Although there are no direct research about The effectiveness of Soy in easing menstrual Symptoms, since soy contains high levels of rich in alpha linolenic acid, an omega-3 fatty acid,I would like to bring you this study.
According to "Menstrual pain in Danish women correlated with low n-3 polyunsaturated fatty acid intake." by Deutch B. (Source:Aarhus University, Denmark.), researcher found that On the contrary certain dietary habits, e.g. low intake of animal and fish products, and intakes of specific nutrients, were correlated with menstrual pain. The average dietary n-3/n-6 ratio of women with menstrual pain was 0.24. It was significantly higher among those with low intake of B12 coincident with low intake of n-3 (0.42, P < 0.001) (chi-square), with low n-3 intake coincident with low n-3/n-6 ratio (0.42, P < 0.005), and finally with low intake of B12 coincident with low n-3/n-6 ratio (0.47, P < 0.001), and concluded that the results were highly significant and mutually consistent and supported the hypothesis that a higher intake of marine n-3 fatty acids correlates with milder menstrual symptoms.