~ Pilot Study Finds Soy Lowers PSA without Reducing Testosterone

A report published online in the European Journal of Clinical Nutrition on June 14, 2006, revealed that adding daily soy to the diet lowers prostate specific antigen (PSA: a marker of prostate cancer), without affecting testosterone, the male hormone which is believed to directly or indirectly contribute to prostate cancer growth.

The high soy intake of men residing in Asia has been hypothesized to be protective against prostate cancer due to the lower occurrence of the disease observed in this population. For the current study, researchers at the University of Hawaii in Honolulu sought to determine the effect of soy on PSA in pilot study of 24 men between the ages of 44 and 69. The subjects were randomized to a diet providing two servings of soy per day selected from five food choices, or their usual diet for three months, followed by a one-month wash out period. Participants then crossed over to the diet they did not previously follow, for another three month period. Blood samples obtained at the beginning and end of each study period were analyzed for serum PSA and testosterone. Soy isoflavone levels were measured in overnight urine specimens collected at five points during the trial, and their excretion rates determined.

Compliance with the study regimen was high, demonstrating the feasibility of incorporating soy into the diet as a protective measure. Mean serum PSA levels were 0.28 nanograms per milliliter lower at the end of the high soy diet compared to levels measured before starting the diet, while men in the low soy group experienced a rise of 0.12 ng/mL. When the groups were compared, there was a 14 percent reduction in serum PSA in men who received the high soy diet, but no difference in testosterone between the two groups.

Although soy has been shown to have estrogenic effects, the finding of the current study suggests the possibility that soy’s protective benefit against prostate cancer is via a nonhormonal mechanism. The authors recommend that “future investigations should be expanded in duration and consider evaluation of additional sex hormone and PSA measurements to explore both hormonal and nonhormonal mechanisms of regular soy intake against prostate cancer.”

Health Concern: Prostate Cancer Overview

Currently, most physicians who focus on PC as their main specialty will recommend routine prostate-specific antigen (PSA) testing starting at age 40. This is important to establish objective findings that indicate a healthy prostate.

Measures to prevent PC must be a routine part of the counsel that general practitioners and internists give their patients. Selenium intake of at least 200 mcg a day should be a consideration in the prevention of PC. Low plasma selenium is associated with a four- to fivefold increased risk of PC. In addition, levels of plasma selenium also decrease with age, resulting in middle-aged to older men being at a higher risk for low selenium levels. Ideally, baseline levels of selenium should be obtained before beginning routine selenium supplementation. It would make sense to begin such a micronutrient and mineral assessment at age 25 and perhaps every 10 years thereafter.

A large-scale study of almost 11,000 men in Maryland showed that the protective effects of high selenium levels, and similarly that of the alpha-tocopherol isomer of vitamin E, were only observed when the concentrations of the gamma tocopherol isomer of vitamin E were also high. In this study, the risk of PC declined with increasing concentrations of alpha-tocopherol, with the highest concentration associated with a 68% PC risk reduction. For gamma-tocopherol, men with levels in the highest fifth of the distribution had a fivefold greater reduction in the risk of developing PC than men in the lowest fifth (p = .002). The observed interaction between alpha-tocopherol, gamma-tocopherol, and selenium suggested that combined alpha- and gamma-tocopherol supplements, used in conjunction with selenium, should be considered in future PC prevention trials.


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