Soy phytoestrogens are widely used as an alternative to hormone replacement therapy for the treatment of menopausal symptoms. In addition, the conclusion reached in 1999 by the US Food and Drug Administration, that foods containing soy protein included in a diet low in saturated fat and cholesterol may reduce the risk of CHD by lowering blood cholesterol levels, has also contributed to increased dietary soy consumption.
Estrogens prevent heart diseases, atherosclerosis, and vascular diseases, by the following mechanisms.[57
] So it is expected that phytoestrogens probably act similarly. However, there is no direct evidence in this regard.
- A favorable impact on serum lipids and lipoproteins
- Increase high-density lipoprotein cholesterol
- Decrease LDL cholesterol
- Decrease lipoprotein (a) (Lp(a))
- Reduce oxidation of LDL cholesterol
- Direct antiatherosclerotic effect in arteries
- Lower uptake of LDL cholesterol in blood vessels
- Prevent cholesterol deposition in vascular lesions
- Direct vascular effects
- Reduce vascular tone
- Preserve endothelial function
- Increase production of nitric oxide from blood vessels
- Increase prostacyclin release
- Inhibit endothelial production of endothelin-1
- Inhibit endothelin-1-induced vasoconstriction
- Direct inotropic action on the heart
- Effects on hemostasis
- Decrease plasma fibrinogen
- Reduce plasminogen activator inhibitor
- Improvement in peripheral glucose metabolism
- Decrease fasting blood glucose
- Decrease serum insulin levels
Antioxidant properties are one of the most important claims for food ingredients, dietary supplements, cosmetics, and anticancer natural products. Phytoestrogen rich plants have established antioxidant activity. Genistein and daidzein isolated from soybean seeds showed stronger antioxidant activity than their glycosides. Red clover extract, a rich source of isoflavones, showed stronger antioxidant activity than soy.[58
] Antioxidant activity of phytoestrogens can restore the tissues from stress or oxidative damage.
Both ERs, α and β, have been shown to be involved in neuroprotective effects of estradiol[59
] and to be expressed in the adult rat hippocampus.[62
] Therefore, the neuroprotective effects of soy extract and genistein in the hippocampus in vivo
may be mediated by ERs. Although soy isoflavones may activate both ER-α- and ER-β-mediated transcription, they are more potent activators of ER-β.[64
] Therefore, it is possible that the neuroprotective effects of soy extract and genistein may be mainly mediated by the activation of ER-β, which is highly expressed in neurons and glial cells in the adult rat hippocampal formation. In addition, estrogenic compounds may have neuroprotective effects that are independent of the activation of ERs.[67
] Indeed, genistein has ER-independent effects, affecting the activity of enzymes, such as protein tyrosine kinases, mammalian DNA topoisomerase I and II, and ribosomal S6 kinase.[68
] By its action on tyrosine kinases, genistein may alter phosphorylation events associated with the activation of neurotransmitter receptors. In addition, genistein may directly interact with neurotransmitter receptors and, in consequence, alter neuronal function and neuronal response to injury. Finally, genistein and soy extracts have antioxidant properties,[69
] which may contribute to their neuroprotective effects.
Phytoestrogen supplements are routinely advertised as an alternative to traditional hormone replacement therapy and as a way to alleviate both the physical and emotional effects of menopause. To date, there is almost no evidence to support these claims, and data from numerous animal studies suggest that soy may act to suppress sexual function and increase anxiety. The impact of soy-rich diets on human emotional and sexual health deserves to be explored in greater detail, and the numerous clinical trials are now underway to examine the effects of a soy-rich diet on bone density, cancer rates, and cardiovascular health.