Results of the review, which examined 18 studies conducted over 26 years, are published today in the Journal of the National Cancer Institute.
"Because soy foods and soy supplements are widely used, we conducted this first true meta-analysis to understand what role soy foods might have on breast cancer risk," said scientists at Georgetown University Medical Center and Johns Hopkins School of Medicine.
"This objective examination of all of the studies tells us that currently, the data are not adequate to provide a clear answer to recommend soy foods to prevent breast cancer," they added.
The epidemiologic studies under review had been conducted between 1978 and 2004, and had revealed that women who eat soy products may have a slightly lower risk of developing breast cancer.
But the recent review has revealed that inconsistencies and limitations among the studies raise doubts over the potential benefits.
These limitations include a difficulty in measuring soy intake accurately, or the possibility that consuming soy may serve as a surrogate for other healthy behaviors.
Indeed, the new findings prompted the scientists to warn that high dose soy supplements, taken by women to reduce their risk of developing breast cancer, could do more harm than good.
Short-term studies of women taking such supplements showed changes in breast cell growth that might actually increase risk for breast cancer, while tests of refined soy products in animals revealed increased tumor growth.
However, on the up side, the scientists said that there is no risk when it comes to adding soy to the diet.
"The important aspect is eating actual soy-based foods like tofu, not highly purified isoflavone supplements. Highly refined components of soy can have very different biological effects than eating tofu or drinking soymilk," said Dr Bruce Trock, associate professor of urology, epidemiology and oncology at John Hopkins.
The scientists now say that definitive studies, which track participants over time and before they get cancer, may take decades before the real impact of soy is known.
But for now, even though evidence of the health link is still inconclusive, the scientists did find that that there was a "small association" between soy intake and a reduced risk of breast cancer.
"While the data were too inconsistent to recommend soy as a breast cancer preventive, there is no evidence to suggest that consumption of soy foods in amounts consistent with an Asian diet is detrimental to breast health," said the authors.
"Any overall health benefits offered by traditional soy foods in the diet, modest though they may be, likely outweigh any risks for the population as a whole," they added.