Soy Protein Isolate: Phytoestrogen Source? (2025)

Soy protein isolate is a carefully processed form of soy protein. It contains phytoestrogens, which are isoflavones that can be found in soybeans. Phytoestrogens have been found to favourably affect the plasma lipoproteins of male rhesus monkeys, and have no adverse effects on their reproductive systems. In this study, monkeys were fed diets that differed only with respect to the protein component, including casein/lactalbumin, alcohol-extracted soy protein isolate, and unextracted soy protein isolate.

Soy protein isolate can be extracted with alcohol to reduce lactins, phytase, and trypsin inhibitors

Soy protein isolate does contain phytoestrogens. In one study, the soy protein with the phytoestrogens intact (Supro 670-HG) contained about 1.10 mg genistein and 0.37 mg daidzein (the two principal soy phytoestrogens) per gram of soy protein isolate. The alcohol-extracted soy protein (Supro 670-IF) contained only 0.12 mg genistein and 0.05 mg daidzein per gram of isolate.

Trypsin inhibitor levels in raw soybeans, soy protein isolates and concentrates, and ready-to-feed, concentrate, and powder forms of soy-based infant formulas were measured by two modifications of the standard enzymatic method for trypsin inhibitor determination. Levels were usually higher following sodium hydroxide extraction compared to glycine-buffered extraction. Soy protein concentrates had inhibitor levels intermediate to those in the raw soy and soy protein isolates tested.

In one study, the monkeys were fed moderately atherogenic diets that differed only with respect to the protein component (casein/lactalbumin, alcohol-extracted soy protein isolate, or unextracted soy protein isolate). This was done to evaluate the relative contribution of the protein moiety versus the alcohol-extractable components of soy on atherosclerosis and its risk factors.

Soy protein isolate contains the phytoestrogens genistein and daidzein

In a study on male rhesus monkeys, it was found that the phytoestrogens in soybeans favourably affected plasma lipoproteins, significantly lowering total and LDL plus VLDL cholesterol, with no adverse effects on the monkeys' reproductive systems. This prompted further studies to ascertain whether soy phytoestrogens could diminish or prevent diet-induced coronary artery atherosclerosis in young male cynomolgus monkeys.

The isolated soy proteins used for these studies were provided by Protein Technologies International. The soy protein with the phytoestrogens intact (Supro 670-HG) contained about 1.10 mg genistein and 0.37 mg daidzein per gram of soy protein isolate. The alcohol-extracted soy protein (Supro 670-IF) contained only 0.12 mg genistein and 0.05 mg daidzein per gram of isolate.

In another study, fifty healthy postmenopausal women aged 50-75 participated in a double-blind, placebo-controlled trial in which they received either soy protein isolate (40 g soy protein, 118 mg isoflavones) or a casein placebo. Urinary isoflavone excretion increased in the soy group and was higher than in the placebo group at the end of three months.

Soy phytoestrogens have been found to lower total and LDL plus VLDL cholesterol in male rhesus monkeys

Soy protein isolate does contain phytoestrogens. In a study, monkeys were fed a diet with soy protein isolate, from which the phytoestrogens had been extracted, as the protein source (0.17 mg phytoestrogens/g isolate). Another group was fed a diet with soy protein isolate, with the phytoestrogens intact, as the protein source (1.5 mg phytoestrogens/g isolate). The monkeys fed the diet with the phytoestrogens intact had significantly lower total and LDL plus VLDL cholesterol concentrations compared with the other two groups.

The study also found that the phytoestrogens (isoflavones) of soybeans both favourably affect the plasma lipoproteins (significantly lower total and LDL plus VLDL cholesterol) of male rhesus monkeys and have no adverse effects on their reproductive systems. This observation prompted further studies to ascertain whether the soy phytoestrogens could diminish or prevent diet-induced coronary artery atherosclerosis in young male cynomolgus monkeys.

The monkeys were fed moderately atherogenic diets that differed only with respect to the protein component (casein/lactalbumin, alcohol-extracted soy protein isolate, or unextracted soy protein isolate). This was done to evaluate the relative contribution of the protein moiety versus the alcohol-extractable components of soy on atherosclerosis and its risk factors. The isolated soy proteins used for this study were provided by Protein Technologies International. The soy protein with the phytoestrogens intact (Supro 670-HG) contained about 1.10 mg genistein and 0.37 mg daidzein (the two principal soy phytoestrogens) per gram of soy protein isolate. The alcohol-extracted soy protein (Supro 670-IF) contained only 0.12 mg genistein and 0.05 mg daidzein per gram of isolate. The alcohol-extracted soy protein isolate contained 90.7% protein and 0.7% fat compared with the unextracted protein, which was 87.0% protein and 4.0% fat.

Soy phytoestrogens have been found to have no adverse effects on the reproductive systems of male rhesus monkeys

Soy protein isolate does contain phytoestrogens. The two principal soy phytoestrogens are genistein and daidzein.

One study fed male rhesus monkeys a diet with soy protein isolate, from which the phytoestrogens had been extracted, as the protein source. Another group was fed a diet with soy protein isolate, with the phytoestrogens intact, as the protein source. The monkeys were fed moderately atherogenic diets that differed only with respect to the protein component. This was done to evaluate the relative contribution of the protein moiety versus the alcohol-extractable components of soy on atherosclerosis and its risk factors.

Another study assessed the biological activity of isoflavone phytoestrogens by analysing the effect of dietary soy isoflavone supplementation on in vivo bioassays of estrogenicity. Fifty healthy postmenopausal women aged 50-75 participated in a double-blind, placebo-controlled trial in which they received either soy protein isolate or a casein placebo. Urinary isoflavone excretion increased in the soy group and at the end of 3 months was higher in the soy group than in the placebo group.

Soy protein isolate has been used in a double-blind, placebo-controlled trial to assess the biological activity of isoflavone phytoestrogens in postmenopausal women

The trial involved 50 healthy postmenopausal women aged 50-75 who participated in the trial. The women received either soy protein isolate (40 g soy protein, 118 mg isoflavones) or a casein placebo. Measurements were taken at the baseline and at 3 months. Urinary isoflavone excretion was measured to reflect compliance. The bioassays of estrogenicity included measurements of hepatic proteins and gonadotropin concentrations.

The baseline characteristics were not significantly different between the soy and placebo groups. However, urinary isoflavone excretion increased in the soy group and, at the end of 3 months, was higher in the soy group than in the placebo group.

Soy protein isolate is carefully processed before ethanol extraction to reduce lactins, phytase, and trypsin inhibitors. The extracted and unextracted proteins are similar for components other than the phytoestrogens and lipid. The soy protein with the phytoestrogens intact (Supro 670-HG) contained about 1.10 mg genistein and 0.37 mg daidzein (the two principal soy phytoestrogens) per gram of soy protein isolate. The alcohol-extracted soy protein (Supro 670-IF) contained only 0.12 mg genistein and 0.05 mg daidzein per gram of isolate.

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