Hemodilution from increased circulating plasma volumes could explain why obese men with prostate cancer have lower serum PSA levels than non-obese men with the malignancy, according to researchers.

A team led by Stephen J. Freedland, MD, of the Duke Prostate Center and Duke University Medical Center in Durham, N.C., studied three cohorts of men (totaling 13,634 subjects) who underwent radical prostatectomy for prostate cancer.

One cohort consisted of 1,373 men treated at five Veterans Affairs Medical Centers that contributed data to the Shared Equal Access Regional Cancer Hospital (SEARCH) database. The other cohorts included 1,974 men treated at Duke Prostate Center and 10,287 men treated at Johns Hopkins Hospital in Baltimore.

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After controlling for various clinical and pathological characteristics, higher BMI was significantly associated with higher plasma volume and lower preoperative PSA concentrations, the investigators reported in the Journal of the American Medical Association (2007;298:2275-2280). The most obese men (BMI of 35 kg/m2 or greater) had 21% to 23% larger plasma volumes and 11% to 21% lower PSA levels than normal-weight men (BMI less than 25 kg/m2).

Moreover, obese men in all three cohorts had similar or higher PSA mass, which the total amount of circulating PSA at the time that PSA level was determined. “This suggests that hemodilution from larger plasma volume may be responsible for the lower PSA values observed among obese men with prostate cancer,” the authors wrote.