Soybeans are the most common source of isoflavones in human foods. The objectives of this study were to determine the effects of Thai soybean variety, planting date, physical seed quality, storage condition, planting location, and crop year on isoflavone content, as well as to analyze the relationship between seed viability and isoflavone content in soybean seeds grown in Thailand. Isoflavone content in Thai soybeans varied considerably depending on such factors as variety, physical seed quality, crop year, planting date (even in the same crop year), and planting location. Most varieties (except for Nakhon Sawan 1 and Sukhothai 1) had significantly higher isoflavone content when planted in early rather than in late dry season. Additionally, seed viability as well as long-term storage at or at ambient condition seemed unlikely to affect isoflavone content in Thai soybean varieties. Isoflavone content in soybean seeds grown in Thailand depends on multiple genetic and environmental factors. Some varieties (Nakhon Sawan 1 and Sukhothai 1) exhibited moderately high isoflavone content regardless of sowing date. Soybeans with decreased seed viability still retained their isoflavone content. 1. Introduction Soybeans (Glycine max (L.) Merrill) are the most common source of isoflavones in human foods, especially in many Asian countries. Isoflavones represent the most common group of phytoestrogens, which are structurally similar to estradiol-17β, the most potent mammalian estrogen [1]. Isoflavones exhibit three aglycone structures (daidzein, genistein, and glycitein), which enter into three -glycoside conjugates (daidzin, genistin, and glycitin), each with a corresponding acetyl and malonyl glycoside conjugate (Figure 1). Since glycitein and its glycoside conjugates account for less than 5–10% of the total isoflavones in soy-based products, most studies have focused on daidzein and genistein and their respective glycoside conjugates [2, 3]. This study adopted a similar approach. Figure 1: Structure of isoflavones in the forms of aglycones (a), -glycosides, 6′′- O-acetyl- -glycosides, and 6′′- O-malonyl- -glycosides (b). Epidemiological and clinical studies of postmenopausal women have suggested that isoflavones decrease their risk of osteoporosis and cardiovascular diseases while alleviating vasomotor symptoms [4–8]. However, the beneficial effects of isoflavones remain controversial. Discrepancies in overall study design, differences in isoflavone type and dosage, variations in participant demographics, and differing outcome parameters might all have contributed
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