The researchers point out that their findings are preliminary and it is too early to say why soy has this effect. However they think that the likely culprits are the phytoestrogens that soy contains. We already know that overweight men have higher estrogen levels which is presumably why they are more affected by the phytoestrogens in soy.
Critics of this study make the point that it looked at men who were already visiting a fertility clinic rather than at men in the general population and that Asian men have been eating lots of soy for generations without any ill effects.
At The YinOva Center we suggest that men, who are trying to conceive, limit their soy to two servings a week. We also suggest switching to fermented soy products, such as miso, tempeh, natto or tamari, which seem to be more beneficial. This is because soy contains phytic acid which can block the absorption of some minerals. Phytic acid is neutralized during the process of fermentation. Consuming fermented soy is also good for digestion because it recolonizes the friendly bacteria in the large intestine.