Phytoestrogens found in foods are very weak relative to estradiol (as much as 1,000) and can compete with estradiol for receptors, hence blocking estradiol from exerting the effects we so desire for.
Grapefruit can indeed affect estrogen, especially when taken orally, as it inhibits enzymes that metabolize estradiol. As a result, estradiol and estrone concentrations will increase. Before deciding to consume grapefruit, check with your doctor as this can also interact with other medications and be dangerous. Vitamin C, I had also read, can increase estrogen concentrations, but I'm not aware of how it does that. I think this was noted with birth control pills.