To the Editor Dr Bosland and colleagues1 found no effect of a soy protein isolate on biochemical recurrence of prostate cancer. I believe there were problems with the choice of both the intervention and placebo products.
The soy isolate was not a true isolate because this would imply a concentrated protein source with minimal additive ingredients. However, the soy isolate also contained sugar (20 g per serving) and participants were encouraged to consume this product with water or fruit juice, which could impede any metabolic advantages of the intervention. Information should have been provided on whether the soy isolate reduced body weight, waist circumference, blood glucose, blood pressure, or cholesterol or testosterone compared with the placebo, as might be expected with a beneficial intervention. These cardiovascular and metabolic parameters have been linked either independently or synergistically with a lower risk and progression of prostate cancer.2
Moyad MA. Soy Protein and Recurrence of Prostate Cancer. JAMA. 2013;310(21):2311-2312. doi:10.1001/jama.2013.278601