Soy foods/isoflavones

Figure 4.3 Chemical structures of Isoflavone glycosides. In soy foods, Isoflavones form 7-0-P-glucosIde conjugates. A Is the simple glucoside commonly found In soy milk, B Is the 6"-0-malonyl-7-0-p-glucoslde from the soybean, and C Is the 6"-0-acetyl-7-O-p-glucoside, a product of toasting of soy flour. D is the 7-0-P-glucuronide that is the principal conjugate in the blood.

Messina, M. and Bennink, M. 1998. Soy foods, isoflavones and risk of colonic cancer a review of the in vitro and in vivo data. Bailliere Clin, Endocrinol. Metab. 12 707-728. 

The primary isoflavones in soybeans (Glycine max) are genistein and daidzein and their respective 7-0- 5-glucosides. There are also small amounts of a third isoflavone, glycitein, and its glucoside, glycitin. In soybeans and in nonfermented soy foods, isoflavone glucosides are esterified with malonic or acetic acid [34]. 

High levels of isoflavone consumption are reached only in typical Asian diets (Chen et al., 1999 Nagata et al., 1997 Adlercreutz et al., 1991), with a peak at approximatively 100 mg/day in the Hong Kong Chinese population (Ho et al., 2000). In Western countries, isoflavone intake is much lower. The most recent evaluations in the USA (de Kleijn et al., 2001), as well as in different European countries, indicate a mean intake of isoflavones of less of 1 mg/day. Even amongst EU consumers eating mainly soy foods it does not exceed 6-10 mg/day (van Erp-Baart et al., 2003). 

In plants and, consequently, in soy food products, isoflavones occur in three main biologically active forms ... 

KIRK p, PATTERSON R E and LAMPE J (1999) Development of a soy food frequency questionnaire to estimate isoflavone consumption in US adults. J Am Diet Assoc. 99 (5) 558-63. 

Before 1990 the composition of isoflavones in soy foods was thought to be largely determined by whether the food had been fermented. Fermented foods (e.g., miso and tempeh) contain the unconjugated isoflavones agycones, while non-fermented food (e.g., tofu, soy flower, and soy milk) contain the conjugated glucoside. Subsequent experiments have demonstrated that fermentation of soy decreased the isoflavone content of the food product, but increased the urinary isoflavonoid recovery, suggesting that fermentation increases availability of isoflavones in soy (Slavin et al., 1998). 

Barnes, S., Kirk, M., and Coward, L., Isoflavones and their conjugates in soy foods extraction conditions and analysis by HPLC-mass spectrometry, J. Agric. Food Chem., 42, 2466, 1994. 

The possible role of isoflavonoids in the prevention of cancer and in particular hormone-dependent cancers such as breast and prostate cancer is currently extensively investi-gated. " " In addition, consumption of soy foods rich in isoflavones has been weakly associated with reduced colon cancer. Colon cancer risk is influenced by estrogen exposure although the mechanism of action has not been fully elucidated, studies with estrogen receptor a (see Section 7.3.3) knockout mice indicate that it may be independent of estrogen receptor... 

Consumption of soy foods (providing 60mg/day isoflavones) for 12 weeks by postmenopausal women has been found to significantly decrease clinical risk factors for osteoporosis (short-term markers of bone turnover) including decreased urinary M-telopeptide excretion (bone resorption marker) and increased serum osteocalcin (bone formation marker). Furthermore, consumption of a soy isoflavone supplement containing 61.8 mg of isoflavones for 4 weeks by postmenopausal Japanese women significantly decreased excretion of bone resorption markers. ... 

Consumption of soy foods for 10 weeks (100 mg/day isoflavones) has been reported to improve human memory in young healthy adults and consumption of a soy isoflavone supplement for 12 weeks (60 mg/day isoflavones) to improve cognitive function in postmenopausal women. By contrast, consumption of soy protein (99mg isoflavones/day) for 12... 

There have been some concerns expressed regarding the possible health consequences in adulthood (endocrinological and reproductive outcomes) of early-life isoflavone exposure from soy-based infant formula. The daily exposure of infants to isoflavones in soy-based infant formulas is 6- to 11-fold higher on a body weight basis than the dose that has hormonal effects in adults consuming soy foods. " However, evidence from adult and infant populations indicates that dietary isoflavones in soy-based infant formulas do not adversely affect human growth, development, or reproduction. 

Knowledge of the pharmacokinetics of isoflavones in soy foods is essential for making recommendations regarding efficacy in clinical studies. Die plasma half-life of daidzein and genistein, measured from their plasma appearance and disappearance curves, is 7.9 h in adults peak concentrations occur 6-8 h after administration of the pure compounds [93] with peak concentrations ranging from 80 to 800 ng/ml (after a single oral dose of 50 mg of pure isoflavones). 

Similar plasma concentrations were detected for daidzein, genistein and equol in adults consuming modest quantities of soy foods containing about 50 mg/d of total isoflavones [91]. 

Analysis of Isoflavones in Soy Foods Basic Protocol 1 Solvent Extraction of Isoflavones in Their Natural Forms 11.6.1... 

SOLVENT EXTRACTION OF ISOFLAVONES IN THEIR NATURAL FORMS FROM SOY FOODS... 

This method focuses on the extraction of soy food samples at room temperature using acidified solvent mixtures. The method is designed to release isoflavones in their natural forms from food matrices based on their polarity and solubility in solvents. Insoluble proteins, carbohydrates, and lipids present in the food are removed from the isoflavone extract using organic solvents, acid, and centrifugation. 

Table 11.6.2 Solvent Mixture for Extracting Isoflavones from Soy Foods"...

Food sample isoflavone extract from soy foods, after clean-up procedure (see Support Protocol 2)... 

The small particle size (4 pm) of the bonded phases and small pore size (60 A) of the Cl8 column efficiently resolve mixtures of similar isoflavone derivatives in the soy food matrix. 

Use the area of each isoflavone peak and the calibration curves to calculate the isoflavone concentrations in the sample. Calculate the final concentration of isoflavones in soy food by considering the dilution factor during sample preparation. 

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