So does this mean soy causes bladder cancer? Lets look at what else the study found. Soy intake was significantly associated with intake of vegetables, fruits, vitamins, A, C and E, fish, protein, fat, carotenoids, and energy. So you could say vegetables, and all the other things listed, are associated with bladder cancer in this study. Does this mean vegetables cause bladder cancer? Of course not. There are many studies showing the health benefits of vegetables. Lets look at what other studies have to say about soy.
Four epidemiological studies show an inverse association between soy intake and risk of breast cancer [1]. Soy intake is also associated with decreased prostate cancer risk [2].
In a study done on mice induced with bladder cancer (by nitrosamines), dietary soy reduced tumors [3]. Soy also inhibited growth of both humans bladder cancer cells outside the body, and implanted murine, or human bladder cancer cells in mice [4].
The bladder cancer/soy study goes on to suggest soy could possibly cause cancer as soy supplements raise IGF-1 levels. High IGF-1 levels have been associated with cancer. Indeed soy can raise IGF-1 levels, but no more so than animal proteins. Soy also raises IGFBP-1. This is probably a good thing [5]. For this reason it's wise to limit soy protein as you probably should with animal protein.
Just like in the study on memory decline and tofu in Indonesia where they found it was an additive that causes the association [6], it's possible some of the soy foods eaten in Singapore have cancer promoting substances added to them. The data in the study supports this possibility as the association was weaker with soy protein and soy isoflavone intake.
As of yet, there is no strong evidence that soy contributes to bladder cancer.
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