Discussion and conclusions

The unique properties of soy isoflavones confirmed in many in vitro studies hold great promise for their possible future use in the treatment of many diseases including cancer. They are summarized and presented in Table 3.

Table 3 Impact of soy isoflavones on human health conditions and diseases
Full size table

So far, the observed estrogenic and cytotoxic activity at dietary doses, induction of cell apoptosis and antiproliferative activity observed at pharmacological doses are among the most important mechanisms by which isoflavones, such as genistein and daidzein, affect cancerogenesis. It is important to expand research and conduct further studies on the molecular mechanisms of action of these compounds, so that they can be used effectively in the treatment of patients beyond oncology. They can bring great value in personalised treatment by local injections in the tumours.

So far, the observed antiproliferative activity (cell-cycle arrest) and cytotoxic activity (apoptosis induction and cell death) are among the most important, though related, mechanisms by which isoflavones such as genistein and daidzein influence carcinogenesis. Clarifying how these dose- and context-dependent effects arise at the molecular level is essential, so that these compounds can be used effectively in the treatment of patients beyond oncology. They may bring significant value in personalised treatment. Therapies whose efficacy has not yet been established, such as oral administration of soy isoflavones in the treatment of malignant neoplasms in humans, cannot ethically be considered as primary treatment modalities for severe diseases. Accordingly, the models described herein are limited to illustrating potential mechanisms that may support cancer therapy and warrant further investigation.

Nevertheless, the precise doses required to optimise the effectiveness of dietary soy isoflavone supplementation in humans remain unreliable. This is primarily attributable to differences in the metabolism of these compounds across diverse populations, along with the presence of ‘equol producers’ and ‘equol non-producers’. In individuals who are ‘equol non-producers’, the efficacy of phytoestrogens is significantly reduced due to not having an adequate intestinal flora. This phenomenon may be the reason why the research performed by Fan et al. revealed that a diet comprising of higher quantities of soy-derived products has been shown to be associated with a significant (approximately 10%) reduction in cancer risk, while it did not reach a statistically important impact on cancer mortality [4]. Moreover, the absence of standardised dosage methods for soy-derived products supplementation poses a significant challenge in conducting objective comparisons of study results from different research centres. Further research on soy isoflavones’ impact on human health is necessary and recommended, especially in clinical conditions, due to the promising results of already conducted studies, presented in this review.

In the context of an ageing population and an increasing annual prevalence of the diseases discussed in this article, nutritional interventions and the promotion of awareness regarding healthy lifestyles appear to be of particular significance in the prevention of the aforementioned diseases. Isoflavone-rich foods, including soybeans, can be regarded as a valuable nutraceutical foodstuff due to their high concentration of bioactive compounds. The term “nutraceutical” is generally understood to mean a purified product derived from a human food source that is intended to provide additional health benefits beyond the basic nutritional value found in food [144]. Isoflavone-rich foods, such as soybeans, can be regarded as a valuable nutraceutical foodstuff due to their abundant estrogenic compounds. However, it should be noted that, despite the growing body of scientific evidence in favour of the beneficial effects of soy isoflavones, further randomised studies are still required to assess the actual therapeutic potential of these substances on a large group of patients.

Furthermore, despite the existence of strong concerns postulating that the consumption of soy products rich in estrogenic compounds is associated with the feminization of males, meta-analyses and clinical data demonstrate that moderate soy consumption is not associated with a reduction in male sex hormone concentrations nor with a deterioration in semen quality [145, 146]. There are a few case studies in the literature which suggest that consuming large quantities of soy milk (an average of 300 mg of isoflavones per day, equivalent to 0.5 to 1.2 L of soy milk) over a period of over two years can lead to erectile dysfunction, decreased androgen levels and gynecomastia [147, 148]. Nevertheless, these reports concern cases of extremely high isoflavone consumption and are not clinically significant in relation to the aforementioned meta-analyses.