Lower PSA in obese men may be due to hemodilution

September 1, 2007

Data in study points to increased plasma volume as the key to the lower PSA concentrations generally observed in obese men.

"Higher body mass index is significantly associated with greater plasma volume and lower PSA levels. We believe that failing to account for hemodilution may result in missed prostate cancers in obese men," explained Lionel L. Bañez, MD, fellow in urology, Duke Prostate Center.

Dr. Bañez, working with Stephen Freedland, MD, and colleagues, identified 1,304 men who underwent radical prostatectomy at five medical centers that make up the Shared Equal Access Regional Cancer Hospital (SEARCH) database. All of the men were treated between 1991 and 2006. None had lymph node metastases.

The retrospective study divided patients into four categories based on their BMI: <25; between 25 and 29.9; between 30 and 34.9; and ≥35.

Clinical data recorded during treatment suggested an inverse relationship between increasing BMI and PSA levels. Mean adjusted serum PSA concentrations decreased in a nearly straight line from 7.45 ng/mL in men with BMI <25 to 6.02 ng/mL in men with BMI ≥35 (p<.001).

Calculated values for total plasma volume showed a corresponding straight line increase associated with higher BMI. Mean plasma volume ranged from 2.93 L in men with BMI <25 up to 3.61 L in men with BMI ≥35 (p=.005).

Researchers also found that the total mass of PSA was relatively uniform, regardless of BMI. Mean adjusted PSA mass-the serum PSA level multiplied by the calculated plasma volume-ranged from 21.89 mcg for men with BMI <25 to 21.52 mcg for men with BMI ≥35 (p= 0.52), Dr. Bañez reported.

Pathology data also showed that more obese men had larger prostates, based on the weight of prostate samples recorded in the SEARCH database. Both higher serum levels of estrogen and free insulin-like growth factor-1 contribute to increased prostate size, he added.

The next step in research will be prospective studies in a screened population of obese men without prostate cancer, Dr. Bañez said. If these results confirm hemodilution as a mechanism responsible for lowered PSA concentrations, clinicians may have to re-evaluate PSA findings based on BMI.