According to the results of a study published in the Journal of the American Medical Association, levels of prostate-specific antigen (PSA) tend to be lower in obese men than in healthy-weight men because PSA is diluted in the larger blood volume of obese men. 

The prostate is a gland of the male reproductive system. It produces some of the fluid that transports sperm during ejaculation. An estimated one in six men in the U.S. will be diagnosed with prostate cancer over the course of his lifetime. Prostate cancer occurs more frequently in older men, in African-American men, and in men with a family history of prostate cancer.

Men 50 years of age or older in the U.S. are often offered prostate-specific antigen (PSA) testing for the early detection of prostate cancer. The PSA test measures proteins that are produced and shed by the prostate. PSA levels tend to be elevated when prostate cancer is present, but levels can also be elevated in benign (non-cancerous) conditions affecting the prostate.

Some studies have suggested that PSA level may be influenced by body weight, with lower PSA values observed in obese men. If this proves to be the case, it may be important to account for body weight when interpreting the results of a PSA test. The level of PSA that raises a suspicion of prostate cancer, for example, may need to be shifted downward for obese men.

To explore the relationships among body size, blood volume, and PSA level, researchers conducted a study among more than 13,000 men who underwent radical prostatectomy for prostate cancer.

Weight was assessed using the body mass index (BMI). BMI involves a comparison of weight to height (weight in kilograms divided by height in meters squared). A BMI between 18.5 and 24.9 is generally considered healthy, a BMI between 25 and 29.9 is considered overweight, and a BMI of 30 or higher is considered obese.

The study assessed PSA concentration (measured as nanograms of PSA per milliliter of blood), as well as the total amount of PSA in circulation (measured as micrograms of PSA).

Men with higher BMI tended to have greater blood volume and lower preoperative PSA concentration. The total amount of PSA in circulation, however, did not vary significantly by body weight.

These results support the idea that PSA is diluted by the greater blood volume of obese men, resulting in a lower PSA concentration in spite of a similar total amount of PSA in circulation.

The researchers note that these results will need to be confirmed in prospective studies of men without prostate cancer.

Reference: Bañez LL, Hamilton RJ, Partin AW et al. Obesity-related plasma hemodilution and PSA concentration among men with prostate cancer. JAMA. 2007;298:2275-2280.

Related News:  Excess Weight Increases Prostate Cancer Mortality (11/14/2007)

Copyright Prostate Cancer Information Center on CancerConsultants.com