoflavones, as compared with milk or other proteins, decreased LDL cholesterol concentrations; the average effect was approximately 3%. This reduction is very small relative to the large amount of soy protein tested in these studies, averaging 50 g, about half the usual total daily protein intake. No significant effects on HDL cholesterol, triglycerides, lipoprotein(a), or blood pressure were evident.

Since the 1999 US FDA soy protein health claim approval, various studies have examined the ability of soy proteins to lower serum cholesterol in order to determine whether the cholesterol-lowering effect is attributable to the protein moiety, the isoflavone component, or due to synergy of a combination of both. It is suggested that earlier reports prior to approval of the soy protein health claim must have overestimated the magnitude of the effect.

Soy protein does contain isoflavones. Soy isoflavones have biological activities unrelated to their interactions with estrogen receptors. For example, by inhibiting the synthesis and activity of certain enzymes involved in estrogen metabolism, soy isoflavones may alter the biological activity of endogenous estrogens and androgens. Soy isoflavones have also been found to inhibit tyrosine kinases, enzymes that play critical roles in the signalling pathways that stimulate cell proliferation.

Additionally, isoflavones can act as antioxidants in vitro, but the extent to which they contribute to the antioxidant status of humans is not yet clear. Plasma F2-isoprostanes, biomarkers of lipid peroxidation in vivo, were significantly lower after two weeks of daily consumption of soy protein containing 56 mg of isoflavones than after consumption of soy protein providing only 2 mg of isoflavones.

The FDA requires that a serving contain at least 6.25 g of soy protein, 25% of the necessary daily amount (25 g), with the expectation that foods containing soy protein would be eaten at least 4 times per day. The FDA also stated that the evidence did not support a significant role for soy isoflavones in the cholesterol-lowering effects of soy protein. In 2000, the American Heart Association (AHA) Nutrition Committee released a scientific advisory on soy protein and CVD. At that time, the conclusion was that it is prudent to recommend including soy protein foods in a diet low in saturated fat and cholesterol.

In the majority of 22 randomised trials, isolated soy protein with isoflavones, as compared with milk or other proteins, decreased LDL cholesterol concentrations; the average effect was approximately 3%. This reduction is very small relative to the large amount of soy protein tested in these studies, averaging 50 g, about half the usual total daily protein intake. No significant effects on HDL cholesterol, triglycerides, lipoprotein(a), or blood pressure were evident.

Soy protein containing isoflavones can lower lipoprotein(a)

Soy protein does contain isoflavones, which are phytoestrogens. In 2000, the American Heart Association (AHA) Nutrition Committee released a scientific advisory on soy protein and CVD, concluding that it is prudent to recommend including soy protein foods in a diet low in saturated fat and cholesterol.

Soy protein and isoflavones have gained attention for their potential role in improving risk factors for cardiovascular disease. In the majority of 22 randomised trials, isolated soy protein with isoflavones, compared with milk or other proteins, decreased LDL cholesterol concentrations by an average of 3%. However, no significant effects on HDL cholesterol, triglycerides, lipoprotein(a), or blood pressure were evident.

In 2003, a study on the effects of soy protein containing isoflavones on lipids and indices of bone resorption in postmenopausal women was published in *Clin Endocrinol (Oxf)*. Another study, published in *Arterioscler Thromb Vasc Biol* in 2002, found that alcohol-extracted, but not intact, dietary soy protein lowers lipoprotein(a) markedly.

Soy isoflavones have been found to have biological activities unrelated to their interactions with estrogen receptors. By inhibiting the synthesis and activity of certain enzymes involved in estrogen metabolism, soy isoflavones may alter the biological activity of endogenous estrogens and androgens. They have also been found to inhibit tyrosine kinases, enzymes that play critical roles in the signalling pathways that stimulate cell proliferation. Additionally, isoflavones can act as antioxidants in vitro, but the extent to which they contribute to the antioxidant status of humans is not yet clear.

Soy isoflavones can act as antioxidants in vitro

Soy protein does contain isoflavones. Soy isoflavones can act as antioxidants in vitro, but the extent to which they contribute to the antioxidant status of humans is not yet clear.

In vitro, soy isoflavones have been found to stimulate the enzymes responsible for xenobiotics metabolism and oxidative stress reduction. They also directly quench free radicals, with genistein and daidzein being the major effective isoflavones in soy products that have ROS quenching activity.

In one study, isoflavones were extracted from soybeans using 80% food-grade ethanol at 80 degrees C for 6 hours. The soy extract contained isoflavones primarily in the form of glucosides. The results showed that the soy isoflavone extract had strong in vitro antioxidant activity. There was a dose-dependent response for the in vitro antioxidant activity at the lower concentrations but not at the higher concentrations.

Another study compared the content of isoflavones in two soy products and demonstrated the antioxidant effect of a soy diet in rats with myocardial infarction (MI). The amount of daidzin was higher in soybean hypocotyl (SH) and genistein in isolated soy protein (ISP). SH presented a 3-fold increase in TAR, while both products exhibited the same TRAP. The rats were fed an ISP diet for 9 weeks.

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